ON THE INTERFACE OF GEOBIOLOGY AND PLANETARY GEOLOGY: EVOLVING STRATEGIES TO EXPLORE FOR LIFE ELSEWHERE IN THE SOLAR SYSTEM

Discoveries in terrestrial biology, paleontology and planetary science have continued to shape our understanding of the nature, distribution and evolutionary potential of life elsewhere in the Solar System, while paving the way for new exploration strategies. Studies of the fossil record have revealed that the biodiversity of our planet has been dominated by microbial life throughout its history. Furthermore, microbes have evolved a stunning array of metabolic capabilities for surviving an equally impressive range of environmental extremes. While conditions for the origin of terrestrial life remain obscure, biosignatures have been found in the oldest well-preserved sedimentary sequences, placing life's origin within a few hundred million years of the onset of habitable conditions. During the late Hadean, on the heels of late, giant sterilizing impacts, surface life appears to have suffered one or more mass extinctions, with subsequent repopulation by subsurface hydrothermal survivors. In short, we have learned that life is ancient, diverse and tenacious in its ability to adapt and survive.

The discovery of a vast, subsurface biosphere, fueled by inorganic chemical energy, has been especially important in opening up new opportunities for the astrobiological exploration of the Solar System. This has enhanced possibilities for the development of a subsurface biosphere on Mars. Exploration by the Galileo mission opened possibilities for subsurface hypersaline brines (perhaps oceans) beneath the surfaces of Jupiter's moons, Europa, Callisto and Ganymede, with plausible sources of energy based on redox disequilibria. Recent discoveries by the Cassini-Huygens mission suggest that Enceladus may also sustain liquid water environments beneath its surface and that Titan may provide alternative environments for life under solvent and energy conditions quite unlike anything found on Earth. Plans to systematically explore these extraterrestrial environments over the next two decades, have helped to catalyze the development of the field of Astrobiology.