Smallest Planet Continues to Shrink

A new study suggests that little, wrinkly Mercury is shrinking faster than expected, the cosmos' version of a grape shriveling into a raisin under the sun, per the AP. Scientists from the German Aerospace Center's Institute of Space Research reported this week that Mercury's shrinkage may be 30% greater than previously believed—a loss in diameter of as much as 14 miles. That's significant for a planet barely 3,000 miles across. Mercury's rough surface, continually reshaped by debris hurled from impact craters, may have previously hidden the true extent of the loss, according to the researchers whose findings appear in the journal Geophysical Research Letters.

The news comes one week after a pair of European and Japanese spacecraft shed its cruising platform and advanced toward Mercury. Known as BepiColombo, the linked craft are expected to enter orbit around Mercury in November before splitting up for a fuller survey. BepiColombo's laser instrument should confirm how much Mercury is withering as a result of internal cooling, says Gaku Nishiyama, the study's lead author, who is taking part in the space mission. Our solar system's smallest and innermost planet has been shrinking ever since its formation 4.5 billion years ago. Its hot iron core—oversized for such a small world—cools and contracts along with the mantle and crust, tightening the surface like a girdle to fit its new slimmer self.