The poor tiny planet of Mercury is becoming even tinier - and more wrinkled.
Nasa'a Messenger spacecraft has determined it has shrunk by seven kilometres since it was formed four billion years ago - a lot in planetary terms.
The shrinking phenomenon is common to all solid planets as hot inner cores lose heat and contract.
Back in the 1970s, Nasa's Mariner 10 mission mapped 45% of Mercury and noted the "wrinkle ridges" formed as the planet cooled.
Unlike the Earth, Mercury only has a single continental plate which is relatively thin meaning these marks are a good indicator of the effects of cooling.
NASA's Messenger probe (the name is short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) has now managed to map the entire thing.
This new data upgraded astronomer's estimates of shrinkage from 1 - 2 kilometres to seven.
Sean Solomon who worked on the project, said: "The discrepancy between theory and observation, a major puzzle for four decades, has finally been resolved.
"It is wonderfully affirming to see that our theoretical understanding is at last matched by geological evidence."