IMPACT BLAST WINDS: ORIGIN OF CERTAIN PERMANENT WIND STREAKS ON MARS

Bright and dark wind streaks across Mars record wind patterns related to atmospheric circulation. In some cases these streaks represent erosion of a surface veneer; in others, they indicate sand grains mobilized by strong vortices shed off of positive relief, such as crater rims. While many streaks change length or orientation over time, others not only remain unchanged but also may indicate a completely different wind direction. These permanent streaks could reflect past circulation patterns in response to conditions related to orbital forcing. Here, however, we propose that some streaks previously attributed to global circulation actually indicate the far-reaching intense blast winds, extending more than 500km away from its parent crater. In this case pre-existing features (crater rims, peaks) extending above the boundary layer generate strong wind vortices that scour mobile surface materials. Although similar in expression to atmospheric circulation wind streaks, the wind intensity (200m/s) is unmatched by even the most intense baroclinic instabilities. Not all large fresh impact craters exhibit radiating wind streaks. Expression depends on the geologic setting (e.g., wind-sensitive surface materials), history of active resurfacing (e.g., high latitude mantling deposits), and/or impactor variables (speed and composition). At very high latitudes (>50°), subtle ejecta flow lobes and linear scours extend to enormous distances (8 to 12 crater diameters), much farther than the continuous ejecta facies. This enigmatic run out formed the basis for suggesting the role of vapor-driven wind flow and the formation of high-latitude pedestal craters (Wrobel and Schultz, 2006). There, the expanding vapor plume likely swept up fines near the crater preceding ejecta emplacement. As a result, blast winds created by impacts into volatile-rich surface materials may enhance run-out distances of ejecta flows.