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

Today the 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 mineralogical composition of surface. 46
Beyond Earth, Are There Modern 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 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 surface environment, albeit extremely cold,
that also is shielded from deadly radiation.
We herein make the assumption that life elsewhere in the solar system will be like terrestrial life
and thereby recognizable to us. For Earth-like life forms to arise and survive in subterranean planetary
habitats would require liquid water; 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 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 the overlying ice crusts might only be 4-30 kilometers thick in
Europa’s case but far thicker for Ganymede and Callisto.
Our 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 resulting 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, searching for plumes and evidence of organics are key goals for this decade.
Discovery of plumes jetting from fractures in the polar plains of tiny Enceladus is a stunning
discovery. Salt-rich grains in the plumes evidence 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, and rock-water reactions, or conceivably biological processes.
Biotic and abiotic organics can be distinguished, for example from 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.
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