Geomorphic Evidence for a Late Hesperian Northern Ocean and its Implications for the Planetary Inventory of Water During the Noachian.

Lobate flow deposits, that appear to have emanated from within the Martian northern plains and propagated from lower to higher elevations along the dichotomy boundary, have recently been identified by Rodriguez et al. (2016) and Costard et al. (2017). Backwash channels are also found in association with these deposits. Such features are strikingly similar to those associated with terrestrial tsunamis - suggesting that the Martian examples may have originated from one or more large marine impacts. The distribution of these landforms is consistent with the location of Contact 2 (elevation -3760), previously identified by Parker et al. (1993) as the possible paleoshoreline of a northern ocean. The occrruence and distribution of these features provides new and compelling evidence of the presence of a northern ocean during the Late Hesperian ( 3 Ga). The volume of water necessary to fill the northern plains to the elevation of Contact 2 is 100 m GEL. However, this is only a fraction of the planetary inventory of water that must have existed at this time. For example, virtually all of the crustal porosity, lying at an elevation below that of the ocean sea level, must have been saturated with groundwater - representing a total volume of 225 ± 75 m GEL. An additional 300 ± 100 m GEL is thought to have been cold-trapped in the cryosphere as ground ice, with another 30 m GEL stored in the more extensive Late Hesparian south polar layered deposits (SPLD). This yields a total global inventory of 655 ± 175 m GEL of H2O during the Late Hesperian. The inventory of water during the Noachian was probably similar, as the amounts released by extrusive volcanism and lost by exospheric escape were comparable. However, the 33% higher geothermal heat flow during the Noachian means that 100 m GEL less water would have been stored as ground ice, making it available as a liquid, to be stored as ground- and surface water. This suggests that a Noachian northern ocean would have been at least 10% more areally extensive than the one that existed at the end of the LH - a conclusion that has important implications for understanding the evolution of the Martian hydrosphere and the potential for the origin and survival of life.