A reluctant conversion

A quotation from Ursula Marvin headed this chapter in the 1992 edition: “A hypervelocity meteorite impact is an extraordinary event – wreaking change instantaneously. Such a process violates every tenet of uniformitarianism. Largely for this reason, hypotheses of impact origin for craters on the Earth and the Moon were vigorously opposed for the better part of the last [19th] century … research has now established, beyond doubt, the authenticity of impact as a geological process, but … a wide chasm still persists between the views of impact specialists and those of terrestrial geologists” [1]. By 1999 a major philosophical advance had occurred so that it was possible for her to write that “Today we realise that collisions in space are the most fundamental process that has operated throughout the history of the solar system. This is a truly revolutionary insight that requires a fresh vision of the basic tenets of geology” [2].

It might be considered surprising to include a separate chapter on this topic in a book that is principally concerned with examining the origin and evolution of the solar system from a cosmochemical perspective. However, it has become clear that collisions between bodies have played a significant role in the evolution of the solar system. These effects have occurred at all times and stages, beginning with the sticking together of grains in low-velocity collisions in the dusty midplane of the nebula, and continuing with the growth of planetesimals from meter- to kilometer- and eventually to planet-sized bodies.