THE EVOLVING PLANETARY TIME SCALE

Formal stratigraphic systems have been developed for the surface materials of the Moon, Mars, Mercury, and the Galilean satellite Ganymede. These systems are based on geologic mapping, which establishes relative ages of surfaces delineated by superposition, morphology, impact crater densities, and other relations and features. Referent units selected from the mapping determine time-stratigraphic bases and/or representative materials characteristic of events and periods for definition of chronologic units. Absolute ages of these units in many cases can be estimated using crater size-frequency data. For the Moon, the chronologic units and cratering record are calibrated by radiometric ages measured from samples collected from the lunar surface by the Apollo and Luna missions. Model ages for other cratered planetary surfaces are constructed by three methods: (1) estimating relative cratering rates with Earth’s Moon, (2) estimating cratering rates directly based on surveys of the sizes and trajectories of asteroids and comets, and (3) for Main Belt asteroids, radiometric ages of large impact events inferred from meteorite studies to correspond to resurfacing events of spectrally associated asteroid as suggested for the asteroid Vesta. Other cratered bodies with estimated surface ages include Venus and the Galilean satellites of Jupiter. However, surfaces with low crater densities must be treated cautiously, evidenced by geologic studies of Venus that have led to competing global catastrophic vs. time-transgressive equilibrium resurfacing histories. Recently published mapping results of Io indicate volcanic processes resurfaced regions of the body on time scales of years to millions of years. Geologic mapping based on newer topographic and image data combined with a global crater database indicates that the earliest Martian highlands are more extensive than previous mapping results had suggested. NASA’s MESSENGER spacecraft is currently imaging the Mercurian surface, half of which was previously unobserved. Documentation of cratering histories of planetary objects in the Solar System will be further enhanced in 2015 by spacecraft reaching the never-before-visited Pluto system by NASA’s New Horizons mission and the asteroid Ceres by NASA’s DAWN mission.