Thematic Articles

The ingredients to make an environment habitable (e.g., liquid water, chemical disequilibria, and organic molecules) are found throughout the solar system. Liquid water has existed transiently on some bodies and persistently as oceans on others. Molecular hydrogen occurs in a plume on Saturn’s moon Enceladus. It can drive the reduction of CO2 to release energy. Methane has been observed in many places: from the dusty plains of Mars, to the great lakes of the Saturnian moon Titan, to the glacial wonderland that is Pluto. Organic molecules are common where volatile elements and reducing conditions prevail: these organic molecules can have diverse origins. Future space missions will attempt to illuminate the “organic solar system” and the role played by possible extraterrestrial life.

Geologically produced (abiotic) molecular hydrogen and methane could be widely utilized by microbial communities in surface and subsurface environments. These microbial communities can, therefore, have a potentially significant impact on the net emissions of H2 and CH4 to Earth’s ocean and atmosphere. Abiotic H2 and CH4 could enable microbial communities to exist in rock-hosted environments and hydrothermal systems with little or no input from photosynthetic carbon fixation, making these communities potential analogs for the earliest metabolisms on Earth (or other planetary bodies). The possible dependence of rock-hosted ecosystems on H2 and CH4 should factor into current and future plans for engineering the subsurface for storage of these compounds as energy fuels.

The presence of H2 and H2O in planetary interiors prompts the need for fundamental studies on these compounds under corresponding conditions. Here, we summarize data on H2 properties in aqueous systems under conditions of high temperature and pressure. We explain how to measure important H2 fugacities in hydrothermal systems. We present available experimental data and thermodynamic models for H2 solubility and vapor–liquid partitioning under hydrothermal conditions. In addition, we introduce the fascinating world of H2–H2O clathrate hydrates under extreme temperatures and pressures. The properties of the H2–H2O system are well established below the critical point of water (374 °C and 22.06 MPa), but far less is known under higher temperatures and pressures, or the effect of salt.

Accumulation of molecular hydrogen in geologic systems can create conditions energetically favorable to transform inorganic carbon into methane and other organic compounds. Although hydrocarbons with a potentially abiotic origin have been proposed to form in a number of crustal settings, the ubiquitous presence of organic compounds derived from biological organic matter presents a challenge for unambiguously identifying abiotic organic molecules. In recent years, extensive analysis of methane and other organics in diverse geologic fluids, combined with novel isotope analyses and laboratory simulations, have, however, yielded insights into the distribution of specific abiotic organic molecules in Earth’s lithosphere and the likely conditions and pathways under which they form.

The capacity for molecular hydrogen (H2) to hydrogenate oxygen and carbon is critical to the origin of life and represents the basis for all known life-forms. Major sources of H2 that strictly involve nonbiological processes and inorganic reactants include (1) the reduction of water during the oxidation of iron in minerals, (2) water splitting due to radioactive decay, (3) degassing of magma at low pressures, and (4) the reaction of water with surface radicals during mechanical breaking of silicate rocks. None of these processes seem to significantly affect the current global atmospheric budget of H2, yet there is substantial H2 cycling in a wide range of Earth’s subsurface environments, with multifaceted implications for microbial ecosystems.

Molecular hydrogen (H2), methane, and hydrocarbons with an apparent abiotic origin have been observed in a variety of geologic settings, including serpentinized ultramafic rocks, hydrothermal fluids, and deep fractures within ancient cratons. Molecular hydrogen is also observed in vapor plumes emanating from the icy crust of Saturn’s moon Enceladus, and methane has been detected in the atmosphere of Mars. Geologic production of these compounds has been the subject of increasing scientific attention due to their use by chemosynthetic biological communities. These compounds are also of interest as possible energy resources. This issue summarizes the geological sources of abiotic H2 and hydrocarbons on Earth and elsewhere and examines their impact on microbial life and energy resources.