Evolution of water reservoirs on Mars: Constraints from hydrogen isotopes in martian meteorites

Martian surface morphology implies that Mars was once warm enough to maintain persistent liquid water on its surface. While the high D/H ratios (∼6 times the Earthʹs ocean water) of the current martian atmosphere suggest that significant water has been lost from the surface during martian history, the timing, processes, and the amount of the water loss have been poorly constrained. Recent technical developments of ion-microprobe analysis of martian meteorites have provided accurate estimation of hydrogen isotope compositions (D/H) of martian water reservoirs at the time when the meteorites formed. Based on the D/H data from the meteorites, this study demonstrates that the water loss during the pre-Noachian (>41–99 m global equivalent layers, GEL) was more significant than in the rest of martian history (>10–53 m GEL). Combining our results with geological and geomorphological evidence for ancient oceans, we propose that undetected subsurface water/ice (≃100–1000 m GEL) should exist, and it exceeds the observable present water inventory (≃20–30 m GEL) on Mars.