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The abundance and distribution of water within Mars through time plays a fundamental role in constraining its geological evolution and habitability. The isotopic composition of Martian hydrogen provides insights into the interplay between different water reservoirs on Mars. However, D/H (deuterium/hydrogen) ratios of Martian rocks and of the Martian atmosphere span a wide range of values. This has complicated identification of distinct water reservoirs in and on Mars within the confines of existing models that assume an isotopically homogenous mantle. Here we present D/H data collected by secondary ion mass spectrometry for two Martian meteorites. These data indicate that the Martian crust has been characterized by a constant D/H ratio over the last 3.9 billion years. The crust represents a reservoir with a D/H ratio that is intermediate between at least two isotopically distinct primordial water reservoirs within the Martian mantle, sampled by partial melts from geochemically depleted and enriched mantle sources. From mixing calculations, we find that a subset of depleted Martian basalts are consistent with isotopically light hydrogen (low D/H) in their mantle source, whereas enriched shergottites sampled a mantle source containing heavy hydrogen (high D/H). We propose that the Martian mantle is chemically heterogeneous with multiple water reservoirs, indicating poor mixing within the mantle after accretion, differentiation, and its subsequent thermochemical evolution.
