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Wee Mercury is getting even smaller

The solar system's smallest planet is still tectonically active.

NASA/JHUAPL/Carnegie Institution of Washington/USGS/Arizona State University

Europa may be getting all the attention today, but Mercury has some pretty exciting news too: According to data collected by NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) probe, the wee planet is still tectonically active, just like Earth, contradicting the previous belief that the planet has been inactive for a very long time.

The data was collected during the final 18 months of MESSENGER's mission, before it plummeted to the surface of Mercury on April 30, 2015. During this period, the spacecraft was able to orbit the planet at a lower altitude, getting much more detailed high-resolution photos than it had previously obtained.

It was in these photos that researchers found for the first time a number of small scarps.

These are a type of geological formation that look like steps, a sort of "wrinkle" in the planet's surface that is caused as it shrinks. Larger scarps had been found on Mercury as early as the 1970s, created as the planet's interior cooled, but this activity had been thought long over.

The size of the newly discovered scarps indicates that they're a lot younger -- which, in turn, means that Mercury is still shrinking to this day. This makes it the only planet in the solar system alongside the Earth that is still tectonically active, according to NASA. (Venus may be active, but it's very hard to tell, since we can't get near the surface easily, and it's probably very different from Earth-like tectonics. Europa and Titan may also have tectonic activity, but they are moons, not planets.)

"The young age of the small scarps means that Mercury joins Earth as a tectonically active planet, with new faults likely forming today as Mercury's interior continues to cool and the planet contracts," said lead author Tom Watters, Smithsonian senior scientist at the National Air and Space Museum in Washington, in a statement.

This is consistent with Mercury's active, internally generated magnetic field. These occur only in planets with molten inner or outer cores. If Mercury's core is still molten and therefore cooling, it would correlate with both the scarps and the magnetic field.

"This is why we explore," said 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."

But don't worry -- there's a limit to how far Mercury can shrink, and it still meets the International Astronomical Union's criteria of planethood, unlike poor Pluto.