Mercury is shrinking, NASA says

Mercury is not just the solar system’s shrimpy kid brother. It’s shrinking.

New measurements taken by NASA’s Messenger spacecraft earlier this year show that the innermost planet has shrunk by more than a mile in diameter over its history. Scientists attribute that to the gradual cooling of the planet’s core.

Messenger is the first spacecraft to study Mercury up close since Mariner 10 in 1975. It made its first close flyby in January, whisking to within 125 miles of the surface. It will swing back for a second encounter in October before settling into a final close orbit in 2011.

The first comprehensive results from the January flyby are being published in today’s issue of the journal Science.

Mercury has long been considered little more than a hot rock, with daytime surface temperatures ranging up to 1,100 degrees Fahrenheit. But Messenger has uncovered a more surprising place, with peaks reaching up to 15,000 feet and vast basins stretching hundreds of miles across the planet’s surface.