Something strange is happening to the smallest planet in our solar system. Mercury is shrinking — slowly, quietly, and by an astonishing amount.
Image: Mercury in enhanced color — NASA/MESSENGER (public domain; color enhanced for contrast)
The wrinkles of a cooling world. When NASA's MESSENGER spacecraft orbited Mercury from 2011 to 2015, its cameras found the surface crisscrossed by giant cliffs called lobate scarps — some stretching hundreds of kilometres, rising up to three kilometres high. These aren't mountains being pushed up. They're the surface crumpling inward as the planet underneath contracts, like an apple skin wrinkling as the apple dries out.
The numbers are bigger than anyone expected. In the 1970s, data from the Mariner 10 flyby suggested Mercury's radius had shrunk by maybe 1–2 km. MESSENGER blew that away: a 2014 study in Nature Geoscience estimated the planet has contracted by as much as 14 kilometres — nearly five times the old figure, and far more than Mercury's small size should have allowed under previous models.
Why it's happening: Mercury is basically a giant iron heart with a thin rock skin. Its metallic core makes up roughly 85 percent of the planet's radius. As that core slowly cools and contracts, the whole planet settles inward — and the brittle crust has no choice but to fold and fracture.
Earth's moon does the same thing, though on a much smaller scale: images from NASA's Lunar Reconnaissance Orbiter showed the Moon has shrunk by roughly 50 metres over the last few hundred million years, producing its own miniature scarps.
Mercury is only about 4,879 km across, yet it's still squeezing. The lesson: planets aren't finished products. Even after billions of years, they're still cooling, contracting, and rewriting their own surfaces.