Crosscutting themes in planetary science deal with issues of profound importance that have been pondered by scientists and non-scientists alike for centuries. They cannot be fully addressed by a single spacecraft mission and will likely not be completely addressed in this decade or the next. The themes are not new; their like can be found in past reports.1,2 They explain why planetary science is an important undertaking, worthy of public support. The committee identifies three themes of particular interest for the next decade; these are stimulated by recent advances in planetary science as well as by their fundamental nature.
• Building new worlds—understanding solar system beginnings,
• Planetary habitats—searching for the requirements for life, and
• Workings of solar systems—revealing planetary processes through time.
Each theme brings its own set of questions, based on current understanding of the underlying scientific issues. Each question represents a distillation of major areas of research in planetary science, and the questions themselves are sometimes crosscutting. Each question points to one or more solar system bodies that may hold clues or other vital information necessary to resolve the questions. Subsequent chapters (4-8) further explore these questions, dissecting them to identify the specific opportunities best addressed in the coming decade by large, medium, and small spacecraft missions, as well as by other space- and ground-based research activities. As outlined in the sections that follow, in situ analyses and ultimately sample return will be required to achieve major breakthroughs in addressing many of these questions.
Building New Worlds
A little over 4.5 billion years ago, a small clump of gas and dust within a giant molecular cloud began to collapse, perhaps triggered by the shockwave from a nearby supernova. The clump was mainly hydrogen and helium gas, slightly enriched with a percent or two of heavier elements—remnants of older generations of stars. Some 100,000 years later, gravity and inertia had shaped the clump into a flattened, swirling disk of material
with a nascent star at its core. After another 50 million years or so, the center of this “protostar” was hot enough that hydrogen fusion began: the Sun was born. Within the disk of debris whirling around the infant star, planet formation began. Gases condensed onto dust and ices, and the ice and dust began to accrete and grow into the precursors of planets: planetesimals. These collided with each other, growing ever larger and more complex. The end result was the diverse suite of planetary bodies seen in the solar system today; planetary systems around other stars are beginning to display even more diversity. Three major questions emerge from this story of the formation and evolution of the solar system:
• What were the initial stages, conditions, and processes of solar system formation and the nature of the interstellar matter that was incorporated?
• How did the giant planets and their satellite systems accrete, and is there evidence that they migrated to new orbital positions?
• What governed the accretion, supply of water, chemistry, and internal differentiation of the inner planets and the evolution of their atmospheres, and what roles did bombardment by large projectiles play?
As the solar system formed, at least one planetary body experienced a remarkable event: life began, proliferated, and developed to the point that humankind now ponders its own origins. Was the origin of life a unique event or was it repeated elsewhere in the solar system or in extrasolar planetary systems? What conditions are required? The fundamental question is broader than whether or not life exists or existed on one particular planetary body like Mars, Europa, or elsewhere. Rather, the question is how life came to exist at all. Although the mechanisms by which life originated are as yet unknown, the processes likely involve the simultaneous presence of organic compounds, trace elements, water, and sources of energy. Demonstrating that other planetary environments are abodes for life will help to elucidate the origins of Earth’s life. To explore this, the following questions about past and present planetary environments that could foster life need to be addressed:
• What were the primordial sources of organic matter, and where does organic synthesis continue today?
• Did Mars or Venus host ancient aqueous environments conducive to early life, and is there evidence that life emerged?
• Beyond Earth, are there contemporary habitats elsewhere in the solar system with necessary conditions, organic matter, water, energy, and nutrients to sustain life, and do organisms live there now?
Workings of Solar Systems
The solar system displays a rich panoply of planetary environments. The known planetary systems around other stars are beginning to display an even greater range of planetary architectures. Comprehending this diversity requires a detailed understanding of the physical and chemical properties and processes that shape planetary interiors, surfaces, atmospheres, rings, and magnetospheres. Relevant interior processes include, for example, chemical differentiation, core formation, and heat transfer throughout planetary history. Impact cratering, tectonism, and volcanism are important geologic processes that have shaped planetary surfaces. Planetary atmospheres hold a record of the volatile evolution of a planet and the interactions among surfaces, weather, and climate. Equally important is to understand the intricate balance of a planet with its environment, an environment crafted and maintained by the host star that dominates the planetary system. Host stars, such as the Sun, have their own life cycle much as planets do, and the changes during that cycle play a profound role in modifying the attendant planets. A variety of critical questions arise about how planetary systems function:
• How do the giant planets serve as laboratories to understand Earth, the solar system, and extrasolar planetary systems?
• What solar system bodies endanger Earth’s biosphere, and what mechanisms shield it?
• Can understanding the roles of physics, chemistry, geology, and dynamics in driving planetary atmospheres and climates lead to a better understanding of climate change on Earth?
• How have the myriad chemical and physical processes that shaped the solar system operated, interacted, and evolved over time?
Table 3.1 summarizes the questions and destinations for the next decade that are discussed more fully in the rest of this chapter; they are examined in much greater detail in Chapters 4 through 8. Table 9.4 in Chapter 9 links these questions and destinations to the committee’s recommended missions.
TABLE 3.1 The Key Questions and Planetary Destinations to Address Them
|Crosscutting Themes||Priority Questions||Key Bodies to Study|
|Building new worlds||1. What were the initial stages, conditions and processes of solar system formation and the nature o he interstellar matter that was incorporated?||Comets, Asteroids, Trojans, Kuiper belt objects (see Chapter 4)|
|2. How did the giant planets and their satellite system ccrete, and is there evidence that they migrated t ew orbital positions?||Enceladus, Europa, Io, Ganymede, Jupiter, Saturn, Uranus, Neptune, Kuiper belt objects, Titan, rings (see Chapters 4, 7, and 8)|
|3. What governed the accretion, supply of water, chemistry, and internal differentiation of the inne lanets and the evolution of their atmospheres, an hat roles did bombardment by large projectiles play?||Mars, the Moon, Trojans, Venus, asteroids, comets (see Chapters 4, 5, and 6)|
|Planetary habitats||4. What were the primordial sources of organic matter, and where does organic synthesis continue today?||Comets, asteroids, Trojans, Kuiper belt objects, uraniaun satellites, Enceladus, Europa, Mars, Titan (see Chapters 4, 5, 6, and 8)|
|5. Did Mars or Venus host ancient aqueou nvironments conducive to early life, and is ther vidence that life emerged?||Mars and Venus (see Chapters 5 and 6)|
|6. Beyond Earth, are there modern habitats elsewher n the solar system with necessary conditions, organi atter, water, energy, and nutrients to sustain life, an o organisms live there now?||Enceladus, Europa, Mars, Titan (see Chapters 6 and 8)|
|Workings of sola ystems||7. How do the giant planets serve as laboratories t nderstand Earth, the solar system, and extrasola lanetary systems?||Jupiter, Neptune, Saturn, Uranus (see Chapter 7)|
|8. What solar system bodies endanger Earth’s biosphere, and what mechanisms shield it?||Near-Earth objects, the Moon, comets, Jupiter (see Chapters 4, 5, and 7)|
|9. Can understanding the roles of physics, chemistry, geology, and dynamics in drivin lanetary atmospheres and climates lead to a bette nderstanding of climate change on Earth?||Mars, Jupiter, Neptune, Saturn, Titan, Uranus, Venus (see Chapters 5, 6, and 8)|
|10. How have the myriad chemical and physica rocesses that shaped the solar system operated, interacted, and evolved over time?||All solar system destinations. (see Chapters 4, 5, 6, 7, and 8)|
What Were the Initial Stages, Conditions, and Processes of Solar System
Formation and the Nature of the Interstellar Matter That Was Incorporated?
A nearby supernova explosion may have initially triggered the collapse of the local molecular cloud and thereby the onset of solar system formation.3 Many primitive bodies—asteroids, comets, meteorites, Kuiper belt objects, Trojan asteroids, and bodies in the distant Oort cloud—still contain intact records of this very early period. Examination of their minerals and their isotopic and molecular chemistry can reveal the physical conditions under which they formed and provide our best view into this earliest chapter of solar system formation. In fact, we may see isotopic evidence of such a supernova explosion in ancient meteorite samples.4 The least-processed of the primitive meteorite samples preserve tiny presolar grains, whose isotopic compositions reflect the nucleosynthetic processes in stars and supernovae that preceded solar system formation.5 These presolar stellar remnants provided key ingredients (e.g., metals and silicates) for the accretion of planets.
In the past decade major progress has been made in linking the compositions of presolar grains in chondritic meteorites to the specific stellar environments where they are formed.6 Unexpectedly, presolar grains were in low abundance in comet samples returned by the Stardust mission, signaling limited understanding as to how and where presolar grains were incorporated into the solar nebula.7 Recent studies of organic matter in these materials are starting to reveal how carbon-based molecules formed in interstellar space are further processed and incorporated. Most of the presolar grains recognized so far are carbon (diamond and graphite) or carbides;8 important questions remain as to the abundance of presolar silicates and oxides and how the compositions of presolar grains and organic molecules differ among comets.
After the Sun formed, the solar nebula gradually began to coalesce and form clumps that, in turn, accreted into planetesimals. In the inner solar system where conditions were hotter, primitive asteroids and meteorites record early events and processes in the solar nebula whereby interstellar solids melted, evaporated, and condensed to form new compounds. Farther out, beyond the “snow line,” it was cooler and volatiles condensed as ices; there the giant planets and their satellite systems began to form. In that region and extending farther out where temperatures were extremely low, minimizing chemical processing, the parent objects of comets formed. They retain the most pristine records of the initial chemistry of the outer parts of the solar nebula. The size distributions of objects in the Kuiper belt reveal the nature of accretion in the outer region that was arrested early, stopping their growth.9,10 Mixing of materials between nebular regions is clearly shown by the diverse components in the Stardust comet samples.11 It also now appears that some differentiated asteroids formed earlier than the primitive chondrites, showing that the accretional sequences were far more complex than once thought.12
Over the next decade important breakthroughs in understanding of presolar materials and early nebular processes will certainly come from applying ever advancing analytical techniques in the analysis of meteorites, interplanetary dust, and Stardust samples. However, greater potential to achieve major steps in understanding of presolar and nebular cosmochemistry would come from the analysis of samples returned directly from the surfaces of comets. Stardust samples have dramatically expanded our knowledge of presolar sources and nebular processes. Eventually, the greatest scientific breakthroughs in addressing these questions will come from studying returned surface samples whose volatiles have been cryogenically preserved.
How Did the Giant Planets and Their Satellite Systems Accrete,
and Is There Evidence That They Migrated to New Orbital Positions?
The terrestrial planets grew only to relatively small sizes owing to the scarcity of metal and silicate grains in the inner solar nebula. However, the ices that condensed from the nebular gas beyond the snow line were more abundant. Planetary scientists witness similar processes ongoing in exoplanetary systems. Thus, the planetary embryos of the giant planets grew rapidly in the first few million years until they became massive enough to capture directly the most abundant elements in the solar nebula, hydrogen and helium. Jupiter’s enormous size is very likely correlated to its position just outside the snow line: water vapor driven out across this boundary would rapidly condense and
pile up; solid particles orbiting outside the snow line experienced a low pressure zone and sped up owing to the reduced drag, thus slowing their migration inward. In this way, Jupiter’s feeding zone was extremely well supplied.
The regular satellites of Jupiter, Saturn, and Uranus orbit in their equatorial planes, suggesting that they formed in subnebular disks like miniature solar systems.13 Neptune’s coplanar satellite system was likely destroyed by the capture of Triton, its large retrograde satellite, probably a renegade Kuiper belt object. Too small to capture much gas gravitationally, the satellites accreted mainly from icy and rocky solids. They might have captured gases in clathrates (i.e., water-ice cages) or in amorphous ices. If their icy solids came directly from the solar nebula, they would retain nebular volatile abundances. Cassini-Huygens data suggest this to be the likely case for Saturn’s moons and Titan in particular.14,15 If they were formed in the gas-giant subnebulae, dependent on the radial temperature profile, some regions would be hot enough to vaporize ices, resetting isotopic thermometers and phases before they re-condensed. Such subnebula processing is speculated for Jupiter’s regular satellites, but crucial measurements are lacking. Untangling nebula versus subnebula processes requires knowing the internal structures of the satellites; abundances of volatile ices; stable isotope ratios of carbon, hydrogen, oxygen, and nitrogen; and abundances of the noble gases. Addressing these key questions will require precise geophysical, remote sensing, and in situ measurements across the outer planet satellites of their internal structures and their compositions from their plumes, sputtered atmospheres, co-orbiting tori, and surfaces.
Many unknowns remain as to how the outer planets formed out of the solar nebula and if and when they migrated into different orbits. This is also an important question with regard to exoplanets. The Galileo probe sent into Jupiter’s atmosphere showed quite surprisingly that the noble gases argon, krypton, and xenon are much more abundant there than in the Sun. Suggested explanations for their concentration include condensation of noble gases on extremely cold nebular solids, capture of clathrate hydrates, evaporation of the protoplanetary disk before Jupiter formed, and outgassing of noble gases from the deep interior enriching them in the atmosphere.16 Each of these hypotheses leads to testable predictions for noble gas abundances in the other giant planets—definitive answers will require in situ probe measurements—critical data that researchers lack for Saturn, Uranus, and Neptune.
Resolving a second major puzzle also mandates probe measurements. Solar system models that placed the formation of Uranus and Neptune at their current positions were unable to produce cores of the ice giants rapidly enough. Modelers concluded that the giant planets must have migrated to new orbits after their formation. It is now thought that during the first half billion years of the solar system, Uranus and Neptune orbited in the region much closer to the Sun; it is even possible that Neptune was inside Uranus’s orbit.17,18 The models suggest that at about 4 billion years ago Saturn and Jupiter entered a 2:1 orbital resonance, increasing Saturn’s eccentricity and thereby driving Uranus and Neptune out into the Kuiper belt, which in turn was driven out to its current location.
Many variations of such scenarios have been hypothesized. However, key evidence is lacking, and a complete understanding of the formation and migration of the four planets that account for 99 percent of the mass in the solar system, excluding the Sun, awaits key measurements at Saturn, Uranus, and Neptune. To distinguish between the array of theorized scenarios for formation and migration of the giant planets and their satellite systems, scientists need to know detailed composition—deuterium/hydrogen and hydrogen/helium ratios, other isotopic ratios, and information about noble gases that can only be obtained in situ from giant-planet atmospheric probes. To address these questions, detailed in situ measurements as acquired by the Galileo probe of the compositions of the atmospheres of Saturn, Uranus, and Neptune are of high priority.
What Governed the Accretion, Supply of Water, Chemistry, and Internal
Differentiation of the Inner Planets and the Evolution of Their Atmospheres,
and What Roles Did Bombardment by Large Projectiles Play?
Planetary researchers now think that the presolar silicate and metallic materials in the hot inner solar nebula accreted quite early, gathering into on the order of a hundred Moon-to-Mars-size planetesimals.19 Owing to the scarcity of such material in the nebula these protoplanets would have ceased growing very early, approximately when the Sun’s T Tauri phase began. Over the next ~100 million years the terrestrial planets grew from the collisional merging of these objects;20 the Moon is hypothesized to have formed in this period by a glancing collision of a Mars-size planetesimal with Earth. If Jupiter and Saturn entered a 2:1 orbital resonance ~4 billion years ago, they
triggered an orbital reshuffling and bombardment that reshaped the inner solar system.21 As Uranus and Neptune surged outward, Kuiper belt objects would have been scattered—many, shed into the inner solar system, could have delivered water and other volatiles to the terrestrial planets as a late veneer.22 Most objects in the asteroid belt were also scattered, some inward, delivering more water to the inner planets. These two impacting populations are hypothesized to have caused the late heavy bombardment that had been suggested in the lunar cratering record; its timing may be linked to the emergence of life on Earth.23,24
Because asteroids and Kuiper belt objects were important ingredients in the recipe for the terrestrial planets, they retain many clues to early evolution of the inner planets. Researchers currently know very little about the composition and physical characteristics of Trojans and Centaurs. Like Centaurs, Trojans may come from the Kuiper belt but may have been formed closer in near Jupiter. Obtaining information by direct spacecraft observations will help constrain existing models of the origins of these bodies. Study of these objects is important because they may contain key information about the parent materials that accreted in the inner solar system. An important science goal for this decade is to begin the scientific exploration of the Trojan asteroids.
The distribution of bodies in the Kuiper belt may provide key evidence about the orbital migration of the giant planets.25 Measuring the time of formation of individual components that constitute comets will constrain the evolution of objects beyond the orbit of Neptune. Refractory inclusions in the Stardust sample from comet Wild 2 suggest that inner solar system material was mixed out into the Kuiper belt zone.26 Determining the hydrogen and other crucial isotopic ratios in multiple comets from samples of their nuclei could help to address major questions about the roles comets played in delivering water and other volatiles to the inner solar system and in particular to early Earth.
Soon after the terrestrial planets formed, their interiors differentiated into rocky crusts and mantles and metallic cores; they continued to dissipate internal energy through mantle convection, magnetic field generation, and magmatism. Earth, the Moon, and Mars all show isotopic evidence that they had differentiated only 10 million to 50 million years after formation; this was very likely the case for Venus and Mercury as well.27 To understand the subsequent evolution of these bodies it is necessary to know their bulk chemistries and internal structures. Geophysical exploration of the internal structure of the Moon and Mars with a global seismic network remains an achievable goal of exceptional scientific importance. Lunar samples indicate that the Moon formed hot with a deep magma ocean; magma oceans may have been common to all terrestrial planets. Analysis of ancient samples excavated from the deep interior during formation of the Moon’s South Pole Aitken Basin could yield deep insights into the earliest stages of Earth-Moon formation and evolution, opening records that have vanished from Earth.
Major questions remain regarding how and when water and other volatiles were delivered to Earth. What fraction of Earth’s volatile inventory was delivered directly by planetesimals during accretion and later outgassed to the surface during differentiation and subsequent volcanism? What fraction was acquired as a late veneer from the impact of comets and volatile-rich asteroids during the late heavy bombardment? Clues to address these questions could be found locked in chemical signatures at the surfaces and in the atmospheres of Earth’s neighbors.
Venus and Mars formed at orbital radii that bracket Earth’s. The isotopic, elemental, molecular, and mineralogic records retained in their surfaces and atmospheres can be studied to reveal radial gradients in the accreting sources of volatiles, the early transport of volatiles into the inner solar system by collisional and gravitational scattering and mixing, and the relative importance of asteroidal and cometary sources in delivery of a late-arriving veneer. For example, Galileo and Venus Express results show that Venus’s highlands may be more silicic, suggesting early eruption of hydrous magmas.28,29 The critical questions of volatile origin for Venus can best be addressed by in situ measurement of the noble gases and molecular and isotopic chemistry in the atmosphere, as well as the geochemistry and mineralogy of its surface. Scientists have gleaned nearly all that can be learned from martian meteorites, likely a highly biased sample based on their young radiometric ages compared to crater counting ages of the martian surface.30 The origin and evolution of volatiles on Mars appear to have a complex, many-staged history. While significant information has been obtained from martian meteorites and current in situ missions on the chemistry of the atmosphere as well as the geochemistry and mineralogy of the surface, many newly identified geochemical environments have not been observed in situ, and significant advances will be obtained through the return of martian samples, which can be studied with the most sophisticated instruments using highly diverse analytic techniques not possible with a single surface mission.
What Were the Primordial Sources of Organic Matter,
and Where Does Organic Synthesis Continue Today?
Organic molecules—crucial to life—are widespread across the solar system. Major progress has been made in the past decade in tracing their origins from the most primitive presolar sources to the active environments and the physical and chemical processes by which they are being created and destroyed today. Tracing their origins and evolution traces ours, for without the complement of organics delivered to or chemically synthesized on early Earth, life here would not exist.
Researchers are beginning to understand how carbon-based molecules were formed in interstellar space and later combined into other complex molecules in the solar nebula and in planetesimals. Satellites of the outer solar system are rich in organics. The complex array of organic molecules and the active chemistry in Titan’s atmosphere and at its surface afford an invaluable laboratory to understand prebiotic chemical processing on a planetary scale.31 Organics in the polar jets of Enceladus signal that the icy satellites harbor organic molecules and processes in their interiors—key indicators if life were ever to emerge in such subsurface environments.32 Evidence for methane, one of the simplest organic compounds, has been reported in the martian atmosphere, leading to testable hypotheses for its origin, whether geochemical or biogenic.33
Interstellar molecular clouds and circumstellar envelopes are space environments where solid-state chemical reactions form a variety of complex molecules. Organic compounds are ubiquitous in the Milky Way and other galaxies and include nitriles, aldehydes, alcohols, acids, ethers, ketones, amines, and amides, as well as long-chain hydrocarbons.34,35 The origin of organic molecules in meteorites is complex; some compounds formed as coatings on presolar dust grains in molecular clouds, and others were altered in the warmed interiors of planetesimals when ices in these bodies melted.36 Their chemistries span a range of molecules including amino acids; these molecules provide a partial picture of the prebiotic components that led to life. But scientists lack critical information on organic components in comets and Kuiper belt objects and on how the compositions of organic molecules may vary among these bodies. What fraction of comet material remains pristine, maintained at low temperatures with little modification? How much mixing occurred across the solar nebula as suggested by high-temperature silicates in Stardust samples? What kinds of reactions occurred between organic compounds and silicate or oxide grains? Analysis of elemental, isotopic, organic, and mineralogic composition of organic-rich asteroid and comet surface samples (eventually, cryogenically preserved samples) using the most advanced analytic laboratory techniques holds the greatest potential for addressing these fundamental questions and tracing the origins and sources of primitive organics that led to life in the inner solar system.
Titan is the richest laboratory in the solar system for studying prebiotic chemistry with a broad range of active organic synthesis. A few percent methane in the thick cold nitrogen atmosphere moves in a global cycle, forming clouds, rain, rivers, lakes, and seas that strikingly resemble Earth’s hydrologic cycle. Exposed to solar ultraviolet radiation and plasma particles in the upper atmosphere, methane and nitrogen are broken into radicals and ions that recombine in multiple stages to form a plethora of hydrocarbons and nitriles, ranging from simple gases to large complex molecules. As these compounds descend they condense as liquids and as a haze of organic aerosols (tholins) that rain onto the surface.
A key question posed by Cassini data is, Do the organics contain amino acids and the building blocks of nucleotides?37 Because the prebiotic chemical pathways of its organic evolution may hold keys for understanding the origin of life on Earth, direct examination of the organic species and active processes and the isotopic both from the surface and in the atmosphere, has become one of the highest scientific priorities for the future.
Icy satellites display abundant evidence for organic chemistry. At Saturn, Iapetus and Phoebe are largely covered by complex dark organics. Ganymede and Callisto and all five uranian satellites exhibit numerous dark organic-rich geologic units. Cassini discovered that the plumes emanating from Enceladus’s interior contain organic compounds that could be primordial, derived from accretion, or that might be generated by chemical reactions such as Fischer-Tropsch reactions (hydrogen and carbon monoxide) or serpentinization reactions (water, carbon dioxide, and silicates).38 As discussed below, Europa probably harbors a subsurface ocean at a shallow depth,
giving it high potential as a habitat for life. Plumes have been observed on several geologically active bodies (Io, Enceladus, and Triton), and it would not be surprising to find them on Europa as well.39 If so, characterization of Europa’s organics could be done in situ as for Enceladus. High-priority science goals that emerge from this key question are to identify organic molecules and characterize processes of organic synthesis in the interiors and at the surfaces of Europa and Enceladus as well as Titan.
Evidence for the possible presence of methane in Mars’s atmosphere is a most remarkable recent report. If methane is confirmed it is likely being continually generated, because the current abundance reported could be photochemically destroyed in only a few hundred years. All of the possible processes that have been suggested would operate in the subsurface: geologic processes including metamorphic reactions of ultramafic rocks with carbonic acid (serpentinization), thermal decay of organics, or even conceivably by extant subsurface microorganisms. New questions and new goals arise. Can the detection of methane be confirmed, and how can researchers test hypotheses for its origin?
The ESA-NASA Mars Trace Gas Orbiter mission now under development for launch in 2016 seeks to answer these questions. It will map key isotopes and trace gases in an attempt to assess the geological or possible biological activity by which methane is evolved. Whether other complex organic compounds could have been produced in early reducing atmospheric conditions, or by mineral-catalyzed reactions, perhaps continuing in the subsurface today, constitutes a most fundamental question in addressing whether life ever arose on Mars. The Mars Science Laboratory will begin to address these questions, making progress toward understanding carbon chemistry and early prebiotic processes. However, definitive answers to these key life-related questions will almost certainly require the return of samples from Mars.
Did Mars or Venus Host Ancient Aqueous Environments Conducive to Early Life,
and Is There Evidence That Life Emerged?
Today the surfaces of Mars and Venus are hostile environments for survival of any life. Venus’s massive carbon dioxide atmosphere exhibits an intense greenhouse effect enhanced by sulfuric acid clouds, resulting in a surface temperature of about 740 K. Mars’s surface environment is a cold desert that is chemically oxidizing; its sparse atmosphere allows intense solar ultraviolet radiation to bathe the surface; the existence of life on Mars’s surface today is likely prohibited. However, during the first half billion years or so of their early histories, the surface environments of Mars and Venus may have been wet, with temperatures and chemistries conducive for life to develop.40 Comparative planetology seeks to understand Earth’s processes and history (in this case, the early Earth) through study of its close neighbors. Beyond liquid water and clement stable environments, Earth-like life would require key organic molecules and energy sources. In exploring these ancient surface habitats and in searching for evidence that they once sustained life, all of these factors must be considered.
In the past decade our picture of ancient Mars has been dramatically advanced. The geology and mineralogy of the oldest terrains (Noachian period) that extend back into the period of intense bombardment provide convincing evidence that there was ample liquid water at the surface. The geologic indicators include high-density drainage networks, delta deposits, sedimentary fabrics formed in standing water, and evaporite deposits. This wet period evidently tapered off during the Hesperian period and had effectively ceased by 3.5 billion years ago.41
Why did this clement period not persist throughout geologic time? Mars Global Surveyor discovered a dynamo-driven magnetic field that could have held off the solar wind and protected the loss of the early thick atmosphere and its abundant water. But the dynamo was not sustainable and the field collapsed after a few hundred million years, close to the end of the late heavy bombardment, perhaps allowing much of the atmosphere to be eroded away.42 Scientists have also uncovered evidence that the chemical conditions in this postulated early warm aqueous period could have been very different from those in subsequent eras. With abundant water in a mainly carbon dioxide atmosphere, it would be expected that widespread carbonates would have formed. Although carbonate has been found in martian meteorites, searches for it exposed at the surface were unsuccessful for decades.
Mars Exploration Rover data show at two globally separate sites that in the early Hesperian the aqueous chemistry was dominantly acidic. Mars Express and Mars Reconnaissance Orbiter identified many isolated regions containing phyllosilicates in older deposits; these would have required more neutral conditions to form.
Perhaps long-lived, widespread volcanism led to a shift to sulfurous acidic conditions, erasing evidence of older carbonate-bearing rocks.43
Recently the Mars Reconnaissance Orbiter and the Spirit rover have found carbonate rocks in ancient strata. These Noachian carbonates must have formed under far less acidic conditions. Whether this shift occurred uniformly on a global scale or more locally, varying from region to region, or from surface to subsurface, remains controversial.44 Old carbonates have important ramifications: they suggest that this could have been the period in Mars’s history that was most conducive for the emergence of life at its surface. Armed with these new perspectives, researchers are poised to search Mars’s ancient records for both the organic building blocks and any evidence that points to the possible emergence and preservation of signs of life.
Theoretical studies suggest that prior to ~4 billion years ago the surface of Venus may have been far cooler than it is today, with liquid water, even oceans, at its surface leading to the possibility of early life.45 This would have been a time when solar luminosity was lower and when Venus’s thick, 100-bar atmosphere with its abundance of carbon dioxide and other greenhouse gases had not fully outgassed from the interior. Subsequently, as the Sun’s luminosity increased, water evaporated and carbon dioxide became ever more abundant, leading to a runaway greenhouse and to the current hot, dry surface environment.
Today Venus’s atmosphere shows a much higher ratio of deuterium to hydrogen than other solar system bodies, providing evidence that ancient water was photodissociated in the upper atmosphere and lost to space, although the rate remains under debate. Venus Express measurements provide evidence that water is still being lost, as the escaping hydrogen and oxygen occur in the 2:1 ratio for water. Characterizing Venus’s early environment, whether it was habitable with liquid water present, is a scientific high priority; this will require measurement of the molecular and isotopic composition of the lower atmosphere and the elemental and mineralogic composition of the surface.46
Beyond Earth, Are There Contemporary Habitats Elsewhere in the Solar System
with Necessary Conditions, Organic Matter, Water, Energy, and Nutrients
to Sustain Life, and Do Organisms Live There Now?
Habitats for extant life at the surfaces of planets are rare. Venus is too hot; the rest of the solar system surfaces, including that of Mars, are exposed to deadly solar ultraviolet and ionized particle radiation. If modern-day habitats for life do exist, most likely they are below the surface. Titan is the exception, the only world beyond Earth that harbors a benign, albeit extremely cold, surface environment that also is shielded from deadly radiation.
The committee herein makes the assumption that life elsewhere in the solar system will be like terrestrial life and thereby recognizable to researchers. For Earth-like life forms to arise and survive in subterranean planetary habitats would require liquid water; the availability of organic ingredients including carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur; and, absent sunlight, some form of chemical energy to drive metabolism. Mars, Europa, and Enceladus hold the greatest potential as modern habitats for Earth-like life, and Titan affords the greatest potential as a prebiotic organic laboratory, conceivably harboring some very different style of life. As our cosmic perspective of the probability of life elsewhere in the universe expands, characterizing potential solar system habitats has become a priority.
Discovery of liquid water in the subsurfaces of the icy Galilean satellites and probably Enceladus has markedly advanced their priority for further exploration in the context of this crosscutting question. The Galileo mission detected internal oceans in Europa, Ganymede, and Callisto from magnetic signatures induced by Jupiter’s magnetic field.47 That the oceans are electrically conductive suggests they are salty and, in fact, signatures of salts have been found in surface spectra. Although the depths and compositions are still poorly constrained, models suggest that the overlying ice crusts might be only 4 to 30 kilometers thick in Europa’s case but far thicker for Ganymede and Callisto.
Current understanding is that the interiors of Ganymede and Callisto are warmed mostly by weak radiogenic heat sources but that Europa undergoes more energetic tidal heating that should result in a much thinner ice cover capping its ocean. These factors combine to make Europa’s ocean the highest priority in the outer solar system to explore as a potential habitat for life. Characterization of its internal ocean and ice shell, and searching for plumes and evidence of organics, are key goals for this decade.
The discovery of plumes jetting from fractures in the polar plains of tiny Enceladus is a stunning discovery. Salt-rich grains in the plumes are evidence of subsurface liquid water.48,49 Cassini also revealed that the plumes exhaust organic molecules, including methane—proffered explanations include thermal decay of primordial organics, Fischer-Tropsch reactions, and rock-water reactions, or conceivably biological processes. Biotic and abiotic organics can be distinguished, for example, by their chirality. Detailed characterization of the molecular and isotopic chemistry of the organics and volatiles in Enceladus’s plumes emerges as an important scientific priority.
Today the subsurface of Mars is likely more hospitable for life than is its ultraviolet-irradiated surface.50 With an average equatorial surface temperature of ~215 K, icy conditions extend globally to depths of kilometers over most of the planet. Still, liquid water might exist near the surface in some special places, particularly as brine solutions. Geologically young lava plains suggest relatively high heat flow and melting of near-surface ice. Although as yet undetected, hydrothermal activity likely also persists and could maintain aqueous habitats at shallow depth.
Researchers lack critical geophysical data about Mars’s interior structure; ultimately seismic measurements will be the best means to reveal volcanic and hydrothermal regimes in the crust. In any case biological habitats could exist in groundwater systems in permeable layers only a few kilometers down. Driven by large excursions in Mars’s axial tilt, recent changes in climate may have increased atmospheric water content and caused substantial surface ice to be transported from the poles to lower latitudes.
In addition to liquid water, subsurface martian life would require organics and energy to drive metabolism. The putative discovery of atmospheric methane has tremendous implications for subsurface habitats and extant life.51 Some active subsurface processes—volcanism, aqueous reactions with rocks, decay of organics—or conceivably microorganisms would be necessary to maintain it. Results of the ESA-NASA Mars Trace Gas Orbiter are key to this question. However, answering with confidence the question of the existence of modern martian habitats and life forms will demand sophisticated laboratory analyses of samples collected from sites with the highest potential as subsurface habitats.
Titan offers the only plausible modern surface habitat beyond Earth that is shielded from radiation. It also provides the richest and most accessible laboratory to explore active organic synthesis on a planetary scale. and nitrogen are energetically decomposed high in the atmosphere, initiating a series of reactions producing a wide variety of hydrocarbons and nitriles, conceivably including amino acids and nucleotides. The existence of methanogenic organisms has even been speculated in the organic-rich deposits that mantle its surface or in its polar lakes and seas.
What energy might drive the metabolic processes? First sunlight, absent sterilizing ultraviolet and particle radiation, reaches the surface. Unsaturated organics such as acetylene and ethane, products of atmospheric reactions, could react with hydrogen, releasing energy at rates comparable to those used by microorganisms on Earth.52 Measurements of the concentration of hydrogen and reactive organics in the surface environment could test such hypotheses. Detailed examination of the nature and interaction of the rich array of solid and liquid organic compounds in Titan’s surface environment is a high priority that would reveal new insights into organic chemical evolution on a global scale and, conceivably, detect ongoing biological processes.
How Do the Giant Planets Serve as Laboratories to Understand Earth,
the Solar System, and Extrasolar Planetary Systems?
Among the mind-stretching advances in space science of the past 10 years is the recognition of the immense diversity of planets orbiting other stars; those confirmed number nearly 500 as of the writing of this report.53 These worlds exhibit an incredible array of planetary characteristics, orbits, and stellar environments. Moreover, some of these planetary systems are found to contain multiple planets. Some exoplanets orbit close to their stellar companions; some have orbits that are highly eccentric or even retrograde. In size and composition known exoplanets range from massive super-Jupiters, mostly hydrogen and helium, to Uranus- and Neptune-size ice giants, dubbed water worlds, down to super-Earths seeming to have ice-rock compositions.54 Discovery and characterization of
watery Earth-size planets are likely within the decadal horizon. New areas of research seek to extrapolate the understanding of the solar system to exoplanets—therefore more complete knowledge of the origin, evolution, and operative processes in our solar system, in particular of the giant planets, becomes ever more urgent.55
Exoplanets exist in a broad range of stellar conditions and illustrate extremes in planetary properties. Many exoplanets “inflated” by close proximity to their star have radii much larger than can be explained by the best thermal history models. Hot Jupiters orbit close in where the internal heat flow is dwarfed by enormous stellar fluxes; others exhibit the reverse, orbiting far from their central stars.56 Analogously, Uranus’s heat flow is a small fraction of the solar flux, but at Jupiter the two are similar.
Exoplanet internal magnetic field strengths are not known. Exoplanets in tight orbits could experience intense magnetospheric interactions with strong stellar winds. In extreme cases a planetary atmosphere could extend beyond the magnetosphere and be rapidly scavenged by stellar winds. Star-planet interactions could take many forms: Venus-like if the internal magnetic field is weak, Earth-like with auroras if the field is strong, or Jupiter-like if the planet is rotating rapidly and the magnetosphere contains plasma. Uranus and Neptune have tilted magnetospheres offset from their centers, configurations that could provide new insights into ice-giant exoplanets.57 Just as giant planets and exoplanets are closely linked, so also do giant-planet ring systems serve as important analogs to help understand exoplanet nurseries in circumstellar disks.58,59,60
The population of ice-giant exoplanets is growing rapidly. Three were detected by transit across their central stars; many more are evident in the early data from the Kepler mission and await confirmation.61 Evidently abundant, these objects are similar in size and composition to Neptune and Uranus—the giant planets about which we know the least. For Jupiter, the Galileo probe provided critical data on isotopes, noble gases, deep winds, and thermal profiles—data lacking now for Saturn, Uranus, and Neptune.
Jupiter fits reasonably well the basic model of giant-planet evolution. Saturn, however, is much warmer than the simple models predict; in fact, Saturn’s ratio of internal heat to absorbed solar heat is greater than Jupiter’s. One long-held theory is that helium rain falls to the deep interior, converting potential energy into kinetic energy and thereby heating the interior and prolonging its warm state. Direct measurement of the helium abundance would test this hypothesis. In conjunction with the Cassini mission, acquiring data on the isotopic composition of noble gases and other key elemental and molecular species would fill enormous gaps in understanding of Saturn’s formation and evolution.
Knowledge of the interior states, chemistry, and evolution of Uranus and Neptune is even more primitive than that for Saturn. More than two decades ago Voyager showed Neptune’s heat flow to be about 10 times and Uranus’s to be about 3 times larger than expected from radioactive heat production—the causes are still unknown. Measuring key elemental and isotopic abundances and thermal profiles in the atmospheres of Saturn, Uranus, and Neptune is essential to advancing understanding of the properties and evolution of gas giants, both in our own solar system and in extrasolar planetary systems.
Cassini is revealing a wealth of dynamical structures in Saturn’s rings. Accretion appears ongoing in Saturn’s F ring, gravitationally triggered by close satellite passages.62,63 Non-gravitational forces like electromagnetism drive dusty rings like Saturn’s E ring, Jupiter’s gossamer rings, and Uranus’s “zeta” ring. The physical processes that confine Uranus’s narrow, string-like rings are a mystery—when solved this could open a new chapter in understanding ring and circumstellar disk processes.64,65 Exploring the rings of Saturn, Uranus, and Neptune is of high scientific priority, not only to deepen understanding of these giant-planet systems but also to obtain new insights into exoplanet processes and their formation in circumstellar disks, albeit of enormously different scale.
What Solar System Bodies Endanger Earth’s Biosphere, and What Mechanisms Shield It?
As the geologic record demonstrates, comets and asteroids have struck Earth throughout its history, sometimes with catastrophic results. Most believe that a roughly 10-km impactor triggered the global-scale extinction at the Cretaceous-Paleogene boundary 65 million years ago (historically referred to as the Cretaceous-Tertiary boundary). Objects smaller than approximately 30 meters in diameter burn up almost completely in Earth’s atmosphere. But larger objects explode in the lower atmosphere or impact the surface and can pose a threat to human life.
The 2010 NRC report Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies addressed the dangers posed to Earth by asteroids (particularly near-Earth objects, or NEOs) and comets.66 The report stated that the risk is small, but that unlike other catastrophic events, such as earthquakes, it not only can be mitigated but also potentially can be eliminated if hazardous objects are detected in time. The report concluded that there were two approaches to completing a congressionally mandated survey of hazardous objects. The more expensive but more expedient method requires both a space-based survey telescope and a suitable ground-based telescope (i.e., a telescope capable of detecting relatively dim objects and also possessing a wide field of view enabling it to survey large portions of the night sky). The more cost-effective method could be accomplished with a suitable ground-based telescope over a longer period of time, provided that non-NEO programs primarily paid for the telescope.
The 2010 astronomy and astrophysics decadal survey report New Worlds, New Horizons in Astronomy and Astrophysics ranked the Large Synoptic Survey Telescope (LSST) as its top-priority ground-based telescope, stating that it “would employ the most ambitious optical sky survey approach yet and would revolutionize investigations of transient phenomena” (p. 223).67 The LSST was given first priority as “a result of its capacity to address so many of the identified science goals and its advanced state of technical readiness” (p. 223). From the perspective of planetary science, the LSST will yield a rich new database that not only can be mined to search for hazardous near-Earth objects but also would be of major scientific value in advancing the exploration of primitive bodies extending out into the Kuiper belt.
Although impact hazards to Earth are real, they are probably actually reduced by the gravitational influence of the giant planets, especially Jupiter. Astronomical surveys tally the number of asteroids larger than a kilometer at about a million. But comet nuclei of this size and larger are probably far more numerous. When these objects are deflected into elliptical orbits that would bring them close to Earth, they often also cross Jupiter’s orbit. Simulations with large samples of orbital encounters show that Jupiter deflects some objects on harmless trajectories that cross into the inner solar system, and that most are ejected out of the solar system. In aggregate, then, Jupiter protects Earth.68
Since the remarkable prediction of the impact of Shoemaker-Levy 9 with Jupiter in 1994 scientists have witnessed three new jovian impacts as of this writing, one in 2009 and two in 2010.69 The orbits and impact rates of Jupiter impactors provide new information to understand how Jupiter deflects hazards toward or away from Earth. Therefore continuous monitoring of Jupiter to capture these events would be invaluable. Today, such work relies on a small number of highly motivated amateur observers; these unfunded volunteers, however, cannot cover Jupiter at all times. Small, dedicated automated planetary monitoring telescopes would be of great value in providing comprehensive surveys to capture future impacts into Jupiter.
Can Understanding the Roles of Physics, Chemistry, Geology, and Dynamics
in Driving Planetary Atmospheres and Climates Lead to a Better Understanding of Climate Change on Earth?
Venus, Mars, Titan, Jupiter, Saturn, Uranus, Io, Pluto, Neptune, and Triton display an enormous range of active atmospheres that in many respects are far simpler than that of Earth—an arguably more difficult atmosphere to model and to understand. The interactions of Earth’s atmosphere, biosphere, lithosphere, and hydrosphere present extremely complex, even chaotic, problems that defy our ability to reliably predict their future or derive their past, on either short or long timescales. Venus, Mars, and Titan provide atmospheric laboratories that exhibit many Earth-like characteristics but operate across the spectrum of temperature, pressure, and chemistry. Likewise, giant-planet atmospheres are also in many respects much simpler to understand than is Earth’s. The processes that drive thick atmospheres can be modeled without the complication of a liquid or solid surface.
Consideration of the full suite of planetary atmospheres immensely broadens the scope of atmospheric science. The goal to understand the full spectrum of planetary atmospheres—the physics, chemistry, dynamics, meteorology, photochemistry, solar wind and magnetospheric interactions, response to solar cycles, and particularly greenhouse processes—drives a richer and more comprehensive perspective in which Earth becomes one example.
Venus and Earth are nearly identical in size and bulk density, but Venus’s massive atmosphere presents an extremely different system when compared with Earth’s. The upper reaches of its hot, dense carbon dioxide
atmosphere, laden with sulfuric acid clouds, circle the planet every 4 days. Venus Express discovered that lightning, auroras, and nightglows light up the planet’s sky.70 Evidence of active volcanism is also suggested, supporting the idea that ongoing volcanic emission of sulfur dioxide feeds the thick sulfuric acid clouds. What mechanisms triggered Venus’s runaway greenhouse climate and on what timescale remain open questions.71 Addressing them can help us better understand the principles of greenhouse atmospheres in general, placing Earth’s in a broader context. For Venus, addressing these questions requires measurements of atmospheric chemistry, notably of the isotopic and noble gas chemistry of the lower atmosphere. Establishing the initial climate conditions and modern states of Venus and Mars can help us to understand how their environments diverged so dramatically from Earth’s.
Mars has perhaps the most Earth-like modern planetary atmosphere, and its earliest climate may have been similar to that of early Earth. Studying it therefore provides opportunities to validate terrestrial climate and global circulation models under very different atmospheric conditions. Mars’s polar layered deposits suggest climate change in the last 10 million years, and dynamical models predict large recent excursions in axial tilt and orbital eccentricity.72 These considerations point to recent climatic change, analogous to ice ages on Earth, detailed records of which are likely preserved in the polar layered deposits. During Mars’s postulated early warm wet climate solar luminosity is thought to have been ~25 percent lower than today. This fact has made it difficult for atmosphere modelers to understand how Mars’s greenhouse effect could have sustained such warm conditions, but the geologic evidence for Noachian rivers and lakes is compelling. The continued investigation of Mars’s climate through time and the study of its modern atmospheric processes from orbit, from the surface, and ultimately from analysis of returned samples remain high-priority science objectives.
Flow within giant-planet atmospheres is organized largely in east-west jet streams. Whereas Jupiter and Saturn exhibit alternating east-west jets, Uranus and Neptune show broad belts of retrograde winds at the equator shifting to prograde with increasing latitude. Vortices, cyclonic and anticyclonic, at many scales spin between the jets and resemble weather features seen on Earth ranging from tornados to hurricanes. North-south circulation continually overturns belt-zone systems in Hadley-like convection cells and by wave forcing.73,74 Major questions remain as to how these motions, visible in the layered cloud decks, couple to the interior structure and deep circulation. The Juno and Cassini Solstice missions may detect gravitational signatures of deep internal flow in Jupiter and Saturn. The most serious gap in the understanding of planetary atmospheres remains for the ice giants, Neptune and Uranus.
The giant planets also provide the only examples of processes common to Earth in which strong internal magnetic fields interact with the solar wind. This includes the fluorescing spectacle of Earth’s northern lights and similar auroral displays seen near the magnetic poles of Jupiter and Saturn. At Jupiter and Saturn the main sources feeding the magnetospheric plasma appear to be Io, Enceladus, and Saturn’s rings, whereas most of the magnetospheric plasma at Earth is trapped solar wind. These interactions pose major consequences for humans; understanding and predicting them are important. The solar wind induces magnetospheric storms that disrupt power and communication systems worldwide. The giant planets provide a wide spectrum of observable magnetospheric processes that can contribute directly to an understanding of the physics at work in Earth’s space environment.
One of the most startling revelations of the past decade is how much the processes ongoing in Titan’s thick global atmosphere and on its surface resemble those of Earth. Both worlds have nitrogen-dominated atmospheres with about the same surface pressure.75 However in Titan’s ultracold meteorology, methane migrates through a global system of clouds, rain, rivers, lakes, seas, and aquifers: the analogy to Earth’s hydrologic cycle is obvious. The mechanics and chemistry of this atmosphere are complex but pale in comparison to the complexity of Earth’s. In the quest to understand greenhouse mechanisms, Titan’s atmosphere manifests both greenhouse warming and anti-greenhouse cooling, puzzling diametric cases in which thermal radiation is sometimes trapped and sometimes radiated to space. The Cassini-Huygens spacecraft arrived at Saturn near the northern winter solstice, and the mission will be extended through the northern summer solstice, allowing unprecedented views of Titan’s seasonal behavior. Continued exploration of this fascinating Earth-like atmosphere, both from orbit and in situ, remains one of the most important objectives for planetary science.
How Have the Myriad Chemical and Physical Processes That Shaped the
Solar System Operated, Interacted, and Evolved Over Time?
In searching for answers to the overarching questions, researchers first seek a deep understanding of the chemical and physical processes that have shaped planetary interiors, surfaces, atmospheres, rings, and magnetospheres through time. Since 1977 when the Voyager spacecraft left Earth, our perspectives regarding the complexity and diversity of the solar system have undergone immense revision and expansion. Ranger, Surveyor, and Apollo data showed that the Moon’s geologic evolution ended long ago. Mariner 10’s visit to Mercury showed a similar picture—an ancient, impact-riddled, and geologically dead world. Mariners 4, 6, 7, and 9 flew by and orbited Mars, and again revealed a mostly cratered volcanic world, but one also with jumbled chaotic terrains, gargantuan canyons, exotic polar deposits, and outflow channels and drainage networks of a watery but ancient origin. Even with Mars’s profusion of geologic processes, it too appeared to be inactive, a frigid desert world.
As had been predicted, Voyager found that Jupiter’s moon Io is the most intensely volcanic object in the solar system; volcanic plumes fountain up to 300 kilometers, and not a single impact crater has been found anywhere on its young volcanic plains. Galileo confirmed that the surface of Io continues to evolve rapidly, discovering molten lakes of silicates and sulfur-rich lavas and active fire fountains. New Horizons provided elegant movies of an eruption in progress. Caught in a celestial dance that also involves Jupiter, Europa, and Ganymede, Io is intensely heated by tides and remains one of the best places in the solar system to study active volcanism and tidal heating.
Turning to the other Galilean satellites, the Galileo mission found all three to have internal oceans. Of the three, the ceiling of Europa’s ocean chamber is thought to be at the shallowest depth because, although less so than Io, it is also tidally heated.76 In addition, because its large rocky interior, upon which the ocean rests, is subjected to both tidal and radiogenic heating, it is reasonable to expect seafloor volcanism and hydrothermal activity that could provide nutrients and energy to support metabolism.77 These factors combine to make Europa, along with Mars, the highest-priority destinations in the solar system as potential planetary habitats.
For the jovian system we have learned to expect the unexpected. Galileo showed Ganymede, the only satellite in the outer solar system known to have an internal magnetic field and a magnetosphere. Galileo’s probe into the jovian atmosphere revealed that the noble gas abundances were very unlike the Sun’s—processes like helium rain falling into Jupiter’s core have been invoked as possible explanations, and more recent observations have shown a dynamic, ever-changing atmosphere, riddled by impacts.78
Saturn’s excessive thermal energy might also signal helium rain; direct measurement of its noble gas abundance and isotopic chemistry is required to get an answer. Cassini confirmed exquisite features in Saturn’s atmosphere: the Voyager-detected hexagonal circumpolar jet rotating around Saturn’s north pole and a hot, hurricane-like vortex with newly discovered well-defined eye wall circles in the south. That a tiny icy moon could be warm enough to maintain liquid water in its interior driving jets out if its broken crust, makes Enceladus a key destination.79,80 Mimicking Earth, Titan has seas of organic sand dunes, hydrocarbon lakes, dendritic river systems, putative icy volcanism, mountain chains, and global fault systems—Titan ranks near the top as a target of future exploration.81 At Neptune Voyager found nitrogen geysers fountaining up into the stratosphere from Triton’s ultracold, 37 K surface—perhaps driven by a greenhouse effect as subliming gas rushes to vents under clear nitrogen ice. Originally researchers thought that only Saturn among the giant planets possesses a ring system. As it turns out so do Jupiter, Uranus, and Neptune, and these systems all differ dramatically. Neptune possesses orbiting arcs forming partial rings; dense dark rings interspersed with broad sheets of nearly invisible dust encircle Uranus;82,83 Jupiter’s gossamer ring orbits as a dusty wreath. Researchers are only beginning to uncover the nature and ages of the ring materials.
We have had tantalizing glimpses of conditions and configurations in the ice giants themselves: oddly tilted and offset magnetic fields, unexplained sources of heat, and supersonic atmospheric motions. Of the major objects of the solar system, these ice-giant worlds are understood least. Because recent discoveries suggest that such bodies may dominate the population of exoplanets, filling this gap in knowledge rates as a high priority.
New lessons from the inner solar system over the past few decades show that it, too, is far more complex and active than previously known—discoveries here are equally exciting. Venus may harbor active volcanic eruptions issuing sulfurous compounds and water vapor to feed the sulfuric acid clouds.84 Mars is also far more active than
thought earlier, with changes on a timescale of only a few years: new impact scars, new landslides, and active processes occurring in gullies.85 Time-lapse movies from the Mars rovers show dust devils racing across the surface. Scientists have new evidence for glaciers on Mars, extending even to the equator in places and active or recent subsurface processes, of hidden origin, generating methane. Mars’s hydrothermal and volcanic activity likely extends through today, but confirmation will require seismic data, a critical area for future investigation. We have found strange “active” asteroids in the main belt jetting dust and gas, behaving like comets. Once the Moon had global oceans of molten lava; today its seismic tremors can elucidate its internal structure and yield secrets of its early origin and evolution.
In summary, we have come full circle in our view of how complex, diverse, and often active the processes are that drive the solar system. In the end, we have come to realize that as we explore, our expectations commonly fall short of what nature has in store for us in the unknown reaches of the solar system and the universe.
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