Study Finds Mercury Shrank 30% More Than Expected

Mercury has been shrinking for about 4.5 billion years. Its hot iron core is very large for such a small planet, and as it slowly cools, the whole planet gets smaller. This pushes up ridges on the surface that look like wrinkles on a drying grape. Scientists measure the shrinkage by counting these wrinkles, but many seem hidden. The researchers found that the roughest areas had fewer wrinkles, probably because debris from asteroid impacts had covered them. Using images from MESSENGER, a spacecraft from NASA (the National Aeronautics and Space Administration) that orbited Mercury for more than four years, the team made a detailed map of the surface. According to the New York Post, it showed far more shrinkage marks than expected.

Mercury is hard to study because it is so close to the Sun, and daytime temperatures reach about 800 degrees Fahrenheit (427 degrees Celsius). So far, only two spacecraft have visited it: NASA's Mariner 10 in the 1970s and MESSENGER. A third, BepiColombo, a joint European and Japanese mission, should enter orbit around the planet in November. Its laser instrument is expected to confirm how much Mercury has shrunk, and the real number could be even bigger. The findings may also help scientists understand other worlds that shrink as they cool. "We are quite excited," Nishiyama said.