Made by a large asteroid impact. Jumbled terrain where seismic waves all converged. Thin wisps of H and He captured from the Solar Wind. In 1991, Radar observations from Earth saw high-reflection signals from Mercury's North Pole. Only ice at or just below the surface can give a return signal that strong.
Smooth ice is a better radar reflector than rock.
Mercury's Interior Mercury is intermediate in size between the Moon and Mars. The lithosphere wrinkles as Mercury's interior cools and contracts. One idea is that during formation Mercury collided nearly head-on with a large body. Collider was smaller than Mercury Impact blew off most of Mercury's mantle. Re-formed planet had a huge iron core left behind.