Refining the Geologic History of Deuteronilus Mensae, Mars Using CTX-based Crater Size-Frequency Distributions

With the availability of new high-resolution images for Mars, crater size-frequency distributions can be extended to smaller craters, providing new insights into the erosional and depositional histories of geologic units as well as refinements of formation ages. Along the dichotomy boundary in the Deuteronilus Mensae region, model ages derived for various geologic units are being used to provide important temporal constraints on the formation of the highland-lowland boundary, development of fretted terrain, and emplacement of lobate debris aprons. Deuteronilus Mensae exhibits a variety of geologically young, ice-related features, including ice-cemented mantling deposits, lobate debris aprons, concentric crater fill, and lineated valley fill that are key indicators of climate conditions. Preliminary results using Mars Reconnaissance Orbiter Context Camera (CTX; ~5 m/pixel) images on the ejecta blankets of several craters including in the Deuteronilus region, including Cerulli crater (32.2°N, 22.0°E), adjacent debris aprons, and mesas are presented here. Analyses of CTX images in the Deuteronilus Mensae region indicate that populations of both superposed and partially buried/modified craters are evident on most surfaces. The size-frequency distribution for fresh craters in the size range ~750 m - 4 km superposed on part of the Cerulli ejecta blanket shows a stable Hesperian surface with depletion of craters at smaller sizes. The ejecta blanket of an unnamed crater (42.1°N, 23.4°E) north of the dichotomy boundary is partially covered by debris aprons extending from adjacent mesas. The size-frequency distribution for this crater ejecta is consistent with an impact event in the Late Hesperian to Early Amazonian and provides a distinct lower constraint on apron emplacement. The size-frequency distribution for debris apron surfaces suggests an Early Amazonian age, consistent with the observed stratigraphy but significantly older than deposits associated with recent obliquity-driven Martian climate change. Depletion of small craters (<250 m) on apron surfaces is more extensive than on ejecta and crater floor deposits, consistent with their interpretation as ice-rich flow features. The size-frequency distribution for mesas surrounded by the debris aprons (for craters ~750 m - 4 km in diameter) suggests a mid-Hesperian age or older, which is also consistent with observed stratigraphy. Depletion of small craters (<750 m) suggests small-scale surface modification.