Time scales for extraterrestrial planetary surfaces permit correlation of geologic events across the solar system. These time scales are primarily based on robotic exploration of planetary surfaces over the past half century but are also critically underpinned by rock samples returned from the Moon and by telescopic observations.
Extraterrestrial chronology began with the Moon through reconstruction of its geologic history by photogeologic mapping. Mapped geologic features having widespread significance include (1) huge basin-forming impacts marked by preserved circular rims hundreds to thousands of kilometers in diameter and surrounding ejecta deposits, (2) vast lava-flow fields (the lunar mare) that document episodes of extensive volcanic resurfacing, and (3) crater ejecta of variable brightness that reveal gradual weathering. Some of these events have been dated radiometrically using returned samples. Furthermore, several dated events formed extensive surfaces whose crater distributions can be used to determine lunar cratering rates, thus making possible age estimates of unsampled surfaces.
Cratering rates are estimated, with various levels of uncertainty, for other surfaces of the inner solar system by applying scaling laws that adjust lunar cratering rates to these bodies. Or they can be estimated directly from skywatch surveys of the sizes and trajectories of asteroid populations. Mars and Mercury, like the Moon, have formal chronostratigraphic systems based on geologic mapping and have abundances of impacts that provide good statistical basis for estimating ages. Venus, however, is sparsely cratered and thus has poor temporal constraints.
Outer solar system mapping investigations have examined variably-cratered icy and rocky surfaces of the larger satellites of Jupiter, Saturn, Uranus, and Neptune. Their cratering histories, however, are not well understood as the contributions of impacts from comets and Kuiper Belt objects and the influence on cratering by interactions of the large planets during early solar system history remain highly uncertain.
Awareness of planetary interactions enhances appreciation for the solar system as a close-knit family of objects. The geologic history of each individual object is partly affected by other members of this family, in particular as recorded by impacts. For example, collisions in the asteroid belt can project asteroids and their fragments into the inner solar system and therefore account for periods of enhanced cratering on terrestrial planets. One such event may be responsible for the Chicxulub impact recorded at the K/T boundary on Earth.