Mercury may be shrinking at a faster rate than scientists previously understood, according to a new study. Researchers from the German Aerospace Center’s Institute of Space Research report that the planet’s diameter could have decreased by as much as 14 miles (23 kilometers) — a loss approximately 30% greater than earlier estimates.
This significant shrinkage, likened to a grape shriveling into a raisin, is attributed to the planet's internal cooling. Mercury's large iron core contracts as it cools over billions of years, causing the mantle and crust to tighten and the surface to wrinkle. Scientists suggest that the planet's rough, cratered surface may have obscured the full extent of this contraction.
By comparing maps of Mercury's geological features with newer data on its surface roughness, the researchers identified areas where shrinkage wrinkles might be hidden beneath impact debris. These obscured regions could account for the underestimated shrinkage.
The findings, published in the journal Geophysical Research Letters, build upon data gathered by NASA’s Messenger spacecraft in the 2010s, which followed up on observations from the Mariner 10 mission in the 1970s. Only these two spacecraft have ever visited Mercury prior to recent missions.
Future observations from the BepiColombo mission, a joint European and Japanese endeavor, are expected to provide further confirmation. The spacecraft entered orbit around Mercury in November and will conduct a detailed survey. One of BepiColombo's instruments is specifically designed to measure the planet's shrinkage resulting from internal cooling, which could validate the new study's findings.
This ongoing shrinkage is a consequence of Mercury's formation about 4.5 billion years ago. Its disproportionately large, hot iron core has been cooling and contracting ever since, leading to the tightening of the planet's surface.