[1] In the 1970s, Mariner and Viking observed features in the Mars northern polar region that were a few hundred kilometers in diameter with 20 mm brightness temperatures as low as 130 K (considerably below CO 2 ice sublimation temperatures). Over the past decade, studies have shown that these areas (commonly called ''cold spots'') are usually due to emissivity effects of frost deposits and occasionally to active CO 2 snowstorms. Three Mars years of Mars Global Surveyor Thermal Emission Spectrometer data were used to observe autumn and wintertime cold spot activity within the polar regions. Many cold spots formed on or near scarps of the perennial cap, probably induced by adiabatic cooling due to orographic lifting. These topographically associated cold spots were often smaller than those that were not associated with topography. We determined that initial grain sizes within the cold spots were on the order of a few millimeters, assuming the snow was uncontaminated by dust or water ice. On average, the half-life of the cold spots was 5 Julian days. The Mars global dust storm in 2001 significantly affected cold spot activity in the north polar region. Though overall perennial cap cold spot activity seemed unaffected, the distribution of cold spots did change by a decrease in the number of topographically associated cold spots and an increase in those not associated with topography. We propose that the global dust storm affected the processes that form cold spots and discuss how the global dust storm may have affected these processes.

Grain flows are an integral part of sand dune migration; they are a direct response to the local wind regime and reflect complex interactions between localized over-steepening of a dune slipface and complex turbulent airflow on the lee slope. Grain flows are primarily responsible for delivering sediment to the base of a dune, thus driving slipface advancement; yet, there are few constraints on their morphological and spatial characteristics or the amount of sediment that is redistributed by these flows. Using a combination of high-resolution terrestrial laser scanning and video recordings, four distinct grain-flow types are identified based on morphology and area on a dune slipface. Grain-flow morphologies range from small, superficial flows to larger flows that affect greater portions of the slipface, moving significant amounts of sediment. Detailed field observations are presented of the dynamics of lee slopes, including measurements of the initiation location, thickness, magnitude and frequency statistics of grain flows, as well as volume estimates of redistributed sediment for each grain flow observed. High-resolution laser scans enable accurate quantification of bulk sediment transfer from individual grain flows and can be used to study grain flows in a variety of environments. A categorization of grain-flow morphologies is presented that links styles of flows with wind strength and direction, turbulent airflow, sediment deposition and environment.

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