Mercury Is Shrinking Steadily

The secret to Mercury's ongoing transformation lies deep within its core. Unlike Earth, Mercury has a disproportionately massive iron core that makes up over 85% of the planet's radius. As this massive metallic heart has slowly cooled over billions of years, it has contracted, pulling the entire planet inward like a baked apple wrinkling as it cools. This process of thermal contraction is the primary driver behind the planet's steady shrinking (Review), setting it apart from its terrestrial neighbors.

The dramatic evidence of this planetary shrinkage is etched across its surface. As the interior shrank, the planet's single, solid crustal plate had to accommodate the smaller volume, causing it to buckle and break. This process formed immense, cliff-like wrinkles known as lobate scarps, some stretching for hundreds of kilometers and rising over a kilometer high. While early probes like Mariner 10 first spotted these features, it was NASA's MESSENGER mission that mapped the entire globe, revealing the true extent of the contraction. Scientists used this complete picture to calculate that Mercury's radius has decreased by approximately 7 kilometers, making it the only planet in our solar system still undergoing such significant global contraction today.