Mercury Is Shrinking Faster Than Scientists Thought, and a Spacecraft Is Headed to Confirm It

Mercury has been quietly shrinking for 4.5 billion years as its iron core cools and pulls the surface inward. A new study from the German Aerospace Center suggests that shrinkage is about 30% greater than earlier estimates, with the planet possibly losing up to 14 miles in diameter. Since Mercury is only around 3,000 miles across, that’s a significant loss for the smallest planet in our solar system.

So why did it take so long to notice? Researchers believe debris from countless impacts over the ages buried many of Mercury’s surface faults, effectively hiding the geological wrinkles that would reveal the true extent of the contraction. The study, published in Geophysical Research Letters, reshapes how scientists understand the planet’s evolution.

The timing couldn’t be better. Europe and Japan’s BepiColombo spacecraft is approaching Mercury now and will enter orbit in November, carrying a laser instrument designed to verify the shrinkage directly. Study lead author Gaku Nishiyama of Hokkaido University says the team is excited and approaching the reality of Mercury’s evolution. The findings could also offer clues about how other rocky planets, including Earth, change as their interiors cool over time.

What would you want a spacecraft to measure if you could send one anywhere in the solar system?