Mid-latitude Glacial Modification of Moreux Crater (44°e, 42°n; 135 Km): Evi- Dence for Polythermal Glaciation Related to Impact-induced Enhanced Thermal Gradients

Introduction: The dichotomy boundary represents one of the most important geological and geophysical features of Mars. We have been assessing the nature of modification processes along the dichotomy boundary at northern mid-latitudes in the Deuteronilus-Protonilus region and have found evidence for extensive regional Amazonian glaciation in lobate debris aprons (LDA) and lineated valley fill (LVF) in the fretted terrain [e.g., 1-6]. The major geological landforms found there support an interpretation that involves cold-based glaciation in valley and plateau icefield landsystems over much of this region earlier in the Amazonion [2-6]. Thus broad environmental conditions (cold hyperarid polar desert) at this time appear similar to those of the present, but changes in orbital parameters [7] led to sublimation and mobilization of H 2 O and its redeposition as frost, snow and ice at low and mid-latitudes [e.g., 8] to produce the observed glacial landsystems. Analysis of these cold-based glacial deposits [2-6] raises the question of whether wet-based conditions ever prevailed in glacial environments on Amazonian Mars. Two factors could lead to wet-based conditions under the cold polar desert conditions thought to have prevailed throughout much of the Amazonian: 1) sufficient ice accumulation to have caused basal melting [e.g., 9-10], or 2) localized heat sources, such as volcanism [e.g., 11-12] causing melting. Here we investigate an additional example of localized thermal anomalies, impact crater formation. We show that the impact of the 135 km diameter crater Moreux, superposed on the dichotomy boundary at ~44°E, 42°N [13] (Fig. 1), provided a thermal anomaly [14-16] sufficient to cause elevated near-surface temperatures and wet-based glacial conditions in the crater interior for a period of time following its formation. We find that the Moreux impact crater environment produced polythermal glaciation in an otherwise cold-based glacial phase along the dichotomy boundary during the Amazonian. Moreux crater setting: Moreux is located at the edge of the plateau of the southern uplands and its southern portion disrupts the regional scarp representing the dichotomy boundary (Fig. 1). To the north, ejecta can be seen on the summits of the plateaus and in the intervening valleys in some places, but in others, lobate debris aprons and lineated valley fill clearly postdate the ejecta, indicating post-Moreux regional glaciation. Thus, Moreux is interpreted to have formed at a time subsequent to the formation of the current dichotomy scarp and the abundant related mesas, but prior to the end of the modification of the …