Masked tectonic structures suggest Mercury contracted 40% more than its visible scars alone indicate

The surface of Mercury is filled with craters, ridges and steep cliffs. Over billions of years, as the planet's interior cooled, the planet also shrank. Just as the skin of a drying fruit wrinkles when what is inside it shrinks, Mercury's rocky surface crumpled into ridges and cliffs as its interior cooled and the planet contracted. Mercury has quite literally grown smaller with age.