  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
Earth's 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 have
existed, and it exceeds the observable present water inventory (~20-30 m GEL)
on Mars.
