Alternatively, we could imagine that for some time the Late Hesperian Martian climate was sufficiently warmed by additional strong greenhouse gases, and thus keeping the ocean at least partly liquid. For instance, reducing gases (e.g. CH 4 and H 2 ) offer a way to warm the surface of ancient Mars above the melting point of water17,18,19,20,21. This effect results in part from the strong collision-induced absorptions of CO 2 -CH 4 , CO 2 -H 2 and H 2 -H 2 pairs17,18,22.

However, the persistence of a deglaciated ocean during the Late Hesperian on Mars would raise several issues. New 3-D Global Climate simulations (see Methods) confirm that a deglaciated northern ocean could be permanently sustained with the assumption of enough greenhouse gases (CO 2 and H 2 here). However, ocean waters would evaporate rapidly and subsequently migrate toward the elevated Martian terrains through the mechanism of adiabatic cooling mentioned above for the case of a frozen ocean. This process is rapid because evaporation and sublimation rates increase exponentially with temperature. For instance, a 200 m deep deglaciated northern ocean would completely evaporate within ∼103 martian years, whatever the obliquity, surface pressure of CO 2 and of additional reducing gases, and whatever the initial temperature (>273 K) of the ocean assumed.

In the simulations, a part of the atmospheric water returns to the surface as rain, near the ocean shoreline (see Fig. 2a). Such precipitation would produce extensive fluvial erosion, in particular in the regions where evidence for tsunami events have been reported7,8, and as long as the deglaciated northern ocean remains. The remainder is sequestered as ice on the elevated terrains (see Fig. 2c). In any case, for the northern ocean to survive, an intense hydrological cycle had to occur in order to replenish the water that was transported to the elevated terrains. Although previous regional maps seem to indicate an absence of such a strong hydrological cycle in the mid and late Hesperian geological record (see23 and references therein), this prediction could be tested in more details through further high-resolution geological investigations, in particular along the proposed paleo-ocean shoreline.

Figure 2 Presents the annual cumulated rainfall, the annual net surface accumulation of water, and the position of permanent ice reservoirs for various 3-D Global Climate simulations. Full size image

To solve this paradox, one hypothesis could be for the ocean to be replenished by groundwater. In this scenario, water that condensed on the elevated Martian volcanic regions would have formed thick glaciers that would undergo melting at their base, possibly introducing the meltwater into subsurface aquifers24. These subsurface liquid water reservoirs could then have provided the water that carved the outflow channels, thus replenishing the northern ocean. Such an hypothesis would be consistent with our 3-D Global Climate simulations (see Fig. 2b) in which water tends to condense preferentially close to the regions that sourced the outflow channels. However it is difficult to reconcile this hypothesis with the estimated lifetime of the ocean. It has been reported that at least two large tsunami events were produced by bolide impacts, resulting in craters 30–50 km in diameter7,8. Based on the crater frequency rates of Rodriguez7, the rate of Late Hesperian marine impacts producing craters ∼30 km in diameter is one every 2.7 million years. Unless the tsunamis were the result of very unlikely occurrences, the ancient ocean would have to survive for a period of at least a few million years to produce the reported two consecutive tsunami events7,8. This is also supported by the detection of glacier valleys cross cuting first, older tsunami deposits and having floors partly covered by younger, second tsunami deposits, indicating that the time gap between the two tsunami events was geologically significant7. We estimate from our 3-D Global Climate simulations (see Fig. 2b) that the net evaporation rate of the ocean is at least 0.6 m per martian year, and that at least 104 km3 of water would be required for replenishment per martian year in order for the ocean to remain stable. Thus, as much as 1.5 × 1010 km3 (e.g. ∼100 km GEL) of water would need to have flown through the outflow channels for a deglaciated northern ocean to survive for 2.7 million years. This amount is several orders of magnitude larger than previous estimates of the total amount of water required to erode all the Martian outflow channels25.