The planforms of low‐angle impact craters in the northern hemisphere of Mars

Abstract

Abstract-We have surveyed Martian impact craters greater than 5 km in diameter using Viking and thermal emission imaging system (THEMIS) imagery to evaluate how the planform of the rim and ejecta changes with decreasing impact angle. We infer the impact angles at which the changes occur by assuming a sin 2 Θ dependence for the cumulative fraction of craters forming below angle Θ. At impact angles less than ~40° from horizontal, the ejecta become offset downrange relative to the crater rim. As the impact angle decreases to less than ~20°, the ejecta begin to concentrate in the crossrange direction and a "forbidden zone" that is void of ejecta develops in the uprange direction. At angles less than ~10°, a "butterfly" ejecta pattern is generated by the presence of downrange and uprange forbidden zones, and the rim planform becomes elliptical with the major axis oriented along the projectile's direction of travel. The uprange forbidden zone appears as a "V" curving outward from the rim, but the downrange forbidden zone is a straight-edged wedge. Although fresh Martian craters greater than 5 km in diameter have ramparts indicative of surface ejecta flow, the ejecta planforms and the angles at which they occur are very similar to those for lunar craters and laboratory impacts conducted in a dry vacuum. The planforms are different from those for Venusian craters and experimental impacts in a dense atmosphere. We interpret our results to indicate that Martian ejecta are first emplaced predominantly ballistically and then experience modest surface flow.

Identification of the Bloody Creek structure, a possible impact crater in southwestern Nova Scotia, Canada

“…Laboratory experiments with impacts into noncohesive sand produced elliptical craters at impact angles of 5° of the horizontal (Gault and Wedekind 1978). More recent experiments with impact into aluminum targets produced elliptical craters at a steeper impact angle of 12° from the horizontal, which is more in keeping with planetary remote sensing studies and also suggests that there is a target strength effect (Bottke et al 2000;Herrick and Hessen 2006). The planetary data indicate that ~4% of the crater populations have an aspect ratio of 1.2 or greater.…”

A geomorphic analysis of Hale crater, Mars: The effects of impact into ice-rich crust

“…Most secondaries that lie within He1 and He2 appear to be filled with ejecta, indicating ballistic ejecta emplacement was followed by surface flow, in agreement with Herrick and Hessen (2006). The rough terrain within He1 northeast of Hale makes it difficult to determine whether this depositional sequence holds in this area.…”

“…The rim and terrace structures are clearly defined in the northern part of the crater. Hale was likely formed by an oblique impact from the southeast, due to its elongated northwest-southeast shape, its asymmetric ejecta blanket (Herrick and Hessen, 2006), its asymmetric central peak complex (peak size and elevation increases from southeast to northwest), and the distribution of secondary craters and streaks. The topographic slope of Argyre basin also influenced Hale's morphology.…”

The Modification Stage of Crater Formation

“…The geometry of the ejecta blanket was often regarded as the only indicator to decipher azimuth and angle for an oblique impact (e.g. Herrick and Hessen, 2006;Chapter 4). The crater outline is in fact insensitive to the impact trajectory and remains circular with the exception for highly oblique impact (<10 °).…”