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

Future Directions for Investigations and Measurements
Important directions for future investigations relating to energy sources for life include 1)
measurement of the oxidant content and increase understanding of its formation mechanisms on the
surface ice of Europa and Enceladus, 2) through remote sensing, improve understanding of geological
processes that might deliver surface oxidants to subsurface liquid water, and, 3) for Titan, improve
measurements of atmospheric and surface chemistry to increase understanding of the biological
availability of chemical energy.
Is There Evidence for Life on the Satellites?
The search for evidence of life is an emerging science priority for the moons of the outer solar
system. Organic material produced biologically is distinguishable from abiotic sources. Studies of the
plume of Enceladus and any organics on the surface of Europa (or in potential Europa plumes) may
provide evidence of biological complexity even if the organisms themselves are no longer present or
viable. Titan has a liquid on its surface—methane, not water—and there are speculations that it may be a
suitable medium for organic life as well. 52
The detection of organic material in the icy plume of Enceladus indicates the possibility of
conditions suitable for biological processes, present or past. On Titan organic molecules are clearly
present and interacting with liquids (certainly liquid hydrocarbons and possibly ammonia water mixtures)
but these interactions are not necessarily of biological origin.
Important Questions
Some important questions relevant to evidence for life on the satellites includes the following:
• Does (or did) life exist below the surface of Europa or Enceladus?
• Is hydrocarbon-based life possible on Titan?
Future Directions for Investigations and Measurements
A key future investigation of the possibility of life on the outer planet satellites is to analyze
organics from the interior of Europa. This requires either a lander in the far term or the discovery of active
Enceladus-style venting which would allow analysis from orbit with a mission started in the next decade.
A detailed characterization of the organics in the plume of Enceladus is important to search for signatures
of biological origin, such as molecules with a preferred chirality or unusual patterns of molecular weights.
A major investigation should be to characterize the organics on Titan’s surface, particularly in liquids, to
reveal any potentially biological processes occurring there.
INTERCONNECTIONS
Connections with Other Parts of the Solar System
The satellites of the outer planets embody processes that operate throughout the solar system.
Io’s hyperactive silicate volcanism provides living examples of volcanic processes that have been
important now or in the past on all the terrestrial planets and our own Moon. Eruptions seen in recent
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