Water born from dry rock

At great depths beneath the Earth's surface, molecular hydrogen (H₂) can react with certain minerals to produce water molecules. The discovery — presented in a study published in Science Advances — opens up new possibilities for our understanding of the subsurface water cycle, as well as for the search for extraterrestrial life.

"Our research shows that molecular hydrogen, which is present deep within the Earth's crust and mantle, can react with anhydrous minerals — that is, minerals containing no water — to generate, through simple redox reactions, both new water and new hydrated minerals," explains Alberto Vitale Brovarone, a Professor at the Department of Biological, Geological and Environmental Sciences at the University of Bologna and first author of the study. "This discovery fundamentally changes the way we think about the water cycle deep within the Earth, with implications for magma formation, seismic activity, and the possibility of life in the deep subsurface." 

We have long known that the availability of water in the lithosphere — the outermost layer of our planet — has been fundamental to the geological evolution of the Earth and to the emergence and survival of life. Yet geological processes can also split water apart, decoupling oxygen from molecular hydrogen, and hydrogen can simply drift off into space.

It was this growing awareness of the presence of molecular hydrogen in the Earth's subsurface that led researchers to investigate whether hydrogen-rich fluids interacting with oxygen-bearing minerals could give rise to the formation of water.

"Rock samples, laboratory experiments, and our thermodynamic modelling all suggest a positive answer" says Vitale Brovarone. “Even trace amounts of water produced by these reactions can dramatically alter the chemical and physical properties of rocks and fluids in the crust and mantle. These chemical reactions could even sustain microbial communities at great depths within the Earth's crust.” 

The researchers also found that this conversion of molecular hydrogen into water within initially dry geological environments can lower the melting point of deep rocks, triggering the formation of magma. At the same time, the appearance of free water within the Earth's interior can change the mechanical behaviour of rocks, making them more prone to deformation and, by extension, to seismic activity. 

The story does not end on Earth. Similar reactions could point to the presence of hydrated minerals (minerals containing water) on other celestial bodies. And the production of water at depth could significantly extend the area in which to search for extraterrestrial life forms. 

The study was published in Science Advances under the title "Unconventional water and hydrous mineral formation from dry minerals and H2 fluids". It was led by Alberto Vitale Brovarone, a Professor at the Department of Biological, Geological and Environmental Sciences at the University of Bologna. Co-authors include Simone Tumiati (University of Milan), Fabrizio Nestola (University of Padua), and Donato Giovannelli (Federico II University of Naples), alongside an international team from France (Université de Lorraine/CNRS), the United States (Yale University and Johns Hopkins University), Germany (BGR), and the European Space Agency.

Photo credits: Jacopo Pasotti for ERC DeepSeep