Mars may be experiencing plate tectonics - a phenomenon previously thought to be confined to Earth.
UCLA scientist An Yin says he's found evidence of the movement of huge crustal plates beneath the planet's surface.
"Mars is at a primitive stage of plate tectonics," he says. "It gives us a glimpse of how the early Earth may have looked and may help us understand how plate tectonics began on Earth."
Yin made the discovery through an analysis of satellite images from NASA's THEMIS spacecraft and from the HIRISE camera on the Mars Reconnaissance Orbiter. He says about a dozen of the 100-odd images show characteristics of plate tectonics.
"When I studied the satellite images from Mars, many of the features looked very much like fault systems I have seen in the Himalayas and Tibet, and in California as well, including the geomorphology," he says.
For example, the images show a very smooth, flat side of a canyon wall - which can be generated only by a fault, he says. Another shows a steep cliff, comparable to those in California's Death Valley.
"You don't see these features anywhere else on other planets in our solar system, other than Earth and Mars," says Yin.
The surface of Mars contains the longest and deepest system of canyons in our solar system, Valles Marineris, which at nearly 2,500 miles long is about nine times longer than Earth's Grand Canyon.
One theory is that it's a big crack in Mars' shell that simply opened up.
"In the beginning, I did not expect plate tectonics, but the more I studied it, the more I realized Mars is so different from what other scientists anticipated. I saw that the idea that it is just a big crack that opened up is incorrect," he says.
"It is really a plate boundary, with horizontal motion. That is kind of shocking, but the evidence is quite clear. The shell is broken and is moving horizontally over a long distance. It is very similar to the Earth's Dead Sea fault system, which has also opened up and is moving horizontally."
What's still unclear, says Yin, is how far beneath the surface the plates are located.
"I don't quite understand why the plates are moving with such a large magnitude or what the rate of movement is; maybe Mars has a different form of plate tectonics," he says. "The rate is much slower than on Earth."