Late Amazonian Glaciation at the Dichotomy Boundary on Mars: New Evidence from MRO for Glacial Thickness Maxima and Multiple Glacial Phases

Lineated valley fill (LVF) in fretted valleys at the dichotomy boundary has been interpreted as glacial in origin. Unknown are 1) the original thickness of the glacier ice, 2) the amount of ice-surface lowering, through sublimation, retreat and ice loss, to its presently observed level, and 3) whether there were multiple periods of glaciation. We address these questions through analysis of an integrated LVF glacial landsystem along the dichotomy boundary at the contact of Protonilus Mensae and Coloe Fossae (41°N, 54.5°E). Analysis of MRO CTX and HiRISE data confirm the presence of a 5-km wide loop-like lobe emanating from LVF that terminates in an elevated box canyon, and the remnant flow patterns do not correlate with present-day topography, indicating that a substantially increased volume of ice was necessary to alter the flow regime and deposit the observed features. Additionally, the upper limit of glacial deposits at this site is topographically consistent with other regional glacial deposits in Coloe Fossae and Protonilus Mensae. The elevation difference between the upper limit of glacial deposits and the current surface of the LVF at the study site is ~920 meters. We interpret this difference to reflect the minimum amount of ice-surface lowering of the valley glacier system during retreat. Consistent with a general lowering of the ice surface are multiple moraines and/or trimlines, and changes in LVF flow patterns, including local flow reversals, as the ice retreated and decreased in thickness. The clear superposition of several lobes out onto the current surface of the LVF indicates that a less extensive phase of glaciation followed the lowering of the valley glacial landsystem. These data suggest that the major Late Amazonian glaciation that produced LVF in this region involved significantly larger amounts of glacier ice than previously thought, and that subsequent, less extensive local tributary glaciation followed.