Evidence for trail forming ejecta boulder falls around fresh simple impact craters at the lunar Orientale multi-ring basin and implications for ballistic ejecta sedimentation on the Moon

Abstract

The topographic slopes of the Moon host different types of mass wasting features, in which boulder falls are abundant globally. Boulder falls are characterized by trails that are produced by rolling, bouncing and sliding of boulders on the lunar surface, in response to ground shaking originating from shallow moonquakes, impact events and other surface processes. Our previous work showed evidence for moonquake-triggered boulder falls at hundreds of sites on the interior of Orientale multi-ring basin. In this study, we document new evidence for ejecta boulder falls around 135 fresh simple impact craters with diameters ranging from 0.1 to 2.5 km in the same basin. Our detailed characterization of ejecta boulder falls around eight selected impact craters reveal their modes of occurrences, morphological properties, formation ages and clues to their impact origins. The ejecta boulder falls have the characteristic boulder trails arranged radially around the host impact craters, shorter trail lengths, narrow trail widths and shallow trail depths, when compared to the moonquake-triggered boulder falls in the same basin. Some impact craters exhibit radial asymmetry of boulder trails around these craters because of oblique impacts, in which longer trails are abundant in down-range directions, while fewer and shorter trails characterize up-range directions. In some cases, local slopes also contribute to increased trail lengths and deviation from radial distribution of trails. Formation ages of a few host impact craters, determined based on conventional crater counting method, are in the range of 0.94–6.4 Ma, and the age of ejecta boulder falls is consistent with the host impact crater ejecta. These characteristics suggest that the trails were produced by the ejecta boulders, during the ejecta blanket formation stage of the host impact craters. The relationship between the host impact crater diameter and the average size of large ejecta boulders present in their ejecta blankets suggests that the host impact craters are primary impact craters. Hence, the initial ejecta boulder fall velocities are expected to be in the ejection velocity range of ∼5–100 m/s. During ballistic sedimentation, these ejecta boulders erode the local target materials including regolith and mix them along their trails. It is also observed that ejecta boulder falls occur around small fresh simple impact craters (<2–3 km) at the Schrödinger and Lorentz basins. Around large size impact craters and basins, ejecta boulder falls produced large-size radial grooves, lineaments and sculptures on the Moon. We conclude that the characteristics of ejecta boulder falls and moonquake boulder falls are distinctly different from each other, reflecting their distinct formation conditions, but these form contemporaneously on the Moon.