Age dating crater counting
The present rate of crater formation can be estimated from telescopic observations of various planet-crossing objects.
These objects include small bodies of asteroidal appearance and the nuclei of comets.
The oldest terrestrial rocks, found in the Precambrian shield of Greenland, are about 3.8 billion years old. The youngest extensive stratigraphic units dated by isotopic methods are the mare basalts, which range in age from about 3.3 to 3.8 billion years.
We have recently shown that the rockiness of large craters' ejecta, derived from the Lunar Reconnaissance Orbiter's Diviner thermal radiometer data, provides a new method for determining the ages of Copernican craters (younger than roughly one billion years old).
This method is not subject to the constraints of traditional crater counting methods using visible images.
Accurate determinations of recent cratering rates on Earth are vital to the estimation of accurate absolute ages from crater densities on the terrestrial planets.
The Earth-Moon system also provides the essential record needed to determine the past variation of this cratering rate (Hartmann, 1972a).