Mercury is slowly shrinking and wrinkling its own crust

Mercury, the smallest planet and the closest to the Sun, is quietly getting smaller. Since its formation about 4.5 billion years ago, its large iron-rich interior has been steadily cooling, and as the inside contracts the outer crust has nowhere to go but inward. Unlike Earth, Mercury has just one rigid shell of rock rather than a patchwork of plates, so as it shrinks the whole surface buckles.

The cooling pushes slabs of crust together until they fracture and one side is shoved up over the other along thrust faults, raising long, curving cliffs called lobate scarps that run for hundreds of kilometres and rise more than a kilometre high. NASA’s Mariner 10 first spotted these features, and the MESSENGER mission later mapped them globally, finding the planet had contracted far more than expected.

Some scarps look geologically fresh, evidence the squeezing has not stopped.

That makes Mercury, like Earth, a tectonically active world, complete with the prospect of its own gentle ‘mercuryquakes.’