Mercury, the smallest and innermost planet in our solar system, has long been known to have shrunk as it cooled over billions of years. This contraction left visible scars on its surface, such as craters, ridges, and steep cliffs, which scientists have used to estimate how much the planet has shrunk. However, new research suggests that Mercury may have contracted significantly more than previously thought—by up to 40% more than its visible features alone indicate. Led by researchers from Hokkaido University and in collaboration with the German Aerospace Center (DLR) and the University of Tokyo, the study found that Mercury’s surface roughness may have hidden key tectonic structures. These structures, known as shortening features, are ridges and scarps formed when the planet’s crust was compressed as its interior cooled. Scientists typically use these features to estimate how much Mercury’s radius has decreased. However, the research team discovered that shortening structures are more commonly found in smoother areas, implying that rougher regions may have obscured some of Mercury’s tectonic history. One major factor contributing to this hidden record is the impact debris from recent collisions. When a large object strikes Mercury, it can scatter material across the surface, creating rough deposits called ejecta. These deposits can cover older tectonic structures, making them harder to detect. For example, near the Rachmaninoff crater, researchers found fewer shortening structures in areas covered by rough ejecta, suggesting that these features may be partially buried. After adjusting for the influence of surface roughness, the estimated contraction of Mercury increased from about 8.3 kilometers to 11.6 kilometers—indicating that the actual shrinkage could be even greater than this revised figure. The findings highlight the need for more precise measurements of Mercury’s surface features. The upcoming BepiColombo mission, a joint venture between Japan and Europe, may help refine these estimates. The mission’s Laser Altimeter (BELA) will map Mercury’s surface at higher resolution, potentially revealing more details about how recent geological events relate to tectonic structures. These insights could also apply to other rocky planets, such as the Moon, which has an even rougher surface than Mercury. Understanding Mercury’s contraction history provides valuable clues about the evolution of planets in our solar system.