Mercury, on the other hand, has only one solid shell for a crust, rather than Earth's many plates that shift about. As Mercury's molten iron core has cooled over the billions of years since the planet formed, it has contracted and the shell of rock surrounding it has cracked and shifted to accommodate the smaller size. Today, the signs of those changes are written all over the face of Mercury.
NEWS: Spacecraft Raises Mercury Mysteries
"Some of these things are really, really big," said Paul Byrne of the Carnegie Institution of Washington and the Lunar and Planetary Institute. "There are some truly gargantuan cliffs on Mercury." Bryne is the lead author of a paper presenting the results in the March 16 issue of the journal Nature Geoscience.
The greater shrinkage corresponds to a model of the planet with a much larger iron core, said McKinnon.
"It's really a giant iron planet" with a thin rock crust, McKinnon said.
As for when all this shrinking happened, that's harder to figure out, said Byrne. Craters now seen on Mercury correspond to what scientists call the Late Heavy Bombardment -- a violent time in the history of the solar system that ended about 3.8 billion years ago. Any shrinkage features that happened before that were likely wiped out by asteroid impacts during that time.