Mercury has shrunk 10 to 30 percent more than earlier maps allowed

Mercury's radius has fallen by more than earlier catalogues of surface wrinkles implied, according to a study published Thursday in Geophysical Research Letters. Gaku Nishiyama of the German Aerospace Center's Institute of Space Research and colleagues used roughness maps to estimate how many lobate scarps lie buried under ejecta from later impacts.

Previous work put radial contraction in a band from about one kilometre to seven kilometres. The new correction adds 10 to 30 percent. Nishiyama's range for total radial contraction is 6.9 to 11.6 kilometres. Diameter loss since formation then sits near 14 to 23 kilometres depending on the end of the range. Mercury's present diameter is about 4,880 kilometres.

The average rate implied by the lower bound is more than 2.7 kilometres per billion years. Nishiyama called that a floor, not a central estimate.

Why crater debris hid the wrinkles

As a rocky planet cools, its interior loses volume and the crust wrinkles. On Mercury those wrinkles appear as lobate scarps, low cliffs that mark thrust faults. Catalogues built from Mariner 10 and MESSENGER images counted the scarps that were still visible. They undercounted scarps in rough terrain, where ejecta from younger craters covers the older faults.

Nishiyama's group measured roughness and asked how many scarps should be present in the rough patches if those patches behaved like the smoother ones. The missing population is the 10 to 30 percent correction. Paul Byrne of Washington University in St. Louis, who has published on Mercury's contraction and was not on this paper, said it is plausible that earlier maps, including his own, missed faults in rugged crust.

What a larger contraction does to the core

Mercury already has a metal core that takes up a larger share of the planet than Earth's core does. More contraction means more cooling. Nishiyama listed the interior consequences: fewer light elements such as silicon in the metal core, a larger inner core, and a higher starting temperature. Those knobs are how modelers reconstruct the first billion years of a small planet sitting next to the Sun.

BepiColombo, the joint European-Japanese orbiter now working through its Mercury campaign, will test some of these numbers with gravity and magnetic data. The scarp correction is a surface argument. The core argument will be settled by how the planet's field and mass are distributed.

For a reader who has seen Mercury described as a shrinking world for twenty years, the new paper is a bookkeeping fix. The scars were always there. Debris from later impacts made them hard to count. Once they are counted, the planet is a colder, more contracted object than the textbooks printed after MESSENGER, and the models of its core have to move with it.