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A. V. Pathare, D. A. Paige (UCLA)
The water ice North Permanent Cap (NPC) of Mars is permeated by dark spiraling lineations that correspond to axisymmetric troughs. The fact that the equatorward-facing (EQWF) slopes of these troughs are much steeper than those of the poleward-facing (PWF) troughs-- typical EQWF and PWF slopes are 10 and 2 degrees, respectively-- almost certainly implies that sublimation is involved in trough formation. Similarly, eolian erosion is most likely responsible for the removal of dust and continued exposure of Polar Layered Deposits (PLD) on steep EQWF trough walls.
Nevertheless, we suggest that glacial flow may ultimately control the distribution and migration of north polar troughs. Previously, we have shown that the rheology of dusty, small-grained water ice is consistent with both the extraordinary youthfulness of the NPC and the multi-million year surface age disparity between the North and South PLD. Here, we present results of our 2D NPC glacial flow modeling, which indicates that NPC flow velocities decrease radially, despite the corresponding increase in average slope due to the deeper and more numerous troughs present at the cap periphery. We show that even for slopes as high as 20 degrees, trough flow velocities will not be substantially greater than the NPC average. We show that this lack of trough closure by flow is a consequence of both the lower basal temperatures at the margins and the lower stress-dependence of grain-size dependent creep (n=1.8), which at low Martian temperatures and stresses dominates dislocation creep (n=4).
We assume that accumulation is restricted to the central 10 percent of the cap, in contrast to other trough evolution models-including Fisher's glacial "accublation" model-- which assume that deposition occurs on PWF walls throughout the cap. However, Viking and MOLA observations are more consistent with a central accumulation zone, as is the relatively small insolation difference between EQWF and PWF walls, which decreases with increasing obliquity. In fact, the results of our 1D radiative-convective calculations indicate that sublimation from PWF walls may actually exceed that of EQWF walls at obliquities greater than 35 degrees. We will incorporate these sublimation calculations into an obliquity-dependent glacial flow model of trough evolution. Although most trough migration models assume that sublimation results in the poleward retreat of troughs, we propose that glacial and orbital effects may actually result in the equatorward advance of troughs, which we will show is more consistent with the MOLA-observed radial increase in average trough depth.