Geomorphic analysis of small rayed craters on Mars: Examining primary versus secondary impacts

摘要

Twenty confirmed impacts over a 7-year time period on Mars were qualitatively and statistically compared to 287 secondary craters believed to originate from Zunil, an ～500 ka, 10-km diameter, primary crater. Our goal was to establish criteria to distinguish secondaries from primaries in the general crater population on the basis of their horizontal planforms. Recent primary impacts have extensive "air blast" zones, distal ray systems (>100 crater radii, R), and ephemeral ejecta. Recent primaries formed clusters of craters from atmospheric fragmentation of the meteoroid body. Secondary craters have ejecta blankets with shorter rays that are consistent with emplacement by low-impact velocities (near 1 km/s). The mean extent of the continuous ejecta blankets was less distal for secondaries (5.38 ± 1.57R) versus primaries (18.07 ± 7.01R), though primary ejecta were less fractal (Fractal Dimension Index (FD_I) < 1.30) and more circular on average (Circularity Ratio (C R) = 0.55 ± 0.25 versus 0.27 ± 0.13 for secondaries). Crater rims were remarkably circular (primaries C_R = 0.97 ± 0.02, secondaries at 0.94 ± 0.05), though secondaries have the lowest values (C_R< 0.9). Secondary crater rims were elongated toward or orthogonal to their primary of origin. Uprange source directions for most secondaries, determined by ejecta planform and crater rim ellipticity, point toward Zunil, although contamination from other primaries is considered in some areas. Ejecta blanket discrepancies between recent primaries and Zunil secondaries are attributable to differences in impact velocity and retention age. After removal of the ejecta blanket, crater rims are generally not diagnostic for determining crater origin. Fragmentation of primaries may play some role in steepening the size-frequency distribution of crater diameters in the 5 m < D < 30 m range.