The History of Water in Martian Magmas From Thorium Maps

Water inventories in Martian magmas are poorly constrained. Meteorite‐based estimates range widely, from 102 to >104 ppm H2O, and are likely variably influenced by degassing. Orbital measurements of H primarily reflect water cycled and stored in the regolith. Like water, Th behaves incompatibly during mantle melting, but unlike water Th is not prone to degassing and is relatively immobile during aqueous alteration at low temperature. We employ Th as a proxy for original, mantle‐derived H2O in Martian magmas. We use regional maps of Th from Mars Odyssey to assess variations in magmatic water across major volcanic provinces and through time. We infer that Hesperian and Amazonian magmas had ∼100–3,000 ppm H2O, in the lower range of previous estimates. The implied cumulative outgassing since the Hesperian, equivalent to a global H2O layer ∼1–40 m deep, agrees with Mars’ present‐day surface and near‐surface water inventory and estimates of sequestration and loss rates. Plain Language Summary Past volcanism on Mars has supplied some of the water that carved ancient river valleys and shaped the chemistry of the Martian near‐surface. However, the amount of water carried by Martian magmas is an open question, in part because igneous rocks and meteorites have often lost their original water contents through degassing. The trace element thorium can be used as a proxy for magmatic water, because thorium and water are transferred in similar proportions to magmas during mantle melting, but thorium does not degas. We use regional maps of thorium from the Mars Odyssey spacecraft to track variations in magmatic water through time and across major volcanic provinces. Key Points Thorium partitions similarly to H2O during Martian mantle melting but does not degas, and is useful as a proxy for primary magmatic H2O Gamma Ray Spectroscopy maps of thorium distribution are used to track variations in primary magmatic H2O through Mars’ history We infer that Hesperian and Amazonian magmas had ∼100–3,000 ppm H2O, implying outgassing of a global H2O layer ∼1–40 m deep