"It's a very regular pattern," Dickson said.
"And everybody agrees that [the gullies are] very, very young in geological terms."
That means the climate of Mars has been much more variable in the past few hundred million years than was once believed.
"We see evidence for this in other areas," Dickson said, such as signs of now-vanished glaciers.
In fact, recent findings suggest that orbital variations cause Mars to change its tilt periodically, creating a cycle of ice ages like the one on Earth.
(Read "Mars, Like Earth, Has Cyclical Ice Ages, Study Says" [September 14, 2007].)
Currently the red planet rotates on its axis at a 23-degree angle toward the sun. But in the past it sometimes hit an angle of 45 degrees or more.
Each time the angle changed, climate would have altered across Mars.
Polar summers would have gotten warmer, melting ice at the poles and creating precipitation in the form of snow that fell at locations where the gullies are now seen.
"That's what the physics says, and we're seeing that in the glaciers and gullies we feel have been formed by melting snow," Dickson said.
Similar climate cycles may also have occurred even further back in the red planet's past.
In another presentation at the conference, Kevin Lewis at California Institute of Technology reported that his team has found signs of recurring climate cycles in Arabia Terra, which is thought to be some of the oldest terrain on Mars.
This sprawling region has light-colored sedimentary deposits that are stacked like eroded layer cakes, often inside ancient craters.
Many scientists believe that early Mars was much wetter, and changes in the way liquid water flowed on the planet would have created such deposits.
Lewis's team used high-resolution photos taken by the Mars-orbiting HiRISE camera to construct a three-dimensional model of certain parts of Arabia Terra to three-foot (one-meter) resolution.
The 3-D map revealed that some areas had hundreds of layers, and that these layers are often of fairly similar thickness.
In one region, for example, Lewis's team found that the layers averaged 118 feet (36 meters) thick, with most of them between 89 feet (27 meters) and 150 feet (45 meters).
The layers also had sub-layers, each about a tenth as thick as the main layer.
Lewis was unable to talk to National Geographic News about his study because of restrictions imposed by the journal in which he hopes to publish his results.
But according to his presentation abstract, the pattern in which these layers were repeatedly deposited indicates that Mars was probably undergoing regular climate cycles early in its history.
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