The largest impact basin on Mercury, 1300 km in diameter. Much of its interior is filled by smooth plains, known collectively as Caloris Planitia. There are many wrinkle ridges and fractures, some of them radial and others concentric, the latter suggesting two or three inner rings. Caloris was probably formed about 4 billion years ago, during the period of late heavy bombardment that affected all the planets.
Subjects: Astronomy and Astrophysics.