A new study suggests Mercury may have contracted 10-30% more than previously thought as its interior cooled over billions of years. Researchers estimate that the planet's radius may have reduced by about 7.2 miles (11.6 km). Using data from NASA's MESSENGER mission to map surface roughness, they found that debris from asteroid and comet impacts may have masked some of the cliffs, wrinkles and ridges formed by the contraction. The findings could shed light on Mercury's metallic core, interior structure, geological history and magnetic field.