The Caloris Basin on Mercury - the surface shows evidence of heavy bombardment from small rocky bodies.

Various processes have shaped the surface of Mercury. Most of them were operational more than 3500 million years ago. During these early times, Mercury's position close to the Sun meant that it experienced more collisions with small rocky bodies than any other terrestrial planet. The bodies would have been travelling particularly fast when they hit the planet and Mercury had no atmosphere to brake their fall. Plains between craters that formed at about this time suggest that the planet may have also experienced extensive volcanic activity before the end of the period of heavy bombardment, 4000 million years ago. The data BepiColombo will return on the shape, height, composition and structure of the surface and interior will reveal whether any of these processes are still acting on Mercury.

Water Ice