A new study shows that small, wrinkled Mercury may be shrinking faster than expected, the cosmos version of a grape shriveling into a raisin in the sun. Scientists at the Space Research Institute of the German Aerospace Center reported Thursday that Mercury’s shrinkage may be 30% larger than previously believed: a diameter loss of up to 14 miles. This is significant for a planet barely 3,000 miles in diameter. Mercury’s rough surface, continually reshaped by debris flung from impact craters, may have obscured the true extent of the loss, according to researchers whose findings appear in the journal Geophysical Research Letters. The news comes a week after a pair of European and Japanese spacecraft left their cruise platform and headed toward Mercury. Known as BepiColombo, the connected spacecraft are expected to enter orbit around Mercury in November before splitting up for more complete study. BepiColombo’s laser instrument should confirm how much Mercury is wilting as a result of internal cooling, said Gaku Nishiyama, the study’s lead author who is involved in the space mission. The shrinkage could be even greater than his team estimates based on measurements from NASA’s Messenger spacecraft in the 2010s. Only one other spacecraft has visited Mercury, NASA’s Mariner 10 in the 1970s. The smallest and innermost planet in our solar system has been shrinking since its formation 4.5 billion years ago. Its red-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 accommodate its new, thinner self. Nishiyama and his team compared maps showing faulting and other geological signs of contraction on Mercury with newer maps that highlight surface roughness. They found that rougher areas had fewer shrinkage wrinkles. The missing wrinkles could be buried under debris from the impact, they said. After taking these darkened regions into account, the researchers provided new insight into Mercury’s contraction that could shed light on other contracting worlds, said Nishiyama, affiliated with Hokkaido University in Japan. “We are very excited,” Nishiyama said in an email, adding that it appears “we are getting closer to the reality of Mercury’s evolution.”
