It has long been known that Mercury has shrunk over time, thanks to telltale wrinkles on its surface. But now it seems as though it has shrunk even more than anyone had realised.
A new study has found that the planet closest to the Sun may have shrunk by between 10 per cent and 30 per cent more than previously thought.
This adds up to a loss of up to 23km (14.5 miles) in radius since the planet first formed about 4.5 billion years ago.
The study, published in the journal Geophysical Research Letters, says the extent of the shrinking has been obscured by debris from the numerous impact craters found on the surface.
The surprising findings confirm the Solar System’s smallest planet, which is about a third of the diameter of Earth, continues to present a puzzle to scientists. However, more of the puzzle may be solved thanks to a new probe that enters Mercury’s orbit in November.
When Mercury was formed by the collision of rocks and asteroids orbiting the Sun, it generated an enormous amount of heat that it has continued to lose since then. Much like an apple left out in the Sun, it became smaller over time, forming ridges and wrinkles.
Those wrinkles are in fact vast landforms with cliff-faces stretching hundreds of kilometres, known as shortening structures. They were first spotted in the mid-1970s when NASA’s Mariner 10 probe made a fly-by of the planet, photographing just under half its surface. By studying the Mariner 10 images, scientists thought Mercury must have shrunk by between 1 and 3km since it was formed.
Then, in 2011, NASA’s MESSENGER probe entered orbit and photographed the entirety of Mercury’s surface in more detail. Scientists subsequently upgraded their estimate, suggesting it had shrunk by up to 7km.
Now, the new findings have upped the estimate once again.
“30 per cent is a little bit surprising, but the corrected amount of contraction actually makes sense to me,” says the study’s lead author Gaku Nishiyama, a planetary scientist at the German Aerospace Center (DLR) Institute of Space Research.
Using data from MESSENGER, Nishiyama and his team created a new map of Mercury’s surface roughness. They realised that the rougher the area, the fewer the ridges. “It made us think that there’s a process obscuring shortening structures,” he says.
That process, they concluded, was the debris created by asteroids striking the planet’s surface over billions of years, hiding the true extent of its shrinkage.
Mercury is not short of significant impact sites – one crater, Caloris Planitia, measures 1,550km (960 miles), which is almost a third of the diameter of the entire planet. Like our Moon, Mercury is pockmarked by impact sites, with craters named after people including John Lennon, the Brontë sisters and Chuck Berry.
Mercury has a large, dense metallic core that, unusually, makes up about 85 per cent of its radius. Understanding how the planet cooled would also provide more answers about the contents of its core.
“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 says.
He and his team may not have long to wait for those answers. In 2018, BepiColombo, a joint European and Japanese mission, launched – and it is now just weeks away from entering Mercury’s orbit. It is expected to start feeding back data from April 2027 but has already made some interesting observations from a distance.
While in orbit, it will take even more detailed images of the surface of the planet than MESSENGER; Nishiyama says BepiColombo may well show his own findings are an underestimate. But the mission could also solve more mercurial mysteries – such as why there appears to be water ice in the polar regions of a planet that can reach 430°C (800°F).
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