Increased hydrogen escape from Mars atmosphere during periods of high obliquity

It is still unknown how much water has escaped from Mars during its history. Hydrogen escape from Mars’s atmosphere probably played a major role in drying the planet, but present-day H loss rates (~3 × 10²⁶ atoms per second on average) cannot explain the geological evidence for the large volumes of liquid water on ancient Mars. Here we used the three-dimensional Mars-Planetary Climate Model to show that H loss rates could have increased by more than one order of magnitude (6 × 10²⁷ atoms per second) during higher spin axis obliquity periods, notably in the last few million years when Mars’s obliquity was about 35° on average. The resulting accumulated H escape over Mars’s history translates into an ~80 m global equivalent layer, which is close to the lower limit of geological estimates, assessing the major role of atmospheric escape in drying Mars.