Mercury Has Shrunk Up to 23 Kilometers in Diameter Since It Formed, 30% More Than Anyone Counted. Crater Debris Was Hiding the Wrinkles.

Mercury's surface is covered in wrinkles, the crumpled scarps and ridges of a planet whose crust has had to buckle as the interior beneath it cooled and shrank over 4.5 billion years. Planetary scientists have used those wrinkles to estimate how much the planet has contracted, and the accepted figure was a loss of 4 to 16 kilometers of diameter. A new study in Geophysical Research Letters says that number is too low by 10% to 30%, because a large share of the wrinkles have been buried under debris from impact craters and never counted.

"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," said Gaku Nishiyama, a planetary scientist at the German Aerospace Center's Institute of Space Research and the paper's lead author. The corrected estimate raises the total contraction to as much as 23 kilometers of diameter, nearly 12 miles more than the low end of the old range.

The logic rests on a simple expectation. A cooling planet should shrink more or less uniformly, so contraction features ought to be spread evenly across the surface. They are not. Nishiyama's team combined existing maps of shortening structures with new global maps of surface roughness, a measure of how lumpy the terrain is, and showed for the first time that the roughest areas of Mercury have the fewest visible wrinkles. "It made us think that there's a process obscuring shortening structures," Nishiyama said. Fresh impacts dig depressions and blanket the surrounding ground in ejecta, and that debris covers older scarps the way a new layer of gravel hides the ruts in a road.

To estimate what was hidden, the team measured how much contraction was needed to produce the ridges and scarps in the smoother, less disturbed regions, then applied that rate to the whole planet, rough patches included. The missing features under the debris account for the extra 10% to 30%. "30% is a little bit surprising, but the corrected amount of contraction actually makes sense to me," Nishiyama said. With the new figure, the observed shrinkage finally matches what thermal models of Mercury's interior predict, a mismatch that had been nagging the field for years.

Even the new number may be an undercount. The data come from NASA's MESSENGER mission, which orbited Mercury from 2011 until it was deliberately crashed into the surface in 2015, and its images can only reliably resolve features larger than about 5 kilometers across. Anything smaller is invisible to the analysis.

That is about to change. BepiColombo, the joint European-Japanese mission and only the third spacecraft ever sent to Mercury, cut loose its transfer engine this month after eight years and 10 billion kilometers in transit and is falling toward orbit. It begins high-resolution mapping in November. Nishiyama is on the mission's science team, and he expects the new images to reveal scarps, ridges and craters in enough detail to settle how much of the planet has actually disappeared.