Degradation of Endeavour crater, Mars

Abstract:

The Opportunity rover is traversing the western rim segments of the 22 km diameter Endeavour crater in Meridiani Planum, with resultant data enabling evaluation of the craters’s degradation state. The crater is Noachian in age, complex in morphology, and largely buried by younger sulfate-rich rocks. Nevertheless, exposed rim segments dubbed Cape York (CY) and Solander Point/Murray Ridge (S/M) ~1500 m to the south reveal breccias interpreted as remnants of the ejecta deposit (Shoemaker Formation or SF) that at CY overlie the pre-impact country rocks (Matijevic Formation or MF).

At CY, relief is ~10 m and consists of 6-7 m of SF over at least several m of MF. By contrast, the MF/SF contact is not visible at S/M despite outcrops some 20 m below and 60 m above the elevation of the contact at CY. This implies some structural offset between the rim segments and suggests up to 70 m section is preserved at S/M. The lack of information about the orientation of the SF at S/M makes the true thickness difficult to establish, though it appears to be 10’s of m more than at CY.

Comparison to similar sized, fresh, complex craters on Mars and the Moon suggests there was originally 100-200 m of ejecta at Endeavour’s rim. Ejecta comprise only 20-25% of the rim relief around lunar craters of similar size, thereby implying an original rim height of up to 500-1000 m at Endeavour. If accurate, then ~400-800 m of the rim remains buried; the higher end of this range is close to the 800-900 m section interpreted elsewhere. If 200 m ejecta were present, then CY and S/M experienced ~190 m and over 100 m erosional lowering, respectively. If 100 m ejecta were present then rim lowering was ~90 m and 10’s of m, respectively. Such differences between rim segments likely relate to changing efficiency of responsible processes and/or varying characteristics of the rocks and indicate portions of Endeavour crater experienced significant degradation.

A paucity of exposed debris shed from SF and MT around the CY and S/M rim segments relegates most erosion to before the surrounding plains deposits were emplaced. Endeavour ranges between 250-500 m deep. By contrast, recent studies of pristine complex craters on Mars suggest the original depth was probably closer to 2000 m, though it is unclear how much older fill may occur beneath exposed sulfate-rich rocks.