Mercury may have shrunk by up to 30 percent more than previously estimated, according to a study published on Thursday in Geophysical Research Letters. Researchers from the German Aerospace Center (DLR) Institute of Space Research found that the Solar System’s smallest planet has lost up to 23 kilometers (14.5 miles) in radius since its formation roughly 4.5 billion years ago, a contraction obscured for billions of years by debris from impact craters.
How Much Has Mercury Actually Shrunk?
Scientists have long known that Mercury’s interior is contracting as the planet cools from its fiery origins, generating vast cliff-faced ridges known as shortening structures. In the mid-1970s, NASA’s Mariner 10 probe photographed just under half the planet’s surface, leading researchers to estimate a shrinkage of 1 to 3 kilometers. When the MESSENGER probe entered orbit in 2011 and mapped the entire surface in greater detail, scientists upgraded that figure to 7 kilometers. The new study pushes that radius loss up to 23 kilometers.
“30 per cent is a little bit surprising, but the corrected amount of contraction actually makes sense to me,” says study lead author Gaku Nishiyama, a planetary scientist at the DLR Institute of Space Research. Nishiyama and his team mapped the planet’s surface roughness using MESSENGER data and determined that rougher areas featured fewer visible ridges. They concluded that impact debris from asteroid strikes over billions of years has hidden the true extent of the planet’s shrinkage.
Impact Craters Obscure Ancient Wrinkles
Mercury is heavily pockmarked by impact sites, including the 1,550-kilometer-wide (960-mile) Caloris Planitia, which spans nearly a third of the planet’s diameter. Paul Byrne, a planetary scientist at Washington University in St. Louis who was not involved in the new study, notes that this fresh look at geological history accounts for the persistent effect of impact bombardment. Byrne presented findings at a 2013 conference indicating Mercury had shrunk by about 11 kilometers, roughly half the new upper estimate.
Did you know? Craters on Mercury are named after people including musicians like John Lennon and Chuck Berry, as well as the literary Brontë sisters.
“If their results hold, and I’m confident they will, then we’ll have taken a step further in investigating planets we can’t yet visit in person,” Byrne says, adding that he believes similar contraction will eventually be documented on other rocky bodies like Mars and the Moon.
Implications for Mercury’s Massive Metallic Core
Mercury features an unusually large, dense metallic core that accounts for about 85 percent of the planet’s radius. Pinpointing the exact degree of contraction alters models of how that core formed and 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 explains.
Further clarity is fast approaching. The joint European and Japanese mission BepiColombo launched in 2018 and is weeks away from entering Mercury’s orbit. While it is expected to start transmitting routine data in April 2027, the probe will capture far more detailed surface images than MESSENGER. Nishiyama notes that BepiColombo could reveal his team’s new figures are actually an underestimate, while potentially solving enduring mysteries like the presence of water ice in polar regions where surface temperatures reach 430°C (800°F).
Frequently Asked Questions
Why is Mercury shrinking?
Mercury is shrinking because it continues to lose the enormous heat generated when it formed from colliding rocks and asteroids 4.5 billion years ago. As the interior cools, the planet contracts, forming ridges and wrinkles across its surface.
How much has Mercury shrunk according to the new study?
The latest research published in Geophysical Research Letters estimates that Mercury has shrunk by 10 percent to 30 percent more than previously thought, translating to a loss of up to 23 kilometers (14.5 miles) in radius.
Why wasn’t this extreme shrinkage noticed earlier?
Debris from billions of years of asteroid impacts has accumulated across the surface, obscuring many of the vast cliff-faced ridges that mark the planet’s contraction.
When will we know more about Mercury’s interior?
The BepiColombo spacecraft, a joint European and Japanese mission launched in 2018, is scheduled to enter Mercury’s orbit soon and begin returning high-resolution data starting in 2027.
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