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Little Mercury May Be Shrinking Faster Than Scientists Expected

2026-09-12 | original

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Why This Matters

A new study in Geophysical Research Letters estimates Mercury has contracted roughly 30% more than previously believed—up to 14 miles in diameter—as its interior cooled, with rugged, impact-churned terrain masking the evidence. The timing matters because the joint European-Japanese BepiColombo mission is closing in on Mercury orbit in November and carries a laser instrument that can test the claim. It's a reminder that planetary models built on limited data can shift substantially once new instruments arrive.

Key Takeaways

Mercury may have lost up to 14 miles in diameter—about 30% more shrinkage than prior estimates—on a planet only ~3,000 miles across.
Impact-cratering debris likely resurfaced and obscured the geological record, causing earlier undercounts based on NASA Messenger data.
BepiColombo, arriving in orbit in November, should verify or revise the figure; only two spacecraft have ever visited Mercury.

Worth a Look

Celestron NexStar 130SLT Telescope — Mercury is notoriously tricky to spot low in the twilight, and a computerized GoTo scope like the NexStar 130SLT can slew right to it for you. Its 130mm reflector gathers plenty of light for planetary viewing, so you can follow the innermost planet yourself while BepiColombo closes in for a much better look.

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A new study suggests Mercury may have shrunk about 30% more than scientists previously thought, losing as much as 14 miles in diameter as its interior cooled and contracted over billions of years. "That's significant for a planet barely 3,000 miles (4,900 kilometers) across," reports the Associated Press. From the report: Mercury's rough surface, continually reshaped by debris hurled from impact craters, may have hidden the true extent of the loss, according to the researchers whose findings appear in the journal Geophysical Research Letters. The news comes one week after a pair of European and Japanese spacecraft shed its cruising platform and advanced toward Mercury. Known as BepiColombo, the linked craft are expected to enter orbit around Mercury in November before splitting up for a fuller survey. BepiColombo's laser instrument should confirm how much Mercury is withering as a result of internal cooling, said Gaku Nishiyama, the study's lead author who is taking part in the space mission. The shrinkage could be even more than his team is estimating based on measurements from NASA's Messenger spacecraft in the 2010s. Only one other spacecraft has ever visited Mercury, NASA's Mariner 10 in the 1970s.

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Explore topics: mercury bepicolombo geophysical research letters nasa messenger mariner 10