Mercury is slowly shrinking, and it's giving the planet wrinkles

• A new study suggests the smallest planet in our solar system is getting even smaller.
• The moon-sized planet is shrinking as its insides cool down.

Mercury, the smallest planet in the solar system, is getting even — and getting more wrinkles as it does, a new study suggests.

Scientists have known that Mercury is considerably smaller than it once was — it has gotten about 14 kilometers (8.6 miles) slimmer as the planet's insides grew cooler.

But it wasn't clear whether the planet was still actively shrinking. A new study has shown that it probably is.

Mercury is wrinkling like an old apple

Like any other planet, Mercury is losing heat. As its internal rock and molten metal have gotten cooler, its insides have shrunk.

Because of this, wrinkles have appeared on the planet's surface — formations known as "scarps," David Rothery, professor of planetary geosciences at UK's Open University and an author on the study, said in a post in The Conversation.

"This is like the wrinkles that form on an apple as it ages," Rothery said.

Scientists have used these scarps to prove the planet has shrunk. The problem is most of these are ancient — about 3 billion years old, Rothery told Insider on Tuesday.

To show that these scarps are still moving today, Rothery and his graduate student Ben Man turned their attention to another geological structure, called grabens.

The advantage of these grabens, which appear as the scarps move about, is that they are quite slight, only about 0.6 miles long and under 300 feet deep. They don't last as long — any older than 300 million years would probably no longer be visible.

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

The study, published in Nature Geosciences on Monday, identified 48 definite and 244 likely grabens on pictures snapped by NASA's MESSENGER probe in 2015.

"That's why we say clearly there's been some movement in about the past three hundred million years, and it's probably going on today still," he said.

Mercury is likely constantly shaking with quakes

The study also suggests Mercury is constantly shaking with quakes, Rothery said.

"Because Mercury is shrinking, you're causing bits of the outer rigid part to be thrust over itself. So that is an earthquake — or a Mercury quake," he said.

These quakes are likely quite big, he said.

"The displacement we see on these fronts is a couple of kilometres. So that's hundreds of really, really big earthquakes, and likely millions of smaller ones.".

"We expect that it's going on all the time, but we've got no way of measuring them until we get some seismometers onto the planet," he said.

The next step for Mercury, Rothery said, will be the arrival of a space probe called BepiColumbo. Launched in 2018, BepiColumbo carries two satellites: one from the European Space Agency and one from the Japan Aerospace Exploration Agency, that will be scrutinising the planet's surface from different angles.

The probe took a picture of Mercury while flying by the planet in 2021, but it is due to return to take higher-resolution pictures in late 2025, early 2026.

The hope is that these pictures could provide even more information about Mercury's geology, for instance by revealing boulder tracks in the soil.

"If you see boulders bouncing down the front of a scarps or the boulder tracks in front of these scarps, you might think well, maybe that's the Mercury quake that's dislodged this boulder," he said.