Mercury is known for being the smallest planet in the Solar System, but, according to NASA, it's also the only one besides Earth that's tectonically active. Unlike Venus and Mars, which are basically cold, dead rocks, images from the unmanned MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft show that Mercury has small, cliff-like formations that suggest that the planet is contracting, which means that it has a hot, cooling interior.

The formations were first seen by the Mariner 10 probe during its flybys in 1974 and '75. They were seen at much closer range and in much higher resolution by the MESSENGER orbiter between 2011 and 2015 and were confirmed to be step-like cliffs hundreds of miles long and over a mile (1.6 km) high. Called thrust fault scarps, they are caused by the planet shrinking, which causes the crust to buckle and crack and thrust the cliffs upward.

But the news isn't that Mercury has scarps. Other planets have those. And the largest and most visible of these on Mercury formed billions of years ago. What is interesting is that MESSENGER found much smaller scarps that are about the size of those found on the Moon. Since the scarps are so small, the constant bombardment of meteors that rain down on Mercury should have obliterated them if they were as old as the larger scarps, so they must be much younger. This means that the Mercurian scarps, like those of the Moon, were formed more recently because the planet is shrinking and is, therefore, tectonically active.

Small graben, or narrow linear troughs, have been found associated with small fault scarps (lower white arrows) on Mercury, and on Earth’s moon(Credit: NASA/JHUAPL/Carnegie Institution of Washington/Smithsonian Institution)

According to NASA, this finding is consistent with the discovery that Mercury has a magnetic field, which is also a property of planets with a still-hot molten core. The space agency says that the planet could suffer from "mercury-quakes," which could be detected by future missions equipped with seismometers.

"This is why we explore," says NASA Planetary Science Director Jim Green. "For years, scientists believed that Mercury's tectonic activity was in the distant past. It's exciting to consider that this small planet – not much larger than Earth's moon – is active even today."

Launched on August 8, 2004, Messenger orbited Mercury until April 30, 2015, when the unmanned probe was destroyed in a controlled impact on the planet's surface.

The research was published in Nature Geoscience.

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