Mercury Is Shrinking

The planet Mercury is shrinking, the LA Times reports. Data from NASA’s Mercury Messenger spacecraft reveal that the planet’s diameter has shrunk by a mile over its history, probably because its core is cooling. Messenger flew in for a close-up in January, and scientists are now piecing together the new information and re-evaluating some long-held beliefs about the planet.

Mercury is the only planet in the solar system besides Earth that has a global magnetic field. On Earth, this is attributable to a flow of liquid iron in the core. Scientists had long believed that Mercury’s core had solidified, but the new data suggest it is still flowing. “We are all surprised by how active that planet is," said a Messenger project scientist. “Its not just a ball of rock and metal.” (More Mercury stories.)