Secondary Craters

Abstract

Telescope and satellite images of the moons and planets of our solar system reveal that large primary impact craters frequently are accompanied by smaller secondary craters of variable size, shape, and distribution (Shoemaker 1962; Melosh 1989; Spudis 1993; Greeley 1994; Fig. 5.1). Roddy (1977) showed that secondary craters also are commonly produced by large man-made explosions. The projectiles that produce secondary craters are inferred to be mainly blocks and clods derived from the target rocks, which are ejected into ballistic trajectories by the primary impact. Planetary secondaries usually are first recognizable beyond the edge of the continuous ejecta blanket, and their geographic range can extend many crater diameters from the primary crater (Melosh 1989). The maximum diameters of secondaries are roughly proportional to the diameter of their primary craters (e.g., lunar secondaries are ∼4% as wide as their primaries). Besides isolated individual secondary craters, clusters (open or closed), chains, loops, gouges, and rays of secondaries are common on large and small planetary bodies. Secondary craters nearest to the rim of the primary crater may have irregular shapes because their impactors interfere with one another, and because their impact velocities are low relative to the velocity of the primary impactor. Distal secondaries usually have more regular shapes, but tend to be asymmetrical in cross section; the crater walls tend to be steepest in the direction toward the primary crater. Ejecta blocks that produce secondary craters may reach several kilometers in diameter; fragment size is inversely proportional to the ejection velocity. The extent of a secondary crater field away from the primary crater is evidently strongly controlled by gravity. Secondary craters tend to cluster closer to their primaries on larger planetary bodies than on smaller ones. According to Melosh (1989), the quantity of ejecta that produces secondary craters is small, typically one to three percent of the total ejecta derived from the primary impact. Thus, most of the large blocks and smaller clasts composing a continuous ejecta blanket do not produce well-defined secondary craters.