The history of Martian water during the Hesperian and Amazonian epochs

Evidence points to there having been more surface and near-surface Martian water in the late Hesperian and early Amazonian than there is at present: (i) The measured enrichment in the D/H isotopic ratio suggests that the total amount of H₂O lost to space during the Amazonian was significant relative to the amount currently locked up in the polar and non-polar ice deposits, and that it was more than expected based on the current rate of loss to space. (ii) The Dorsa Argentea Formation (DAF), surrounding the South Polar Layered Deposits (SPLD) and having an area larger than the SPLD, is thought to be the remnant of a Hesperian-era polar cap that may have contained ∼20 m Global Equivalent Layer (GEL) or more of water that is not thought to be present in the DAF today. We explore the consequences of this greater surface water inventory, how it ties in with the exchange of water between the different non-atmospheric reservoirs and the loss of water to space from the Hesperian epoch up to the present. The combination of polar and atmospheric processes and the likelihood of the axial obliquity typically having been greater than today's 25.2° earlier in history would have resulted in more surface and near-surface water and therefore larger H₂O polar caps in the late Hesperian, enhanced atmospheric water content, and an enlargement of regions having stable ground ice so that global near-surface ice may have been the norm. In addition, the increased solar extreme ultraviolet (EUV) flux, solar flares, and coronal mass ejections (CMEs) during the earlier epochs would have enhanced loss of H and O to space above the current loss rate, so that much or all of the water from the DAF may have been lost to space. Coupling between solar processes, atmospheric and upper-atmospheric processes, and geological processes produces a self-consistent scenario for the evolution of water.