[1] 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 Mars Global Surveyor in the southern hemisphere is consistent with a~500 m thick Global Equivalent Layer (GEL) 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 also consistent with the serpentinization of a~500 m thick water GEL. We reassess the role of nonthermal 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.

We present the magnetic properties of the Noachian Martian breccia NWA 7034. Among the 25 unpaired Martian meteorites studied to date, NWA 7034 has a unique magnetic mineralogy. It contains about 15 wt % of iron oxides as magnetite that has experienced cation substitution and partial alteration to maghemite, with about a quarter of the oxides being pure maghemite. It also contains oxyhydroxides in the form of superparamagnetic goethite. The presence of maghemite and goethite makes NWA 7034 the most oxidized Martian meteorite. The overall magnetic assemblage is partly linked to near-surface hydrothermal alteration. The high concentration of magnetic phases with high laboratory unblocking temperatures makes NWA 7034 a plausible analogue source lithology for the strong magnetization of the Martian Noachian crust. Near-surface hydrothermal alteration can enhance the remanence of Martian rocks and account for local, high magnetic anomalies of shallow source.