Mercury May Have Shrunk Far More Than Scientists Realized—and Its Cratered Surface Was Hiding the Evidence

Mercury has been shrinking for billions of years, but scientists may have underestimated how dramatic that transformation has been. A new study suggests that Mercury’s cratered surface has concealed a significant fraction of the faults created as the planet cooled and contracted. Researchers estimate that Mercury may have shrunk by up to 7.2 miles (11.6 kilometers) in radius, which is 30% more than previous estimates. The study, published in *Geophysical Research Letters*, led by Dr. Gaku Nishiyama of the German Aerospace Center (DLR), examined the tectonic record of Mercury and found that the surface roughness may have obscured the true extent of contraction.

The study found that fault scarps and other shortening structures are preferentially identified in smoother terrain, while rougher regions contain substantially fewer detectable structures. This suggests that the tectonic deformation is not uniformly distributed across Mercury. The researchers argue that the discrepancy may be due to the surface being heavily cratered, which can hide the true geological features. They developed a correction using comparatively smooth regions, increasing the estimated shrinkage from 5.2 miles to 7.2 miles. The correction accounts for a potential bias where scientists see more evidence of contraction in areas where the surface is easier to preserve and identify.

The findings could alter models of how Mercury’s interior evolved and raise questions about contraction on other rocky worlds. The study highlights that the surface roughness of Mercury may have hidden significant contractional structures, which could affect how scientists interpret the planet’s geological history. The revised estimate favors models with a thinner regolith and lower sulfur content in Mercury’s core. The research also suggests that similar effects may be at play on other planetary bodies, such as the Moon and Mars, where surface roughness may obscure geological features. The ESA-JAXA BepiColombo mission will provide a more detailed view of Mercury’s topography, potentially revealing geological relationships that were previously undetectable.

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