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

satellite interiors should include use of subsurface sounding from orbit (e.g., using radar) to investigate
the presence of near-surface water and perhaps the ice-ocean interface on Europa. In the far term in situ
measurements from the surface would provide additional information on the surface composition and
environment, and subsurface structure (via seismology or magnetometry). Improved compositional
measurements of gas and dust ejected from the Enceladus plume (and potential Europa plumes) would
provide valuable insights into the presence of liquid water at the plume source.
What are the Sources, Sinks and Evolution of Organic Material?
Life as we know it is made of organic material (i.e., complex carbon-based molecules). Organic
molecules can be abiotically produced in the laboratory and it is well known that the solar system and the
interstellar medium are rich in non-biological organics. The satellites have much to teach us about the
formation and evolution of complex organics in planetary environments, with implications for the origin
and evolution of terrestrial life.
Perhaps the clearest example of organic synthesis in our solar system is on Titan, where Cassini
and Huygens have provided abundant new information. 46 Methane and nitrogen in the atmosphere are
decomposed by particle and solar radiation, starting a chemical reaction cycle that produces a range of
gaseous organic molecules, with molecular weights up to and exceeding 5000, and a haze of solid
organics and liquid condensates.
Once on the surface the organics accumulate and apparently are responsible for the huge dunes
seen in Titan’s equatorial regions. The atmospheric organics probably accumulate in the lakes seen in the
polar regions.
Cassini has revealed that the plume of Enceladus hosts a rich organic chemistry, including
methane and a rich suite of hydrocarbons. 47 The source of the organics is not clear. Possibilities include
thermal decay of organics brought in with the accreting material, Fischer-Tropsch type synthesis in a
subsurface environment, rock-water reactions that can produce hydrogen, and finally, if most
speculatively, methanogenic microorganisms.
Europa may have organics on its surface but this has not been conclusively demonstrated, and the
radiation environment makes the survival of organics uncertain over a few million years. 48 If organics are
found on the surface of Europa the next step would be to determine if these organics may have derived
from the underlying ocean and if so, whether they might be biological in origin.
Important Questions
Some important questions about the sources, sinks and evolution of organic material include the
following:
• What is the nature of the atmospheric processes on Titan that convert the small organic gasphase
molecules observed in the upper atmosphere (such as benzene) into large macromolecules and
ultimately into solid haze particles?
• What is the fate of organics on the surface of Titan and their interaction with the seasonally
varying lakes of liquid hydrocarbons?
• Are organics present on the surface of Europa, and if so, what is their provenance?
• What is the source of the organic material in the plume of Enceladus?
Future Directions for Investigations and Measurements
Observations of the surface of Europa should include the capability to determine the presence of
organics, for instance by reflectance spectroscopy or low-altitude mass spectroscopy of possible out-
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