Mercury Is Shrinking — the Whole Planet Is Getting Smaller

Planets are supposed to be finished. Baked, cooled, done — geology happens on them, not to them.

Mercury didn’t get the memo. The closest planet to the Sun is actively shrinking.

The evidence is written across its face. Mercury’s surface is wrinkled with enormous cliff-like scarps — some over 3 kilometers tall and hundreds of kilometers long — that cut across craters like stretch marks in reverse. They’re compression features: the kind of wrinkles you get when a sphere’s skin becomes too big for its insides.

Here’s the mechanism. Mercury is a planetary oddity — it’s basically a cannonball with a thin rocky coat. Its iron core makes up about 85% of the planet’s radius (Earth’s is closer to 55%). For 4.5 billion years, that huge core has been slowly cooling, and as metal cools, it contracts. The rigid crust riding on top has no choice but to buckle, crack, and fold to fit the shrinking planet inside it.

The measurements from NASA’s MESSENGER spacecraft, which orbited Mercury from 2011 to 2015, put numbers on it:

• Mercury’s diameter has contracted by as much as 14 kilometers since its surface formed
• Some scarps appear geologically young — cutting through recent features — meaning the shrinking likely continues today
• The planet may even still experience “Mercury-quakes” as the crust snaps under the strain

It’s a strangely mortal image: the innermost planet slowly deflating like a cooling baked apple, its skin wrinkling with age.

ESA and JAXA’s BepiColombo mission is now studying Mercury. One of its jobs: figuring out just how alive — or how far into its long, slow collapse — the little planet really is.