Taking the 2015 Astrobiology Strategy (NASA 2015) as its starting point and building on that foundation, Chapters 2, 3, 4, and 5 of this report emphasized additional insights from recent advances in the search for signs of life—intellectual (e.g., conceptual insights and frameworks, modeling), empirical (e.g., key discoveries and observations, technology development, and novel technologies), and programmatic. Building on those updates, in this chapter, the committee identifies the most promising key research goals and questions in the field in which progress is likely in the next 20 years, and discusses pathways by which of the key goals can be addressed by U.S. and international space missions and ground telescopes in operation or in development.
The stage for the emergence of life was set long before the rise of prebiotic chemistry. Condensation of the solar nebula, disc formation, stellar activity, planetary accretion and differentiation, and the composition and impact frequency of asteroids and comets all determine the conditions within which life might emerge and survive. All of these factors, and more, are pivotal to ensuring the presence of the necessary environmental and geological conditions and elements that give rise to prebiotic molecules and then biotic chemistry on a planet or a planetary body. Looking forward, emergence-of-life research will remain focused in the solar system and will require a broadening of perspective that integrates answers to the following questions:
- Habitability of the early Earth—What processes and parameters were critical to Earth’s habitability as the Sun and the young solar system coevolved? How can this knowledge inform investigations for the habitability of other bodies, including exoplanets?
- Carbon and volatile inventory of the prebiotic Earth—What characteristics of the carbon and volatile inventories of prebiotic Earth, and of the solar system architecture that delivered them, are relevant to the emergence of life?
- Conditions on the prebiotic Earth—Can the many, co-varying parameters giving rise to emergence-of-life conditions in prebiotic Earth environments be better constrained?
Habitability of the Early Earth
What processes and parameters were critical to Earth’s habitability as the Sun and the young solar system coevolved? How can this knowledge inform investigations for the habitability of other bodies, including exoplanets? Many phenomena related to the early evolution of the solar system were critical to the habitability of the terrestrial planets in general, such as Venus and Mars, as well as Earth and probably a number of the satellites around the gas giants, including Europa, Callisto, Enceladus, Titan, and possibly others. Thus, understanding the creation of a habitable environment for Earth in terms of the emergence of life is relevant for the other bodies in the solar system and beyond as well. These include physical as well as chemical parameters. Starting at the beginning of planetary accretion from clumping of the dust/particle/gas nebula, distance from the Sun conditioned the formation of rocky planets closer to the Sun, within just over 1.5 AU, and gas/ice planets further out, and beyond Jupiter at 5.2 AU, with the main asteroid belt objects orbiting between Mars and Jupiter.
Although intensely studied, the physical and mechanical processes leading to the initial creation of rocky objects are not clearly defined. For example, it is unknown if accretion was localized or involved the whole of the protosolar disc, how the temperature gradient within the disc affected chemical differentiation, and to what extent the formation and timing of formation of Jupiter and Saturn affected the composition of the reservoirs of material for accretion in the inner and outer solar system (e.g., Kruijer et al. 2017). Modeling of planetary formation by the group of Morbidelli et al. (2016), for example, suggests that the formation of Jupiter effectively created a “fossilized snow line” Sun-ward of which the accreted bodies were effectively volatile poor. This raises the question of how and when volatiles were transported into the inner solar system.
Carbon and Volatile Inventory of the Prebiotic Earth
What characteristics of the carbon and volatile inventories of prebiotic Earth, and of the solar system architecture that delivered them, are relevant to the emergence of life? Small bodies, such as asteroids, planetesimals, and comets, are likely to have delivered organics and volatiles to Earth that were necessary for the emergence of life. Models suggest that the volatile-rich carbonaceous chondrites could have been scattered into the inner regions very early during the accretion process, with water accreted on the early Earth within ~100 Ma (Raymond and Izidoro 2017). Questions of interest include the following: What is the relationship between comets and the primitive asteroids in the outer solar system that gave rise to carbonaceous chondrites? Do they represent end members of a continuum?
Debate continues about the timing and effects of instabilities in the orbits of the giant planets, which affected cometary bombardment and importation of volatiles including water and organics to the inner planets. Evidence of this cometary bombardment comes from xenon isotope study of 67P/Churyumov-Gerasimenko performed by the instruments on the European Space Agency’s (ESA’s) Rosetta spacecraft, which shows that about 22 percent of atmospheric xenon comes from comets but that this accretion was a late veneer, after planetary differentiation (Marty et al. 2017). Regarding the contribution from later cometary and carbonaceous chondritic components in terms of timing, composition, and quantity, present estimates suggest that only ~2 percent of the late veneer is cometary in origin.
Are there relics of prebiotic chemistry remaining on Earth, Mars, or elsewhere that might be informative? Where might these be and how might we detect, analyze, and correctly interpret them? Studies of volatiles on satellite moons, such as Titan, and on comets and asteroids provide valuable details for understanding the heritage, formation, and possible delivery of organics to settings conducive to the emergence of life. Studies incorporating data from the atmosphere of Titan show that, even in a more neutral atmosphere, organics, and possibly even an organic haze, could have formed in the atmosphere of early Earth (Trainer 2013). These investigations can be extended to include continued study of organics in available extraterrestrial material on Earth (micrometeorites, carbonaceous chondrites, and returned samples), as well as investigation of the organic inventory on a planet, such as Mars. Although the oxidizing and irradiated conditions existing at the surface of Mars destroy the organics, especially the more volatile components, the exogenous component could be preserved at depths below 1.5 to 2 m (Kminek and Bada 2006) targeted by the European-Russian ExoMars 2020 rover (Vago et al. 2017).
Conditions on the Prebiotic Earth
Can the many, co-varying parameters giving rise to emergence-of-life conditions in prebiotic Earth environments be better constrained? There is increasing focus on the role that specific early Earth environmental conditions played in the development of prebiotic chemistry. The fundamental chemistry of life is based on oxidation/reduction reactions—that is, the chemistry of electron transfer. The oxidation/reduction reactions that drive prebiotic chemistry rely either on chemical and thermal disequilibria generated as Earth cools or by solar ultraviolet radiation. In either case, a primary product is often hydrogen. This process can be supported by the thermochemical alteration of olivine to serpentine (serpentinization), by radiolysis of water, or by populating the antibonding orbitals of transition metals in aqueous phase by photons. The hydrogen can be used to form reduced carbon and, potentially, to reduce dinitrogen gas to ammonium. Reactions with transition metal containing minerals (e.g., iron sulfur clusters) can potentially lead to prebiotic catalytic reactions.
Attempts are being made to arrive at a more realistic understanding of the variety of local environmental conditions that could have affected the formation of critical prebioic molecules and, eventually, the emergence of life. Westall et al. (2018) reviewed current understanding of the geological/geochemical environment of early Earth, together with various environmental scenarios that have been proposed for the emergence of life. Each of these scenarios presents certain advantages and disadvantages, but at the present time, there are insufficient data to conclude that any one scenario stands apart as being most conducive to life’s origins. There is a lack of scientific understanding regarding how life originates on a suitable planet, other than the recognition that it involves the interplay of environmental fluxes, both energy and raw materials, to drive plausible chemical pathways.
Nonetheless, substantial advances have been made in recent years with regard to potential scenarios for the origin of life on Earth, fostered by consideration of plausible early Earth conditions (Dass et al. 2016). For example, the potential importance of hydrothermal systems for the emergence of life has long been understood (e.g., Baross and Hoffman 1985). But recent experiments that sought to re-create these environments in the laboratory have shown how hydrothermal systems can support the synthesis of prebiotically relevant compounds such as methane and amino acids (Suzuki et al. 2015; Kobayashi et al. 2017). Other experiments that adopt a systems chemistry approach have shown how selective crystallization and other geochemical fractionation processes can sequester key intermediates for the prebiotic synthesis of biomolecules (Patel et al. 2015).
In future, the experimental approach to prebiotic chemistry will have an increased focus on integrating the multiparameter space of early Earth environments and their covariance (Dass et al. 2016; Westall et al. 2018). This opens the possibility of exploring the environmental mechanisms that foster accumulation, differentiation, and preferential selection of what was likely to be a highly diverse mix of prebiotic precursors. Furthermore, multiparameter focus will help elucidate the environmental and chemical sequences that led to the emergence and selection of more complex and refined prebiotic molecules. On longer timescales, better understanding of the intimate ties between planetary processes and life’s evolution can be applied to better understand the coevolution of the planet and its biosphere, the expansion of life’s limits through adaptation to changing planetary environments, and the evolution of biological complexity. An understanding of these coevolutionary processes can then be applied to the search for life outside the solar system taking into account increased knowledge of dynamic and evolving habitability and the conditions and mechanisms that might lead to atmospheric biosignatures, such as an inventory of thermodynamically unlikely gas mixtures such as seen on Earth today.
Pathways to Understanding the Evolution of the Early Solar System and Prebiotic Earth
Understanding the prebiotic inventory of exogenous materials, and how they were and are distributed through the solar system, is and will continue to be challenging. To make progress in this line of research requires study of the original material in situ on the small body or in returned samples. Sample analysis will need to be paired with investigations of the effects of the space environment, such as radiation, changes in temperature, and effects of a vacuum, on analog and recovered materials. For returned samples, studies pertaining to sample alteration due to atmospheric entry will also be important. In the past, questions surrounding small body composition and contribution to the organic and volatile inventory of the solar system have been addressed by in situ and sample
return missions to comets (e.g., Rosetta and Stardust) and asteroids (e.g., Hayabusa 1). Looking into the next 20 years, this will continue to be a robust area of research with great potential for discovery.
In the near term, the Japan Aerospace Exploration Agency’s (JAXA’s) Hayabusa 2 mission will return samples that may be organic rich or contain hydrated materials from the asteroid Ryugu. Shortly thereafter, in 2023, the National Aeronautics and Space Administration’s (NASA’s) Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx), will return to Earth samples from the asteroid Bennu that are believed to be rich in informative volatiles (Lauretta 2017). In the longer term, a comet nucleus sample-return mission was recently selected by NASA for further study. Its proposed goal is to return to comet 67P/Churyumov-Gerasimenko, which was originally visited by Rosetta. One of the mission goals is to capture a sample of organics and volatiles and return it to Earth for analysis (Squyres 2018).
Both for the emergence of life on Earth, and especially when considering the possibility of life beyond Earth, it is essential to consider a range of geophysical and geochemical scenarios. Laboratory studies of the various scenarios will continue to be important, especially when backed by an understanding of the geological record and modeling efforts that help to constrain parameters such as temperature, pressure, ultraviolet flux, and the concentration of key starting materials. It also is useful to consider what areas of parameter space are out of bounds, either because they are implausible for the planet being studied or because they defy the principles of chemical reactivity. Progress in this area will depend on the collaborative interaction among scientists from multiple disciplines and an integrative view of how to explore putative early Earth scenarios via experiments, field work, and modeling with an open mind regarding possible pathways to life. Early Earth investigations in fact provide important “test-bed” opportunities in which community developed initiatives such as a Comprehensive Framework for Life Detection can be most tractably tested and refined. Such “missions” to Early Earth can catalyze vital conceptual progress and feed forward into mission planning and implementation in the search for life elsewhere in the universe.
Planetary habitability depends as much on planetary evolution and solar system dynamics as it does on genomic mutations and ecological successions. Planetary conditions enabled life. When life emerged, planets and putative planetary biospheres evolve together through time such that the initial conditions from which life arose are not necessarily the same as those that subsequently gave rise to local-scale or planet-wide habitability. This coevolution can be direct and causative, indirect, fundamentally disconnected, or stochastic. While Earth is presently our singular example of planetary and biosphere coevolution, nearby terrestrial planets have their own geologic history. Together, the geologic records of the rocky planets in this solar system can be applied to better understand how their planetary dynamics and potential biospheres did or did not coevolve. These principles apply to habitable exoplanets as well, although we are far from having data to apply them. Comparative planetology can extend these lessons beyond the inner solar system. Informed by recent trends in assessing planetary habitability, the following questions are likely to guide this field of research for the next 20 years.
- Predictable elements of planetary evolution—What elements of planetary evolution are predictable and independent of biosphere evolution?
- Feedback between biosphere and geosphere—What feedbacks exist between the biosphere and geosphere, including during long periods of quiescence?
- Periods of catastrophic change—How do periods of catastrophic change reflect the balance of influence between planetary dynamics and the biosphere?
Below, we provide a description of the key research goals and enabling missions and technologies needed to answer these questions.
Predictable Elements of Planetary Evolution
What elements of planetary evolution are predictable and independent of biosphere evolution? Evaluating those aspects of Earth’s evolution driven exclusively by planetary dynamics is difficult because of accompanying
changes in the global biosphere, its impacts on surface and subsurface environments, and incomplete understanding of feedbacks between biosphere and planetary processes. As such, Earth represents one planetary trajectory—one with a sustained and active biosphere. Mars and Venus serve as examples of other planetary trajectories—ones expressing interactions between planetary surface processes, interiors, and atmospheres potentially without extant life or a known, sustained biosphere. These trajectories are pivotal to identifying which aspects of the planetary conditions are predictable and occur independently of biosphere evolution. By extension, they can be used to identify which planetary dynamics are affected by a global biosphere, and how such feedbacks are recorded. Taken together, understanding these interactions will enable the development of models that span the possibilities of planetary trajectories. New research suggests Venus may have been habitable early (e.g., Way et al. 2016) with oceans that may have extended into the past billion years, suggesting a rapid shift in conditions. Therefore, Mars and Venus are ideal candidates to develop models of the evolution of rocky planets under different forcing. Together, Earth, Mars, and Venus can be used to develop trajectories in planetary evolution and punctuated or sustained habitability and to understand how the endogenic and exogenic factors that change the planet locally, regionally, and globally combine to drive shifts in the environment.
Feedback between Biosphere and Geosphere
What feedbacks exist between the biosphere and geosphere, including during long periods of quiescence? Earth’s own history provides the only current opportunity to identify the feedbacks between planetary dynamics and biosphere evolution over global and geologic time scales. The Hadean Earth was starkly different from today’s familiar surface. Since then, evolution in the biosphere has clearly driven major changes to the geosphere, most notably the Great Oxidation Event (GOE) at ~2.45 Ga. However, long periods of time appear to be relatively quiescent—for example, the so-called (but actually not-so) Boring Billion years (1.8 to 0.8 Ga ago) during which it has been suggested the oceans may have been largely sulfidic (Poulton et al. 2004). Although pivotal changes were occurring during these periods (e.g., Mukherjee et al. 2018) that set the stage for periods of subsequent rapid, catastrophic change, the feedbacks between discrete evolutionary events in the geosphere and biosphere are poorly understood. Whether this is inherent to the feedbacks themselves, for instance if they became decoupled, buffered, or stochastic, if it is an artifact of the incomplete record, preservation bias, or if the feedbacks during these periods simply occurred below the detection threshold of modern methods, is not known.
Moving forward, understanding these long stretches of stability will become as important as understanding the catastrophic events. When considering other planetary surfaces, Earth’s own record shows that quiescence or stability is, geologically, the more prevalent state. As such, quiescent states are also likely important on other planets and could be well preserved and revealed through exploration. Further, periods of quiescence hold the key to understanding the influence of stable organisms on the planet’s geosphere. For example, many organisms would have had to find refuge in the oxygenated photic zone if the Proterozoic oceans were indeed sulfidic. Studying such periods will not only inform whether or not a continuous biosphere maintains a planet’s habitability, but also will help identify potential biosignatures by which such organisms might be recognized.
Periods of Catastrophic Change
How do periods of catastrophic change reflect the balance of influence between planetary dynamics and the biosphere? Although periods of quiescence may hold the key to systematically understanding the feedbacks between biosphere and planetary dynamics, geochemical and geological evidence simultaneously suggest that life’s innovations and catastrophic environmental change may be coupled. This has been shown for punctuated events and extended phases of planetary evolution that involved rapid change as has been suggested, for example, during Snowball Earths. Snowball (or slushball) Earths it is argued were precipitated by continental configuration and changes to planetary insolation, which together accelerated ice accumulation over the continents and plunged Earth into icehouse conditions. Despite this drastic change in global environmental conditions driven by planetary dynamics, life persisted. Not only did life persist, it is thought to have been pivotal in the recovery of Earth’s climate. A rapid, second rise in atmospheric oxygen followed the Neoproterozoic icehouse periods and appears to be associated with the radiation and expansion of eukaryotic algae (Brocks et al. 2017). The close association
of these events suggests that changes in the biosphere (most likely precipitated by icehouse conditions) in turn altered Earth’s environment, to what extent is presently unclear. It has also been suggested that such events may have played a role in triggering the Cambrian radiation, for instance (Canfield et al. 2007). These examples demonstrate that the interplay of planetary conditions and the biosphere can be critical to planetary and life’s evolution. The relative influence of planetary dynamics versus the biosphere also changes with time. In contrast to periods of quiescence, which may represent a balanced influence of planetary evolution and life’s influences, periods of catastrophic change may represent an imbalance in influence. Further research on both of these modes will significantly advance understanding of the coevolution of life and its environment on both this planet and on other planetary bodies including exoplanets.
Pathways to Exploring Dynamic Habitability and Comparative Planetary Trajectories
Incomplete records, preservation biases, and detection limits as discussed in Chapter 4 all pose challenges to exploration of Earth’s deep past and the feedbacks between life and environment expressed in its record. Field-based research, experimentation, as well as models, provide an important path forward. Systems models are growing increasingly sophisticated. Models that capture interactions between the ocean, atmosphere, and weathering cycles or metabolic and ecological parameterizations have the potential to evolve into models capable of tracking both geological and biological conditions that might enhance or detract from habitability.
Understanding the coevolution of life and Earth’s environment over time is important for understanding planetary trajectories and, by extension, predictive models of potentially habitable planets in the solar system, such as ocean worlds. During its early stages, Earth itself was an alien planet by current standards, and might not have been recognized as habitable despite being widely inhabited. Thus, Earth provides a foil to planets in the solar system currently thought to be uninhabited, notably Venus and Mars. In turn, these planets can be used to aide in the identification of global-scale feedbacks between life and its environment here on Earth by providing abiotic baselines. This will require modeling the trajectories of Mars and Venus, both of which followed similar early planetary conditions but resulted in drastically different outcomes.
On Mars, the InSight mission, as well as the ExoMars rover to be launched in 2020, have instrumentation for making gravity, radar, and seismic measurements. Both will help to reveal the planet’s internal structure, feeding into models of planetary evolution and dynamics. Without a series of planet-wide geophysical platforms however, full understanding of the martian interior structure could remain elusive.
On Venus, a path forward is less clear. The Russian Venera-D mission, proposed to launch in 2026, may be a precursor to a landed mission, although this remains undecided. Many Venus missions, focusing on science from the atmosphere to the surface, have been proposed through the New Frontiers and Discovery programs, but have as yet to be selected.
In the next 20 years, this line of research may provide the ability to predict how a different early evolution of life may have led to a modern Earth surface drastically different from today’s. Or, conversely, how a different early trajectory in Earth’s dynamics could have changed the evolution of life today. Such a model would draw together understanding of fundamental, planetary geodynamics learned from comparing rocky planets that may or may not be inhabited, juxtaposed with understanding the feedbacks between the biosphere and planetary dynamics as evidenced in the rock and paleogenomic records on Earth. Those fundamental advancements would then be able to inform understanding of other models of life—even, potentially, non-terran life—and biosignature science essential to the search for life on ocean worlds and exoplanets.
Earth may represent only one end-member of a life-hosting planet, even among bodies with similar initial conditions and geophysical processes. The deep subsurface on Earth, Mars, terrestrial planets in other systems, and the ocean worlds all have a diversity of environmental conditions that share some degree of similarity, and could be habitable in similar ways. Examples include subterranean water reservoirs on Mars and other planets, and modern day oceans in contact with rock on the ocean worlds of the solar system.
Can subsurface life exist in the absence of surface life? That is, can we distinguish whether the surface biosphere is an outgrowth of the subsurface biosphere, or is the colonization of the subsurface facilitated by surface phototrophy? Growing sophistication in our understanding of life and its trajectory on this planet could reveal much about how life could persist on exotic worlds. The exploration of these worlds gives us the chance to search for a second genesis of life and even to study an alternative biochemistry if enabled by advances in astrobiology.
The committee has identified four key lines of research that need to be addressed over the next 20 years:
- Adaptation to extreme environments—How does life adapt when subject to its environmental and energetic limits?
- Chemosynthetic and rock-hosted biospheres—How can marine and continental subsurface terrestrial analogs help define what a chemosynthetic or rock-hosted biosphere might look like on another rocky planet, or on an ocean world?
- Habitable environments in the martian subsurface—What is the spatial and temporal distribution of subsurface water, the sources and sinks for methane and other reduced gases such as hydrogen, and the relevant water-rock reactions capable of sustaining habitable environments in the subsurface on Mars?
- Habitability of ocean worlds—What are the chemical inventories and sources of energy that could generate habitability on ocean worlds, and what processes sustain these inventories?
Adaptation to Extreme Environments
How does life adapt when subject to its environmental and energetic limits? Our understanding of the limits of life is continuously updated as we document and discover microbial communities thriving in nominally “extreme” environments, where perceived extremes of temperature, pressure, pH, salinity, energy, etc. are actually preferred conditions for the organisms that thrive in them. From slow life living in energy-starved environments, to those that thrive in extremes of temperature, radiation, and pressure, life on this planet has become well adapted to its conditions. Understanding the genetic tracers of adaptation could, simultaneously with the rock record, inform how life has adapted, and help retrace the history of life on Earth. For low temperature organisms, for instance, at the coldest temperatures recorded for survivability and activity, many organisms also have extraordinary radiation tolerance. Does this tolerance derive from the ability to repair cellular and DNA damage, or is it a relict of prior environmental stress (e.g., survival in desiccating environments)? While significant progress has been made in understanding the adaptations of organisms to one parameter (e.g., temperature, low water activity), there is an urgent requirement to understand life’s response and ability to adapt to multiple parameters (combined effects of extreme temperature and pressure for instance).
How long can dormant or slow-growing cells remain viable? Reports that cells from 250 Ma salt deposits had been resuscitated (Vreeland et al. 2000) are now largely attributed to later contamination (Graur and Pupko 2001). Studies of 600-ka-old permafrost showed that the DNA in the cells contained in it could still be replicated, but not in older permafrost (Johnson et al. 2007). These studies suggest that low-level gene repair in a very slowly metabolizing cell is better for cell viability than dormancy, but these are some of many issues that remain unresolved and controversial.
How does life adapt to environments in which habitable conditions are heterogeneous in space and time? Some of the most notable examples of slow-growing microbial systems are those hosted in Earth’s continental or ocean crust, or in deep-sea sediments. Marine sediments have been shown to host viable cells down to depths of nearly 2 km, but metabolic turnover rates may be as slow as one cell division every thousand years (Kallmeyer et al. 2012; Ciobanu et al. 2014). How do organisms adapt to such slow time scales and what are the approaches to interrogate life occurring on a scale so fundamentally different from the scale of human investigations (Trembath-Reichert et al. 2017)? In fractured continental crust, water infiltrating deep fractures has entrained microbes over long periods of time to depths of between 3 and 4 kilometers (Onstott 2016). Due to water–rock reactions such as serpentinization and radiolysis, individual fractures, sometimes as small as 1 cm, may be a relative oasis in what is an overall oligotrophic environment. The discovery of preservation of habitable fluids on billion-year time scales in Earth’s deep continental crust (Holland et al. 2013; Warr et al. 2018) have defined new frontiers in
research on habitability and microbiology that can help address the question of long-term survivability of both extant ecosystems and preservation (or lack thereof) of biosignatures of past subsurface life.
Chemosynthetic and Rock-Hosted Biospheres
How can marine and continental subsurface terrestrial analogs help define what a chemosynthetic or rock-hosted biosphere might look like on another rocky planet, or on an ocean world? Given the prevalence of life on Earth that exists without direct influence of the Sun, alongside the harsh surface conditions encountered on the most compelling bodies in the solar system, subsurface environments in general may prove to be more habitable than planetary surfaces. The realization that habitable zones in the subsurface can host complex ecosystems that exist independently of surface energy sources has encouraged the search for fossil biosignatures in a wide range of subsurface geological settings on Earth. These places host a subsurface paleontology that includes a variety of biomediated microfabrics and textures (see Hofmann and Farmer 2000, 2008). In association with distinctive suites of morphological biosignatures, biogeochemical indicators of past life (see Chapter 4 and references therein), and concentrations of bioessential transition metals (that are required to sustain enzyme functions) may be investigated. Exploring such suites of bioindicators is arguably the most productive approach for subsurface exploration, and is most useful when conducted in tandem with strategies to determine how to efficiently translate this information to other planets and moons (for example via spectral indicators of concentrated materials, or targeted flyby or in situ missions).
Importantly, given the different evolutionary pathways that such planets may have experienced, different types of ecosystems might be expected. Over the next 20 years, growing sophistication in understanding exotic conditions on Earth could provide key information for elucidating how a chemosynthetic or rock-hosted biosphere might operate on another rocky planet or ocean world. Exploration of the deep sea floor by drilling projects and in situ vehicles, and parallel investigation of the rock record through deep time will expand our picture of the evolution of subsurface habitability. Although early anaerobic Earth is likely the largest terrestrial analog environment in which rock-hosted chemotrophic life once flourished, analog studies of modern terrestrial systems still provide the most readily actionable activities to address questions about the processes governing subsurface habitability and the nature, diversity, and preservation of both extant and extinct subsurface communities.
Given the recent focus on the habitability of ocean worlds, research will need to continue on energy-rich environments, such as hydrothermal vents and marine sediments. An increasing focus on terrestrial subsurface communities, including oligotrophic, rock-hosted, continental subsurface communities, however, has even greater potential for novel discoveries at present because in many cases so much less is known. Similarly subducted minerals possibly containing direct and indirect evidence of biological processes can be targeted (Hazen et al. 2008). Lessons learned from these environments facilitate process-based thinking about life that can be directly relevant to mission planning. Astrobiology is not just about the search for extraterrestrial life, but also the broader scientific understanding of how habitable planets form, what makes them habitable, and the processes that sustain life.
Habitable Environments in the Martian Subsurface
What is the spatial and temporal distribution of subsurface water, the sources and sinks for methane and other reduced gases such as hydrogen, and the relevant water-rock reactions capable of sustaining habitable environments in the subsurface on Mars? Continued advances in understanding extreme life on Earth coupled with discoveries from the martian surface and near subsurface environments have driven a revolution in thinking about Mars’s habitability. Not only the spatial and temporal distribution of surface and subsurface hydrology, but also the characteristics of that water through time have challenged preconceptions about surface and subsurface habitats, including the potential for ephemeral niches and isolated refugia, and related topics such as the production of reduced gases such as methane (Ehlmann and Edwards 2014; Ehlmann et al. 2015, Grotzinger et al. 2015; Goudge et al. 2016; Ehlmann et al. 2016; Webster et al. 2018).
The high radiation flux on the surface of Mars may be the driver for life to seek refuge underground. Discoveries of the persistence of life in Earth’s subsurface, of the ability of life to adapt to extremely slow growth
rates under anaerobic and oligotrophic conditions (Ciobanu et al. 2014; Trembath-Reichert et al. 2017), and of preservation of habitable fluids on billion-year time scales have further defined new frontiers in research on martian habitability and microbiology. These new frontiers, and the diversity of discoveries that support them (see Chapter 4) have yet to be fully leveraged by the Mars exploration program. However, technological advances and the future missions they enable will begin to address these new frontiers. Several instruments on NASA’s Mars 2020 are designed to address the past habitability of Mars. In particular, the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals and X-ray fluorescence spectrometer. Samples cached by Mars 2020 will be returned to Earth for analysis on a future, but currently unscheduled, mission.
Despite the advances that continue to be made in understanding terrestrial subsurface communities, accessing the subsurface systems on other planets to advance the goals of astrobiology remains challenging. Technological advances to address this challenge, however, are on the horizon. ESA’s ExoMars 2020 rover will take the first steps toward that goal with its objectives to search for past and extant life. That search will include the geological and environmental context of the surface, and sampling down to a depth of 2 m (Vago et al. 2017; Vargo 2018). While the radiation damage from solar ultraviolet and energetic particles on Mars is literally “skin deep,” the secondary effects from galactic cosmic rays can destroy organics down to ~1.5 to 2 m (Kminek and Bada 2006; Pavlov et al. 2012). This will be the first instance of drilling to any substantial depth into the surface materials of another planet. Furthermore, it will have scientific value of collecting samples below, or at the lowest depth, the penetration of ionizing radiation while also reaching below the depth of surface oxidation. It is hoped that samples from below 2 m will contain preserved organics that may, potentially, provide signatures of life. This material will be analyzed by the Mars Organics Molecule Analyzer, which includes a laser desorption and a gas chromatograph-mass spectrometry mode (Goesmann et al. 2017).
While considerable community activity is focused on developing drilling technologies (Antilla 2005; Stamenkovic et al. 2018), it is important to recognize that there are a wide variety of strategies for accessing subsurface samples and investigation of subsurface processes. Indeed drilling is not the only approach for studying the subsurface. Geophysical instrumentation provides a broad array of noninvasive strategies to advance understanding of subsurface processes. For example, the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission is intended to provide data relevant to the martian subsurface. InSight’s heat probe is intended to penetrate the subsurface to several meters and together with its seismometer will provide information on planetary seismic activity (“marsquakes”), and on heat (and possibly volatile) transport directly relevant to astrobiology (Banerdt et al. 2013).
Similarly, the ExoMars 2020 rover will carry the Water Ice and Subsurface Deposit Observation on Mars ground-penetrating radar (GPR). The radar will sound the subsurface with a vertical resolution on the order of centimeters down to a depth of approximately 3 m. In addition to being of scientific value in and of themselves (for reconstructing surface deposit structure and constraining subsurface materials, potentially including fluids, volatiles, or clathrates), radar observations can be fed forward to identify drilling targets and hazards as well as optimize drilling operations.
Additional noninvasive techniques for studying the subsurface, and for gathering samples, exist. Geomorphological features such as seeps, scarps, impact craters, as well as fractured terrains, lava tubes, and ice caves all provide alternative means of accessing samples that originated in the subsurface (e.g., white paper submissions Blank et al. 2018, Davila et al. 2018, Vance et al. 2018). If obtained, such samples, although altered by contact with the surface, are still likely to contain information on subsurface processes. High resolution orbital images have already provided numerous examples of locations (e.g., Boston 2010; Oehler and Etiope 2017) that could be followed up with ground-penetrating radar and seismic sounding observations. Small satellites delivered as secondary payloads are capable of monitoring large areas of the planet at lower costs and can complement rover exploration missions by accessing sites that rovers cannot reach.
Looking to the future, an integration of the multiple approaches described above could address the role of subsurface processes in governing habitability, the preservation of habitable environments, and the preservation of biosignatures. This is a growing need identified in recent studies (e.g., MEPAG 2015; MEPAG HSO-SAG 2015). Subsurface geophysical, geochemical, geological, hydrological, and potentially biological processes are vital to understanding habitability from local to global scales. The remarkable discoveries of methane and its variability
on Mars (Webster et al. 2015, 2018) underscore the relevance and urgency of such subsurface-focused activities for astrobiological investigations of both the rocky planets, and the new frontiers provided by recent discoveries on ocean worlds and exoplanets.
Habitability of Ocean Worlds
What are the chemical inventories and sources of energy that could generate habitability on ocean worlds, and what processes sustain these inventories? The ocean worlds of the outer solar system are compelling both due to their potential for extant life as well as for exotic life as we do not know it.
For ocean worlds such as Europa and Enceladus, where saline oceans exchange material and energy with an ice shell and potentially active seafloors, the question remains to what extent water-rock reactions may have progressed, whether they are still ongoing, whether planetary interiors were ever or are still active, and how interactions between their oceans and surfaces mediated by their ice shells might support active biospheres. Estimates for the salinity and pH range of these oceans span a range of environments on Earth, from neutral and low-salinity to much more extreme conditions, which need to be better constrained, along with knowledge of the origins and limits of life, in order to understand their potential habitability, and finally whether they are inhabited. Discovery of life here that originated separately but in potentially similar conditions to that on Earth would transform our knowledge, and requires access to the ice and liquid water processed through and below it.
For Titan, measuring the details of potentially complex prebiotic organic chemistry, and understanding the extent to which surface reservoirs interact with the ice and ocean below provide keys to understanding the kinematics and organization of an exotic chemical playground in which a separate origin could be possible. In any of these worlds, the distribution of carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur (CHNOPS), the lifetime and pathways of critical compounds from the surface to the subsurface, and the degree of interaction between various reservoirs within these worlds is unknown.
Spacecraft mission and concept development are being guided by and will continue to benefit from the exploration of Earth’s subsurface systems because terrestrial water-rock interactions have implications for the production of reduced gases on Enceladus and Europa. Recent discoveries of metabolisms fuelled by natural radiogenic energy, such as the radiolysis of water at the rock/water interface used by sulphate-reducing bacteria deep in Earth’s crust (Lin et al. 2006; Li et al. 2016) are relevant to the development of strategies for searching for life on icy, ocean worlds.
Missions to the oceans of Europa, the lakes of Titan, or the plumes of Enceladus, with instruments capable of analyzing long-chained organics, could revolutionize our understanding of ocean worlds. Lake floaters, small fissure explorers, and other surface and subsurface explorers could deliver advanced instrumentation to search for biosignatures on these planets. The ocean worlds cannot be explored in the same way as the terrestrial surface or the surface of Mars, because the conditions are different, and the planets have been subject to different dominant processes. The development of instruments to explore them needs to be be informed by research discoveries in the context of subsurface habitability, as key science questions for ocean worlds will require subsurface access, through the ice or into lakes where putative biospheres could thrive.
Vigorous program support and intense focus on the icy moons could, in the next 20 years, go as far as planning a subsurface expedition to an ocean world. Exploration under the ice on Europa or another ocean world has attracted the interest of not only NASA but also ESA. Both agencies have funded large-scale analog science and engineering activities to make these missions possible in the not so distant future. Achievements in underwater and under and through ice missions and scientific analysis on Earth can drive the development of science questions, innovative instrumentation for in situ analyses, and both engineering and operations scenarios needed to make long-term subsurface missions possible. If driven by astrobiology and science community participation, in a new interdisciplinary and interagency landscape that prizes innovation and achievement, such missions are possible within the next few decades. NASA’s Europa Clipper and ESA’s Jupiter Icy Moons Explorer (JUICE) will be launched in the near future. A Europa lander is also under consideration for launch to Europa in the 2020s.
In order to search for life on the icy satellites, it will be necessary to sample the plumes and material deposited on the icy crust in the immediate vicinity of the plume exit points. The biologically interesting material will be
rapidly degraded by radiation. The nature of the missions to these icy satellites orbing the high radiation belts of their host planets makes these missions highly challenging from a technological point of view. The success of the Cassini-Huygens mission to Titan, albeit with a significantly lower radiation load, shows that these challenging missions can be accomplished. In the longer term, however, both in situ measurements and sample return will be needed for a complete understanding of these interesting planetary bodies. Technological advances in terms of protected and cryogenic sample return capsules need to be developed.
In the outer solar system, conventional photosynthetic processes may be highly limited, if possible at all. Thus, any biosphere is more likely to derive its energy from alternate ranges of the electromagnetic spectrum (e.g., by using infrared rather than visible radiation) or be similar to chemosynthetic or rock-hosted communities on Earth. Therefore, the ocean worlds of the outer solar system are the likeliest to answer questions about what alternative biospheres might look like on another planet and what processes of energy cycling they use. Europa Clipper, currently slated to launch in early- to mid-2022, is the first systems-level mission to an ocean world, and the first motivated primarily by characterizing the moon’s potential for habitability. This Jupiter-orbiting, multi-flyby spacecraft will carry instruments for sampling plume or sputtered surface materials to assess chemical composition and the nature of Europa’s nonice materials. The spacecraft will also carry the capability for magnetic sounding of the ocean and core, near-infrared spectroscopy of surface materials, imaging, and radar sounding of the ice shell. Not only will this suite of instruments study the surface of the moon and begin to assess the structure of the ocean’s icy shell, including perched water bodies and putative water-filled fractures and shallow brine zones, it will also be able to characterize the nature of the sea floor and the moon’s rocky core via gravity, allowing better models of the global energy budget. ESA’s JUICE mission to Ganymede will conduct similar investigations to potentially reveal the nature of its interior, perhaps revealing how ocean worlds with deep high-pressure icy mantles could yet maintain habitability.
Candidate lander missions to the ocean worlds currently under study for possible flight in the 2020s include a Europa lander concept (Hand et al. 2016) and a Titan rotorcraft (Lorenz et al. 2018). The Europa lander, as currently envisaged, would sample in the shallow subsurface to extract ice samples, analyzing them for composition and potential biomarkers, as well as characterizing the ice shell structure using seismometers. A lander with the ability to get below the likely sterilized upper surface layers would increase the fidelity of information about the habitability of Europa. The rotorcraft concept would explore Titan’s surface and lakes by flying across and making multiple landings to explore Titan’s atmosphere, diverse surfaces, and organic chemistry. Prior versions of these types of missions also exist, including more expansive instrumentation and deeper drilling by the 2012 Europa lander concept and the proposed Discovery mission that was intended to land in and float across one of the moon’s large seas.
While landing missions on ocean moon surfaces remains a mid-term goal, nearer-term opportunities using technology that is already well developed exist to constrain the chemical inventories on ocean worlds, to evaluate the processes that sustain these inventories, and to search for signs of life. Alternative methods have been envisaged such as measuring chemical complexity or capturing samples from plumes of material ejected from these worlds. For example, in the discussion of life detection at Enceladus, there is debate as to whether remote plume characterization by biosignatures that consider molecular composition, organic complexity, and Gibbs free energy is sufficient. The alternative is direct sampling plume material during a flyby and analyzing that sample onboard or returning it to Earth. Plume characterization via remote-sensing or by in situ analysis of material captured during a flyby requires appropriate instrumentation packaged in a manner consistent with typical power, mass, volume, data rates, and cost limitations inherent in any mission to the Saturn system.
In situ characterization or sample-return options require the collection of sufficient material, in its original state, to undertake the requisite analyses. On Earth, the common practice for DNA extraction or microbial characterization in the oceans—which are known to teem with life—is to filter multiple liters of water to gain enough material to get good signals. At Enceladus eruption rates, and assuming 107 cells/cc, in order to expect to collect a cell from a plume an estimated 12,000 km of plume flybys would be needed, before even considering what has happened to the sample by the time it reaches a spacecraft (Hand 2015; Lorenz 2016).
Return options will necessitate long flight times—i.e., to Saturn and back to Earth in the case of an Enceladus plume sample-return mission—and samples collected may be compromised if they are collected when flying
through the plume at too great a velocity. For Titan, the development of an agnostic approach to searching for life will be critical in order to interpret any samples from its hydrocarbon lakes, underscoring the progress on biosignatures required and discussed in detail in Chapter 4.
The foregoing missions will provide a new wealth of data to deepen our understanding of how habitable systems arise, and provide links to lessons from Earth. Comparisons between the ocean worlds and Earth will further the discussion of systems-level interactions across a wide range of scales and a broad set of conditions pivotal to astrobiology. Increased understanding of ocean world habitability is almost certain, and perhaps progress can be made in measuring statistical and direct biosignatures from an ocean world through particle and compositional analyses.
Revolutions in small satellites could also provide opportunities to explore the nearby environment or surfaces of the ocean worlds. CubeSats could accompany larger missions to measure chemistry in plumes or act as small surface payloads. Small instruments are already under development that could fly on these missions such as imagers, dust detectors, and miniature mass spectrometers (see Chapter 5). At the same time, the James Webb Space Telescope (JWST) and the proposed next-generation space-based observatories—e.g., LUVOIR, HabEx, and OST—planned to support exoplanet and cosmic origins sciences, together with existing ground-based telescopes—e.g., the Atacama Large Millimeter Array (ALMA)—and the various GSMT projects under development could provide observations relevant to studies of the ocean worlds in between dedicated spacecraft missions. Observations of auroras and exospheres make atmospheric, plume, and internal activity remotely observable (see Chapter 3). The observatories may well provide key measurements of the composition of the Galilean moons that have been difficult to characterize from Galileo data. These observatories can provide monitoring and characterization of known ocean worlds, such as Neptune’s Triton, and search for evidence of oceans within the moons of Uranus. In addition, such facilities can search for evidence of putative ocean worlds within the Kuiper Belt and Oort clouds, for which few detailed observations are in hand, and which could motivate future spacecraft missions.
While no current missions to the moons of Uranus or Neptune are under development by NASA, the science of these bodies is a major driver of interest in the Ice Giant found in extra-solar planetary systems (see, for example, Hofstadter and Simon 2017). A better understanding of the solar system’s Ice Giants will help us understand the hundreds of Neptune-sized exoplanets detected by Kepler and other facilities. While Saturn’s moons Rhea, Dione, and Mimas may be ocean worlds, Neptune’s Triton almost certainly is and, as such, is a particularly promising target for future missions to the outer solar system. With a dynamically changing surface (e.g., Bauer et al. 2008) including the first detection of cryovolcanism in the solar system from the 8 km methane geysers (e.g., Kargel 1995), Triton appears to be resurfacing, and with little topography observable, an ocean below its ice shell is strongly suspected. Moreover, as a captured Kuiper Belt Object, Triton is an archetype of potential ocean worlds like Pluto in the far reaches of the solar system. Thus, the Outer Planets Assessment Group (OPAG) Roadmap for Ocean Worlds suggested prioritizing Neptune, due to the potential habitability of Triton. With an Ice Giant mission advocated as a high priority mission within the 2013 Visions and Voyages planetary science decadal survey, the potential selection of such a mission could accelerate ocean-worlds science in the coming decades. Similarly, a return to Pluto or dedicated ocean moons missions made possible by open New Frontiers calls in future opportunities could expand greatly the number of known and characterized oceans within the solar system.
The next 20 years promise to revolutionize the field of exoplanet astrobiology, providing the first observations of habitable zone terrestrial planets and starting the search for life beyond the solar system. We will explore the environments of terrestrial planets orbiting M-dwarf stars, which likely undergo a very different evolutionary path than planets in the solar system. The nature and habitability of these alien worlds will be a key step in understanding the probability of life in the universe. Future direct imaging missions would allow us to probe the surface environments of terrestrial planets orbiting stars like the Sun to search for oceans and signs of life. The committee has identified five key exoplanet research questions to be addressed over the next 20 years. Key research areas will include the following:
- Formation of habitable planetary systems—How do habitable planetary systems form and what are their architectures?
- Factors influencing habitability—What are the characteristics and processes that affect planetary habitability?
- Evolution of terrestrial planets—How do terrestrial planets evolve around different stellar types?
- Nearly habitable exoplanets—Do nearby stars host habitable planets?
- Life on other planets—Is there evidence of life on other planets?
Below, the committee provides a description of the key research goals and enabling missions and technologies needed to answer these questions (see Table 6.1).
Formation of Habitable Planetary Systems
How do habitable planetary systems form and what are their architectures? Developing an overall picture of the planet formation processes that can lead to habitable exoplanets continues to be a key goal in astrobiology research. Obtaining statistics of exoplanetary systems can not only illuminate how common potentially habitable planets are, but also how the planetary architecture (the types and orbits of other bodies in the system) can impact the potential habitability of a terrestrial planet. To address the galactic prevalence of potentially habitable exoplanets, definitive constraints on the number of low mass planets as a function of stellar type, distance, radius, and mass are needed. To understand better the impact of the planetary system architecture on potentially habitable planets the overall architecture of systems with potentially habitable planets are needed. Improved statistics on demographics, the number and distribution of different types of planets (as seen Figure 6.1) will enable stronger tests of terrestrial planet formation models. Dynamical simulations can probe possible migration pathways that lead to final system architectures that may impact habitability. Observations of protoplanetary and debris disks are needed to provide a picture of planet formation and help identify volatile reservoirs and volatile transport throughout the early disk environment.
Factors Influencing Habitability
What are the characteristics and processes that affect planetary habitability? A planet’s habitability is a complex interplay between intrinsic planetary processes, such as interior evolution, outgassing, magnetic field strength, and atmospheric composition. The planet’s habitability is also impacted by its interaction with the host star, both radiatively and gravitationally, via atmospheric escape, climate and photochemistry, and via orbital and tidal evolution. The planetary system, including the presence of sibling planets, also impacts habitability by enabling or inhibiting volatile delivery, and inducing orbital evolution. Key research goals in the next 20 years for planetary habitability will involve synthesis of modeling, observations, and laboratory work to understand how the interactions between planet, star, and planetary system impact planetary habitability, and how these processes and planetary habitability evolve dynamically over time. This knowledge will be needed to improve our ability to vet and rank potentially habitable planets for follow up, helping us identify those planets that are more likely to be habitable and harbor life.
Evolution of Terrestrial Planets
How do terrestrial planets evolve around different stellar types? The recent discovery of likely terrestrial-size or mass planets orbiting nearby M-dwarf stars has opened up a new era of comparative planetology that will allow us to better understand the diversity of evolutionary paths for terrestrial planets. While Venus and Earth provide evidence of strongly divergent evolutionary paths for similar mass planets, understanding how planets can acquire, retain, or lose habitability will be better informed with observations of a range of terrestrial planet atmospheres. Whether observations of terrestrial exoplanets reveal habitability or not, assessment of the planet’s characteristics, and its interactions with other components of its planetary system, including the host star, will help elucidate those processes that can enhance or reduce a planet’s habitability. A large set of observations of
TABLE 6.1 Current and Future Technology Programs and the Measurements That Will Address the Key Astrobiology Questions for Exoplanet Research Programs
|Key Research and Development Goals over the Next 20 Years for Pathways to Search for Life Beyond the Solar System|
|Enabling Technologies and Techniques||How Do Habitable Planetary Systems Form and What Are Their Architectures?||What Are the Characteristics and Processes that Affect Planetary Habitability?||How Do Terrestrial Planets Evolve Around Different Stellar Types?||Do Nearby Stars Host Habitable Planets?||Is There Evidence of Life on Other Planets?|
|Transiting Exoplanet Survey Satellite|
|Nearly all-sky precision relative photometry to detect transiting planets||Demographics of nearby exoplanetary systems for relatively short-period planets||Multiplanet systems, dynamical interactions||Demographics of terrestrial planets for a broad range of stellar host types||Frequency of potentially terrestrial planets orbiting M dwarfs||Identify optimal targets for JWST follow up to search for the potential habitability of terrestrial planets in the habitable zones of nearby, bright M dwarfs|
|James Webb Space Telescope|
|Transit spectroscopy||Atmospheric composition, volatiles||Variation of atmospheric composition of low-mass planets with planet mass, distance from star, and host star properties||Atmospheric composition for M-dwarf terrestrial planets as a function of orbital distance||Existence and composition of atmospheres for M-dwarf terrestrials||Search for biosignatures for a handful of M-dwarf habitable zone terrestrials|
|Secondary eclipse and thermal phase curves||Atmospheric composition, and day-night temperature contrasts for more massive planetary companions||Atmospheric composition and day-night temperature contrasts for hotter and larger terrestrials||Possible measurements of atmospheric composition via near- and mid-infrared spectroscopy for a handful of M-dwarf habitable-zone planets||Search for biosignatures for M-dwarf planets|
|High-contrast photometry and spectroscopy of protoplanetary disks||Disk demographics and composition of young stars and nascent planetary systems||Disk dynamics, volatile delivery||Variation of disk properties with stellar age and mass|
|Ground-Based <10 m Facilities|
|Precision radial velocities||Mass measurements, long-period planets||Architectures and dynamics of multiplanet systems||Detection, minimum masses, and orbits of low-mass, habitable-zone planets around nearby low-mass stars with a range of masses||Detection of terrestrial planets around nearby low-mass stars and possibly Sun-like stars. Density, surface gravity of transiting M-dwarf, habitable-zone terrestrials|
|Key Research and Development Goals over the Next 20 Years for Pathways to Search for Life Beyond the Solar System|
|Enabling Technologies and Techniques||How Do Habitable Planetary Systems Form and What Are Their Architectures?||What Are the Characteristics and Processes that Affect Planetary Habitability?||How Do Terrestrial Planets Evolve Around Different Stellar Types?||Do Nearby Stars Host Habitable Planets?||Is There Evidence of Life on Other Planets?|
|High-contrast direct imaging with extreme adaptive optics||Wide planet statistics, young planetary systems||Multiplanet systems, direct detection of nonhabitable-zone planets on wide orbits||Demographics of nearby planetary systems via direct detection as a function of stellar mass|
|High-resolution spectroscopy, with adaptive optics||Mass measurements for nontransiting planets||Detection of atmospheric constituents for massive planets||Potential detection of the atmospheres of a limited number of small planets around nearby stars||Biosignature searches for very nearby habitable-zone, M-dwarf planets|
|Transit detection and monitoring||Long-term monitoring of transiting systems for planetary companions||Long-term monitoring of transiting systems for planetary companions||Detection of low-mass planets (including terrestrial planets) around nearby low-mass stars||Detection of nearby habitable-zone planets orbiting the smallest stars|
|Sub-millimeter imaging||Protoplanetary disks, volatiles, ices|
|Mini- and Nanosatellites (CubeSat)|
|Time-resolved photometry||Demographics of exoplanets||Stellar activity||Environments of exoplanets as a function of host-star mass and age, detection of transits of low-mass planets identified via radial velocities||Stellar activity and ultraviolet photometry for M dwarfs as a function of stellar mass and age; mass, radius, and density measurements for transiting low-mass planets|
|Wide Field Infrared Survey Telescope|
|Direct imaging spectroscopy
|Circumstellar disks, jovian planets||Volatile deliveries||Atmospheric characterization of jovian planets||With starshade, detection of habitable-zone terrestrials around a few bright stars|
|Microlensing||Planetary system demographics at orbital distances of > 1AU, including free-floating planets||Determination of the frequency of giant planets beyond the ice line||Masses for stars and planets||Improved estimates of frequency of habitable-zone terrestrials|
|Key Research and Development Goals over the Next 20 Years for Pathways to Search for Life Beyond the Solar System|
|Enabling Technologies and Techniques||How Do Habitable Planetary Systems Form and What Are Their Architectures?||What Are the Characteristics and Processes that Affect Planetary Habitability?||How Do Terrestrial Planets Evolve Around Different Stellar Types?||Do Nearby Stars Host Habitable Planets?||Is There Evidence of Life on Other Planets?|
|Giant Segmented Mirror Telescopes (GSMTs)|
|High resolution spectroscopy (including radial velocities)||Stellar-activity indicators and characterization Planetary mass Atmospheric composition of terrestrials in transmission and reflected light||Planetary mass for habitable-zone planets from M-dwarf to Sun-like stars Atmospheric composition for M-dwarf habitable-zone planets in transmission and reflected light||Potential biosignatures for M-dwarf planets|
|Direct imaging and spectroscopy||Young planet observations, solar system analog jovian planets||Disk dynamics, multiplanet dynamics||Atmospheric characterization||Nearby reflected light habitable-zone planets (M dwarfs) or thermal imaging (G dwarfs)||Biosignatures for select targetsa|
|Future Space-Based Missions|
|Direct imaging (ultraviolet, visible, and near-infrared)||High-spatial-resolution disk imaging, planetary demographics, planetary system architectures||Orbits and atmospheric composition of ice and gas giants Stellar ultraviolet characteristics||Terrestrial atmospheric composition from direct imaging spectroscopy (Sun-like stars) and transmission spectroscopy (M dwarfs) Stellar ultraviolet characteristics for photochemistry, ocean, and atmospheric loss||Terrestrial atmospheric composition of habitable-zone terrestrials from direct imaging spectroscopy (Sun-like stars) and transmission spectroscopy (M dwarfs) Planetary rotation rate, photometric mapping, possible ocean detection (Sun-like stars)||Spectroscopic detection of biosignatures, false positives and other environmental context for G-dwarf and other more Sun-like stars Constraints on the frequency of living worlds for larger numbers of planets observed|
|Extreme ultraviolet||Stellar characterization for planetary atmospheric and ocean loss||Characterization of stellar variability for transit detection methods, stellar characterization for atmospheric and ocean loss, photochemistry|
|Enabling Technologies and Techniques||How Do Habitable Planetary Systems Form and What Are Their Architectures?||What Are the Characteristics and Processes that Affect Planetary Habitability?||How Do Terrestrial Planets Evolve Around Different Stellar Types?||Do Nearby Stars Host Habitable Planets?||Is There Evidence of Life on Other Planets?|
|Mid- to far-infrared||Protoplanetary disk characterization||Transit spectroscopy of terrestrial M-dwarf planets
Emission spectroscopy and phase curves for terrestrial planets
|Transit spectroscopy of habitable-zone M-dwarf planets||Transit spectroscopy of potential biosignatures for M-dwarf planets|
|Advancements in Modeling Techniques|
|Planetary, habitability and biosignature modeling||Planet formation modeling to constrain volatile delivery and initial terrestrial planet composition||Interior/outgassing/atmosphere models and stellar, orbital and planetary evolution models to predict properties of secondary atmospheres and the likelihood of a surface ocean||Stellar, orbital, atmospheric evolution models to understand atmosphere and ocean loss and characterize planetary processes that could mimic biosignatures||Evolution models to predict possible environmental states and coupled climate/photochemical and radiative transfer modeling to predict observational characteristics for habitable-zone terrestrials||Interdisciplinary modeling to develop a comprehensive framework for biosignature assessment in the context of the environment (incl. false positives and negatives)|
a Assuming that direct imaging capabilities are being fed by a high-resolution spectrograph. This option is being explored for the TMT and GMT second-generation instruments.
possible terrestrial planet environments will provide a baseline for interpretation of environments and potential biosignatures. For example, ocean and atmospheric loss has been posited as a means to generate extremely large amounts of atmospheric oxygen abiotically (Luger and Barnes 2015), but it is unclear how long this oxygen would persist in a planetary atmosphere against atmospheric, surface, chemical, and even magma ocean loss processes (Schaefer et al. 2016). Observations of a range of terrestrial exoplanetary atmospheres under different levels of stellar insolation and for stars of different ages will help constrain the likelihood of ocean loss as a potential false positive. Indeed, because the host star has a significant impact on planetary habitability, and the star’s activity and luminosity evolve considerably, it will be important to determine and observe stellar activity indicators in systems of all ages and to understand evolutionary pathways, particularly for M-type stars, to feed back into the overall picture of the evolution of habitable terrestrial planets.
Nearly Habitable Exoplanets
Do nearby stars host habitable planets? Another key science goal in the next 20 years will be identifying nearby potentially habitable exoplanets, and determining whether they are habitable. While several promising candidates have been found, including TRAPPIST-1 e, f and g (Gillon et al. 2017; Anglada-Escudé et al. 2016) (Figure 6.2), Proxima Centauri b (Figure 6.3), and Luyten Half-Second catalog 1140 b (Dittmann et al. 2016), larger numbers of targets will be needed to constrain the probability of habitability and to better understand the diversity and distribution of habitable environments. In the near term, the question of the habitability of M-dwarf planets will
be a major theoretical and observational goal, as it is both observationally more tractable than observing planets orbiting Sun-like stars, and more statistically significant for understanding the distribution of habitable planets in the galaxy. Terrestrial planets, which are small, are more readily observed and characterized when orbiting small M-dwarf stars, and yet M-dwarf stars, although the most common type of star in the galaxy, present many challenges to habitability for their planets. To understand whether these planets are habitable will require a coordinated effort between modelers and exoplanet and stellar observers to determine if M-dwarf planets can retain their atmospheres and oceans, and to understand the composition of M-dwarf planet atmospheres. In the longer term, observations of planets orbiting more Sun-like stars, including true Sun-Earth analogs, will extend our understanding of habitability to different stellar types, and provide a direct comparison with Earth. To make the best determination of habitability possible, improved capabilities and techniques will be needed to study not only a planet’s upper atmosphere, but also the planetary surface and near-surface atmosphere. As transit spectroscopy cannot do this; direct imaging of exoplanets will be required. Larger sample sizes will also be valuable to provide an increased chance of finding a habitable environment, or placing more stringent limits on the dearth of habitable environments in the galaxy.
Life on Other Planets
Is there evidence of life on other planets? In the near term, the search for signs of life will start with a handful of known habitable-zone planets orbiting M dwarfs, but the longer-term goal will be a more thorough survey of many more planets across a range of stellar types. To increase the chances of success in the search for life, research to support the interpretation of any potential biosignature observed needs to be undertaken. An important aspect of this research is the identification of new potential biosignatures. Also important are efforts to increase the robustness of the interpretation of potential biosignatures in the context of the planetary and stellar environment. A key specific research goal includes the identification of novel biosignatures, including agnostic biosignatures that are not tied to a known metabolism. Another goal is to understand, for new and existing biosignatures, how
to best characterize a planet’s environment and host star to assess the potential for both false positives and false negatives, and to recognize the observational markers of key stellar and planetary characteristics that can be used to rule out false positives or negatives.
Pathways to Search for Life Beyond the Solar System
To address the key science goals and questions outlined above, advances in interdisciplinary interactions will be needed to leverage expertise from planetary science, astronomy, oceanography, chemistry, and biology to address habitability as the outcome of the dynamic interaction between planet, star, and planetary system. These efforts to explore planetary environmental parameter space will enable identification of the most promising targets for biosignature searches, and guide the observation plans for upcoming ground- and space-based observations (Table 6.1). To complement these theoretical efforts ground- and space-based observational facilities will expand our knowledge of exoplanet demographics, refining estimates of the prevalence of habitable-zone planets, and identifying nearby targets for exploration.
Starting in the 2020s, ground-based facilities and NASA’s JWST will obtain the first observations of habitable-zone planets orbiting M dwarfs using high-resolution and transmission spectroscopy. Such observations will address questions about the nature and habitability of the most common type of habitable-zone planet in the galaxy. Such studies will also provide an observational test of the habitable zone concept. Specific observational goals will be to ascertain if terrestrial exoplanets can retain an atmosphere, and if so, the diversity of these atmospheres and their implications for planetary habitability and life. In the 2035 timescale, all three giant segmented mirror telescope (GSMT) programs have planned second generation instruments that will be capable of direct imaging of terrestrial planets as well spectroscopic capabilities for exploring biosignatures. A large space-based, direct-imaging facility will be capable of searching for oceans on terrestrial exoplanets and for surveying a statistically significant sample of habitable terrestrial planets.
Over the next two decades a variety of large- and small-scale space missions, ground-based observatories, and new technologies will be developed that will be capable of addressing core astrobiology exoplanet questions. Subsequent sections address a selection of key activities.
The Transiting Exoplanet Survey Satellite (2018-2020)
The Transiting Exoplanet Survey Satellite (TESS) (Figure 6.4) was successfully launched in April 2018. It will perform an all-sky survey, searching for transiting planets orbiting stars that are much closer to the Sun than the very distant targets surveyed by the Kepler space telescope. TESS will obtain high-cadence lightcurves of 200,000 nearby bright stars to search for transiting planets amenable to detailed follow-up observations. Though not designed as a statistical mission, the data collected by TESS will improve our knowledge of the demographics of short-period planets.
Nearby targets are more amenable to detailed follow-up observations that will reveal planetary characteristics and environments. TESS is anticipated to find ~15,000 new planets, a handful of which (~10) will be terrestrial-sized exoplanets orbiting in the habitable zones of M-dwarf hosts (Sullivan et al. 2015; Barclay et al. 2018). These latter targets can be prioritized for observation with the Ariel, James Webb Space Telescope (JWST; see below) or with ground-based GSMTs (see below). TESS will also place definitive constraints on the number of short-period planets as a function of stellar type, distance, and radius. Obtaining masses for these planets will rely on support from ground-based telescopes capable of deriving planetary masses using radial velocity (RV). The masses, when combined with the sizes determined from transit, will provide crucial constraints on planetary bulk composition, which is needed to determine if these planets are likely to be terrestrial. As such, the availability of RV support is a primary means to maximize the science return from TESS and to optimize selection of those planets most likely to be habitable for further observation. Statistical data from TESS’s and ESA’s Gaia astrometry mission will also provide information on overall planetary architecture, especially when coupled with radial velocity monitoring or direct imaging that can detect additional more distant or nontransiting planetary companions. Improved demographics with robust statistics will enable stronger tests of terrestrial planet formation models.
James Webb Space Telescope (2021-2026)
JWST (Figure 6.4) will provide our first chance to assess terrestrial exoplanet habitability and search for signs of life on planets orbiting M-dwarf stars (Meadows et al. 2018). JWST observations using transmission spectroscopy in the visible and near-infrared, as well as secondary eclipse and phase curve observations at thermal wavelengths, can be used to search for the presence of a high-molecular weight (e.g., oxygen or carbon dioxide) atmosphere. If such is confirmed, JWST can also be used to search for atmospheric gases, including biogenic ones (Schwieterman et al. 2016), in a planet’s upper troposphere and stratosphere. Highly irradiated terrestrial planets close to their star will be the most easily observed, but in favorable cases, some potentially habitable planets such as TRAPPIST-1e, may be probed, albeit with several hundred hours of exposure time (Meadows et al. 2018). Thermal observations using secondary eclipse can be used to determine planetary temperature, and to search for atmospheric gases. For nontransiting habitable-zone planets such as Proxima Centauri b, thermal phase curves may reveal day-night temperature contrasts that indicate whether or not an atmosphere is present (Kreidberg and Loeb 2016), and molecules can also be sought using these observations. Because transmission spectroscopy is likely more sensitive than secondary-eclipse emission spectroscopy, the former will be the preferred mode of characterizing habitable-zone planets with JWST. However, this technique will not be able to observe the planetary surface or probe the near-surface atmosphere, making habitability assessment more challenging (Meadows et al. 2018).
More generally, JWST will help us better understand the evolution of terrestrial planets orbiting M dwarfs, including putting constraints on atmospheric and ocean loss processes, and potentially providing an observational test of the habitable zone concept via observations of the seven planets spanning the habitable zone in the TRAPPIST-1 system. In the process it will also teach us about the variability of M dwarfs at near-infrared wavelengths, and about observations required to interpret exoplanet data taken by JWST and ground-based telescopes. Mid-infrared spectroscopic measurements of young disks with JWST also have the potential to identify the signatures of volatile elements as a function of radius. Mid-infrared spectroscopic measurements of forming stars with JWST also have the potential to identify the signatures of volatile elements as a function of planetary separation. The combination of moderate spectral resolution with the high spatial resolution of JWST offers the ability to map pre-stellar cores, protostars, and young disks down to the regions where planet formation is thought to occur. This wavelength range will allow for spectroscopic measurement of the precursors of organic molecules and ices.
Current Large Ground-Based Facilities (2018-2025)
Upcoming instrumentation and surveys on existing ground-based facilities will be able to expand the demographic discovery space (Figure 6.1) of terrestrial planets around solar-like stars and less massive K and M-stars. The high-resolution Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) on the Very Large Telescope (VLT) will begin radial velocity surveys in late 2018, and is projected to have a Doppler precision capable of detecting Earth-mass planets in the habitable zones of solar-type stars (Hernandez et al. 2017). The Keck Planet Finder (KPF) is the next generation high resolution fiber-fed spectrograph that is currently being developed for Keck Observatory to conduct high-precision radial velocity exoplanet measurements (Gibson et al. 2016). The higher signal-to-noise afforded by 8-10m diameter telescopes (compared to their 4m-class counterparts) with ESPRESSO and KPF will be also able to probe radial velocities of fainter stars and continue to expand our understanding of planetary architectures and formation.
Direct imaging capabilities using coronagraphs and extreme adaptive optics, like the Gemini Planet Imager and the Spectro-Polarimetric High-Contrast Exoplanet Research, are now being further developed for new instrumentation programs in the coming decade. These next-generation direct-imaging instruments on 8-10m class telescopes will be essential for technology development that will feed into the future GSMTs. Observations of planets in star forming regions with nonredundant masking on 8-10 m class telescopes have the opportunity to show ongoing accretion of large planetary companions, providing hints of formation and volatile transport occurring throughout natal disk environments. Detecting the light from nearby systems, like Proxima Centauri b and potentially new nearby planetary systems discovered by TESS, will become prime, though exceedingly difficult, targets for 8-10m direct imaging cameras (Lovis et al. 2017). The VLT imager and spectrometer for the mid-infrared instrument is
to be upgraded with new wavefront sensors and coronographic capabilities for direct imaging of planets in the Alpha Centauri system. In addition, instruments are currently being designed for near-infrared direct imaging coupled with high resolution spectroscopy using Keck Observatory with the Keck Planet Imager and Characterizer (Mawet et al. 2017). Rapid technological development of infrared detectors, wavefront sensors, deformable mirrors, coronagraphs, and fiber-injection units now allow such test-bed systems to be built for 8-10m telescopes that will be capable of spectroscopy of low-mass exoplanet atmospheres.
ALMA will continue to play a crucial role in characterizing both young and debris disks. Current observations of disks with ALMA have yielded a detailed array of morphologies and molecule measurements (e.g., Andrews et al. 2016; Pèrez et al. 2016). The structures seen in both young disks (Figure 6.5) and debris disks provide hints of possible planet formation in progress, as well as potentially signaling ongoing phase transitions that may play a key role in determining where planets form and of what their atmospheres are composed. ALMA measurements are deeply probing the chemical processes in these disks, including the detection of complex molecules that could play a crucial role in prebiotic chemistry (e.g., Öberg et al. 2015; Bergner et al. 2018). Continued studies with ALMA of a large variety of disks will aid in the understanding of disk chemistry as a function of spectral type and morphological evolution as a function of age, potentially yielding signposts of habitable planet formation. The proposed Next Generation Very Large Array (ngVLA) would complement ALMA’s capabilities by providing increased angular resolution and access to longer wavelengths. The ngVLA, with baselines up to 60 times those of ALMA and a 10-times greater sensitivity, will be proposed for prioritrization by the next astronomy decadal survey.1
The WFIRST (Wide Field Infrared Survey Telescope) mission entered Phase B in May 2018, with a currently estimated launch in the late 2020s. The nominal mission decision includes a coronagraphic instrument which will be used primarily to characterize the atmospheres of known Jupiter analogs detected via radial velocity using low-resolution spectroscopy. Simulations have shown that the mission could be photometrically sensitive to a few nearby super-Earth planets. The accomplishment of these goals requires significant advances in space coronagraphic design, serving as a precursor to larger space missions to measure the atmospheres of terrestrial planets. Additional required developments include the advancement of spectral retrieval techniques and models for reflected light atmospheric measurements of Jupiters and super-Earths (e.g., Lupu et al. 2016). WFIRST is also designed to be compatible with a potential future starshade. Such a development would offer access to imaging Earth-like planets in nearby systems and would represent a critical technological leap for future missions.
Ground-based Giant Segmented Mirror Telescope (2028-2035)
The technological development occurring on current ground-based optical facilities is essential for the pathway of exoplanet and biosignature research using the future ground-based GSMT (Figure 6.6). There are three worldwide endeavors that aim to have a 25-40 m optical telescopes in operation by 2028-2030: the Giant Magellan Telescope (GMT) with a 25-m aperture at Las Campanas Observatory in Chile; the Thirty Meter Telescope (TMT) with a 30-m aperture at either Mauna Kea, Hawaii, or Roque de los Muchachos on La Palma in the Canary Islands; and the European Exteremely Large Telescope (E-ELT) with a 39-m aperture on Cerro Armazones, Chile. The GMT first light instrument will be a high-resolution spectrograph (Szentgyorgyi et al. 2016) that will be capable of measuring precision radial velocities of Earth-sized planets and of searching for atmospheric biosignatures, such as O2. At first light, TMT and E-ELT will have near-infrared integral field spectrographs and imagers that will be capable of spectroscopically characterizing atmospheres of jovian-sized planets that are at large separations from their young host stars.
All three GSMTs projects are conducting conceptual studies and associated technology development activities with the goal of deploying second generation instrumentation that will have direct imaging and spectroscopic capabilities for exoplanet and biosignature studies.2 The test-bed instrumentation and technological developments that are occurring on current 8-10m class telescopes and coronagraph designs are essential for reaping the benefits from GSMT direct-imaging capabilities. These second generation exoplanet instrumentations are currently being designed to use high dispersion coronagraphy techniques at near-infrared wavelengths, which have the potential of reaching the necessary 108 contrast and sensitivities for studying rocky planets around M dwarfs and nearby stars. Near-infrared high-resolution spectroscopy coupled with this mode will allow the study of molecular species such as O2, H2O, CH4, and CO2 in these exoplanetary atmospheres. At mid-infrared wavelengths with coronagraphy, GSMTs will have the capability of directly imaging the thermal emission of rocky planets around nearby solar-like stars, and potentially with low spectral resolutions the ability to trace H2O and CO2.
Complementing the much larger facilities and missions, CubeSats, nanosatellites of less than 10 kg mass, support relatively inexpensive missions with short development lead times, and can provide a nimble and flexible solution to specific science goals and technology demonstrations. Two CubeSat missions have been launched or funded that are relevant to exoplanet astrobiology: The Arcsecond Space Telescope Enabling Research in Astrophysics (ASTERIA), which was launched in late 2017, and the Star-Planet Activity Research CubeSat (SPARCS), to be launched in 2021.
2 Examples of concepts for second-generation instrument for the three GSMT projects under way include the following: the Planetary Camera and Spectrograph (PCS) for the E-ELT (see https://ao4elt3.sciencesconf.org/12804/document and https://spie.org/Publications/Proceedings/Paper/10.1117/12.2056842); the Planetary Systems Imager (PSI) for the TMT (see https://spie.org/Publications/Proceedings/Paper/10.1117/12.2314331 and https://spie.org/Publications/Proceedings/Paper/10.1117/12.2314173); and the Giant Magellan Extreme Adaptive Optics System for the GMT (see https://magao-x.org/gmagao-x/).
ASTERIA is designed primarily to demonstrate that a CubeSat can collect photometric data, and process photometric light curves. The mission’s secondary goals include measurement of stellar rotation periods, characterizing the activity of exoplanet’s parent stars, and providing simultaneous photometric measurements in support of ground-based radial velocity measurements.
SPARCS is a funded mission that will provide comprehensive measurements of the time-dependent spectral slope, intensity and evolution of M-dwarf stellar ultraviolet radiation, including flare activity. These measurements are crucial to assess atmospheric retention and planetary habitability, and interpret atmospheric compositions and biosignatures for planets orbiting low-mass stars. SPARCS will monitor stellar activity in ~25 M-dwarf stars with ages spanning 20 Ma to 5 Ga in two ultraviolet photometric bands simultaneously. These observations will help illuminate the evolution of stellar activity and the history of potentially habitable systems that may soon be the targets of biosignature probes. These observations can only be performed in space, and the ultraviolet-capable Hubble Space Telescope is a common-user facility that cannot support the dedicated observing campaigns required to monitor stellar activity over one to three stellar rotations. SPARCS will be capable of target of opportunity observations to support ultraviolet characterization of new habitable-zone planet host stars identified by TESS.
Space-Based Next Generation Flagship Missions (2035 and Beyond)
NASA is currently funding four concept studies for flagship missions to fly after JWST, and these missions will be ranked by the next astronomy decadal survey. They are: (1) the Large Ultraviolet Optical Infrared Surveyor (LUVOIR),3 a large aperture (8-15m) general observer facility that will be capable of direct imaging of exoplanets; (2) the Habitable Exoplanet Observatory (HabEx),4 a smaller aperture (4-6m), more exoplanet focused direct imaging mission; (3) the Origins Space Telescope (OST),5 a mid-infrared moderate aperture telescope that is a successor to JWST; and (4) Lynx, an X-ray Observatory.
The direct imaging missions, LUVOIR and HabEx (Figure 6.7), will have the capability of suppressing the light from the parent star and surveying up to hundreds of stars in the solar neighborhood to search for habitable-zone terrestrial planets. These missions will then be able to image and obtain direct imaging spectra of nontransiting terrestrial planets within the habitable zones of a handful to several dozen more Sun-like (F, G, K) stars. The larger aperture LUVOIR will also be able to directly image planets orbiting nearby M dwarfs. Consequently, these telescopes will allow the study of planets orbiting stars more like our own, and will complement what might have been learned by that point about M-dwarf planets by JWST and ground-based telescopes. These direct imaging observations will provide a significant increase in our capability of characterizing exoplanets for habitability and life by enabling an observational probe through the entire atmospheric column of a planet, as well as by direct imaging of the planetary surface. Notably, direct imaging can be done for nontransiting planets, and thus can be used to search for planets around all of the nearby stars. This technique is more sensitive to near-surface water and biosignatures than are the transmission observations that will be obtained by JWST. Spectra will be obtained from 0.2-1.8 μm by LUVOIR and HabEx. HabEx spectroscopic capabilities will increase if it is flown with a starshade, which would allow it to observe planets close to their star that would otherwise have been the purview of LUVOIR only. Transmission observations from the ultraviolet to the near-infrared are also possible with these telescopes. HabEx will explore the nearest stars to search for signs of habitability and biosignatures. LUVOIR, with its larger aperture, will survey more stars to constrain the frequency of habitability and biosignatures for up to 50 terrestrial habitable-zone exoplanets, and to enable enhanced comparative planetology and produce a statistically meaningful sample of exoEarths. Even if they do not find evidence for life, either mission would enormously increase our knowledge of terrestrial planet atmospheres by expanding the database beyond the four rocky planets in the solar system.
The Origins Space Telescope is currently conceived to be a general observer, mid-infrared 6m telescope, similar to JWST. Like HabEx and LUVOIR, it will support a broader astrophysics community and will be capable of observing
transits of terrestrial planets in the habitable zone of M dwarfs. Emission observations via thermal phase curves or secondary eclipse of transiting exoplanets may also be possible. Lynx will provide important observations of potential exoplanet host stars, helping to monitor stellar activity and the space radiation environment for habitable-zone planets.
Synergies with Solar System Missions
In understanding terrestrial exoplanets, synergies will exist with solar system missions that seek to understand terrestrial planetary processes and the history of habitability on Venus and Mars. This relevance to the astrobiology of exoplanets is, in general, a bonus. That is, the relevance of the small subset of solar system missions discussed in this section to astrobiology is in addition to the primary science these missions were selected to undertake. While Venus, Earth and Mars likely formed from the same initial inventory of the solar nebula material, and isotopic and geological evidence has suggested that each planet supported surface liquid water in the past, their atmospheres and climates have diverged over the past 4 billion years. Comparative terrestrial planetology will help inform the processes that support, maintain, and destroy planetary habitability, helping us interpret habitable-zone exoplanets. Missions to Venus like JAXA’s current Akatsuki mission that take steps to improve our understanding of current processes like lightning and volcanism on Venus inform these processes. Future Venus missions that may also provide insights into the early evolution of Venus’s atmosphere, the current outgassing rates, atmospheric escape, and photochemistry will provide clues to the runaway greenhouse process, and the evolution of atmospheres for highly irradiated terrestrial exoplanets. Similarly studies of martian atmospheric loss processes by the Mars Atmosphere and Volatile Evolution spacecraft at Mars have been used to inform calculations for atmospheric lifetime for terrestrial planets orbiting M-dwarf stars, a key question in exoplanet habitability (Brain et al. 2017).
With the detection of exomoons on the horizon (Teachey et al. 2017; Rodenbeck et al. 2018), as well as the prevalence of possible water worlds around other stars, lessons learned from the ocean worlds of the solar system, including Earth, Europa, Ganymede, Enceladus, and Titan, will bear important information that can be integrated into exoplanetary missions. The Europa Clipper mission will measure the composition, environment, and geophysical characteristics of Europa along with its ocean. This mission, through gravity, may finally answer whether activity at the seafloor has ever occurred on this moon, an archetype for other ocean worlds. ESA’s JUICE mission will focus on Ganymede while also studying Jupiter, Europa, and Callisto, and has as its central goal to understand the “emergence of habitable worlds around gas giants,” directly feeding into exoplanet studies. Ganymede itself, as a planet-sized ocean world, is also an analog for other potential ocean worlds whose size is great enough to produce high pressure ice phases at the deep sea floor. JUICE will provide information on whether Ganymede can be considered habitable, and inform the exoplanet community on the range of exotic conditions experienced on ocean planets. Any future Enceladus or Titan missions would provide similar context. Since the great observatories planned for the coming decades, both on the ground and in space, will observe the ocean worlds with the same instruments as we search for inhabited worlds beyond the solar system, such observations may prove complementary.
Although significant progress has been made since publication of the 2015 Astrobiology Strategy, those advances have revealed that much more work needs to be done before the majority of biosignatures are well enough understood to resolve outstanding controversies regarding the earliest evidence of life on this planet, let alone on planets, moons, or exoplanets beyond Earth. There is a pressing need for a comprehensive set of standards to guide the evaluation and testing of remote and in situ biosignatures in their environmental context and to take into account the probabilities of false positives, false negatives, and levels of uncertainty. Further, the range of potential biosignatures—both remote and in situ—is in need of reevaluation to take into account extreme or even non-terran life. Given recent trends in biosignature research and detection technology, as well as gaps in existing knowledge, over the next 20 years the following questions will prove critical:
- Novel biosignatures—How are novel biosignatures identified?
- Interpreting biosignatures—How can confidence in the interpretation of biosignatures be increased?
- Detecting biosignatures—Given preservation biases and false negatives, which biosignatures have the highest probabilities for detection?
- Achieving consensus—How can biosignature detection and interpretation be standardized as a probabilistic outcome such that community consensus is achievable when a purported sign of life is detected?
How are novel biosignatures identified? The suite of in situ and remotely detectable biosignatures is at present modest and needs to be expanded. In particular, accelerated efforts to understand alternative chemistries or metabolisms and their likely impact on the environment are warranted. Microbial life on Earth uses a well-defined set of electron donors and acceptors, largely determined by their environmental availabilities (Falkowski et al. 2008; Falkowski 2015). On planetary bodies within the solar system, these chemistries have led to a wide variety of complex organic molecules that may be preserved in the lithosphere and can be detected directly, although issues of false negatives, false positives, and differentiation from signatures produced by nonbiological processes remain a critical challenge.
Methods exist to compute the energy yields of redox reactions, even those reactions not known to be used by terrestrial life (Amend and Shock 2001). The caveat is that knowledge of in situ temperatures and concentrations of redox compounds is essential to enable meaningful evaluation of these potential metabolic drivers, and an understanding that the reactions lead to gaseous signatures is desirable. Novel biosignatures of metabolism may also be identified by studying metabolic processes on Earth in more detail, both by exploring Earth’s past to identify when these processes became sufficiently pervasive to provide an environmental context for the biosignature (Magnabosco et al. 2018), and by surveying alternative redox partners that plausibly could be produced by life. For remote-sensing biosignatures, it is important to identify means by which life’s complex interaction with its environment can be identified at a planetary scale, for example via atmospheric (gases and aerosols), surface, or temporal processes (day-night and seasonal processes).
While identification of biosignatures from specific known or past metabolisms can profitably continue to be pursued for both in situ and remote-sensing biosignatures, it will also be important to expand our understanding of agnostic biosignatures which identify complexity in the environment that is unlikely to have been produced by nonbiological processes. For in situ life searches, examples include molecules with a sufficiently large number of steps required for their formation that they are unlikely to occur via abiotic processes (Marshall et al. 2017). Considerable work will be needed to extend this approach to remote biosignatures, but examples include global-scale complex chemical networks that are unlikely to have arisen by chance, or chemical disequilibria in planetary atmospheres that signify surface fluxes that are unlikely to be due to geological or photochemical processes alone.
How can confidence in the interpretation of biosignatures be increased? Equally important as the discovery of new biosignatures is the need to increase confidence in existing biosignatures. In part, this confidence will stem from an ability to identify false positives and false negatives, and to determine complementary measurements to rule them out, or to identify multiple lines of evidence to strengthen the conclusion that a given phenomenon or product is indeed due to life. Additionally, understanding the variety of environmental contexts that can either strengthen or weaken biosignature credibility will be crucial. The following are identified as gaps in present understanding, each of which will require further research:
- Following the template for biosignature assessment set by false positive studies for abiogenic oxygen (Meadows et al. 2018; Catling et al. 2018), identification of geological, photochemical, and other planetary processes for production, and quantification of likely fluxes, for abiogenic methane, nitrous oxide, methyl chloride, and other biosignature gases;
- Laboratory, field, and modeling studies that help to understand isotopic fractionation in both biological and nonbiological systems;
- Analysis of biogenic sedimentary fabrics and their abiotic mimics; and
- Recognition of abiogenic processes that can mimic microscopic “cellular” objects, sedimentary fabrics, biomineralization, bioalteration, isotopic signatures, and complex spatial organization of molecules.
Given preservation biases and false negatives, which biosignatures have the highest probabilities for detection? False negatives that occur due to obfuscation or destruction of biosignatures provide an equally difficult and important challenge to address. Compounding the effect of physical and chemical processes that may reduce the signal of a biosignature to the lower limit of detection, preservation biases may alter or entirely erase the signature. Current gaps in understanding of false negatives and preservation biases include:
- Suppression or buffering of the rise in atmospheric concentrations of oxygen and other potential remote-sensing biosignature gases due to geological and surface processes;
- Recognition of environmental parameters that can alter remotely sensed biosignature gases, especially those due to star-planet interactions or the effect of aerosols, which may ultimately influence target exoplanet selection;
- Alteration of biogenic gases that are encapsulated in ices or minerals;
- Degradation of mineralogic and isotopic signatures;
- Environmental perturbation of isotopic fractionation, especially when abiotic reactions do not proceed to completion;
- Destruction of materials by ionizing radiation, ultraviolet light, heat, pressure, tectonism, and aqueous chemistry;
- Alteration of organic molecules, microscopic structures, and sedimentary fabrics due to mineral encapsulation, chemical modification, or aqueous delivery of silica, salts, sulfides, or chemical reducing agents; and
- Low-energy environments, with barely perceptible biological activity that would be difficult to detect above the baseline signal of abundant abiogenic processes. Such environments are prevalent in Earth’s subsurface and at the surface (e.g., Wilhelm et al. 2018).
How can biosignature detection and interpretation be standardized as a probabilistic outcome such that community consensus is achievable when a purported sign of life is detected? The potential value of a biosignature for life detection derives from a combination of the above considerations and a comprehensive framework for biosignature assessment. It reflects not only the intrinsic value of the biosignature, but also the associated propensity for both false negatives and false positives, which together create an uncertainty and likelihood for detection unique to each biosignature. As a result, without standard assessment criteria and uncertainty calculations by which to evaluate each biosignature, it is difficult for the scientific community to agree upon the robustness of a biosignature interpretation, even when the environmental context and multiple lines of evidence are integrated. As discussed in Chapter 4, this ambiguity can affect interpretations of potential biosignatures, as amply demonstrated by enduring controversies concerning the geologic and life record of early Earth. Resolving such ambiguities will require the development of comprehensive, probabilistic frameworks for the assessement of all newly proposed and existing biosignatures (e.g., Catling et al. 2018; Walker et al. 2018). Studies of biosignatures in the rock record of early Earth provide an ideal initial test bed for the development and “field testing” of biosignature criteria, standards, and uncertainties to facilitate consensus within the biosignature community when signs of life are detected on other planets.
Pathways to Standardizing Biosignature Interpretation and Uncertainty
Methods employed in the standardization of biosignature assessment and uncertainty have the potential to provide an objective path forward in the search for life beyond Earth. In the coming years, there is an urgent need
to develop standardized methods for assessing the predictive value of biosignatures. This does not mean focusing solely on biosignatures that pertain to terrestrial organisms. That approach, although most readily undertaken, would risk false negatives by ignoring the possibility of life unlike our own. Stated more aspirationally, there is value in pursuing agnostic biosignatures that pertain to life in a more general sense, while keeping in mind the need to avoid false positives that may result from being too general. The challenge for biosignature science is to strive for a comprehensive, quantitative foundation that uses multiple lines of evidence and environmental context to provide the most robust life detection framework involving interdisciplinary laboratory, field and modeling work, as well as community efforts to develop a consensus on assessment to apply to the search for life beyond Earth.
Biosignature Searches in the Solar System
Missions to Mars, Venus, and the ocean worlds will provide opportunities to search for biosignatures, and provide insight into the planetary processes that may also lead to biosignature false positive and negatives. Besides Earth, Mars has been studied in greatest detail among terrestrial worlds, considering that it had the potential to sustain microbial life in the past, and could still have, contingent on liquid water in the martian subsurface as recently reported for the base of the martian polar caps by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (Orosei et al. 2018). Though detection of organics, in particularly recent confirmation of methane in the atmosphere (Webster et al. 2015, 2018) and organics in ancient mudstones (Freissinet et al. 2015; Eigenbrode et al. 2018) of Mars, have been made with the Curiosity rover, the data are inconclusive as to their origin. The Trace Gas Orbiter (TGO) of the ESA-Roscosmos ExoMars program, which is entering its scientific phase, is designed to map the distribution of methane on Mars, and if possible its temporal and spatial variability. TGO will also measure other related trace gases. Though life as we know it generates methane, so that nearly 95 percent of methane on Earth is biological in origin, existing data for Mars do not reveal its origin. The TGO type of data are crucial for beginning to determine whether methane on Mars is biologic or geologic in nature, and whether it was produced only in the past and stored, or is being produced even today in the subsurface where liquid water may be present and water-rock reactions ongoing. As on Mars (and Earth), methane is a key biomarker for habitable exoplanets. Besides methane, a number of other potential biomarkers including oxygen (or ozone, as proxy for oxygen), nitrous oxide, etc. as well as environmental context are essential to make an unambiguous claim of life on a habitable exoplanet.
The 2020 ExoMars lander/rover is equipped with a drill to access samples from up to ~1.5 meters depth, which is below most of the depth of penetration of galactic cosmic rays, thus increasing the chances of finding well-preserved organics, complementing Curiosity, which can drill down to only ~6 cm. ExoMars measurements may reveal the origin of the martian organics. NASA’s Mars 2020 rover will cache samples for later return to Earth by a sample-return campaign (Box 6.1), allowing in depth investigation of martian rocks.
As a highly irradiated terrestrial planet Venus can provide clues to several key photochemical and catalytic processes that affect the abiotic formation and destruction of O2 in terrestrial planetary atmospheres. Future Venus missions that focus on atmospheric chemistry, as well as ongoing work on modeling Venus photochemical processes may provide important insights into the environmental processes needed to interpret any detection of O2 in an M-dwarf planetary atmosphere.
Biosignature Searches of Exoplanets
JWST and the ELTs will provide our first chance to look for signs of life on exoplanets, and search for false positives in highly irradiated terrestrial atmospheres, including for JWST the possible detection of O2, O3, hydrocarbon haze (Arney et al. 2018), C2H6 (Domagal-Goldman et al. 2011) and the false positive indicators O2-O2, CO and CO2 (Schwieterman et al. 2016), and O2 and CH4 for existing ground-based telescopes using high-resolution spectroscopy (Lovis et al. 2017). These observations will be extremely challenging, however, as JWST’s capabiltiies are insensitive to the deep atmosphere and surface where a range of biosignatures are most readily observed. In the case of ground-based high-resolution spectroscopy, it will be difficult to observe potential false positive indicators like the broad absorption of O2-O2 in abiotic O2 atmospheres. More thorough searches for biosignatures
in the deep atmospheres of exoplanets and on their surfaces will be possible with large space-based direct imaging telescope concepts that are currently under development.
The increasing number of international partnership opportunities, as well as private-public partnerships (see Chapter 7) and innovative technologies (see Chapter 5) means NASA has the opportunity over the next 20 years to reconceive the scale, frequency, and risk level of missions and instruments to advance astrobiology.
- How can small spacecraft enable innovative, more frequent, astrobiologically relevant missions, and those that may be more accommodating of higher risk?
- How can standard small spacecraft buses; communication and navigation systems; entry, descent, and landing systems; de-orbit, descent, and landing systems; measurement techniques; and science instruments be developed to allow planetary missions to be more frequent and cost effective?
- How can instrument suites in which the failure of a single instrument would not be life threatening to a mission allow more comprehensive ways to search for life in other worlds?
- How could the development of onboard image processing systems, requiring orders-of-magnitude more computing power and memory allocation than available on current spaceflight systems, leverage systems currently being developed by private industry?
- Could investments in biomedicine, food security and defense be leveraged in the development of instruments for in situ life detection?
Small satellites were the first to be launched into Earth’s orbit at the beginning of the space age, but satellite sizes increased dramatically immediately afterward because the instruments, flight, power, and telecom systems required by more sophisticated missions were substantially larger. In the last few decades, the miniaturization of electronics, flight, and telecom systems has been enabling small satellites to accomplish increasingly more sophisticated missions. Indeed, NASA just launched the first deep space mission consisting of two small spacecraft (Mars Cube One), as secondary payload to the InSight Mars lander mission. Mars Cube One (MarCO) is a pathfinder mission that is testing new miniaturized communications and navigation technologies.
Small spacecraft technology, especially in the area of hardware miniaturization and software developments, is expected to develop rapidly during the next 20 years, with the rapid increase in the number of small spacecraft being developed and launched into Earth’s orbit by public and private companies (Figure 6.8). This has the potential to have a positive impact on planetary science and particularly astrobiology missions by allowing the development of low-cost, opportunistic secondary payloads. The miniaturization of space instruments allows secondary payloads that can be more innovative and cost effective, but may incorporate higher levels of risk than those used in traditional missions. For example small satellites will allow orbital transponders and receivers to be placed in orbit to relay data collected during the critical phase of the primary mission back to Earth (as done by MarCO), as illustrated in Figure 6.9, and will allow small satellites to probe the atmospheres of planets and moons. These small satellites will have direct impacts on atmospheric, climate, surface, and subsurface studies of solar system bodies.
A small satellites mission for investigating Europa’s subsurface ocean has been conceptualized as secondary payload to the Europa Clipper mission (Figure 6.10). Meanwhile, in the upcoming decades, SpaceX is expeditiously moving forward with ambitious Mars missions that could deploy fleets of small satellites as secondary missions, while Blue Origin has been developing concepts for lunar missions that could also carry small satellites. Advancements in machine learning software and hardware can be leveraged to continue the integration of miniaturization technologies in space missions. This will likely have a direct impact on astrobiological missions as discussed in Chapter 5.
Required Development in Next 20 Years
The United States is a leader in the development of innovative space missions that enable scientific discoveries that inspire and engage the public around the world (e.g., NASEM 2017). It is essential that the development of these missions continue not only because of their scientific value, but also because they inspire and contribute
to the education of new generations of scientists, engineers, and technologists. The following sections examine some small-scale activities that may have big payoffs in the coming decades.
Small Satellite Technologies
Knowledge of the atmospheric density and wind profiles as a function of altitude is critical to the planning of entry, descent, and landing of Mars missions requiring precise landing for the collection of optimum samples for astrobiology studies. This is important because the absence of traces of life at any one Mars landing site, for example, does not mean absence of any life on Mars, if the punctuated habitability of the planet is taken into account (e.g., Westall et al. 2015). Thus, multiple in situ and sample return missions are necessary to search for traces of life on Mars. Most of the potentially inhabited niches on Mars are on a scale too small to be readily observed from space (unless they are as large as Home Plate, i.e., approximately 90 meters across). This means that options to include relatively small scale missions be examined.
Satellite-to-satellite (crosslink) radio occultation at Mars (and other planetary bodies with atmospheres) could provide data for detailed studies of atmospheric dynamics and climate. Radio occultation measurements by three or more smallsats could provide dense global coverage in periods of a week or less, enabling greatly improved understanding of global atmospheric processes for many planetary bodies across the solar system (Ao 2017). At
the giant planets and Titan, where traditional radio occultation to Earth is limited to dawn or dusk, crosslink radio occultation measurements could provide the first coverage of the diurnal cycle.
The maturation of radio occultation technology is necessary because current smallsat technologies such as the Iris transponder were developed for communication and do not produce the necessary phase observables for occultation science. This constrains the measurements to X-band (7-8 GHz), making unambiguous measurements from the near surface to the ionosphere difficult (e.g., Withers 2010). Moreover, systems developed for communication are much more complex and expensive than a dedicated radio occultation instrument, requiring more power, volume, and mass allocations than a dedicated instrument. This makes current technologies unaffordable for use in fleets of smallsats. Hence, in order for cost effective smallsats capable of performing radio occultations to be possible, key strategic knowledge gaps that prevent them from being implemented need to be filled. This can be done efficiently by leveraging existing global positioning system (GPS) radio occultation instrument packages developed for smallsats. These instruments can be transformed into instrument packages for use in deep space, with the addition of a transmitter to provide a reference signal to replace the GPS signals used at Earth.
If future planetary missions are to be more frequent and cost-effective than they are currently, technology developments in the following areas are required:
- Standard small spacecraft buses;
- Communication and navigation systems;
- Entry, descent, and landing systems;
- De-orbit, descent, and landing systems;
- Measurement techniques; and
- Science instruments.
Such developments would reduce fabrication and qualification costs in comparison with those of current systems. Moreover, they would allow the development of instrument suites for multiple missions, making the failure of single instruments less of a risk to the overall program.
An example of an astrobiologically relevant smallsat mission is the Europa Clipper Cubesat Mission, a concept developed by JPL and the University of Michigan. According to this concept, during a close flyby of Europa, Clipper will eject a CubeSat that will then orbit Europa at an altitude of about 1,500 km, 1 europan radius. The CubeSat could conduct multifrequency magnetic induction sounding over two 85.2-hour orbital periods. This would allow the determination of the salinity and depth of Europa’s ocean, quantities not measured by the Europa Clipper. Once Clipper returns to the region, 15 days later during its long looping elliptical orbit of Jupiter, the CubeSat will transmit all data to Clipper and complete its mission.
In Situ Life Detection
Investments in the biomedicine, food security, and defense sectors have led to large developments in instruments capable of in situ life detection. Comparably modest investments can transform these commercial instruments into spaceflight hardware capable of addressing key astrobiology science goals, ensuring appropriate planetary protection requirements (Box 6.2) are met, and providing the computational and memory resources needed to support on-board data analysis. Relevant instruments that could be matured in the next two decades to accomplish astrobiological goals include, among others, miniaturized mass spectrometers, sample-compatible DNA sequencers, and optical microscopes. Analyses carried out onboard in support of these instruments may include searching images and other large data sets for features of interest using the exceptional computational capability developed by private industry and government in the past decades.
For example, onboard image processing—which could augment in situ sample analysis—demands orders-of-magnitude increases in computing power and memory allocation compared with what is currently available for landed space missions. Furthermore, it is anticipated that onboard data analysis will become necessary because of the large constraints in data volume placed on surface missions to the ocean worlds, and possibly beyond. In these instances, increased resistance to and shielding from radiation may also be necessary.
Finally, the in situ search for life could also benefit from significant increases in bandwidth of the communication back to Earth. Because of this, moving forward optical communications with spacecraft in deep space will become increasingly necessary. In addition to data transfer, a larger communication bandwidth would allow onboard software to be updated more extensively and more frequently, as scientists and engineers on the ground analyze mission data and improve onboard processing techniques as they gain experience with mission operations.
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