According to a new study based on traces left on the planet's surface, Mercury may have reduced in size by 10% to 30% more than scientists previously estimated. Researchers calculated that since its formation about 4.5 billion years ago, Mercury has lost nearly 19 kilometers of its diameter.
The overall calculation indicates that the total shrinkage could reach 23 kilometers, exceeding the previous estimate, which ranged from 4 to 16 kilometers. This difference might have gone unnoticed because Mercury's surface is constantly subjected to impacts from asteroids and other objects, which create craters and scatter debris, obscuring the geological structures that record the compression.
For scientists, determining the degree of Mercury's compression is significant, as this process can provide clues about the composition of the planet's interior. Gaku Nishiyama, the group leader and a researcher from the German Aerospace Center's (DLR) Institute of Space Research, explained that 'greater compression means that Mercury may have a larger metallic core, fewer light elements like silicon mixed with the metallic core, or a higher initial temperature.'
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To study this phenomenon, researchers combined old geological maps of Mercury with new observations of the entire planet's surface roughness. The results showed an interesting pattern: the most rugged areas of Mercury had the fewest structures related to compression. This led the team to hypothesize that impact-generated debris might be covering the signs of compression.
Nishiyama noted: 'This made us consider that there is a process masking the shortening structures.' The researchers then analyzed ridges and cliffs in areas least affected by debris scattering. Based on these zones, they were able to estimate what portion of the structures might be missing in the more rugged regions.
The resulting finding pointed to an additional shrinkage of 10–30% throughout Mercury's history. With this correction, the total change in the planet's diameter could amount to 23 km instead of the current 4–16 km. Nishiyama added: '30% is somewhat surprising, but the corrected amount of shrinkage makes sense to me.'
However, this estimate may not represent the absolute limit of Mercury's actual compression. The calculation was based on data obtained by NASA's MESSENGER spacecraft, whose mission concluded in 2015. This instrument could distinguish structures approximately 5 km and larger, implying that smaller structures might have remained undetected.
The situation is expected to change in November 2026 when the BepiColombo mission begins observing Mercury's surface with much higher resolution. New data could potentially reveal compression structures that have not yet been identified. The study was published on Thursday (10) in Geophysical Research Letters.