Mercury's wrinkled surface has been hiding its own history. A new study finds the solar system's smallest planet may have contracted 10% to 30% more since it formed than previously estimated — a total loss of up to 14.5 miles (23 km) of diameter, rather than the 2.5 to 10 miles (4 to 16 km) in current estimates.
The research, led by Gaku Nishiyama of the German Aerospace Center's (DLR) Institute of Space Research and published September 10 in Geophysical Research Letters, attributes the underestimate to debris. Mercury cooled and shrank over 4.5 billion years, and its rocky crust crumpled in response, forming "wrinkle" ridges and scarps. But billions of years of impacts have roughened the surface and buried those features — much as fresh gravel hides the ruts in a road.
Nishiyama's team combined existing maps of contraction features with new maps of surface roughness across the entire planet, finding for the first time that the roughest terrain holds the fewest visible wrinkles. Using the contraction implied by scarps and ridges in smoother regions, they extrapolated how much shrinking is likely hidden under the rough patches.
The revision matters because contraction constrains the planet's interior. "More shrinking means Mercury could have a larger metal core, less light elements like silicon mixed into the metal core, or a higher starting temperature," Nishiyama said. The team notes the corrected figures bring Mercury's observed shrinkage into better agreement with physical models of how much the planet should have cooled.
The new number may still be conservative. The data comes from NASA's MESSENGER mission, which ended in 2015 and can only reliably resolve features wider than about 3 miles (5 km). BepiColombo — only the third mission ever sent to Mercury — begins collecting higher-resolution scans in November 2026, with Nishiyama on the science team.
