Outcrops on lunar crater rims: Implications for rim construction mechanisms, ejecta volumes and excavation depths

Abstract

The raised rims of impact craters consist of ejecta deposited onto structurally uplifted target rocks. Although it is commonly accepted that ejecta makes up 50-75% of the total rim height, no previous measurements on well-preserved, naturally occurring impact craters exist. Relying on data from the Lunar Reconnaissance Orbiter, I report the first direct measurements of the rim-forming constituents from 21 lunar craters ranging in diameter from 2.2 km to 45 km. Results show that the ejecta deposit accounts for no more than~20% of rim relief, and structural uplift is the primary mechanism of rim development in both simple and complex craters. Thick, quasi-horizontal outcrops of coherent bedrock indicate that this uplift is the result of subsurface injection of debris or melt into the surrounding target rocks. Crater growth, at least during the latest portion of the excavation stage, therefore, proceeds mainly through injection-rather than ejection-of material. These results suggest that ejecta volumes and excavation depths may be factors of 3 to 4 less than previously considered.

The 3‐D geological model around Chang'E‐3 landing site based on lunar penetrating radar Channel 1 data

“…The northerly flow direction suggested here is consistent with the north component of the Em lava flow direction (long yellow arrow in Figure 3) presented in previous studies [Schaber, 1973;Schaber et al, 1975]. Moreover, C 1 crater ejection deposition may relate to more surface materials [Sharpton, 2014] atop stratum a-d in the west, resulting in the increased thickness we infer there. …”

Gravity field of the Orientale basin from the Gravity Recovery and Interior Laboratory Mission

“…This ejecta volume represents a downward revision from a previously published estimate (70). It is also consistent with a recent study of lunar crater outcrops that shows that ejecta accounts for no more than 20% of crater rim relief, with structural uplift the major contributor to rim development; as a consequence ejecta volumes and excavation depths may be factors of 3-4 less than previous estimations (71).…”

Evidence for an explosive origin of central pit craters on Mars

“…Raised rims can be formed by both the addition of ejecta (e.g. White and Ross, 2011) and structural uplift (Sharpton, 2014), although the latter indicates that the uplift is the greatest contributor to raised impact rims for impacts. The average grain size for ejecta decreases with radial distance from the crater, such that the largest clasts or blocks are proximal to the crater rim (e.g.…”