Mercury has spent billions of years slowly getting smaller, but scientists now believe the tiny planet may have shrunk much more than earlier estimates suggested. New research indicates that Mercury’s contraction could be as much as 30% greater than previously calculated, potentially revealing important clues about the planet’s enormous metal core, its early history, and what has been happening deep inside the closest planet to the sun.
A Small Planet Has Lost More Ground Than Expected
Mercury is already the smallest planet in our solar system, measuring roughly 3,000 miles, or 4,900 kilometers, across. Yet over its long history, it has gradually become even smaller.
A new study led by scientists at the German Aerospace Center’s Institute of Space Research estimates that Mercury may have contracted 10% to 30% more than previous calculations suggested. The planet may have lost as much as 14.5 miles, or 23 kilometers, from its diameter since it formed. Earlier estimates generally placed the loss at about 2.5 to 10 miles, or 4 to 16 kilometers.
The findings were published in the journal Geophysical Research Letters and offer researchers a new way to understand Mercury’s geological evolution.
For a planet that is already so small, those extra miles matter. They could change scientists’ understanding of Mercury’s internal structure and how the planet has cooled during its 4.5-billion-year history.
Why Has Mercury Been Shrinking for Billions of Years?
Mercury’s shrinking story begins when the planet formed about 4.5 billion years ago.
Like the other rocky planets, Mercury came together through violent collisions involving rocks and asteroids orbiting the young sun. Those impacts produced tremendous amounts of heat, leaving the newly formed planet with a hot interior.
Mercury has been losing that heat ever since.
As its interior cools, it contracts. The mantle and crust above it must adjust to the smaller interior, causing the surface to buckle, crack and form ridges and cliffs. These geological wrinkles have become some of the most important evidence scientists use to estimate how much Mercury has contracted.
The amount of shrinkage also tells researchers something about what lies beneath the surface.
“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,” explained Gaku Nishiyama, a planetary scientist at the German Aerospace Center’s Institute of Space Research and lead author of the study.
Mercury’s Battered Surface May Have Hidden the Evidence
One of the biggest challenges has been figuring out how much of Mercury’s contraction is actually visible.
Mercury has endured countless impacts during its history. Craters and the debris thrown out by those collisions have repeatedly changed its surface. That rough terrain may have covered some of the ridges and faults created as the planet contracted.
Nishiyama and his colleagues compared existing geological maps showing evidence of contraction with newer maps measuring the roughness of Mercury’s surface.
A pattern emerged. The roughest parts of Mercury tended to contain fewer visible contraction features. The researchers concluded that many of the missing wrinkles may still exist but are buried beneath debris created by later impacts.
By accounting for those hidden regions, the researchers arrived at their larger estimate for Mercury’s overall contraction.
“30% is a little bit surprising, but the corrected amount of contraction actually makes sense to me,” Nishiyama said.
The revised figures bring the geological evidence closer to what physical models suggest should have happened as Mercury cooled.
NASA’s Messenger Provided the Clues
Much of what scientists currently know about Mercury comes from NASA’s Messenger spacecraft.
Messenger became the first spacecraft to orbit Mercury in 2011 and spent more than four years studying the planet before its mission ended in 2015. It mapped Mercury’s surface, investigated its composition and geological history, studied its magnetic field, and helped confirm the presence of water ice in permanently shadowed areas near the poles.
The new contraction study relies on measurements collected during that mission, but Messenger’s data have limitations. Researchers can reliably use the existing information to measure contraction features larger than roughly 3 miles, or 5 kilometers, across.
That means smaller ridges, scarps and other signs of contraction may still be missing from the current picture. As a result, even the new estimate of Mercury’s shrinkage could eventually prove too low.
BepiColombo Could Reveal What Messenger Could Not
Scientists may not have to wait long for better answers.
BepiColombo, the joint European-Japanese mission to Mercury, is approaching the planet after an eight-year journey through the inner solar system. The mission is expected to enter orbit around Mercury in November 2026. It will then use two orbiters to investigate the planet and its surrounding environment in far greater detail.
Among its instruments is the BepiColombo Laser Altimeter, known as BELA. It will make detailed measurements of Mercury’s topography and surface roughness, giving researchers an opportunity to identify geological features that previous spacecraft could not clearly resolve.
Those observations could show whether still more evidence of Mercury’s contraction has been buried or overlooked.
BepiColombo will be only the third spacecraft mission to visit Mercury. NASA’s Mariner 10 made three flybys in the 1970s, followed decades later by Messenger’s orbital mission.
A Shrinking Mercury Could Help Explain Other Worlds
The research matters beyond understanding one unusually small planet.
If rough terrain and impact debris can hide evidence of contraction on Mercury, similar problems could affect estimates for other rocky bodies. Scientists studying the Moon and other worlds may also need to consider whether visible geological features tell the complete story of how those bodies changed internally over billions of years.
For Mercury, the revised estimate provides another piece of a complicated planetary puzzle. Its unusually large metal core, thin rocky outer layers and long history of cooling have made it an important place for scientists trying to understand how rocky planets form and evolve.
Nishiyama said the new findings are bringing researchers closer to that goal.
“We are quite excited,” Nishiyama said, adding that it feels like “we are approaching the reality of Mercury’s evolution.”
As BepiColombo begins its close study of Mercury, scientists should soon have their best opportunity yet to test that picture. A planet that appears quiet from millions of miles away still carries billions of years of geological history across its wrinkled surface, and some of that history may remain hidden beneath the scars of countless impacts.
