  The long-standing debate on the existence of ancient oceans on Mars has been
recently revived by evidence for tsunami resurfacing events that date from the
Late Hesperian geological era. It has been argued that these tsunami events
originated from the impact of large meteorites on a deglaciated or nearly
deglaciated ocean present in the northern hemisphere of Mars. Here we show that
the presence of such a late ocean faces a paradox. If cold, the ocean should
have been entirely frozen shortly after its formation, thus preventing the
formation of tsunami events. If warm, the ice-free ocean should have produced
fluvial erosion of Hesperian Mars terrains much more extensively than
previously reported. To solve this apparent paradox, we suggest a list of
possible tests and scenarios that could help to reconcile constraints from
climate models with tsunami hypothesis. These scenarios could be tested in
future dedicated studies.
