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Mercury's size has decreased by 11km since birth: Here's why
19 Aug 2025

Mercury, the smallest planet in our solar system, is still slowly shrinking.

A new study published in AGU Advances gives a clearer picture of how much the planet has contracted since its formation 4.5 billion years ago.

The research estimates that Mercury's radius has decreased by between 2.7km and 5.6km, refining earlier estimates that varied from 1km to as much as 7km.

Why is the planet shrinking?
Planetary evolution

As Mercury's interior cools, its rocky shell has had to adapt.

This process has given rise to massive thrust faults, which are cliff-like scarps where parts of the crust have been pushed up.

Scientists have studied these fault systems and measured how much contraction the planet has undergone.

However, earlier methods that relied on landform height and length often produced inconsistent results.

New method to measure shrinkage
Innovative approach

To tackle this problem, researchers Stephan R. Loveless and Christian Klimczak developed a new method.

Instead of counting every fault, they focused on how much the largest fault in each dataset could account for contraction and extrapolated that across the planet.

They tested this technique on three different datasets: nearly 6,000 faults, another with 653, and a smaller set of 100.

What did the study find?
Measurement results

The new method revealed that shrinkage from faulting alone accounts for about 2-3.5km. When combined with other cooling-driven processes, the total contraction reaches up to 5.6km.

This refined understanding of Mercury's contraction offers a valuable tool for studying tectonics on other rocky planets such as Mars, which also has large fault systems marking its surface.

Mercury shrinks because its iron-rich core loses heat faster
Planetary dynamics

Mercury shrinks because its iron-rich core loses heat faster than Earth's. As the core contracts, the crust adjusts to a smaller volume beneath it.

The total diameter (twice the length of radius) has reduced by nearly 11km since formation.

This method could also study tectonics on rocky worlds like Mars. Researchers hope it offers more insights into planetary evolution over time.