Mercury has been slowly shrinking for billions of years. But scientists studying the smallest planet in the solar system may have overlooked just how dramatic that transformation has been because some of the evidence has literally been buried.
A new study suggests Mercury’s battered, crater-covered surface has concealed a significant fraction of the faults created as the planet cooled and contracted. After accounting for those hidden geological structures, researchers estimate Mercury may have shrunk by as much as 7.2 miles (11.6 kilometers) in radius—up to 30% more than previous estimates based on the same tectonic record.
The findings could alter models of how Mercury’s interior evolved and raise an interesting possibility beyond the planet itself. For instance, scientists may also be underestimating how much other rocky worlds, particularly the Moon, have contracted.
“Combined with the contractional strain map from Broquet and Andrews-Hanna (2026a), we show that radial contraction may have been underestimated by 10%–30%,” researchers write. “Our radial contraction correction should be considered as a lower bound.”
In the study published in Geophysical Research Letters, researchers led by Dr. Gaku Nishiyama of the German Aerospace Center (DLR) examined a longstanding mystery in Mercury’s geology.
As a rocky planet loses heat, its interior contracts. Much like the skin of a drying fruit, the planet’s rigid exterior must accommodate a shrinking interior, producing faults and enormous cliff-like features known as lobate scarps, along with wrinkle ridges and other shortening structures.
Scientists therefore use the crustal shortening recorded by these structures as a geological ruler to determine how much Mercury has contracted.
The problem is that those structures aren’t distributed across Mercury the way scientists might expect.
If cooling caused the entire planet to contract, the resulting deformation should be comparatively widespread. Instead, previous mapping has revealed regions filled with large scarps alongside enormous areas where surprisingly few contractional structures have been identified.
Dr. Nishiyama and his colleagues investigated whether at least part of that discrepancy could be an illusion created by Mercury’s extraordinarily battered surface.
Mercury’s Craters May Be Hiding Its Wrinkles
The researchers constructed a global map of Mercury’s surface roughness using topographic measurements acquired from NASA’s MESSENGER mission and a global digital terrain model produced from spacecraft imagery.
They then compared that roughness with previously cataloged tectonic structures.
Findings showed that fault scarps and other shortening structures were preferentially identified in smoother terrain, while rougher regions contained substantially fewer detectable structures. The relationship remained apparent across geographic scales extending to roughly 93 miles or about 150 kilometers.
That is counterintuitive because tectonic ridges themselves create topographic roughness. If roughness and plate movements were unrelated, areas with scarps might be expected to be slightly rougher than the surrounding terrain.
Instead, the researchers found the opposite.
One explanation is evident around relatively young impact craters such as Rachmaninoff. Large impacts blasted enormous quantities of debris across the surrounding landscape, producing thick blankets of material and secondary craters. In some locations, long tectonic structures appear to lose relief as they approach these deposits—consistent with the older structures being progressively hidden under impact material.
Other faults may still exist but become difficult to recognize against heavily cratered terrain. Once the surrounding topography becomes rough enough, a weathered ridge only a little higher than the background landscape can effectively disappear into the geological noise.
There is also a third possibility.
Heavily cratered regions may contain thick, porous layers of broken rock called regolith. During planetary contraction, some deformation could be absorbed as pores collapse rather than producing large faults visible from orbit. Alternatively, deformation could create faults too small for current observations to resolve. The researchers say available evidence cannot yet rule out that possibility.
Mercury Could Have Shrunk by More Than 7 Miles
Whatever mechanism is responsible, Dr. Nishiyama and his colleagues argue the relationship creates a systematic bias, where scientists see more evidence of contraction where Mercury’s surface makes that evidence easier to preserve and identify.
The team developed a correction using comparatively smooth regions, where tectonic structures appear less affected by this bias.
Applied to an earlier global estimate of 5.2 miles (8.3 kilometers) of radial contraction, the correction increased Mercury’s estimated shrinkage to roughly 7.2 miles or about 11.6 kilometers.
Depending on which tectonic structures are included, the researchers estimate surface roughness could have caused previous calculations to underestimate Mercury’s contraction by roughly 10% to 30%. They also caution that their corrected figure may itself represent a lower limit.
Ultimately, Mercury’s shrinking surface effectively records what has happened deep inside the planet.
The amount of contraction constrains how quickly Mercury lost heat, the thickness of its insulating regolith, the abundance of heat-producing elements, and even the composition and freezing history of its enormous metallic core. The revised estimate, for example, favors models involving a thinner regolith and lower sulfur content in Mercury’s core than some previous estimates.
Scientists may soon get a much sharper look.
The ESA-JAXA BepiColombo mission’s laser altimeter will eventually measure Mercury’s topography at scales as small as about 330 feet or 100 meters, possibly revealing geological relationships that MESSENGER could not resolve.
And Mercury may not be the only world whose geological record has been hiding in plain sight.
The Moon’s highlands are even rougher than Mercury’s cratered plains—by roughly 40% at the little-over 6-mile (10-kilometer) scale examined by the researchers. Yet evidence suggests the Moon contracted less than some thermal models estimate. If rough terrain similarly hides lunar faults, part of that difference might reflect structures scientists haven’t been able to see.
In other words, Mercury’s missing wrinkles may expose a wider problem in planetary geoscience. The more battered an ancient world becomes, the more effectively its later surface can erase the visible record of what happened before it.
“Investigating roughness alongside tectonic landforms may therefore refine our understanding of the lunar thermal evolution as well,” researchers conclude. “Mars could also be subject to this roughness effect, while topographic smoothing due to fluvial and aeolian processes may introduce more complex correlation between surface roughness and shortening structures.”
Study: Nishiyama, G., Broquet, A., Tosi, N., Preusker, F., Stark, A., Hussmann, H., & Hauber, E. “Underestimation of Planetary Contraction Due to Obscuration by Surface Roughness: The Case of Mercury.” Geophysical Research Letters 53, (2026).
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com
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