Water flowed on Mars as recently as several hundred million years ago when sunlight melted a thin layer of glacier ice, researchers now say.
The evidence lies in dozens of channels on Mars carved by melting glacier water during the cold, dry period that has dominated the red planet for the past 3.5 billion years, researchers said. Such youthful evidence surprised scientists, because it suggests that running water existed on Mars much more recently than previously found.
"We think of [post-Noachian] Mars as really, really cold and really, really dry, so the fact that these exist, in those kinds of conditions, is changing how we view the history of water on the planet," said study leader Caleb Fassett, a planetary geologist at Brown University in Providence, RI.
A vast ocean on ancient Mars covered more than one-third of the Martian surface more than 3 billion years ago, according to a study released earlier this month. But evidence of liquid water on Mars since the planet's Noachian era, a time period that ended 3.5 billion years ago, has remained scarce until now.
Fassett and colleagues at Brown University and Portland State University discovered more recent evidence in the form of channels that stretch for a mile or two and can have a width of more than 150 feet (nearly 46 meters). They also linked ice deposits to the channels — called glaciofluvial valleys — carved out by the melt-water.
Such channels arose on the interior and exterior of Mars craters in the planet's middle latitudes during the recent Amazonian epoch on Mars, when sunlight melted a thin layer of ice on the top of glaciers.
Some geologists from Brown and Boston University have spotted similar conditions on Earth, too. They occur in the Antarctic Dry Valleys, where glacier surfaces melt during the summer, they said.
"You're freezing cold and there's glacial ice everywhere, and it gets just warm enough that you get a river," said James Dickson, a research analyst also at Brown University.
Fassett and his colleagues surveyed 15,000 images taken by the Context Camera (CTX) aboard NASA's powerful Mars Reconnaissance Orbiter, which has mapped about 40 percent of the planet so far. But that's just the beginning of their search for more glaciofluvial valleys.
The research is detailed in a recent issue of the science journal Icarus.