Minimum velocity for impact ejecta to form secondaries on terrestrial bodies

Abstract

The minimum velocity (v) for impact ejecta to form secondary craters (secondaries) remains enigmatic, but it is a crucial parameter in untangling the fate of impact ejecta on planetary surfaces. By cataloging the distances (L) of the nearest secondaries from centers of various-sized (D) primary craters (primaries) on the Moon, Mars and Mercury, we find that v can be as small as ~25 m/s, and an unified power-law relationship of L = 1.86D0.93 (both in meters) works for both simple and complex craters, regardless of different surface gravity and target properties. This relationship also successfully predicts occurrences of secondaries formed by craters on Venus. The constraint on v explains the common concurrences of structural disturbances in crater walls and continuous ejecta deposits caused by landing of cogenetic ejecta, suggesting that ejecta forming self-secondaries do not need near-vertical ejection angles and tertiary craters should be abundant on terrestrial bodies. Mapping of the distances of secondary craters from their primary crater on the Moon, Mars, and Mercury indicates a unified power law relationship, regardless of gravity and target properties and suggests the minimum impact velocity is lower than expected.