The saline flavor of deep continental energy

The emergence of life on Earth, as well as its preservation at extreme conditions within the Earth’s crust, has required energy sources other than photosynthesis [1]. Extensive community effort has been deployed over the past decades to identify abiotic energy sources within the crust as potential analogues of the prebiotic Earth and extraterrestrial habitable bodies [2,3]. Among them, energy sources produced by the interaction of rocks and fluids are considered central. Over the past decades, direct observations of rock-sourced energy forms such as molecular hydrogen (H2) and light abiotic hydrocarbo ns such as CH4 have been identified in hydrothermal vents at the seafloor and on continents. These molecules can also promote the abiotic formation of prebiotic con- densed organic substrate [3,4]. Among the processes identified at the origin of H2 and its combination with carbon to form light abi- otic hydrocarbo ns, the aqueous alteration of redox-sensitive ele- ments in rock-forming minerals, most notably Fe, is central [3]. Processes such as serpentinization—the aqueous alteration of ultramafic rocks in the stability field of serpentine minerals—have been identified in multiple localities worldwide as drivers for active H2-CH4 venting and seepage [3,5]. The same processes are searched on other objects of the Solar System and beyond for their potential to supply energy to extraterrestrial life forms [6]. In the felsic continental crust typical of the Earth, water radiolysis—the dissociation of water caused by ionizing radiation involved in the radiogenic decay of elements such as U, Th, and K characteristic of a felsic crust—has also been shown to produce H2 and light hydrocarbons functional to deep-seated microbial life in aquifers as old as 2.6 billion years [7].