Are we alone in the universe? Sages and scientists, philosophers and poets have posed variants of this question since time immemorial. Today, we are formulating research programs that may someday provide an answer. We are in this enviable position thanks to the intertwining of three scientific and associated technological threads developed over the past 400 years.
- Astronomical sciences—The era of modern astronomy began in the 16th and 17th centuries when Copernicus, Brahe, Galileo, and Kepler enabled the delineation of orbits of planets and satellites in the solar system. Today, their successors are doing the same for planets orbiting other stars (exoplanets). Moreover, astronomers have shown that the elements required for life (e.g., carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur) are present across our galaxy and countless others, raising the possibility not only for habitable environments beyond the solar system, but also for life itself.
- Geological sciences—Development of the theory of uniformitarianism by Hutton and Playfair at the end of the 18th century began a revolution in our understanding of Earth’s structure, evolution, and age. Today, techniques such as isotopic dating, electron microscopic imaging, and seismological analysis, coupled with theories such as plate tectonics, have produced a deep understanding of the nature of Earth as a planet while continuing to uncover new insights and frontiers. With the coming of the space age, geologists began applying knowledge gained from the study of Earth to other bodies in the solar system. This has evolved into an interdisciplinary quest to evaluate the habitability of bodies both in this solar system and in planetary systems around other stars.
- Biological sciences—The twin 19th-century developments of Darwin’s theory of evolution and Mendel’s theory of genetics, coupled with the contemporaneous microbiological insights of Pasteur, brought understanding of the nature of life on Earth into the modern age. In the early-20th century, Haldane and Oparin independently proposed that the origin of life was a natural consequence of environmental conditions on the early Earth—an idea later supported by the experiments of Miller and Urey. The last half of the 20th century saw a revolution in the understanding of biological systems exemplified by the deciphering of the genetic code and the discovery of the three domains of life. The discovery of deep-sea hydrothermal vents and their associated ecosystems, powered by chemical reactions, suggested new venues for life’s origins and novel habitable zones in Earth’s subsurface, while advances in biotechnology provided new approaches to life detection.
The development of robust space technologies in the second half of the 20th century catalyzed the cross-fertilization of ideas from astronomy, geology, and biology and their application to the search for habitable environments and life beyond Earth. The first international conference on the origins of life took place in Moscow in 1957, the same year as the launch of the first artificial satellite. Astronomers, geologists, biologists, and others rapidly saw the potential of access to space as a new venue for research. Astronomers would no longer be restricted to observing only at those wavelengths that penetrate Earth’s blurring atmosphere. Planetary geologists could study extraterrestrial bodies up close rather than from afar. Similarly, some biologists saw space as a place to test ideas about prebiotic chemistry and the origins of life.
The founding of the Committee on Space Research (COSPAR), the National Aeronautics and Space Administration (NASA), and the Space Studies Board (SSB) of the National Academies of Sciences, Engineering, and Medicine in 1958 accelerated the creation of a multidisciplinary space science community. Planetary science had already developed a distinct, multidisciplinary identity of its own by this time. In 1959, astronomer Gerard Kuiper went to the University of Arizona to found the Lunar Laboratory (eventually, the Lunar and Planetary Laboratory/Department of Planetary Science), separate from the departments of astronomy and geology. In the same year, astrophysicist Thomas Gold went to Cornell University to establish the Center for Radiophysics and Space Research to do planetary science and ionospheric research separate from an astronomy department. Just a few years later, planetary science was established at the California Institute of Technology by geologist Robert Sharp, transforming the Division of Geological Sciences into the Division of Geological and Planetary Sciences.
NASA, in particular, was seeking experiments for its spaceflight missions. Bodies such as the SSB were keen to provide NASA with advice. COSPAR and other scientific organizations provided venues at which members of the international space science community could exchange ideas. At about the same time as the planetary science community was developing its own distinct identity, Joshua Lederberg and other like-minded researchers interested in the search for life beyond Earth soon banded together to establish the new scientific discipline of exobiology.
NASA funded its first exobiology project in 1959 and established an Exobiology Program the following year. In response to a request from NASA, the SSB published an extensive series of reports on life in the universe (Table 1.1). The first report (NRC 1966) concluded the following:
The biological exploration of Mars is a scientific undertaking of the greatest validity and significance. Its realization will be a milestone in the history of human achievement. Its importance and the consequences for biology justify the highest priority among scientific objectives in space—indeed in the space program as a whole. (p. 15)
The SSB’s report called for the development and launch of an Automated Biological Laboratory (ABL) to Mars as early as 1971.
NASA’s implementation of the ABL recommendation took the form of the twin Viking lander/orbiters to Mars in 1975. Each Viking lander carried a comprehensive suite of scientific instruments, including three specifically designed to search for signs of martian life. While both landers and orbiters were great scientific successes, they proved to be a programmatic dead end. The failure of the Viking gas chromatograph-mass spectrometer to find organics in the martian soil, coupled with generally ambiguous results of the life detection experiments, led most in the community to conclude that there was not evidence of life on Mars. Viking’s perceived failure set back exploration of the Red Planet by two decades.
The early 1990s were not a happy time for NASA. The agency suffered a series of budget cuts. The mirror of the Hubble Space Telescope (HST) was flawed. Major missions in development were descoped (e.g., Cassini) and others cancelled outright (e.g., the Comet Rendezvous/Asteroid Flyby). Moreover, in 1993 Mars Observer was lost 3 days prior to entering orbit. Just a few years later, however, space science discoveries began to make headlines on an almost weekly basis. Late 1995 and the first half of 1996 were particularly notable for the following news stories (Ehrenfreud et al. 2004, p. 452):
TABLE 1.1 Selected Reports by the National Academies on the Search for Life in the Solar System and Beyond
|1966||Biology and the Exploration of Mars||Highlighted the biological importance of Mars exploration and recommended spacecraft missions focusing on biology|
|1977||Post-Viking Biological Investigations of Mars||Assessed results from the Viking landers and recommended that future “detailed biological studies . . . be conducted on samples returned to Earth.”|
|1990||The Search for Life’s Origins: Progress and Future Directions in Planetary Biology and Chemical Evolution||A comprehensive review of theories of and venues for the origin of life and strategies for detecting life in the solar system and beyond (including a discussion of technosignatures).|
|1994||An Integrated Strategy for Planetary Sciences: 1995-2010||Highlights the importance of the search for the origins of life and the study and characterization of exoplanets.|
|2001||Astronomy and Astrophysics in the New Millennium||First astronomy decadal survey to mention exo/astrobiology and recognize the search for life as a key strategy for addressing “the fundamental goal of astronomy and astrophysics.”|
|2002||Signs of Life||A wide-ranging examination of biosignatures and techniques used to measure biosignatures.|
|2003||New Frontiers in the Solar System||Highlighted the role of astrobiology in providing “common thread for [addressing] some of the most exciting intellectual questions of our time . . .”|
|2005||The Astrophysical Context for Life||A review and assessment of the role played by astronomy and astrophysics in addressing the goals of astrobiology.|
|2007||An Astrobiology Strategy for the Exploration of Mars||An in-depth review of Mars as an abode of past and/or present life and strategies for its future study.|
|2007||The Limits of Organic Life in Planetary Systems||An initial examination of the possibility of non-terran life (i.e., life as we do not know it), its nature, and detectability.|
|2010||New Worlds, New Horizons in Astronomy and Astrophysics||One of the report’s three primary scientific objectives is seeking hahabitable planets|
|2011||Vision and Voyages for the Planetary Sciences in the Decade 2013-2022||One of the report’s three scientific themes is exploring planetary habitats. Recommends spacecraft missions to explore astrobiologically significant environments on Mars, Europa, and Enceladus.|
|2017||Searching for Life Across Space and Time||The proceedings of a workshop highlighting the current understanding of biosignatures detectable via in situ and remote-sensing techniques.|
- Identification of the first confirmed exoplanets;
- Results from NASA’s Galileo spacecraft suggesting that a liquid water ocean existed below the icy surface of Jupiter’s satellite, Europa;
- Discovery of the extent of Antarctica’s Lake Vostok and the realization that it was a terrestrial analog of a potentially habitable extraterrestrial environment;
- Observations of protoplanetary disks with the newly repaired HST; and
- Discoveries of the diversity of novel microbial lifeforms existing in extreme terrestrial environments.
The pace of popular interest in space-related activities reached a peak with the publication of a paper suggesting the presence of microbial fossils in the martian meteorite ALH84001. Although subsequent scientific analyses did not substantiate this claim, the wave of public interest in topics relating to the possibility of life beyond Earth reached to the highest levels. Most notably, President Clinton responded by supporting a congressional call for a “Space Summit.” Such a summit would
Allow the Administration to work with congressional leadership to develop a broad consensus on a balanced NASA program for the future . . . [and] provide an opportunity to discuss the recent evidence that life may have existed on Mars, as well as other significant advances in space science and technology. (Gibbons 1996)
In preparation for the summit, the SSB organized a workshop in October 1996 to discuss the implications of ALH84001 and other recent scientific advances relating to the search for the origins of life, planetary systems, stars, galaxies, and the universe (Ehrenfreud et al. 2004, p. 453). Key participants in the workshop subsequently briefed Vice President Gore on the workshop’s key finding that recent “breakthroughs are astonishing returns being reaped from years of investments in many science disciplines. Now is the time to leverage that investment and pursue the quest for origins into the 21st Century” (Canizares and Sargent 1996).
The outcome of these high-level discussions came a few months later with the inclusion of the “Origins Initiative” in the administration’s fiscal year 1998 budget proposal for NASA. In addition to the inclusion of significant funding increases for spacecraft missions in astrophysics and planetary science (particularly for Mars and Europa), the billion-dollar initiative included the establishment of a new program in astrobiology. The Astrobiology Program would subsume the existing Exobiology Program and have as its initial central feature the founding of a NASA Astrobiology Institute.
The 1998, 2003, and 2008 Astrobiology Roadmaps
Soon after the establishment of the NASA Astrobiology Institute (NAI) in 1998, work began on the drafting of the first astrobiology roadmap. The purpose of this document was to provide guidance for research and technology development relevant to astrobiology across NASA’s space and Earth science programs, as well as its human spaceflight program. More specifically, it was to provide a scientific framework for the nascent NAI and to serve as a reference for NASA astrobiology funding opportunities more generally. The following three fundamental questions formed the roadmap’s foundations (NASA 1998):
- How does life begin and evolve?
- Does life exist elsewhere in the universe?
- What is life’s future on Earth and beyond?
Since all three questions represented long-term aspirations rather than issues addressable in the near-to-mid-term, the roadmap presented a series of 10 more specific goals.
The 1998 roadmap underwent a complete revision in 2003. The 2003 Roadmap was narrower in scope than its predecessor, particularly in its focus on microbial life and its elimination of objectives concerning human
spaceflight. It retained the three fundamental questions, reduced the number of goals to seven, and provided details regarding those goals in 18 specific objectives. The 1998 roadmap was distributed by NASA via the Internet and in informal media. The 2003 and 2008 roadmaps were published as articles in the journal Astrobiology (Des Marais et al. 2003, 2008).
The 2015 Astrobiology Strategy
In 2013, NASA initiated an ambitious activity to revise and extend the Astrobiology Roadmap. Through a series of in-person and virtual meetings, NASA and its contractor, Knowinnovation,1 collated and combined input from 100-plus leading members of the astrobiology community to create the 2015 Astrobiology Strategy (NASA 2015; hereafter referred to as the 2015 Astrobiology Strategy).
Unlike the preceding astrobiology roadmaps, the 2015 Astrobiology Strategy was organized around a series of major research themes spanning astrobiology, as follows:
- Identifying abiotic sources of organic compounds;
- Synthesis and function of macromolecules in the origin of life;
- Early life and increasing complexity;
- Coevolution of life and the physical environment;
- Identifying, exploring, and characterizing environments for habitability and biosignatures; and
- Constructing habitable worlds.
The current committee’s statement of task was to take the 2015 Astrobiology Strategy as its starting point and build on its foundation, emphasizing key scientific discoveries, conceptual developments, and technology advances since its publication. Rather than revisiting aspects that were already well covered in the existing document, the committee’s work necessarily focused on additional insights from recent advances in the field—intellectual (e.g., conceptual insights and frameworks, modeling), empirical (e.g., observations, discoveries, novel technologies), and programmatic. Therefore, it is useful to examine each of the 2015 Astrobiology Strategy research themes in more detail because they and the key questions deriving from them are revisited and built on in subsequent chapters.
Identifying Abiotic Sources of Organic Compounds
The key research areas identified within the 2015 Astrobiology Strategy’s thematic area on identifying abiotic sources of organic compounds are as follows:
- What were the sources, activities, and fates of organic compounds on the prebiotic Earth?
- What is the role of the environment in the production of organic molecules?
- What is the role of the environment on the stability and accumulation of organic molecules?
- What constraints can the rock record place on the environments and abiotic reactions of the early Earth?
Synthesis and Function of Macromolecules in the Origin of Life
The key research areas identified within the 2015 Astrobiology Strategy’s thematic area on the synthesis and function of macromolecules in the origin of life are as follows:
- What is the chemistry of macromolecular formation reactions?
- How does information transmission and chemical evolution occur?
- What are the chemical alternatives? How and why do they occur?
- What is the role of environment?
- Macromolecular function—how did physicochemical effects develop over time?
- What are the advanced steps of macromolecular function?
- What led to macromolecular complexity?
Early Life and Increasing Complexity
The key research areas identified within the 2015 Astrobiology Strategy’s thematic area on early life and increasing complexity are as follows:
- Origin and dynamics of evolutionary processes in living systems—theoretical considerations;
- Fundamental innovations in earliest life;
- Genomic, metabolic, and ecological attributes of life at the root of the evolutionary tree;
- Dynamics of the subsequent evolution of life; and
- Common attributes of living systems on Earth.
Coevolution of Life and the Physical Environment
The key research areas identified within the 2015 Astrobiology Strategy’s thematic area on the coevolution of life and the physical environment are as follows:
- How does the story of Earth—its past, present, and future—inform us about how the climates, atmospheric compositions, interiors, and biospheres of planets can coevolve?
- How do the interactions between life and its local environment inform our understanding of biological and geochemical coevolutionary dynamics?
- How does our ignorance about microbial life on Earth hinder our understanding of the limits of life?
Habitability and Biosignatures
The key research areas identified within the 2015 Astrobiology Strategy’s thematic area on identifying, exploring, and characterizing environments for habitability and biosignatures are as follows:
- How can we assess habitability on different scales?
- How can we enhance the utility of biosignatures to search for life in the solar system and beyond?
- How can we identify habitable environments and search for life within the solar system?
- How can we identify habitable planets and search for life beyond the solar system?
Constructing Habitable Worlds
The key research areas identified within the 2015 Astrobiology Strategy’s thematic area on constructing habitable worlds are as follows:
- What are the fundamental ingredients and processes that define a habitable environment?
- What are the exogenic factors in the formation of a habitable planet?
- What does Earth tell us about general properties of habitability (and what is missing)?
- What are the processes on other types of planets that could create habitable niches?
- How does habitability change through time?
The 3 years between the publication of the 2015 Astrobiology Strategy and drafting of the current report have seen significant scientific, technological, and programmatic advances in the quest for life beyond Earth. For example, in January 2015, the Kepler spacecraft had identified about 1,000 confirmed exoplanets and its follow-on mission, the Transiting Exoplanet Survey Satellite (TESS), was still in development. Today, Kepler’s observations have confirmed the existence of some 2,600 exoplanets, and TESS is now in orbit. These discoveries include complex multiplanet systems and Earth-size bodies. Closer to home, small particles of silica and molecular hydrogen have been observed within the plume of material emenating from the south pole of Enceladus, suggesting ongoing hydrothermal activity on this moon of Saturn. Similarly, strong seasonal variations in the trace amounts of methane observed in the martian atmosphere provide tantalizing hints about possible biological or geologic activity on Mars. While on Earth, the studies of the noble gas components in subsurface waters has demonstrated that such aqueous environments can be preserved on a billion-year timescale. The preservation of a habitable environment on Earth and by extension to all rocky planets for a billion years or more has major implications for the search for life in the solar system and beyond.
Finding: Given the considerable rate of advancement in astrobiological science since the 2015 NASA Astrobiology Strategy was published, significant strategy updates and new discoveries can be addressed in this report.
Against the backdrop of these and other rapid advances, Congress directed NASA (see Appendix A) to enter into an arrangement with the National Academies of Sciences, Engineering, and Medicine to develop a science strategy for astrobiology to address the search for life’s origin, evolution, distribution, and future in the universe. In response to the congressional mandate, NASA’s associate administrator for the Science Mission Directorate approached the SSB with a request to carry out this study (see Appendixes A and B).
It is often said that astrobiology is a highly interdisciplinary science, but this simple statement may not fully capture the degree to which the field requires integration and synthesis of an enormous range of subject matters and disciplines including application across deep time. The search for life in the universe requires a rigorous open mindedness, constant questioning of paradigms, and nimble reevaluation of criteria and search strategies in response to intellectual innovation and conceptual advances, and technological advances and discoveries both expected and unanticipated. For example, the origin of life is one of the most profound questions addressed by astrobiology. Research on this question frequently takes the form of “bottom-up” investigations, which focus on prebiotic synthesis in various plausible early Earth environments that lead to the molecules necessary for life’s emergence. Alternatively, research in this area is “top-down” and focuses on the emergence of the most fundamental aspects of extant life, such as the genetic code and the assembly of primitive enzymes. Yet understanding the prebiotic pathways that led to life’s emergence on Earth is only one aspect of the more universal search for life through both time and space. That endeavor requires more cross-cutting research that focuses not only on life itself, but also on the role life plays in evolving planetary processes and how these processes in turn influence the evolution of life—for example, by providing access to chemical pathways that dissipate planetary-scale, time-dependent variations in heat, enthalpy, and entropy (i.e., thermodynamic disequilibrium). Further, to extend the study of the origin of life to celestial bodies beyond Earth, these perspectives require careful integration with inferences about the environments and conditions that may have existed on extraterrestrial bodies both within and beyond the solar system at any putative time when life may have arisen. Finally, emergence-of-life studies are also concerned with understanding how such life might have left its signature.
Studies such as these require not only the coming together of scientists from an extraordinarily wide range of disciplines, but also a true synthesis of their varied perspectives and approaches to produce a model describing both processes and products that are as complete as possible. Such models also need to satisfy the scientific criteria of experimentation, form testable hypotheses, allow modification of hypotheses based on observation and measurement, and embody forward predictive power. Astrobiology is thus inherently a systems-level science in
which an integrated view across many disciplines is essential for major progress on fundamental questions—Life affects its Environment. At the same time, the Environment affects Life (NASA 2015).
Vision and Voyages for Planetary Science in the Decade 2013-2022 (NRC 2011) captured much of this by identifying the necessity in understanding how the myriad chemical and physical processes that define the solar system, and specifically planetary formation, evolve over time to impact habitability and the potential for life. This report extends that thinking a step further and emphasizes the coevolution of planetary bodies and life, both in the solar system and beyond. Systems science is a holistic approach to a problem that considers as many of the constituent parts and dynamics of the system as is possible. There are many pertinent examples for astrobiology. For the study of the origin of life, this approach derives information from preexisting conditions, the interplay of the planetary and chemical environment, and the subsequent coevolution of life and the environment. For the exploration of planets, systems science approaches offer the chance to better select landing sites by, for instance, evaluating how an environment may have sustained life over long timescales and concentrated evidence of that life in the rock record. For exoplanets, such an approach enables an assessment of how a given atmospheric signature is related to the planet and potential biosphere, given the multiple potential sources that may have given rise to that gas. As an example, methane is produced by geological processes as well as living systems, and oxygen can be produced photochemically in atmospheres without biogenic sources of oxygen.
The 2015 Astrobiology Strategy emphasized the evolution of astrobiology to more effectively link “astro” and “life,” in the sense that the intersection of life and environment is what makes a planetary body habitable. The coevolution of life and the physical and chemical environment is a core feature of astrobiology.
Finding: Astrobiology system science is the integrative study of the interactions within and between the physical, chemical, biological, geologic, planetary, and astrophysical systems as they relate to understanding how an environment transforms from nonliving to living and how life and its host environment coevolve.
Coevolution of life and its physical environment is indeed a powerful lens and strategy for investigation of the origin of life, for understanding early Earth and the evolution of life on Earth, for the search for life, and for the characterization of habitable environments and search for life in the solar system and beyond. The committee’s charge is to take account of and build on NASA’s current 2015 Astrobiology Strategy and outline key scientific questions and technology challenges in astrobiology in the light of recent discoveries and conceptual development and models. To address this charge, a considerable amount of space (the next three chapters) is devoted to a systems-science approach to the major developments in key scientific questions and challenges in the context of three evolving conceptual frameworks.
- Chapter 2 is organized around the concept of dynamic habitability; that is, the habitability of an environment is not a binary (yes/no) characteristic but is a continuum that evolves over time.
- Chapter 3 is devoted to comparative planetology and multiparameter habitability assessment.
- Chapter 4 discusses the identification and interpretation of biosignatures.
In each of these three chapters, the committee has outlined areas where significant scientific progress has been made in the last few years and has led to new understanding and the formulation of new questions and/or research themes not addressed in, or substantially developed since, the 2015 Astrobiology Strategy. References are provided throughout to provide a snapshot of the breadth of the field, but are not meant to be exhaustive. Where possible review papers are provided to serve as an entry point to the literature. Due to the limits of the statement of task and the amount of time for this endeavor, the reader is referred to the general literature for a complete list of research papers. Please note that in Chapter 3, rather than the traditional separate treatment of in situ and remote-sensing techniques, the committee has endeavored to integrate the discussion of in situ and remote-sensing, despite the fact that this attempt to break down barriers between different communities and their favorite techniques may introduce some unavoidable duplication of material.
The remaining chapters are organized as follows:
- Chapter 5 illustrates how the systems science approach to astrobiology developed in Chapters 2, 3, and 4 is supported and contextualized within a dynamic and evolving technology and programmatic landscape.
- Chapter 6 integrates the advances and opportunities in Chapter 5 to identify the most promising key research goals in the field of the search for signs of life in which progress is likely in the next 20 years and to discuss which of the key goals can be addressed by U.S. and international space missions and ground telescopes in operation or in development.
- Chapter 7 contains a discussion of how expanded partnerships—for example, interagency, international, and public/private—can further the study of life’s origin, evolution, distribution, and future in the universe.
For readers interested in the committee’s responses to specific aspects of its charge, the committee offers the following mapping:
- Take account of and build on NASA’s current 2015 Astrobiology Strategy (see Chapter 1).
- Outline key scientific questions and technology challenges in astrobiology, particularly as they pertain to the search for life in the solar system and extrasolar planetary systems (see Chapters 2, 3, 4 and 5).
- Identify the most promising key research goals in the field of the search for signs of life in which progress is likely in the next 20 years (see Chapter 6).
- Discuss which of the key goals could be addressed by U.S. and international space missions and ground telescopes in operation or in development (see Chapter 6).
- Discuss how to expand partnerships (interagency, international, and public/private) in furthering the study of life’s origin, evolution, distribution, and future in the universe (see Chapter 7).
- Make recommendations for advancing the research, obtaining the measurements, and realizing NASA’s goal to search for signs of life in the universe (see Chapters 2, 4, 5 and 7).
The committee notes that achieving a clean mapping between specific aspects of the charge (e.g., bullet items 3 and 4) necessitated the repetition of some material from Chapters 2, 3, 4, and 5 in Chapter 6. The committee believes that such repetition is acceptable and appropriate because the average reader is more likely to be interested in the committee’s views on a specific topic than they are to read the report from cover to cover. To reduce repetition the reader is referred to the earlier chapters for the main literature references on key concepts, and referencing is more streamlined in Chapter 6.
Canizares, C.R., and A.I. Sargent, eds. 1997. The search for origins: Findings of a space science workshop. Available as Appendix A.2 of National Research Council, Space Studies Board Annual Report 1996, National Academy Press, Washington, D.C., 1997.
Des Marais, D.J., L.J. Allamandola, S.A. Benner, A.P. Boss, D. Deamer, P.G. Falkowski, J.D. Farmer, et al. 2003. The NASA Astrobiology Roadmap. Astrobiology 3(2):219-235.
Des Marais, D.J., J.A. Nuth III, L.J. Allamandola, A.P. Boss, J.D. Farmer, T.M. Hoehler, B.M. Jakosky, et al. 2008. The NASA Astrobiology Roadmap. Astrobiology 8(4):715-730.
Ehrenfreund, P., W.M. Irvine, T. Owen, L. Becker, J. Blank, J.R. Brucato, L. Colangeli et al., eds. 2004. Astrobiology: Future Perspectives. Astrophysics and Space Science Library Series, Vol. 305. Springer, The Netherlands.
Gibbons, J.H. 1996. Assistant to the President for Science and Technology, Letter to D.S. Goldin, Administrator, NASA, on September 25.
NASA (National Aeronautics and Space Administration). 1998. Astrobiology Roadmap. NASA Astrobiology Program, Washington, DC. https://nai.nasa.gov/media/roadmap/1998/introduction.html.
NASA. 2015. NASA Astrobiology Strategy 2015. NASA Astrobiology Program, Washington, DC.
NRC (National Research Council). 1966. Biology and the Exploration of Mars. National Academy Press, Washington, DC.
NRC. 2011. Vision and Voyages for Planetary Science in the Decade 2013-2022. The National Academies Press, Washington, DC.