Mercury is shrinking faster than scientists previously thought

Wrinkles on Mercury’s surface suggest the planet closest to the sun is shrinking faster than scientists previously thought. The new study, published in the journal Geophysical Research Letters, shows that Mercury is shrinking 10 to 30 percent faster than previously thought. The paper’s authors calculated that the planet’s diameter has decreased by nearly 19 kilometers since its formation.

BepiColombo Mercury Probe. Image source: ESA/BepiColombo/MTM

The researchers believe that such significant compression was overlooked because the continued bombardment of the planet’s surface by cosmic objects caused the formation of craters, hiding traces of this process. According to scientists, determining the extent of Mercury’s compression is key to studying the composition of Mercury’s interior.

“Stronger compression could indicate that Mercury had a larger metallic core containing fewer light elements, such as silicon, or that Mercury had a higher initial temperature.”Said Gaku Nishiyama, leader of the research team from the Planetary Science Institute of the German Aerospace Center (DLR).

Like all rocky planets in the solar system, Mercury formed during a turbulent era about 4.5 billion years ago, when space rocks and asteroids collided and merged with each other. These collisions generate heat, and Mercury gradually loses heat over time. As the interior cools, the planet shrinks. This resulted in the formation of geological folds (ledges and ridges) in the outer layers of Mercury’s rock.

As the planet cools, it shrinks relatively uniformly, which would make similar structures that emerge in the process a common phenomenon. However, asteroid impacts throughout Mercury’s history not only created craters that hide traces of its geological evolution, but also scattered debris across the surface. This makes it harder to see the folds in the context of the many geological structures that have accumulated over billions of years.

On March 29, 2011, NASA’s Messenger probe captured a new crater on the surface of Mercury during its first orbit around the planet. Image source: NASA/JHUAPL/CIW

The team compared existing geological maps of Mercury with new data on the topography of the entire planet’s surface. It turns out that the areas with the most uneven topography have the least wrinkles. “This leads us to believe that there is a process that hides the structures that form as a result of compression.”Nishiyama noted that he believes the fragments scattered across the most uneven areas of the mercury’s hidden folds formed due to compression.

The team then used data from ridges and scarps in areas less prone to debris dispersal to estimate the extent of compression. The results suggest that Mercury may have shrunk by 10-30% during its 4.5 billion years of existence due to a lack of surface roughness data. Therefore, its diameter may be reduced by 23 kilometers, rather than 4-16 kilometers as previously thought. “The 30% number is a bit surprising, but the adjusted compression number seems reasonable to me.”Nishiyama pointed out.

An illustration depicting NASA’s Messenger spacecraft above the surface of Mercury. Image source: NASA/JHU-APL

Nishiyama believes even these latest figures are likely to be underestimates. The assessment is based on information obtained by NASA’s Messenger spacecraft, whose mission ended in 2015. The device can only distinguish objects larger than 5 kilometers.

In November 2026, the BepiColombo mission will begin scanning Mercury’s surface at higher resolution, which may reveal more signs of Mercury’s compression.