Study Finds Mercury Shrank 30% More Than Thought

Mercury has been shrinking ever since it formed around 4.5 billion years ago. Its hot iron core, unusually large for such a small planet, is slowly cooling, and as it contracts, the mantle and crust tighten around it, pushing up ridges on the surface that resemble wrinkles on a drying grape. Scientists measure the shrinkage by counting these features, but many appear to be hidden. The researchers compared maps of the wrinkles with maps of surface roughness and found that the roughest areas showed fewer of them, suggesting that debris thrown out by asteroid impacts had buried the evidence. Using paired images from MESSENGER, a spacecraft from NASA (the National Aeronautics and Space Administration) that orbited Mercury for more than four years in the 2010s, Nishiyama also built a highly detailed map of the terrain, which, according to the New York Post, revealed far more shrinkage marks than previously believed.

Studying Mercury remains a challenge: it is the closest planet to the Sun, often lost in its glare, and daytime temperatures reach about 800 degrees Fahrenheit (427 degrees Celsius). Before now, only two spacecraft had visited it: NASA's Mariner 10 in the 1970s and MESSENGER. A third, BepiColombo, a joint European and Japanese mission, is expected to enter orbit around the planet in November. Nishiyama, who is taking part in the mission, said its laser instrument should confirm how much Mercury has shrunk. The true figure could be even larger than the team's estimate. The findings may also help scientists understand other worlds that contract as they cool. "We are quite excited," Nishiyama said, adding that the team feels it is "approaching the reality of Mercury's evolution."