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 first 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 an active, deep biosphere on Mars today. Exploration of the outer Solar System supports the presence of saline brines (perhaps oceans) beneath the icy crusts of Europa, Callisto, and possibly Ganymede, along with plausible energy sources for life based on chemical disequilibria between oxidized and reduced compounds. It also appears that interior zones of liquid water may also exist on Enceladus, a moon of Saturn, while hydrocarbon oceans of liquid methane discovered on Titan may provide alternative solvents for novel life forms completely unlike anything found on Earth. Ongoing efforts to systematically explore potentially habitable environments elsewhere in our Solar System have helped catalyze the development of astrobiology, an emerging interdisciplinary science that seeks to understand the origin, evolution, distribution, and future of life in the cosmos. Geobiology, which studies interactions of biological and physical-chemical systems and how they have evolved over the history of Earth, is a central focus of astrobiology, providing fertile ground for the growth of conceptual models and new technological tools needed to implement the search for extraterrestrial life elsewhere in the Solar System.

Field geological research, as traditionally practiced on Earth, is an extremely flexible science. Although field geologists plan their traverses ahead of time—nowadays with the advantage of remote-sensing data—initial plans are continually modified in response to observations, such that traverses evolve over time. This research modality differs from that utilized in extreme environments on Earth (e.g., on the ocean floor), on the Martian surface by the mobile laboratories Spirit and Opportunity , and by the Apollo astronauts during their explorations of the Moon. Harsh and alien conditions, time constraints, and resource limitations have led to the development of operational modes that provide a constrained and usually lower science return than traditional field geology. However, emerging plans for renewed human exploration of the Moon, Mars, and near-Earth asteroids serve as an opportunity to invent a new paradigm for advanced planetary field geology that embraces coordinated human and robotic research activities. This approach will introduce an operational flexibility that is more like that of traditional field geology on Earth. In addition, human and robotic collaborations, combined with the integration of new “smart” tools, should provide an augmented reality that leads to even greater science return than traditional field geology. In order to take full advantage of these opportunities when planetary field geology again becomes practical, it is imperative for field geologists on Earth to begin right now to learn how best to incorporate advanced technologies into their research. Geologic studies of analog sites on Earth that employ new technology-enabled strategies rather than traditional research methods provide ideal opportunities to test and refine emerging designs for advanced planetary field geologic studies, as well as to gain new insights into terrestrial geologic processes. These operational experiments will be most informative if they embrace the entire geologic research process—including problem definition, field observation, and laboratory analysis—and not simply field work. The results of such comprehensive research can be used to inform the design of a maximally effective training regimen for future astronaut explorers.