Mercury's hidden features raise estimates of how much the planet has shrunk

How much a planet has shrunk may depend on how many traces of that shrinkage survived on its surface. On Mercury, cooling of the interior compresses the outer rocky layers, which deform into long ridges and scarps—steps in the terrain produced by rock displacement. Gaku Nishiyama and colleagues asked whether counting only visible structures leaves out an important part of that history.

The study, published September 10 in Geophysical Research Letters, compared a global map of Mercury's roughness with maps of compressional structures and the contraction inferred from them. Here, roughness means how much terrain varies over short distances, not how a stone feels to the touch. The team used data from NASA's MESSENGER mission to investigate whether more uneven terrain also coincided with fewer recognized signs of shrinkage.

The comparison revealed precisely that pattern: the catalog records fewer compressional structures in the roughest regions. Material thrown out of impact craters can cover ancient scarps or make them harder to identify. Around Rachmaninoff crater, impact deposits coincide with areas where these structures are less common. The finding supports the interpretation that part of the record is hidden; it is not a direct observation of each buried fault.

The researchers used the relationship between roughness and visible structures to correct the global estimate. In the estimate attributing the structures to global cooling, reported by Hokkaido University, radial contraction rises from 8.3 to 11.6 kilometers. Radius is the distance from the center to the surface: in this estimate, the corresponding difference in diameter would be 23.2 kilometers. These are reconstructed values for accumulated geological evolution, not a change measured between two recent photographs or a prediction of how much the planet will lose in the coming years.

The amount of contraction helps test models of Mercury's interior. Different core compositions and heat-loss histories can produce different reductions in size. By recovering a potentially missing portion of the surface record, the study changes the constraint these models must satisfy; it does not uniquely determine the planet's composition or initial temperature. Becky Ferreira's independent analysis, published in 404 Media on September 12, likewise places the result in the context of revising the geological past.

The next test is to look for smaller structures and better distinguish the effects of impacts. The BELA laser altimeter on the European-Japanese BepiColombo mission is expected to measure terrain at finer scales. Those data could show whether the current correction adequately recovers what remained hidden. For now, the contribution is to make an observational bias explicit: terrain with few recognized features does not necessarily record little contraction.