It is now generally recognized that Mars has had fluid flows, and a number of riverbeds, lake beds, etc have been identified, but there are also maps on the web of a proposed Northern Ocean. It has also been proposed that there has been polar wander, and this Northern Ocean was more an equatorial one when it was there about 3.6 billion years ago. The following is a partial summary from my ebook “Planetary Formation and Biogenesis”, where references to scientific papers citing the information can be found.
Various options include: (with bracketed volumes of water in cubic kilometre): a northern lake (54,000), the Utopia basin, (if interconnected, each with 1,000,000), filled to a possibly identified ‘shoreline’ (14,000,000), to a massive northern hemisphere ocean (96,000,000). Of particular interest is that the massive channels (apart from two that run into Hellas) all terminate within an elevation of 60 m of this putative shoreline.
A Northern Ocean would seem to require an average temperature greater than 273 degrees K, but the faint sun (the sun is slowly heating and three and a half billion years ago, when it is assumed water flowed, it had only about two thirds its current output) and an atmosphere restricted to CO2/H2O leads in most simulations to mean global temperatures of approximately 225 degrees K. There is the possibility of local variations, however, and one calculation claimed that if global temperatures were thirty degrees higher, local conditions could permit Hellas to pond if the subsurface contained sufficient water, and with sufficient water, the northern ocean would be possible and for maybe a few hundred years be ice free. A different model based on simulations, assuming a 1 bar CO2 atmosphere with a further 0.1 bar of hydrogen, considered that a northern ocean would be stable up to about three billion years. There is quite an industry of such calculations and it is hard to make out how valid they are, but this one seems not to be appropriate. If we had one bar pressure of carbon dioxide for such a long time there would be massive carbonate deposits, such as lime, or iron carbonates, and these are not found in the required volumes. Also, the gravity of Earth is insufficient to hold that amount of hydrogen and Mars has only 40% of Earth’s gravity. This cannot be correct.
This northern ocean has been criticized on the basis that the shoreline itself is not at a constant gravitational potential, and variations of as much as 1.8 km in altitude are found. This should falsify the concept, except that because this proposed ocean is close to the Tharsis volcanic area, the deformation of forming these massive volcanoes could account for the differences. The magma that is ejected had to come from somewhere, and where it migrated from would lead to an overall lowering of the surface there, while where it migrated to would rise.
Support for a northern sea comes from the Acidalia region, where resurfacing appears to have occurred in pulses, finishing somewhere around 3.65 Gy BP. Accumulation of bright material from subsequent impacts and flow-like mantling was consistent with a water/mud northern ocean. If water flows through rock to end in a sea, certain water-soluble elements are concentrated in the sea, and gamma ray spectra indicates that this northern ocean is consistent with enhanced levels of potassium and possibly thorium and iron. There may, however, be other reasons for this. While none of this is conclusive, a problem with such data is that we only see the top few centimeters and better evidence could be buried in dust.
Further possible support comes from the Zhurong rover that landed in Utopia Planitia (Liu, Y., and 11 others. 2022. Zhurong reveals recent aqueous activities in Utopia Planitia, Mars. Science Adv., 8: eabn8555). Duricrusts formed cliffs perched through loose soil, which requires a substantial amount of water, and also avoids the “buried in dust” problem. The authors considered these were formed through regolith undergoing cementation through rising or infiltration of briny groundwater. The salt cements precipitate from groundwater in a zone where active evaporation and accumulation can occur. Further, it is suggested thus has occurred relatively recently. On the other hand, ground water seepage might also do it, although the water has to be salty.
All of which is interesting, but the question remains: why was the water liquid? 225 degrees K is about fifty degrees below water’s freezing point. Second, because the sun has been putting out more heat, why is the water not flowing now? Or, alternatively, as generally believed, why did it flow for a brief period than stop? My answer, somewhat unsurprisingly since I am a chemist, is that it depends on chemistry. The gases had to be emitted from below the surface, such as from volcanoes or fumaroles. The gases could not have been adsorbed there as the planet accreted otherwise there would be comparable amounts of neon as to nitrogen on the rocky planets, and there is not. That implies the gases were accreted as chemical compounds; neon was not because it has no chemistry. When the accreted compounds are broken down with water, ammonia forms. Ammonia dissolves very rapidly in water, or ice, and liquefies it down to about 195 degrees K, which is well within the proposed range stated above. However, ammonia is decomposed slowly by sunlight, to form nitrogen, but it will be protected when dissolved in water. The one sample of seawater from about 3.2 billion years ago is consistent with Earth having about 10% of its nitrogen still as ammonia. However, on Mars ammonia would slowly react with carbon dioxide being formed, and end up as solids buried under the dust.
Does this help a northers sea? If this is correct, there should be substantial deposits of nitrogen rich solids below the dust. If we went there to dig, we would find out.