PERFORMANCE Signal 52
Study Estimates Mercury's Shrinkage May Exceed Prior Figures By Up To 14 Miles
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A new study in Geophysical Research Letters estimates Mercury may have lost up to 14 miles of diameter on a planet barely 3,000 miles across, roughly 30% more shrinkage than prior figures had indicated, and attributes the gap to cratering that hid the surface record of interior contraction.
The revised figure gives the BepiColombo mission's laser altimeter a concrete target to verify once the joint European-Japanese spacecraft enters orbit around Mercury later this year. If confirmed, the upward revision reframes the working estimate of how much the planet's interior has cooled and reshaped its surface. Until BepiColombo returns data, the number is a reanalysis of Messenger-era topography, not an independent measurement.
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The new estimate puts Mercury's cumulative diameter loss at up to 14 miles, roughly 30% more than prior figures, based on data from NASA's Messenger spacecraft in the 2010s.
Researchers led by Gaku Nishiyama argue that impact cratering has roughened Mercury's surface enough to hide the markers of interior contraction, biasing earlier estimates downward.
The BepiColombo laser altimeter, carried by a joint European-Japanese mission now in cruise, is expected to independently verify the figure when the spacecraft reaches orbit in November.
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The headline change is a baseline number: Mercury is now estimated to have lost up to 14 miles of diameter, about 30% more than the working figure in prior literature. The shift is not driven by fresh ranging to the surface but by reinterpreting topography already returned by NASA's Messenger mission in the 2010s. That makes the revision a reanalysis waiting on an independent measurement rather than a confirmed observation. The lead author, Gaku Nishiyama, has flagged the BepiColombo laser altimeter as the instrument that should resolve the disagreement, which gives the new number a built-in falsification path.
Accepting the larger contraction figure depends on accepting that impact cratering has roughened the surface enough to hide the markers of a cooling interior. The study's logic is that the roughened terrain obscures the loss rather than the loss being smaller, which leaves the previous counts working from a biased sample. A researcher who treats mapped contraction features as a complete census will see little reason to revise; one who treats cratering as obscuring the underlying record will land close to the new 30% upward adjustment. The material flags that the new figure could still be too small, leaving the upper bound open rather than closed.
The estimate rests entirely on Messenger-era topography and not on any direct ranging to the surface since NASA's Mariner 10 flybys in the 1970s. That puts it roughly two generations of measurement technology behind what BepiColombo is carrying. The model also cannot separate contraction that occurred early in Mercury's history from any that is still occurring, because the surface record accumulates both signals together. Anything finer than a global diameter change is out of reach of the current dataset.
BepiColombo shed its cruise platform one week before this study was published and is on track to enter orbit around Mercury in November, after which its two orbiters will separate for a fuller survey. Its laser altimeter is the instrument explicitly tasked with measuring how much the planet is still withering, which means the revised number has a near-term test. Until that data returns, the new figure is best read as the working hypothesis of one research group rather than a community consensus.
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