Last year, NASA’s MESSENGER spacecraft created one of Mercury’s newest craters when it slammed into the planet’s surface at a blistering 8,750 miles per hour. But the observations made during its mission around the closest planet to the sun continues to lead to new discoveries.

Today, new NASA-funded research suggests Mercury is joining an exclusive club in our solar system. The planet is tectonically active. Mercury joins Earth as the only currently active planets in the solar system.

Small scarps are key

Scarps are cliff-like features that look sort of like stair steps. They are formed as Mercury’s interior cooled causing the planet to contract. As the planet shrinks, its crust breaks and pushes upward along faults creating cliffs (scarps). The largest ones can stretch hundreds of miles long and reach one mile high.

We first discovered these cliff-like features when the Mariner 10 flew by Mercury in the 1970s. But Mariner 10 only saw large scarps, which point to the planet being active – but at some point in the distant past.

MESSENGER’s final 18 months saw the spacecraft swing closer and closer to the planet’s surface. High-res, low-altitude images revealed new sets of scarps that are much smaller than the larger scarps. And because these scarps are still around after the steady pounding of asteroids and comets, scientists believe they must be young features.

“The discovery of the small, very young fault scarps on Mercury is like finding a sapling of a tree thought to be long extinct,” said lead author Tom Watters, Smithsonian senior scientist. “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.”

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Let’s take a look at what MESSENGER found.

Mercury scarps

The top image shows Enterprise Rupes (white arrows), one of the largest scarps on Mercury. It’s about 600 miles long and rises more than a mile above the surface. The bottom image shows one of the smaller scarps MESSENGER spotted as its mission came to an end. It’s barely six miles long and rises less than 100 feet. Scientists compare them to small fault scarps seen on the Moon.

Current tectonic activity on Mercury means there would be Mercury-quakes happening right now. Too bad we can’t see evidence of that. We need future missions to place a few seismometers on the surface.

The next mission to Mercury is a joint mission by the European Space Agency and the Japan Aerospace eXploration Agency called BepiColombo. The mission was going to incorporate a lander armed with a seismometer, but it was cut due to budgetary constraints. Instead, an orbiter will continue to study Mercury from above.

BepiColombo is slated to launch in April 2018 and will meet up with Mercury in December 2024.

Image credits: NASA

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