Dependence of secondary crater characteristics on downrange distance: High-resolution morphometry and simulations.

On average, secondary impact craters are expected to deepen and become more symmetric as impact velocity ( v _i) increases with downrange distance ( L). We have used high-resolution topography (1-2 m/pixel) to characterize the morphometry of secondary craters as a function of L for several well-preserved primary craters on Mars. The secondaries in this study ( N = 2644) span a range of diameters (25 m ≤ D≤400 m) and estimated impact velocities (0.4 km/s ≤ v _i≤2 km/s). The range of diameter-normalized rim-to-floor depth ( d/ D) broadens and reaches a ceiling of d/ D≈0.22 at L≈280 km ( v _i= 1-1.2 km/s), whereas average rim height shows little dependence on v _i for the largest craters ( h/ D≈0.02, D > 60 m). Populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent: planform elongations ( L< 110-160 km), taller downrange rims ( L < 280 km), and cavities that are deeper uprange ( L< 450-500 km). Populations of secondaries with lopsided ejecta were found to extend to at least L ∼ 700 km. Impact hydrocode simulations with iSALE-2D for strong, intact projectile and target materials predict a ceiling for d/ D versus L whose trend is consistent with our measurements. This study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations. This has applications to understanding the chronology of planetary surfaces and the long-term evolution of small crater populations. [ABSTRACT FROM AUTHOR]