A small crater formed by ejecta thrown out from a larger impact crater. Secondary craters tend to cluster in a ring around the main crater, the greatest number lying slightly more than about one crater diameter away on the Moon, but rather closer on Mercury because of its higher surface gravity. Ejecta falling nearer than about half the crater diameter is travelling too slowly to form craters, and piles up as an ejecta blanket. Secondary craters may be widely dispersed over the body, if it has no atmosphere. Those far from the primary crater tend to be fairly circular, whereas those nearby may be very irregular in shape, a result of the lower velocities of the nearer ejecta.
Subjects: Astronomy and Astrophysics.