The degradational history of Endeavour crater, Mars

Abstract

Endeavour crater (2.28°S, 354.77°E) is a Noachian-aged 22km-diameter impact structure of complex morphology in southern Meridiani Planum. The degradation state of the crater has been studied using orbital data from the Mars Reconnaissance Orbiter and in situ data from the Opportunity rover. Multiple exposed crater rim segments range in elevation from ∼10m to over 100m above the level of the embaying Burns Formation. The crater is 200–500m deep and the interior wall exposes over ∼300m of relief around the southern half of the crater. Slopes of 6–16% flank the exterior of the largest western rim segment. On the west side of the crater, both pre-impact rocks (Matijevic Formation) and Endeavour impact ejecta (Shoemaker Formation) are present at Cape York, but only the Shoemaker Formation (up to ∼140m section) outcrops at Cape Tribulation.Study of similar sized pristine craters Bopolu and Tooting (with complex morphology) and use of metrics for describing the morphometry of martian craters suggest the original rim of Endeavour averaged 410m in elevation, but relief varied about ±200m around the circumference. A 250–275m section of ejecta (±50–60m) would have comprised a significant fraction of the rim height. The original crater was likely 1.5–2.2km deep and may have had a central peak (no obvious evidence is present) between 200 and 500m high.Comparison between the predicted original and current form of Endeavour suggests 100–200m of rim degradation ranging from nearly complete ejecta removal in some locations to preservation of a thick ejecta section in others. Differences in rim relief are at least partially due to degradation and not just original rim relief and (or) due to offsets along rim faults. Most degradation occurred prior to deposition of the Burns Formation which is ∼200m thick outside the crater, but likely thicker inside the crater.Aeolian stripping of the Burns Formation continues today via prevailing winds and lesser mass wasting is important on steeper walls. However, analogy with degraded Noachian craters south of Meridiani suggests fluvial processes were most important in early degradation and is consistent with the nearly complete removal of ejecta from some rim segments, gaps in the rim, formation of Marathon Valley, and interpretation of a pediment flanking the western rim. Slope processes likely accompanied incision that may have accounted for tens of metres rim lowering near Marathon Valley to more than 100m at Cape York.