The shape and appearance of craters formed by oblique impact on the Moon and Venus

Abstract

Abstract-We surveyed the impact crater populations of Venus and the Moon, dry targets with and without an atmosphere, to characterize how the 3-dimensional shape of a crater and the appearance of the ejecta blanket varies with impact angle. An empirical estimate of the impact angle below which particular phenomena occur was inferred from the cumulative percentage of impact craters exhibiting different traits. The results of the surveys were mostly consistent with predictions from experimental work. Assuming a sin 2 Q dependence for the cumulative fraction of craters forming below angle Q, on the Moon, the following transitions occur: <~45 degrees, the ejecta blanket becomes asymmetric; <~25 degrees, a forbidden zone develops in the uprange portion of the ejecta blanket, and the crater rim is depressed in that direction; <~15 degrees, the rim becomes saddle-shaped; <~10 degrees, the rim becomes elongated in the direction of impact and the ejecta forms a "butterfly" pattern. On Venus, the atmosphere causes asymmetries in the ejecta blanket to occur at higher impact angles. The transitions on Venus are: <~55 degrees, the ejecta becomes heavily concentrated downrange; <~40 degrees, a notch in the ejecta that extends to the rim appears, and as impact angle decreases, the notch develops into a larger forbidden zone; <~10 degrees, a fly-wing pattern develops, where material is ejected in the crossrange direction but gets swept downrange. No relationship between location or shape of the central structure and impact angle was observed on either planet. No uprange steepening and no variation in internal slope or crater depth could be associated with impact angle on the Moon. For both planets, as the impact angle decreases from vertical, first the uprange and then the downrange rim decreases in elevation, while the remainder of the rim stays at a constant elevation. For craters on Venus <~15 km in diameter, a variety of crater shapes are observed because meteoroid fragment dispersal is a significant fraction of crater diameter. The longer path length for oblique impacts causes a correlation of clustered impact effects with oblique impact effects. One consequence of this correlation is a shallowing of the crater with decreasing impact angle for small craters.

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

“…As impact angle decreases on both planets, first the uprange and then the downrange rim go to zero elevation, while the crossrange rim elevation remains unchanged. The Herrick and Forsberg-Taylor (2003) surveys are consistent with the Bottke et al (2000) results for grazing impacts, but the number of craters surveyed are far fewer because Herrick and ForsbergTaylor (2003) considered only well-preserved craters. At small crater diameters on Venus (<15 km), meteoroid breakup and dispersal becomes a significant fraction of crater diameter (Herrick and Phillips 1994), and a variety of crater shapes and planforms can be observed that are consistent with clustered, oblique impact experiments (Schultz and Gault 1985).…”

“…In this work, we use a methodology similar to that used in Herrick and Forsberg-Taylor (2003). We surveyed a large number of craters within a given area in a manner that allowed us to attribute the variations in crater appearance to impact angle.…”

“…Understanding how the cratering process changes for increasingly oblique impacts is critical to understanding planetary impact crater formation and to utilizing the cratering record to study the geologic history of a planet. As illustrated in laboratory experiments (e.g., Gault and Wedekind 1978;Schultz 1992b) and through comparison of lunar and Venusian craters (Schultz 1992b;Herrick and Forsberg-Taylor 2003), the nature of asymmetries associated with oblique impacts and their angle of occurrence can vary dramatically with target properties. For example, the MoonVenus comparison (Herrick and Forsberg-Taylor 2003) highlighted the differences between impacts in a vacuum and those occurring on a planet with a dense atmosphere.…”

“…As illustrated in laboratory experiments (e.g., Gault and Wedekind 1978;Schultz 1992b) and through comparison of lunar and Venusian craters (Schultz 1992b;Herrick and Forsberg-Taylor 2003), the nature of asymmetries associated with oblique impacts and their angle of occurrence can vary dramatically with target properties. For example, the MoonVenus comparison (Herrick and Forsberg-Taylor 2003) highlighted the differences between impacts in a vacuum and those occurring on a planet with a dense atmosphere. Detailed examination of oblique impact forms on Mars has the potential to provide unique constraints on the general cratering process and the particular mechanism of Martian rampart crater formation.…”

“…Rather than interpreting their results as indicating an excess of low-angle impacts on all three planets, Bottke et al (2000) concluded that planetary craters become elliptical at higher impact angles (~12°) than in the Gault and Wedekind (1978) experiments. Herrick and Forsberg-Taylor (2003) surveyed crater shapes and ejecta planforms on Venus and the Moon and inferred the progression with decreasing impact angle on those two bodies. The progression in ejecta planform and crater shape and the angles at which various transitions occur on Venus and the Moon are broadly consistent with those observed in experimental work in a vacuum (Gault and Wedekind 1978) and in an atmosphere (Schultz 1992b), respectively.…”