A new analysis of puzzling gullylike features on Mars offers further evidence that water flowed on the Red Planet's surface, perhaps as recently as several hundred thousand years ago. The findings bolster the case that melting snow from a departed Martian ice age carved these gullies, rather than shifting sands or other "dry" phenomena.

The paper, published today in the journal Geology, examines one of the many gullies on crater and valley walls visible in satellite images of the Martian surface. The cause and age of these channels are the subject of debate.

To help settle the matter, the paper's authors relied on a bit of geologic good luck to date one of the features. Images snapped by the HiRISE (High Resolution Imaging Science Experiment) camera onboard the Mars Reconnaissance Orbiter showed a typical gully system about 0.62 mile (one kilometer) long with a deltalike fan at its bottom—similar to features on Antarctic Dry Valleys and on dry mountains after rain in the U.S. Southwest.

The researchers noticed an array of craters dotting the delta's western portion [lower left in the photograph] and suspected that a large meteorite impact had peppered the area with ejected rocks to form these so-called secondary craters. Sure enough, the pattern of pockmarks led like a trail of bread crumbs to a large impact crater some 62 miles (100 kilometers) southwest. Infrared imaging assisted in matching the impact events.

"We had hoped to find the source of these secondary craters, and voilà, we found a link to this big crater," says lead study author Samuel Schon, a graduate student at Brown University's Planetary Geosciences Group.

To estimate the age of planetary surfaces, geologists look to craters. Earth, with its active tectonics and dynamic weather, has few impact craters on its relatively young surface; Earth's weatherless, geologically dead moon, however, is heavily cratered and has very old surfaces (except where ancient lava flows formed large, flat, dark areas called maria, or "seas"). Because much of the rest of the studied Martian gully system is craterless [at right in the photograph], those areas are believed to be geologically younger than the cratered portions, Schon says.

By dating the primary Martian crater that sprayed the gully to 1.25 million years old—which qualifies as very young in Mars's 4.6-billion-year history—Schon and his colleagues demonstrated that an estimated three subsequent outflows in the gully system must be younger than the first, which was bombarded by ejecta.

"Dating these gullies has been a big problem," says Michael Meyer, lead scientist for NASA's Mars Exploration Program. "But because these guys dated the nearby crater, you have proof that the water activity is recent."

Today, virtually all water on Mars is believed to be trapped underground or frozen near the poles, where the Phoenix lander found ice mere inches under the soil this past summer. Unresolved research, though, has hinted at very recent water activity on Mars—within the past decade. A tantalizing ribbon of light-colored material mimicking water flow appeared in satellite images of a crater wall sometime between 1999 and 2005. But materials scientists have shown that avalanching sands can also create this effect, and later studies cast doubt on water as the culprit.

For this newly studied gully system, the evidence of multiple outflows discredits drifting sands, and the classic alluvial pattern of the delta does not fit with sedimentary shifts, Schon says. Jack Holt, a geophysicist at the University of Texas at Austin, agrees "melting an ice deposit caused by an ice age seems like a more feasible scenario."

Mars has an axial tilt (similar to Earth's) that causes seasons, but the Red Planet wobbles more on its axis than Earth does because it lacks the gravitational stabilization that our relatively large moon provides. Coupled with a more elliptical orbit than Earth, Mars likely has major swings in climate and temperature over short spans of geologic time. Other recent discoveries, including massive Martian glaciers still present under blankets of crustal debris in the mid-latitudes, support theories of past ice ages on the Red Planet.

"This new study is yet more strong evidence for widespread deposition of ice during a different climatic regime," says Holt, who led the glacier work.

Due to frigid surface temperatures and low atmospheric pressure, liquid water could not persist for long on Mars's surface nowadays. But scientists continue to hunt for signs of liquid water in the recent and distant past—not least for the clues they may provide about the possible development of extraterrestrial life when Mars's climate was more hospitable.

"We think the heyday for water on Mars was over three billion years ago," Schon says. "With this gully system, we're talking about a relative trickle compared to then, but nonetheless, this happened, and now we know when."