Mercury is shrinking because its iron core is slowly cooling down. As the interior loses heat, the metal contracts, and the entire planet gets smaller, like a grape drying into a raisin. Since Mercury formed about 4.5 billion years ago, its radius has shrunk by an estimated 2.7 to 5.6 kilometers. That process is still happening today.
A Giant Iron Core Driving the Process
Mercury’s core is enormous relative to its size. The iron core has a radius of about 2,074 kilometers, roughly 85% of the planet’s total radius. For comparison, Earth’s core is only about 55% of its radius. This means Mercury is, in a sense, a giant ball of iron with a thin rocky shell wrapped around it.
All that metal has been radiating heat into space for billions of years. As the core cools, the iron atoms pack more tightly together, and the core physically shrinks. The rocky mantle and crust sitting on top of it have no choice but to follow. But rock doesn’t shrink as neatly as cooling metal. The outer shell becomes too large for the interior it’s sitting on, and something has to give.
How the Surface Buckles and Cracks
When Mercury’s interior contracts beneath its crust, the outer layers are squeezed into a smaller space. This creates enormous compressive stress, similar to what happens when you push the sides of a cardboard box inward. The crust resists for a while, but eventually the stress exceeds the strength of the rock, and it fractures.
These fractures don’t split the surface apart. Instead, one slab of crust gets shoved up and over another along angled fault lines, forming cliffs called lobate scarps. Some of these cliffs stretch for hundreds of miles and rise more than a mile high. Smaller features called wrinkle ridges also form where the surface buckles and folds under compression. Together, these features are the visible fingerprints of a planet that’s been slowly compressing itself for billions of years.
Scientists using data from NASA’s MESSENGER spacecraft mapped nearly 6,000 of these ridges and scarps across Mercury’s entire surface. That global count revealed the planet has contracted far more than earlier estimates suggested. The Mariner 10 mission in the 1970s only photographed about 45% of Mercury, so early calculations significantly underestimated the total shrinkage. MESSENGER’s complete mapping showed the contraction was up to seven times greater than those original figures.
How Much Has Mercury Shrunk?
Pinning down an exact number has been tricky. Early estimates based on the visible fault scarps ranged anywhere from 1 to 7 kilometers of radial contraction. More recent analysis has narrowed that range. When researchers applied consistent methods across multiple datasets, they found the shrinkage from faulting alone comes to roughly 2 to 3.5 kilometers.
But faulting isn’t the only way a cooling planet accommodates shrinkage. Some contraction gets absorbed by processes that don’t leave obvious surface scars, like subtle folding or changes in rock density. When those factors are added in, the total radial shrinkage since Mercury’s formation likely falls between 2.7 and 5.6 kilometers. That may sound modest for an entire planet, but it translates to a significant reduction in surface area, enough to reshape the landscape on a global scale.
Mercury Is Still Shrinking Today
This isn’t purely ancient history. During the final low-altitude phase of the MESSENGER mission, scientists discovered a population of tiny thrust fault scarps that had never been seen before. These small cliffs have only tens of meters of relief and are just a few kilometers long, far smaller than the grand lobate scarps. Their pristine appearance is the key detail: they cut across impact craters rather than being buried by them, and they’re associated with small trench-like features called graben. All of this points to an age of less than 50 million years, which in geological terms is practically yesterday.
The discovery, published in Nature Geoscience, indicates that Mercury is tectonically active right now. The planet’s interior is still losing heat, still contracting, and still generating enough stress to crack the surface. The cooling is happening slowly at this point, billions of years after formation, but it hasn’t stopped.
Why Mercury Shrinks More Than Other Rocky Planets
Earth’s interior is also cooling, but our planet isn’t visibly shrinking the way Mercury is. The difference comes down to composition and size. Mercury’s oversized iron core means a much larger fraction of the planet is made of metal that contracts significantly as it cools. Earth has a proportionally smaller core, a thicker mantle, and active plate tectonics that redistribute internal stress across moving crustal plates. Mercury has no plate tectonics. Its crust is a single, unbroken shell, so all of that compressive stress builds up globally until it creates new faults or reactivates old ones.
Mercury is also much smaller, which means it loses heat faster relative to its volume. A smaller body has more surface area per unit of interior, so heat escapes more efficiently. The combination of a huge iron core and a small planetary body makes Mercury the most dramatically shrinking planet in the solar system.