The divergent fates of primitive hydrospheric water on Earth and Mars

• Published in: Nature (London) 2017-12, Vol.552 (7685), p.391-394
• Main Authors: Wade, Jon, Dyck, Brendan, Palin, Richard M., Moore, James D. P., Smye, Andrew J.
• Format: Article
• Language: English
• Subjects: 704/2151/431; 704/445/209; 704/445/845; Earth; Humanities and Social Sciences; Hydrogeology; Hydrological research; letter; Mantle (Geology); Mars (Planet); multidisciplinary; Science; Surface water
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature25031

Modelling the reactions of water with the crusts of early Earth and Mars sheds light on how water was transported through their crusts to give the surfaces we see today. Different fates of surface water on Earth and Mars 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. Despite active transport into Earth’s mantle, water has been present on our planet’s surface for most of geological time 1 , 2 . Yet water disappeared from the Martian surface soon after its formation. Although some of the water on Mars was lost to space via photolysis following the collapse of the planet’s magnetic field 3 , 4 , 5 , the widespread serpentinization of Martian crust 6 , 7 suggests that metamorphic hydration reactions played a critical part in the sequestration of the crust. Here we quantify the relative volumes of water that could be removed from each planet’s surface via the burial and metamorphism of hydrated mafic crusts, and calculate mineral transition-induced bulk-density changes at conditions of elevated pressure and temperature for each. The metamorphic mineral assemblages in relatively FeO-rich Martian lavas can hold about 25 per cent more structurally bound water than those in metamorphosed terrestrial basalts, and can retain it at greater depths within Mars. Our calculations suggest that in excess of 9 per cent by volume of the Martian mantle may contain hydrous mineral species as a consequence of surface reactions, compared to about 4 per cent by volume of Earth’s mantle. Furthermore, neither primitive nor evolved hydrated Martian crust show noticeably different bulk densities compared to their anhydrous equivalents, in contrast to hydrous mafic terrestrial crust, which transforms to denser eclogit