Ancient Mars may have had a liquid ocean despite freezing temperatures

Mars may have had a liquid water ocean 3 billion years ago, even if the temperature at the surface was below freezing.

There is strong geological evidence that Mars once had an ocean, such as ancient shorelines, but it is unclear what conditions could have made possible all the features seen on the planet today. If it was warm enough for a liquid ocean, there should be valleys scarred by rivers, but these haven’t been observed. If the climate was too cold, there would have been land ice, which doesn’t fit with our observations of rock deposits from historical tsunamis.

Now, Frédéric Schmidt at the University of Paris-Saclay in France and his colleagues have found that a liquid ocean could have existed with an above water temperature of just below freezing. In this scenario, the ocean is kept warm enough to remain liquid by water circulation that could give it a temperature of around 4.5°C

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Schmidt and his team used a model that simulates how Earth’s oceans and atmosphere interact, but changed the parameters to match Mars’s ancient environment, such as its atmospheric gas makeup and a lower sun power. As well as a liquid ocean, the model also suggests there may have been moderate rainfall along the ocean shores and a largely frozen southern region.

The ancient climate features that the model produced were similar to Earth’s billions of years ago, and would have contained some of the key ingredients for microbial life.

“If we could travel in time to 3 billion years ago, we could live on this ancient Mars with just a spacesuit for oxygen,” says Schmidt. “Pressure, clouds, liquid water, ocean, rain, snow and glaciers: all of them were very similar to Earth today. Only oxygen was missing.”

The study shows that an ocean at this stage in Mars’s past is plausible, says Sanjeev Gupta at Imperial College London, but it is only a simulation. “The authors pull together observations from other studies with evidence for an ocean to tie into their results, but proof of an ocean does not come from the modelling. We would need stronger geological evidence for an ocean,” he says.

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