ROLE OF GEOBIOLOGY IN THE ASTROBIOLOGICAL EXPLORATION OF THE SOLAR SYSTEM

Recent discoveries in microbiology, Precambrian paleontology and planetary science have dramatically reshaped our understanding of the nature, distribution and evolutionary potential of terrestrial life, paving the way for new strategies in the exploration for life elsewhere in the Solar System. Advances in geomicrobiology have revealed the importance of microbial processes in global biogeochemical cycles and in the evolution of Earth environments over time. Microbial life is now known to occupy a stunning array of environmental extremes, seemingly limited by only the distribution of liquid water, nutrients and energy sources. The discovery of a vast subsurface biosphere, fueled by inorganic chemical energy, has been especially important in opening up new horizons for the astrobiological exploration of Mars, as well as icy satellites of the outer Solar System. Although the environment of life's origin remains uncertain, molecular studies suggest that the last common ancestor of life probably lived in hydrothermal environments, and utilized simple forms of chemical energy. This is consistent with geological evidence for early Archean environments on the Earth, as well as with model-based predictions of late, giant impacts that would likely have exterminated all mesophilic (and photosynthetic) surface life. Such hypotheses, while controversial, have contributed to the view that life could be much more broadly distributed in the Solar System than previously thought. We now believe it possible that life became established in surface environments on Mars during the first half billion years of the planet's history when liquid water was widespread there. Furthermore, a subsurface hydrosphere (suggested by models and geomorphic evidence) could support an active, deep biosphere on Mars today. Exploration of the outer Solar System supports the existence of saline brines (oceans?) beneath icy crusts of Europa, Callisto and Ganymede, with plausible energy sources for life, based on the predicted disassociation of water by surface radiation and the decay of radioactive potassium. Current plans to systematically explore these and other potentially habitable environments in the Solar System over the next decade, have helped to catalyze the development of a new interdisciplinary science called Astrobiology.