ASYMMETRIC STRUCTURE OF LUNAR IMPACT CRATERS DUE TO OBLIQUE IMPACTS? M.Goeritz

Introduction: It has long been a matter of debate whether there exist any characteristic features in crater morphology diagnostic for oblique impacts [e.g. 1,2]. The vast majority of impact craters are almost circular in plain view, although it is well known that most impacts occur at an oblique angle of incidence [3]. Impact events may be approximated as a stationary point source of energy and momentum buried at a certain depth in the target, analogous to the detonation center of an explosive source [e.g. 4]. In accordance to this analogy the observed crater structures on planetary surfaces are relatively symmetric. Nevertheless there exist many morphological features in crater structures deviating from perfect symmetry and may be used to reconstruct the direction and obliquity of impact: The most conclusive feature to determine the direction of impact is the asymmetric shape of the ejecta blanket. In an oblique impact the preferential concentration of ejecta occurs downrange and a “forbidden” ejecta zone develops uprange (Fig. 1,2). These observations are based on remote sensing studies [5], laboratory impact experiments [6] and numerical modeling [7]. However, ejecta blankets are rarely preserved in particular at terrestrial craters and other criteria are required to determine the direction of impact. Based on the remote sensing and laboratory experiments [8] proposed enhanced rim collapse, and offset of the central peak, downrange breaching, and an enlarged central uplift as being indicative for oblique impacts. However, [9] studied Venusian craters and found that an uprange offset is statistically unwarranted for assessing an oblique impact. They conclude that asymmetries occur due to heterogeneities in the target. Numerical modeling of oblique impacts and subsequent crater collapse [10,11] and field studies of the subsurface at eroded terrestrial crater structures [12,13] revealed structural asymmetries as a criteria to decipher impact incidence. Moreover, detailed study of material flows during crater collapse in numerical models hints at structural asymmetries in the surface expression of complex craters in oblique impacts [10,11,14]. In this study we use Lunar Orbiter and Clementine data to investigate deviations of the position of the central peak in complex craters from the geometric centre of the crater. Methods: To avoid target heterogeneities as a possible explanation for observed offsets of central uplifts we restricted our study area to the basalt flooded lunar mare basins. Complex crater morphologies occur on the Moon for craters > 10-15 km [15], whereas craters >~80 km exhibit peak rings. We chose only craters within this size range (20-80 km). A second criteria for the selection of our crater database was the preservation-stage of the ejecta blanket. Mapping of the ejecta distribution is crucial for the reconstruction of the impact direction. First a mosaic of Clementine data [16] (resolution: 200 m/px) was used to search and preview craters that fall into the given size range and exhibit a well-preserved ejecta blanket. Additional information like the preservation of the ejecta blanket and, in particular, higher resolution images (60 m/px) were taken from the Lunar Orbiter Digitalization Project [17]. To avoid distortion of craters due to their location at high latitude we used a sinusoidal projection. A total number of 140 craters were pre-selected but due to in part poor preservation of ejecta blankets, and low quality of the images only 20 craters could be used for further investigation.