Study suggests Mercury's contraction may be 30% greater than previously thought

Scientists at the German Aerospace Center’s Institute of Space Research reported Thursday that Mercury’s reduction in size may be about 30% larger than earlier calculations, amounting to a loss of roughly 14 miles (23 kilometers) in diameter. The planet, which measures about 3,000 miles (4,900 kilometers) across, has a heavily cratered surface that can conceal evidence of contraction, the researchers noted in a paper published in Geophysical Research Letters.

The findings arrive as the European-Japanese BepiColombo spacecraft approaches Mercury, with plans to enter orbit in November and deploy a laser instrument to measure the planet’s shrinkage caused by internal cooling. Lead author Gaku Nishiyama, who is also part of the BepiColombo mission, said the instrument should verify the extent of the contraction.

Mercury has been gradually contracting since its formation 4.5 billion years ago as its oversized iron core cools and the mantle and crust contract. By comparing fault maps and newer surface-roughness maps, the team found that the roughest regions show fewer visible contraction wrinkles, suggesting that impact debris may have buried some of the deformation.

Nishiyama said the revised estimate could improve understanding of other planets that experience similar cooling-induced shrinkage.