Introduction

The absolute dating techniques of Chapter 5 rely on very precise laboratory analyses of rock samples. For Earth, an abundance of accessible samples exists. However, with respect to the rest of the solar system, only meteorites, small bits of asteroidal and cometary debris - interplanetary dust particles (IDP), and samples from the Moon have been delivered to terrestrial laboratories for age analyses. One class of meteorites, the Shergottites-Nakhlites-Chassigny (SNC), may have been ejected from Mars by collision with one or several asteroids. Aside from these cases, we have no known samples of material from large bodies in the solar system and thus cannot date major geologic events on the surfaces of the bodies in an absolute fashion.

Instead, scientists use relative dating techniques to infer time histories of the moons and planets in the solar system, and they rely primarily on the record of bombardment, or cratering, of the surfaces of these bodies. We describe this technique and the physics of cratering in the present chapter. In addition to providing a foundation for inferring key aspects of the solar system's history, this discussion provides a good foundation for the presentation in Chapter 8 of relative age dating on Earth, which relies on geologic processes other than cratering but for which the principles are much the same.

Process of impact cratering

Impact cratering is a process in which a high-speed projectile collides with a solid surface, forming an excavated region called a crater. Impact craters, and the closely related form of craters caused by massive explosions, such as nuclear detonations, can be distinguished from those produced by other processes, such as volcanism or collapse due to groundwater withdrawal, by their distinctive appearance (Figure 7.1).