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.

Previous evidence for an ancient northern ocean on Mars has come in the form of a somewhat controversial interpretation of certain landforms as shorelines, the striking smoothness of the northern terrain, and the presence of surfaces that appear to be sedimentary deposits.

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 orbited the Red Planet since 2003. (MARSIS stands for Mars Advanced Radar for Subsurface and Ionosphere Sounding.) "We mapped the intensity of the surface echo all over the planet," says lead study author Jérémie Mouginot, an associate project scientist in Earth system science 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. "The only interpretation we see is sediments," Mouginot says. "These sediments came from a flow or a shallow ocean that put the sediments there."

The sedimentary interpretation of the formation agrees with data obtained by another sounding radar on NASA's Mars Reconnaissance Orbiter, which surveyed the region a few years ago. That orbiter's SHARAD (Shallow Radar) 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 covered a large region of the northern plains, but 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. (Mouginot notes that there may also have existed a more primitive ocean on Mars, which could be responsible for the supposed shorelines.) "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 refroze and been buried underground or escaped as vapor.

The radar reflectivity of the Vastitas Borealis formation could be explained by sediments either standing alone or mixed with ice. So it is possible that some subsurface remnant of the ocean itself persists. But that seems a fairly remote possibility, according to planetary scientist Norbert Schörghofer of the Institute for Astronomy of the University of Hawaii at Manoa, who was not part of the research team. "It's much more likely that it's sediment, because it's just very hard to keep ice there for a long time," he says. "Physically, the ice should most likely have disappeared."

The new radar data offer support—but not incontrovertible ground truth—for the long-held vision of an expansive body of water covering the Martian north. "The ancient ocean hypothesis will take awhile to prove to a high scientific standard, because it's a bit buried, so to speak, today," Schörghofer says. 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."