We observe ancient shorelines, indicating the presence of a large standing body of liquid water, but little to no erosional activity in the form of rivers/valleys. We observe rock deposits from tsunamis that occurred in the past; but tsunamis would not have been possible had the ocean been completely frozen. This paradox has been under the scanner of many planetary science studies (see, for example, Turbet and Forget (2019)).
What explains these apparent discrepancies? A liquid ocean might have been possible if temperature was just above the freezing point i.e. 4.5 °C. At first there seems to be a clear mutual exclusivity between warm-and-wet and cold-and-dry. The conditions on Mars 3.4-3.0 Ga (billion years ago; a period also known as the Late Hesperian in Mars’s geologic history) could either have been this or that.
A possible solution to this puzzle could be that the climate was cold-and-wet. So far, studies had shown that was not possible. A 2016 review argued that the ancient climate of Mars was largely cold, because of low solar radiation, and could not have sustained an ocean. On the other hand, another 2021 study maintained that the planet had a warming mechanism capable of harbouring lakes and rivers.
In this study, Schmidt et al. (2022) reconstructed the past climate of Mars using a three-dimensional model that was based on an Earth climate model. This model for Mars is capable of estimating the interaction between atmosphere and ocean circulation as well as surface water. Furthermore, the model also accounted for the fact that the Sun’s brightness was 79% of what it is today. Glacial flux, i.e. the runoff of glacial melt to the ocean, was considered as well.
It was found that in the cold-and-wet scenario, the globally averaged surface temperature is below 0 °C. But, the northern ocean surface temperature is ~7 °C and well above the freezing point. The southern hemisphere, on the other hand, is under ice cover. The presence of an ocean in the northern hemisphere of the Red Planet is explained by a few things.
One, while the average surface temperature for the whole planet is well below 0 °C, the ocean remains above the freezing point because of its low altitude (the more the altitude, the lesser the temperature; and vice versa). The other contributing factor is ocean circulation, that transports heat. 40 showed how ocean gyres, a group of ocean currents, transported heat towards the poles. ‘Surface temperature increase due to the active circulation of the ocean,’ and the consequent ‘warming is globally present at all latitudes but locally higher near the northern polar ocean, from 1 ◦C up to 4.5 ◦C,’ Schmidt et al. (2022) state.
Another reason for the above-freezing point temperatures lies in the chemical composition of the atmosphere. The atmosphere of Mars was – and still is – dominated by carbon dioxide. The blanket of carbon dioxide also had 10% hydrogen gas (H2) in good measure that produces a greenhouse effect, thereby increasing surface temperatures. The paper argues that hydrogen gas could have been a product of volcanic activity. While hydrogen gas produced from volcanic activity is ‘not expected to persist for more than 1 million years,’ it has been established that volcanic activity would have spanned a period of 3.8-0.2 Ga.
Finally, it was found that 60% evaporation in the ocean water is pretty much balanced by precipitation. There is also ‘significant glacier return flow’ from the cold highlands, where precipitation occurs largely in the form of snow, back to the ocean. These explain why we have an ocean, but not many river valleys – which would have been the case had the climate been warm and wet. By contrast, the southern hemisphere was covered almost entirely by extensive, thick, ice sheets.
The findings of the model are well corroborated by the observed geology, such as ancient shorelines or presence of widespread sedimentary rocks in the northern plains of Mars. This was also a time marked with ‘extensive [glacial] outflow channels,’ that would have stemmed from deglaciation or the collapse of icy reserves of water at high altitudes (aquifer disruption).
A few questions, however, still remain. One, did life exist on the planet some 3 billion years ago? One cannot be sure. The hydrological cycle is similar to today’s Earth, after all, replete with pressure and temperature conditions, liquid water, ocean, snow, glaciers. In fact, some of the regions of Mars bear resemblance to Alaskan glaciers and Antarctic ice streams. And as this study demonstrates, there was a large standing body of water, and for a fairly long duration to allow for life. However, this may not have been sufficient, Schmidt says, as the atmosphere was still starved for oxygen.
Lastly, where did all the water go? There is the possibility of breakdown of water molecules into hydrogen and oxygen and formally finally being ejected into space. Additionally it has been proposed, by a 2021 study, that ancient Martian water is either present as ice under the surface, or has been consumed by rock and mineral forming processes, making the crust very hydrated.
“The author is a research fellow at the Indian Institute of Science (IISc), Bengaluru, and a freelance science communicator. He tweets at @critvik”