If 3.6 billion years ago, Earth were not a inhospitable place, covered with oceans of magma, constantly bombarded by meteorites, and inhabited by anaerobic microorganisms living in extreme environments, instead of what we have now, the probes and rovers we are sending to Mars would have discovered a holiday paradise.
Recent research on the red planet has just revealed that Mars may have hosted in the past sandy beaches bathed in sunlight with gentle waves. A Chinese rover that landed on Mars in 2021, in an area known as Utopia Planitia, in the Martian northern hemisphere, has just detected underground beach deposits in an area believed to have once hosted a sea, providing new evidence that almost half of the planet was occupied by oceans. "We found evidence of wind, waves, and lots of sand: a beach suitable for vacations," says Benjamin Cárdenas, assistant professor of geology at Pennsylvania State University, and co-author of the study.
The Zhurong rover was operational for a year, between May 2021 and May 2022. It traveled 1.9 kilometers perpendicular to a rugged area believed to have been part of an ancient coast from 4 billion years ago, when Mars had a thicker atmosphere and a warmer climate. Along its journey, the rover used a ground-penetrating radar, similar to those used to detect pipelines, and probed up to 80 meters below the surface.
The images showed thick layers of material along the entire route, all pointing upwards, towards the supposed coast, at an angle of about 15 degrees, almost identical to the angle of Earth's beach deposits. On our planet, these deposits would have taken millions of years to form, suggesting that Mars had a long-lasting water mass with waves to distribute sediments along a sloping coast.
The radar also determined the size of the particles in these layers, matching those of sand, and not resembling ancient wind-blown dunes that characterize Mars.
"They don't look like sand dunes, impact craters, or lava flows. That's when we started thinking about oceans," said Michael Manga, professor of earth and planetary sciences at the University of California, Berkeley. "The orientation and slope of these formations are similar to what an ancient coastline would have had over a long period of time."
The research team estimated that the ocean could have formed as a result of floods about 3.7 billion years ago, then froze and gradually disappeared about 3.4 billion years ago.
"This reinforces the hypothesis that this region of Mars was habitable in the past," noted Hai Liu, professor at the School of Civil Engineering and Transportation at Guangzhou University and lead member of the scientific team of the Tianwen-1 mission, which included the first Chinese explorer on Mars, Zhurong.
According to the Chinese and American authors of the article, just published in Proceedings of the National Academy of Sciences, the existence of beaches implies the existence of a large ice-free ocean on Mars, as well as rivers that carried sediments to this ocean, although Mars is now too cold for water to flow in liquid form.
"Coasts are excellent places to look for evidence of past life. It is believed that early life on Earth began in places like this, near the interface between air and shallow waters," says Cárdenas.
Images taken by the Viking probe in the 1970s led to the first speculation about the existence of an ocean on Mars, probably at a time when the planet had a denser atmosphere that could retain heat and, therefore, liquid water. The Viking probe images showed what appeared to be a coastline around a large part of Mars' northern hemisphere and a depression that could be an ancient seabed.
However, the coastline was so irregular, with ups and downs of up to 10 kilometers, that planetary scientists doubted this hypothesis because coasts, like those on Earth, should be flat. Other mysteries, such as what happened to the water, also cast doubt on this theory, as Mars' polar caps do not contain enough water to fill an ocean of that size.
However, later missions to Mars provided evidence that, while much of the planet's water likely escaped into space along with Mars' atmosphere as the planet cooled, much went underground, either as ice or combined with rocks to form new minerals.