Mercury may have shrunk considerably more than scientists previously calculated, with a new study suggesting that billions of years of impact debris have hidden some of the geological evidence.
The research, published in Geophysical Research Letters, estimates that Mercury may have contracted 10% to 30% more than earlier measurements suggested. The revised figure means the planet could have lost as much as 14.5 miles, or 23 kilometers, from its total diameter since it formed.
The difference does not mean Mercury recently began shrinking more quickly. Instead, researchers believe parts of the evidence left behind by its long-term contraction have been buried or obscured by the planet’s heavily cratered surface.
Mercury Has Been Contracting for Billions of Years
Mercury formed roughly 4.5 billion years ago during the turbulent early history of the solar system, when rocky material repeatedly collided and gradually came together to form planets.
Those collisions generated enormous amounts of heat.
Over billions of years, Mercury’s interior has slowly cooled. As that happened, the planet contracted.
Its solid outer crust could not simply shrink smoothly along with the interior. Instead, portions of the surface buckled and were pushed over neighboring terrain.
That process created large cliffs known as lobate scarps, along with wrinkle ridges and other tectonic structures that can still be seen across Mercury today.
For decades, planetary scientists have used these structures as evidence of the planet’s contraction. By studying their height, length and distribution, researchers can estimate how much smaller Mercury has become since its formation.
Impact Debris Changed the Picture
Gaku Nishiyama of the German Aerospace Center’s Institute of Space Research and his colleagues examined whether those visible structures tell the whole story.
Mercury has one of the most heavily cratered surfaces in the solar system.
Every major impact can throw rock and debris across the surrounding landscape. Over enormous stretches of time, that material can bury, damage or make older geological formations much harder to identify.
That creates a problem for scientists trying to calculate Mercury’s shrinkage.
If contraction-related scarps and ridges are hidden underneath younger impact debris, estimates based only on visible structures will naturally come out too low.
The research team tested that possibility using maps created from observations by NASA’s MESSENGER spacecraft.
MESSENGER orbited Mercury from 2011 until 2015 and provided the first detailed global look at the planet’s surface.
The researchers compared maps showing known contraction structures with measurements of surface roughness.
A pattern emerged.
Regions with rougher, more impact-damaged terrain tended to show fewer visible signs of crustal shortening than smoother areas.
The researchers argue that this does not necessarily mean those areas contracted less. Instead, many of the structures may simply have become more difficult to see.
After accounting for this hidden geological record, the team estimated that Mercury’s total contraction may have been underestimated by between 10% and 30%.
The Revised Estimate Closes an Old Gap
The finding could help explain a long-standing difference between observations of Mercury’s surface and computer models of the planet’s interior.
Models describing how Mercury cooled over time have generally suggested that the planet should have contracted more than estimates based on visible scarps indicated.
If impact debris has hidden some of those structures, the disagreement becomes smaller.
The result could also improve scientists’ understanding of Mercury’s interior.
Mercury is unusual because its metallic core is extremely large compared with the overall size of the planet.
The amount of contraction a planet experiences depends partly on its internal composition, temperature and cooling history.
A larger contraction estimate could therefore help researchers place tighter limits on Mercury’s internal structure and how the planet evolved after its formation.
Nishiyama and his colleagues note that the revised estimate may still not capture every sign of contraction.
Smaller Surface Features May Still Be Missing
MESSENGER transformed scientists’ understanding of Mercury, but its images still have limits.
According to the study, some smaller contraction structures may be difficult to identify reliably in the existing global data.
That means additional scarps or ridges could still be present without appearing clearly enough to be included in current measurements.
Researchers may soon have a much better opportunity to look for them.
The ESA-JAXA BepiColombo mission is now in its Mercury arrival phase after nearly eight years of travel through the inner solar system.
Its Mercury Transfer Module successfully separated from the spacecraft on September 3, 2026, marking the first major step in the arrival sequence.
The two science orbiters are scheduled to enter orbit around Mercury on November 21, 2026.
They will separate in December before moving toward their individual science orbits. Routine science operations are scheduled to begin in April 2027.
BepiColombo Could Sharpen the Estimate
BepiColombo carries instruments designed to examine Mercury’s surface, topography, magnetic environment and interior in greater detail.
Among them is a laser altimeter capable of producing precise measurements of the planet’s surface elevation.
Higher-quality observations could reveal scarps, ridges and impact structures that were too small or difficult to identify using MESSENGER data.
That will give researchers another opportunity to test the new contraction estimate and determine whether Mercury has shrunk even more than the latest study suggests.
Nishiyama is also part of the BepiColombo science team, meaning the researchers behind the latest work may soon have access to much better data for investigating the same question.
For now, the study shows how a planet’s violent impact history can interfere with scientists’ attempts to reconstruct its deeper geological past.
Mercury’s craters are not simply scars left by ancient collisions. Some may also be covering the evidence scientists need to understand how dramatically the planet changed as its interior cooled.
The finding comes as researchers continue using better observations and monitoring to revise earlier estimates across different areas of science. In Nigeria, for example, NCDC has identified five high-burden states for Lassa fever, helping health authorities focus preparedness efforts where the disease is having its greatest impact.
Mercury has been shrinking for billions of years. Scientists now have stronger evidence that the true scale of that contraction was partly hidden in plain sight beneath one of the solar system’s most battered surfaces.
The SBCC Editorial Team researches, writes, and reviews news and explainers using credible sources and official information.
