Role of geobiology in the astrobiological exploration of the Solar System

Abstract

Discoveries in geobiology have dramatically shaped our understanding of the nature, distribution, and evolutionary potential of terrestrial life, paving the way for new exploration strategies to search for life elsewhere in the Solar System. Genomic studies, applied over a broad range of geological environments, have revealed that the vast proportion of species on Earth are microbial. Studies of the fossil record indicate that this has been the case for >75% of our planet’s history. Microbial life has been shown to occupy a stunning array of environmental extremes, seemingly only limited by the distribution of liquid water and its chemical activity, nutrient availability, suitable energy sources, radiation, etc. Advances in geomicrobiology have revealed important contributions of microbial processes to many global biogeochemical cycles, and in the evolution of Earth’s atmospheric and surface composition. The discovery of a subsurface biosphere, fueled by inorganic chemical energy and able to tolerate extremes in temperature and salinity, 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 where it utilized simple compounds of carbon, hydrogen, and sulfur as sources of chemical energy. This general view is consistent with what we know about late Hadean to early Archean environments on the Earth, as well as model-based interpretations of late, giant impacts that could have exterminated early mesophilic (and possibly photosynthetic) surface life forms, leaving behind only deep subsurface chemotrophic thermophilic microbial communities to re-populate the biosphere. These and related discoveries have contributed extensively to the view that life could be much more broadly distributed, within the Solar System and beyond, than once thought. We now believe it possible that life may have become established in surface environments on Mars during the fi rst half billion years of the planet’s history, when liquid water was widespread there. Furthermore, a subsurface hydrosphere on Mars (suggested by both models and geomorphic evidence) may have provided a continuously habitable zone for life over most of Martian history and could still support