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."