Explain the role of impacts in planetary evolution, including both giant impacts and more modest ones.

Impacts in early planet formation led to heating of the protoplanets, allowing for differentiation of materials and outgassing of lighter elements. Later, larger impacts probably led to some of the unusual characteristics of planets, such as formation of the Moon around Earth, Mercury's consisting of mostly core material and not as large a mantle or crust as the other planets, and the slow, retrograde rotation of Venus. Later impacts led to craters on all the solid worlds and to the deposition of volatile substances on worlds in the inner solar system that lacked them. Such impacts contributed to changes in the early planetary atmospheres.