Like Earth, Mercury is tectonically active. As evidenced by images captured by NASA's Messenger probe, Mercury's surface is host to fault scarps.

The geologic phenomenon, never before seen on Mercury, suggests the planet is still contracting. Prior to the latest discovery, detailed in the journal Nature Geoscience, scientists believed Earth was the only planet in the solar system with active tectonic plates.

Scarps are the sudden shifts in elevation caused by the displacement of land along an active fault line. Because the scarps seen on Mercury are so small, researchers believe they're relatively young.

"The young age of the small scarps means that Mercury joins Earth as a tectonically active planet in our solar system, with new faults likely forming today as Mercury's interior continues to cool and the planet contracts," Tom Watters, a researcher with the Center for Earth and Planetary Studies at the Smithsonian's National Air and Space Museum, said in a news release.

Messenger images have previously confirmed the presence of much larger, older scarps, first identified during Mariner mission flybys in the 1970s. Those scarps stretched a few hundred miles in length and were several thousand feet tall.

The newly discovered scarps, imaged during Messenger's closest flyby, are only a few dozen feet tall and a mile or so long.

"The discovery of the small, very young fault scarps on Mercury is like finding a sapling of a tree thought to be long extinct," Watters explained. "The small scarps are the tectonic saplings that with enough contraction can grow in size to become the giant redwoods of Mercury's fault scarps."

The latest revelations fit neatly with astronomers' previous discovery that Mercury's magnetic field is several billion years old and that it's still-hot outer core is still cooling. The two findings suggest the planet also experiences seismic activity.