Much of Mars’ ancient water was buried in the planet’s crust, not lost to space.
Several oceans’ worth of ancient water may reside in minerals buried below Mars’ surface, report researchers. The new study, based on observational data and modeling, shows that much of the red planet’s initial water — up to 99% — was lost to irreversible crustal hydration, not escape to space.
The findings help resolve the apparent contradictions between predicted atmospheric loss rates, the deuterium to hydrogen ratio (D/H) of present-day Mars, and the geological estimates of how much water once covered the Martian surface.
Ancient Mars was a wet planet — dry riverbeds and relic shorelines record a time when vast volumes of liquid water flowed across the surface. Today, very little of that water remains, mostly frozen in the planet’s ice caps. Previous studies have assumed that the lost water escaped to space over several billion years, an assertion supported by the currently observed atmospheric D/H ratio. However, measurements of the current rate of atmospheric water loss are too low for atmospheric escape alone to explain all Martian water loss.
Eva Scheller and colleagues show how large volumes of water could have instead become incorporated into minerals that were buried in the planet’s crust. Using observational constraints from orbiting spacecraft, rovers, and Martian meteorites, Scheller et al. developed a water budget and D/H model that considers atmospheric escape, volcanic degassing and crustal hydration through chemical weathering. By simulating Martian water loss through geological time and for a range of plausible conditions, the authors discovered that Mars had lost most of its water — between 40-95% — over the Noachian period (~4.1 – 3.7 billion years ago).
The results suggest that between 30 and 99% of Mars’ initial water was incorporated into minerals and buried in the planet’s crust, with subsequent escape to space of the remainder accounting for the currently observed D/H ratio.
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Reference: “Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust” by E. L. Scheller, B. L. Ehlmann, Renyu Hu, D. J. Adams and Y. L. Yung, 16 March 2021, Science.