This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Mercury's surface is riddled with wrinkles, evidence of a once-larger planet that has shrunk over time. A new study finds that Mercury may have contracted 10% to 30% more than previously thought—a loss of nearly 12 miles (19 kilometers) of total diameter since the planet formed—due to debris from impact craters obscuring signs of the planet's shrinking.
The study will appear in Geophysical Research Letters on Sept. 10, 2026. Like the other rocky planets in our solar system, Mercury formed through a flurry of violent collisions between rocks and asteroids orbiting the sun around 4.5 billion years ago. The energy from these impacts generated huge amounts of heat, which Mercury has been losing ever since.
Like a balloon left out in the cold, Mercury's interior shrank as it cooled, and the outermost rocky layers compensated for the planet's changing size by crumpling and cracking to form tectonic features such as scarps and ridges. The degree of shrinking is important for helping researchers infer key features of Mercury's interior and evolution. "More shrinking means Mercury could have a larger metal core, fewer light elements like silicon mixed into the metal core, or a higher starting temperature," explained Gaku Nishiyama, a planetary scientist at the German Aerospace Center (DLR) Institute of Space Research and lead author on the study.
Cooling should shrink a planet roughly uniformly, making "wrinkle" features, known as shortening structures, similarly common everywhere. But the formation of new impact craters creates depressions and covers the landscape in debris, making the surface rougher and rendering signs of shrinkage harder to spot among billions of years of geologic features. Nishiyama worked with his team to combine previous maps of geologic evidence for contraction with new maps of surface roughness—a measure of how lumpy a landscape is—for the entire planet's surface.
For the first time, they showed that the roughest areas on Mercury have fewer visible wrinkles. "It made us think that there's a process obscuring shortening structures," said Nishiyama. He thinks impact debris in rough areas could be covering up the shrinkage wrinkles, like freshly added gravel hiding the ruts in a road.
Nishiyama and his team used the contraction required to form shrinkage ridges and scarps in less disrupted areas to estimate how much contraction likely occurred planetwide, including under rough patches. They found that missing features in rough areas could amount to 10% to 30% more shrinkage over Mercury's lifetime—a total change of up to 14.5 miles (23 kilometers) in the planet's diameter rather than the currently estimated 2.5 to 10 miles (4 to 16 kilometers). "30% is a little bit surprising, but the corrected amount of contraction actually makes sense to me," explained Nishiyama.
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