Lessons from the Deep Earth for the Search of Life in the Solar System and Beyond
Space Telescope Science Institute (STScI) Bahcall Auditorium 3700 San Martin Drive Baltimore, MD 21218
12:00 PM - 2:30 PM
Barbara Sherwood Lollar (Department of Earth Sciences, University of Toronto)
The recent National Academies Report - the 2018 Astrobiology Science Strategy for the Search for Life in the Universe emphasized, among other major themes, the need for an expanded focus on investigation of subsurface environments and subsurface processes for our understanding of planetary evolution, habitability and the search for life. Our research program at Toronto focuses on Earth analog systems – in particular, deep fracture waters preserved on geologically long time scales in the Precambrian cratons of Canada, Fennoscandia, and South Africa. Science has long relied on fluid inclusions - microscopic time capsules of fluid and gas encased in host rocks and fracture minerals - to access preserved samples of ore-forming fluids, metamorphic fluids, and remnants of the ancient atmosphere and hydrosphere. Until recently, groundwaters were thought to reflect only much younger periods of water-rock interaction (WRI) and Earth history, due to dilution with large volumes of younger fluids recharging from surface hydrosphere. In the last 10-20 years, global investigations in the world’s oldest rocks have revealed groundwaters flowing at rates > L/min from fractures at km depth in Precambrian cratons. With mean residence times ranging from Ma to Ga at some sites, and in the latter case, geochemical signatures of Archean provenance, not only do these groundwaters provide unprecedented samples for investigation of the Earth’s ancient hydrosphere and atmosphere, they are opening up new lines of exploration of the history and biodiversity of extant life in the Earth’s subsurface.
Rich in reduced dissolved gases such as CH4 and H2, these fracture waters have been shown to host extant microbial communities of chemolithoautotrophs dominated by H2-utilizing sulfate reducers and, in some cases, methanogens. Recent estimates of global H2 production via WRI including radiolysis and hydration of mafic/ultramafic rock (e.g. serpentinization) show that the Precambrian continents are a source of H2 for life on par with estimates of H2 production from WRI calculated for the Earth’s marine lithosphere. To date this extensive deep terrestrial habitable zone has been significantly under-investigated compared to the marine subsurface biosphere. Beyond Earth, these findings have relevance to understanding the role of chemical water-rock reactions in defining the potential habitability of the subsurface of Mars, as well as that of ocean worlds and icy bodies such as Europa and Enceladus. This talk will address some of the highlights of recent exploration of the energy-rich deep hydrogeosphere, and connections to deep subsurface life on Earth and to planetary exploration and astrobiology.