Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust

Abstract

Burying Mars ancient water in the crust Mars once had oceans of liquid water on its surface but little of that water remains today in the planet s ice caps and atmosphere. This discrepancy is usually interpreted as loss of water to space, supported by the atmospheric deuterium/hydrogen (D/H) ratio, but this has been difficult to reconcile with other constraints. Scheller et al. propose that water could instead have been incorporated into minerals in the planet s crust, which were later buried (see the Perspective by Kurokawa). They simulated the evolution of the D/H ratio and atmospheric loss rates for a range of plausible conditions, finding that 30 to 99% of Mars initial water was buried in the crust. Science, this issue p. 56; see also p. 27 Much of the water in Mars’ ancient oceans was incorporated into minerals and buried in the planet’s crust, not lost to space. Geological evidence shows that ancient Mars had large volumes of liquid water. Models of past hydrogen escape to space, calibrated with observations of the current escape rate, cannot explain the present-day deuterium-to-hydrogen isotope ratio (D/H). We simulated volcanic degassing, atmospheric escape, and crustal hydration on Mars, incorporating observational constraints from spacecraft, rovers, and meteorites. We found that ancient water volumes equivalent to a 100 to 1500 meter global layer are simultaneously compatible with the geological evidence, loss rate estimates, and D/H measurements. In our model, the volume of water participating in the hydrological cycle decreased by 40 to 95% over the Noachian period (~3.7 billion to 4.1 billion years ago), reaching present-day values by ~3.0 billion years ago. Between 30 and 99% of martian water was sequestered through crustal hydration, demonstrating that irreversible chemical weathering can increase the aridity of terrestrial planets.