A new global reading of Mercury's surface shows that impact debris concealed signs of contraction and left earlier counts too low. The correction puts the planet's diameter loss since formation at about 19 kilometres, 10% to 30% above previous estimates. The result narrows models of its cooling interior and gives the approaching BepiColombo observations a revised benchmark for more detailed surface measurements.
As Mercury's interior cools, its crust is squeezed, leaving ridges and cliff-like faults called lobate scarps as surface traces of shrinkage. Researchers found that rough ejecta scattered by large impacts covered some of these structures. Earlier global counts, built from features that remained visible, therefore underestimated the total shortening the planet has undergone.[1], [2]
The team linked contractional structures visible in less disrupted terrain to surface roughness, then extended the estimate across obscured regions. The correction points to roughly 9.5 kilometres of radial contraction, or about 19 kilometres of diameter loss. That raises total shrinkage 10% to 30% above previous estimates, and the researchers say the true loss may be slightly greater.[2], [1]
The amount of contraction provides an observable measure of how much Mercury has cooled. A larger value changes the room available for explanations such as a larger metallic core, fewer light elements such as silicon in that core, or a hotter starting state. BepiColombo's finer surface scans may sharpen the global estimate by distinguishing younger and smaller structures that remain hidden in current maps.[1], [2]
