The fate of early Mars' lost water: The role of serpentinization

The fate of water which was present on early Mars remains enigmatic. We propose a simple model based on serpentinization, a hydrothermal alteration process which may produce magnetite and store water. Our model invokes serpentinization during about 500 to 800 Myr, while a dynamo is active, which may have continued after the formation of the crustal dichotomy. We show that the present magnetic field measured by MGS in the Southern hemisphere is consistent with a ~500 m thick Global Equivalent Layer of water trapped in serpentine. Serpentinization results in the release of H 2 . The released H atoms are lost to space through thermal escape, increasing the D/H ratio in water reservoirs exchanging with atmosphere. We show that the value of the D/H ratio in the present atmosphere (~5) is consistent with the serpentinization of a ~500 m thick water GEL. We reassess the role of non-thermal escape in removing water from the planet. By considering an updated solar wind-ionosphere interaction representation, we show that the contribution of oxygen escape to H isotopic fractionation is negligible. Our results suggest that significant amounts of water (up to a ~330-1030 m thick GEL) present at the surface during the Noachian, similar to the quantity inferred from the morphological analysis of valley networks, could be stored today in subsurface serpentine. 1. The study Like Earth, Mars has been endowed with large amounts of water during accretion, equivalent to the content of several terrestrial oceans, corresponding to a several 10 km thick Global Equivalent Layer. The present inventory of observable water on Mars, mainly within the polar caps, is quite smaller, in the range from ~20-30 m. The mega-regolith capacity is large, with up to ~500 GEL m potentially trapped in the cryosphere, and hypothetically several additional hundreds of meters (up to ~500 m) of ground water surviving at depth below the cryosphere [1]. A ~500 m thick GEL is generally assumed to be required to explain the formation of outflow channels [2], and most of this water could be trapped today as water ice, and possibly deep liquid water, in the subsurface, and also possibly under the form of hydrated minerals.