Mercury Has Been Shrinking

The solar system's smallest planet is getting even smaller, but there's not much danger of Mercury disappearing completely: The planet has shrunk as many as 8.6 miles in diameter over the last 4 billion years, according to new research, making its current diameter roughly 3,032 miles. The planet diminished as it cooled over the years, a realization scientists made in the mid-1970s based on photographs taken by the Mariner 10 probe. But recent images from NASA's Messenger satellite have allowed scientists to more precisely map how much the planet's surface has changed as the single rocky plate in its surface contracts, National Geographic finds. "We have the big picture now," says the study's lead author, Paul Byrne.

"We see the landscape literally crumpling up," one scientist says. "Massive slabs of rock are sliding over one another." The most breathtaking effect of the shrinkage has been the creation of huge features called scarps created as rock is thrust upward. "There's a structure called Enterprise Rubes in the southern hemisphere that is a single scarp system" more than 600 miles long and nearly 2 miles high in places, Byrne tells the BBC. "Imagine standing in front of it. It's Mercury's version of a mountain belt. It would be a very dramatic landscape." Byrne says the new finding will allow scientists to investigate related questions, like when the shrinking began, and why so much of it has occurred. (Last year scientists spotted a planet smaller than Mercury.)