Scientists have long suspected that Mercury is actively shrinking over billions of years — but new research suggests that that process may still be going on, leaving cracked-up wrinkles behind.

In an essay for The Conversation, planetary scientist David Rothery, a professor at the UK's Open University and co-author of a new paper published in the journal Nature Geoscience, explained that the wrinkles on Mercury's crust may be the result of continued planetary cooling and contracting.

Since the 1970s, scientists have had telescopic evidence that Mercury's crust had shrunk due to internal cooling and created wrinkly ridges known scientifically as "scarps." As the planet compressed an estimated total of about 8.8 miles in diameter over the last 3.8 billion years, the wrinkles formed, as Rothery described, like those that appear on an old apple.

To see if Mercury might still be getting smaller, Rothery and Open University PhD student Ben Man turned their attention to another geographical feature of the closest planet to the Sun: "grabens," taken from the German word for "ditch," which are like small cuts on the backs of the ridge-like scarps that line the planet's surface.

Grabens, which are generally just over half a mile long and less than 300 feet deep as compared to the up to 1.25 mile-high wrinkly ridges on which they sit, occur as the scarps stretch due to "Mercury quakes,"  and are akin to the cracks that might appear if one tries to bend a piece of toast, as Rothery aptly illustrated.

These comparably small cracks, which the researchers posited would have smoothed over if they were as old as the scalps themselves, led Man and Rothery to believe that Mercury is still shrinking.

"Because there are such small structures," Rothery told Insider, "they wouldn't survive for very long."

The British scientist is, of course, speaking in terms of a planetary scale — the majority of grabens are estimated to be less than roughly 300 million years old, a fraction of the three billion years or so since Mercury's wrinkles were formed.

Between 2011 and 2015, NASA's MESSENGER probe sent back photos of hundreds of possible grabens and 48 scarps that definitely had grabens after it orbited Mercury. Since that probe, scientists have begun wondering whether Mercury might still be shrinking, and as NASA noted in a 2017 post, some grabens might be as "young" as 50 million years old.

Scientists are hoping to learn more about the phenomenon thanks to the European and Japanese BepiColombo probe mission, which was launched in 2018. The spacecraft has flown by the planet twice and should arrive back at Mercury in 2026, at which point, it'll be able to take higher-resolution photos than MESSENGER to verify if the 244 "probably" grabens are what they seemed to be.

That could give scientists a much more detailed look at the phenomenon and perhaps conclude once and for all if the smallest planet in the solar system is still seismically active — and therefore continuing to shrivel up like an old apple.

More on planetary goings-on: James Webb Spots Numerous Pairs of Planets, Hanging Out With No Stars

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