The hills of Mercury are alive with earthquakes, according to a study published Monday in Nature Geoscience. The diminutive planet joins Earth as the only other known tectonically active planet in the Solar System.
“We have to take another look at how rocky planets are evolving,” said Thomas Watters, Smithsonian planetary scientist and study co-author.
The discovery was made thanks to NASA’s MESSENGER probe, which orbited the closest planet to the sun from 2011 to 2015. MESSENGER spotted fault scarps, which are cliff-like features that indicate a history of contractions in the planet’s crust. These scarps are huge, with the largest up to 1,000 kilometers — 621 miles — long.
The smoking gun came when Messenger spotted the presence of small scarps, measuring less than 10 kilometers (6.2 miles). “Small landforms are not going to survive for terribly long” due to the asteroid bombardment on Mercury, Watters said, meaning the tectonic activity responsible for them must have occurred recently.
Large fault scarps had been discovered previously by Mariner 10, a NASA probe sent to Mercury in the 1970s, but small scarps had never been observed in the past.
Another cool finding: The tiniest planet in our celestial neighborhood is shrinking. Watters and his colleagues found the planet is becoming smaller due to a cooling core.
Early signs suggest Mercury harbors a single tectonic plate across the entire planet, unlike Earth which sports a mosaic of plates. And this uniform shell appears to be contracting.
“As the interior of the planet cools, it contracts and shrinks,” said Watters. But even though the core is cooling, he added, Mercury’s core remains warm enough to drive movement of these plates. This feature contrasts with Mars, which is tectonically dead.
Mercury’s cooling core is likely accompanied by seismic activity. After detecting evidence of recent slip events on the rocky planet’s large faults, Watters believes Mercury might boast stronger earthquakes than the magnitude-5 events found on our moon.
To confirm “Mercuryquakes,” scientists will need readings from seismic stations on the surface, Watters said.
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