Denning Crater is a crater in the Sinus Sabaeus quadrangle of Mars at 17.7° south latitude and 326.6° west longitude. It is about 165 km in diameter and was named after William F. Denning, a British astronomer (1848–1931).
Recent small crater on floor of Dennin Crater, as seen by HiRISE. Arrow shows group of secondary craters from ejecta falling down.
When a comet or asteroid collides at a high rate of speed interplanetary with the surface of Mars it creates a primary impact crater. The primary impact may also eject significant numbers of rocks which eventually fall back to make secondary craters. The secondary craters may be in clusters. All of the craters in the cluster would appear to be equally eroded; hence they would all seem to be of the same age. If these secondary craters formed from a single, large, nearby impact, then they would have formed at roughly the same instant in time. The image below of Dennin Crater shows a cluster of secondary craters.