When we think of Mars, we think of a red planet, barren, covered in dust and rocks and ancient lava flows. There’s an ongoing debate about the existence of liquid water under the polar ice caps. We’ve tracked seasonal changes with a host of satellites. And we have a small collection of rovers wandering the surface, sampling, analyzing, and collecting rocks. There are sedimentary rocks that were clearly created due to the flow of water. We’ve seen pictures of stream channels and river deltas, and Perseverance landed in what we think was an ancient crater lake bed.

And yet, no life. Granted, Percy is the first rover really tasked with looking for signs of life*, and even it’s not allowed to sample where we think life could still be if it were there. We seem, on the whole, to be hopeful that we’ll find evidence of life or past life because of that wet history of Mars, even if most of the water is gone, whether it has escaped the thin atmosphere or gone underground.

Now, in a new paper published in the Proceedings of the National Academy of Sciences, a NASA-funded simulation of past Mars has revealed that the red planet was wetter for longer than previously thoughts – about 500 million years longer. The research into Mars’ past conditions is not an easy task, as co-author Michael Way explains: Discerning the climate of Mars approximately three billion years ago is challenging because the Martian surface features do not seem to fully support either a warm and wet or cold and dry climate during that time. A warm and wet climate would have produced extensive erosion from flowing water, but few valley networks have been observed from this age. A too-cold climate would have kept any northern ocean frozen most of the time. A moderate cold climate would have transferred the water from the ocean to the land in the form of snow and ice. But this would prevent tsunami formation, for which there is some evidence.

This new simulation found not only a cold and wet Mars was possible, but there might have even been a stable ocean in the northern hemisphere where the atmosphere was dense and warm. That would make it possible for the water to evaporate but then fall back to the surface as rain or snow. In nearby highland regions, the snow could accumulate enough to become glaciers, and those glaciers would flow back down to rejoin the ocean.

And that northern ocean means a mechanism for circulating warmer water to the north pole, raising temperatures there as well.

Factor in the evidence that Mars’ atmosphere was thicker in the past, and you can now keep Mars warm for a longer period of time than previously thought. A warmer, wetter planet for an extra 500 million years gives you that much more time and the right conditions for life to develop.

The evidence is piling up for a habitable past Mars, and I’m excited to contemplate what future rock samples might find.

*This statement is incorrect. Per NASA’s page on the Viking 1 and 2 missions: Besides taking photographs and collecting other science data on the Martian surface, the two landers conducted three biology experiments designed to look for possible signs of life. These experiments discovered unexpected and enigmatic chemical activity in the Martian soil, but provided no clear evidence for the presence of living microorganisms in soil near the landing sites. According to scientists, Mars is self-sterilizing. They believe the combination of solar ultraviolet radiation that saturates the surface, the extreme dryness of the soil and the oxidizing nature of the soil chemistry prevent the formation of living organisms in the Martian soil.

More Information

NASA press release

“Circumpolar ocean stability on Mars 3 Gy ago,” Frédéric Schmidt et al., 2022 January 25, PNAS