In the eyes of many planetary scientists, the surface of Mars’s northern hemisphere has long looked like it once contained an ocean. Now it is “sounding” that way, too.

A European spacecraft equipped with sounding radar that bounces radio waves off the Red Planet to investigate its makeup has identified what appear to be sedimentary deposits in the Martian north. The sediments, which could be mixed with ice, would represent the remains of a shallow ocean that existed some three billion years ago, according to a study published in January in Geophy­sical Research Letters.

The new research is based on a series of radar soundings by the MARSIS instrument on the European Space Agency’s Mars Express orbiter, which has circled the Red Planet since 2003. “We mapped the intensity of the surface echo all over the planet,” says lead study author Jérémie Mouginot, a geophysicist at the University of California, Irvine. In the Vastitas Borealis formation, a geologic deposit near the Martian north pole that has long been suspected of being sedi­mentary in origin, the radar reflectivity was quite low—lower than would be expected if the formation were volcanic rather than sedimentary.

Mouginot’s interpretation is in sync with data obtained by another sounding radar on NASA’s Mars Reconnaissance Orbiter, which surveyed the region a few years ago. That spacecraft’s SHARAD instrument suggested that the Vastitas Borealis formation comprised a sub­stantial sedimentary layer overlying volcanic plains.

Based on the extent of the sediments identified by Mars Express, the ocean would have overlain a large region of the northern plains, though not for very long. Around three billion years ago Mars appears to have had enough geothermal activity to melt a large amount of ground­water and feed a shallow ocean, perhaps 100 meters deep. (There may also have existed an earlier ocean, Mouginot adds.) “I think what we had here is some epi­sode of flash flooding or something like that that covered the northern plain,” Mouginot says. But the environment would have been too cold and too dry to sustain a large body of water over geologic timescales. Within a million years or so the ocean would have refrozen and been buried underground or escaped as vapor.

The new radar data offer support—but not incontrovertible ground truth—for the long-held vision of an expansive body of water spread over the Martian north. “The ancient ocean hypothesis will take a while to prove to a high scientific standard because it’s a bit buried, so to speak, today,” says planetary scientist Norbert Schörghofer of the Institute for Astronomy at the University of Hawaii at Manoa, who was not part of the research team. And one can always wonder about additional interpretations of the radar echoes, which provide a relatively non­specific diagnosis of a given material. All the same, “it’s another piece of evidence for an ancient ocean,” he says. “I’m starting to believe it.”

This article was published in print as "Swimming on Mars."