Vision and Voyages for Planetary Science in the - Solar System ...

heavy bombardment? Clues to address these questions could remain locked in chemical signatures at the
surfaces and in atmospheres of Earth’s neighbors.
Venus and Mars formed at orbital radii that bracket Earth’s. The isotopic, elemental, molecular,
and mineralogical 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. We 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 appears to have a complex, many-staged
history. While we have gleaned significant information 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 analytical techniques not possible with a single surface
mission.
PLANETARY HABITATS: SEARCHING FOR THE REQUIREMENTS OF LIFE
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 last 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 the early Earth, life here would not exist.
We 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 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 solidstate
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 we lack critical information on organic components in
comets and Kuiper belt objects and how 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
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