Our Earth was generally believed to be the only planet that exhibited both tectonic and seismic activity.  But a new analysis of photographs of Mercury shows its surface is active.

NASA's robotic space probe named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) snapped several high definition photos near the planet's north pole in the final months of its mission.  These revealed that "the planet's past 4 billion years of tectonic history have been dominated by contraction expressed by lobate fault scarps that are hundreds of kilometers long" - in other words, long fault lines that are comparable to the San Andreas fault in California. 

"They're huge," said Thomas Watters of the Smithsonian Institute, who led the new study.  "So we know Mercury had contracted, but we didn't know if that contraction had happened millions or billions of years ago, because large features with kilometers of relief will not disappear," he added.

Among the ways that scientists can tell that some of the faults are new is because they cut across older impact craters.  Mercury's surface is under constant bombardment from meteors.  That fact that any of the faults were visible showed they were new because they hadn't been erase by impacts yet. 

These are formed by the cooling of Mercury's core, something a lay person might not have guessed given the tiny planet's proximity to the Sun.  Scientists assumed that Mercury's core cooled and solidified billions of years ago.  But recently, MESSENGER's magnetometer produced evidence of ongoing magnetic field activity.