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

We lack critical geophysical data about Mars’s interior structure; ultimately seismic
measurements will be our 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 climate changes 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 Trace Gas Orbiter are key to this question. However, answering the question of the
existence modern martian habitats and life forms with confidence 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. Methane and nitrogen are energetically decomposed high in the atmosphere,
initiating a series of reactions producing a wide variety 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.
WORKINGS OF SOLAR SYSTEMS:
REVEALING PLANETARY PROCESSES THROUGH TIME
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 last ten 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-sized ice giants, dubbed water worlds, down to super-
Earths seeming to have ice-rock compositions. 54 Discovery and characterization of watery Earth-sized
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 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 our 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.
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