Mercury shrinking up to 30% faster than thought, German-led study finds

A German Aerospace Center-led team reports that Mercury has contracted up to 30% more than previously believed, losing as much as 23 km of diameter since it formed 4.5 billion years ago. Impact debris had hidden the planet's wrinkles. The finding recasts our understanding of Mercury's core and comes weeks before Europe's BepiColombo mission arrives.

Bottom line — Mercury may have lost 23 km of diameter, forcing a rethink of its core composition ahead of BepiColombo's November arrival.

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• Researchers compared prior geological maps with new surface-roughness maps and found the roughest terrain had the fewest shrinkage wrinkles, suggesting debris was hiding them, per the study in Geophysical Research Letters.
• Lead author Gaku Nishiyama says more shrinkage implies either a larger metal core, fewer light elements like silicon mixed into it, or a higher starting temperature.
• Previous estimates based on NASA's MESSENGER data put the contraction at 4–16 km; the new analysis suggests up to 23 km, and the authors say even that could be an underestimate.
• The BepiColombo mission, a joint European-Japanese project, is expected to enter Mercury orbit in November and has a laser altimeter that can resolve surface changes as small as 20 cm, according to TIME.
• Paul Byrne, a planetary scientist at Washington University in St. Louis who was not involved, says the results are likely to hold and that the moon and Mars may show similar shrinkage.
• The updated figures were derived by combining MESSENGER images into 3D terrain maps using stereophotogrammetry, said Nishiyama, who is also affiliated with Hokkaido University.

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