Water, Shallow and Deep, and the Emergence of a Deep Biosphere

In the early Earth, water was a rare commodity except next to the surface where intense bombardment was making life preservation difficult. Hydration of the Earth's interior, which was strongly depleted even in elements much less volatile than water, such as alkaline elements, was postponed until the onset of plate tectonics. It is now well understood that reaction between water and the ferrous components of hot mafic minerals and melts was a major source of hydrogen that early life could eventually use to sustain metabolism and evolve. Due to its rather strong gravity field, the Earth lacked the feldspathic crust present on smaller bodies such as the Moon. On the Moon, such a conductive boundary layer slowed down solidification of the magma ocean. On the Earth, crust was only preserved because surface water reacted with hot and molten silicates to produce buoyant hydrous minerals (serpentine, talc, amphibole), the leftovers of hydrogen generation. Such a hydrous layer was probably very thin (a few km) and vulnerable to impacts. At the surface of a quiescent planet, hydrogen and its byproducts methane and ammonia were quickly removed upon equilibration with H2O, CO2, and N2, gases that also represented a permanent threat to early organisms. The efficiency of the hydrogen factory and the preservation of a deep biosphere were noticeably increased by meteoritic bombardment. Even medium-sized projectiles raise pressure up to a few tens of GPa. Under these conditions, serpentine and the hydrous mafic minerals are transformed into high-pressure hydrous phases such as superhydrous phase B and DHMS D. Buried high-pressure polymorphs of serpentine produced by impacts would rapidly turn unstable and become a significant source of water, and therefore of hydrogen, which may have been critical in the emergence of a deep biosphere.