CONCENTRIC CRATER FILL IN THE NORTHERN MID-LATITUDES OF MARS: FORMATION PROCESSES AND RELATIONSHIPS TO LINEATED VALLEY FILL AND LOBATE DEBRIS APRONS

Hypotheses for the formation of martian concentric crater fill (CCF) range from ice-free processes (e.g., aeolian fill), to ice-assisted talus creep, to ice-rich debris covered glaciers. Based on analysis of new CTX and HiRISE data, we find that concentric crater fill (CCF) is ice-rich and is a significant component of Amazonian-aged martian glacial systems. We present mapping results documenting the nature and extent of CCF along the martian dichotomy boundary over -30˚ – 90˚ E latitude and 20˚ - 80˚ N longitude. On the basis of morphological analysis we classify CCF landforms into “classic” CCF and “degraded” CCF. Classic CCF is most typical in the middle latitudes of the range analyzed, while a range of degradation processes results in the presence of degraded CCF landforms at high and low latitudes.

We evaluate formation mechanisms for CCF and interpret the landforms to be relict debris covered glaciers rather than ice-mobilized talus, or dunes. We examine filled crater depth diameter ratios and conclude that in many locations, hundreds of meters of ice may still be present under desiccated surficial debris. This conclusion is consistent with the abundance of “ring-mold craters” on CCF surfaces that suggest the presence of near-surface ice. Analysis of breached craters and distal glacial deposits suggests that in locations, CCF-related ice was once several hundred meters higher than its current level, with sublimation during the most recent Amazonian lowering the ice surface. Crater counts on ejecta blankets of filled and unfilled craters suggests that CCF formed most recently between ~60-300 Ma, consistent with the formation ages of other martian debris-covered glacial landforms such as lineated valley fill (LVF) and lobate debris aprons (LDA).

Morphological analysis of CCF in the vicinity of LVF and LDA suggests that CCF is a part of integrated LVF/LDA/CCF glacial systems. Instances of morphological continuity between CCF, LVF, and LDA are abundant. The presence of formerly more abundant CCF ice, coupled with the integration of CCF into LVF and LDA glacial systems suggests the possibility that CCF represents one component of a significant Amazonian mid-latitude glaciation on Mars.