According to a recent study published in Geophysical Research Letters, Mercury has been shrinking more significantly over the past 4.5 billion years than scientists previously estimated. This discovery offers new insights into the thermal and chemical evolution of the solar system’s smallest planet and may reshape models for other rocky worlds.
While Mercury is modest in size—measuring just 3,023 miles across, or roughly 3.6% of Jupiter’s diameter—its geological history provides critical clues about planetary formation. Previous calculations suggested the planet had contracted between 2.5 and 10 miles in diameter as its interior cooled after formation. However, new analysis indicates that shrinkage may be up to 30% greater than those figures, adding approximately 4.5 miles to the total contraction estimate.
The findings rely heavily on data from NASA’s MESSENGER spacecraft, which orbited Mercury from 2011 to 2015 after launching in 2004. Equipped with a Mercury Laser Altimeter and a dual-imaging system, MESSENGER mapped surface features known as shortening structures. These include wrinkles, ridges, and lobate scarps—cliff-like formations created when the planetary crust pushes upward along faults due to contraction.
Despite these tools, accurately gauging the planet’s full shrinkage proved difficult. The laser altimeter could only operate effectively when the spacecraft was within 930 miles of the surface, a proximity achieved only near the north pole during parts of its elliptical orbit. At other times, MESSENGER was as far as 9,500 miles away. Additionally, Mercury’s ancient shortening structures have been partially obscured over billions of years by asteroid impacts that covered them with craters and debris.
To overcome these challenges, Gaku Nishiyama of the German Aerospace Center Institute of Space Research and his colleagues employed a stereophotogrammetric technique. By combining two-dimensional images taken from slightly different angles, the team generated detailed digital terrain models. This approach allowed them to infer the presence and extent of buried shortening structures even in rougher, crater-filled regions that had previously skewed estimates downward.
“We find a lack of shortening structures in rough regions,” the researchers wrote. “This suggests that roughness-related processes obscure pre-existing structures, biasing previous contraction estimates downward.”
The revised contraction rate has implications for understanding Mercury’s internal composition. A greater degree of shrinkage implies the planet’s core may contain less sulfur or silicon than previously thought, or that it started at a higher temperature, leading to more dramatic volume changes as it cooled. “More shrinking means Mercury could have a larger metal core, less light elements like silicon mixed into the metal core, or a higher starting temperature,” Nishiyama stated.
These findings may also apply to other rocky bodies in the solar system, such as the Moon and Mars. If similar contraction patterns exist elsewhere, these worlds may still be actively cooling and shrinking today. However, the Moon’s heavily cratered surface makes identifying surviving shortening structures particularly challenging.
Future data from the BepiColombo mission, a joint venture by the European and Japanese space agencies, promises to refine these estimates further. Launched in 2018, BepiColombo arrived at Mercury in December and began scientific operations in April 2027. Its advanced laser altimeter can resolve surface changes as small as 20 centimeters, and its less eccentric orbit will provide broader coverage of the planet’s surface. As Nishiyama noted, this new data will help open a clearer window into how Mercury has been shaped throughout its history.
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