Water is thought to have been present on Earth’s surface for most of geological time, but it disappeared from the surface of Mars soon after the planet’s formation. Jon Wade and co-authors quantify the relative volumes of water that could be removed from each planet’s surface via the burial and metamorphism of hydrated mafic rocks from the crust. They show that the metamorphic mineral assemblages in Martian lavas, which are richer in ferrous oxide than are Earth’s lavas, can hold about 25% more water. These assemblages are able to transport water to greater relative depths within the interior of Mars. The existence of a buoyant mafic crust and hotter geothermal gradients on Earth probably reduced the potential for upper mantle hydration early in geological history, leading to water being retained close to its surface.
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