Mercury is shrinking faster than we thought. What’s going on inside of it?
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Wrinkles on the surface of Mercury have revealed that the closest planet to the sun is shrinking faster than scientists thought.
New research suggests that Mercury is shrinking between 10% and 30% faster than had been previously estimated. The team behind this research calculated that the planet has lost nearly 12 miles (19 kilometers) of its total diameter since it formed.
The researchers think that the extremity of this shrinking has gone unnoticed previously because bombardment from space rocks over time has created craters that obscured the evidence of this process.
Scientists say determining the extent of the shrinking of Mercury is key to investigating the composition of the planet’s interior.
“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,” team leader Gaku Nishiyama, of the German Aerospace Center (DLR) Institute of Space Research said in a statement.
Mercury’s rocky past
Like all rocky planets in the solar system, Mercury was formed 4.5 billion years ago during a violent and turbulent time in the solar system’s history, as rocks and asteroids collided with each other and were bound together.
These impacts generated heat, which Mercury has been losing ever since. As the interior of Mercury cooled, it also shrank. This caused geological wrinkles in the form of scarps and ridges to develop in the rocky outer layers of Mercury.
This cooling should result in Mercury shrinking in a roughly uniform way, and that would make these shortening structures common across the planet. However, asteroid impacts throughout its history have not only created craters that obscure signs of its geological history, but have also spread debris across the surface of Mercury. This makes wrinkles tougher to spot among the accumulation of billions of years’ worth of geological features.
The team combined previous maps of Mercury’s geology with new observations of the entire planet’s surface roughness. They found the roughest terrain of Mercury featured the fewest wrinkles.
“It made us think that there’s a process obscuring shortening structures,” said Nishiyama, who thinks that impact debris in the roughest regions of Mercury is covering shrinking wrinkles.
The team then used the ridges and scarps in regions less affected by the dispersal of debris to estimate the amount of contraction. This indicated that missing features in rough areas could amount to 10% to 30% extra shrinkage over the 4.5 billion-year lifetime of Mercury. That amounts to a total change in diameter of up to 14.5 miles (23 kilometers) rather than the currently estimated 2.5 to 10 miles (4 to 16 kilometers).
“30% is a little bit surprising, but the corrected amount of contraction actually makes sense to me,” Nishiyama said.
There may be more surprises in store. Nishiyama believes that the updated figures could still be an underestimate. This estimate is based on data collected by NASA’s MESSENGER probe, which ended in 2015. MESSENGER could only distinguish features larger than around 3 miles (5 kilometers) across.
In November 2026, the BepiColombo mission will start conducting scans of Mercury’s surface in much higher resolution than this, and that could reveal even more shrinkage features.
The team’s research was published on Thursday (Sept. 10) in the journal Geophysical Research Letters.
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