ALLUVIAL FANS NEAR THE CURIOSITY LANDING SITE

Many publications and talks have identified fans in Gale crater. Here we focus on a fan system just north of the Curiosity landing site. As noted by previous workers, the south-facing slope of Gale at the landing site has a clearly developed fan system (about 70 km2 in surface area). Here we recognize a distinction between a western and eastern fan system. The western fan is slightly steeper and smaller than the eastern fan, and is more deflated. It also has numerous elongated, but discontinuous ridges, well expressed at the distal fan edge, which appear to be inverted channels, the longest of which extends from the head of the fan for almost 3 km downslope. The eastern fan is much smoother, with subtle ridges that slope parallel to one another; they may be slightly inverted channels. The fan’s head is in a trough with a slope of about 7%, but quickly drops to 3% and then declines to approximately 1% forming a concave up profile. The lowest contours of the eastern fan show a steepening, or local convexity. The western fan terminates mostly on a local elevated ridge, but the eastern fan enters into a distinct enclosed basin about 14.5 km long and 4.4 km wide with a maximum depth of about 30 m. Contour analysis indicates possible drainage from some of the western fan to the enclosed basin, as well as from areas east of the eastern fan. We hypothesize that because material from the source canyon enters the fan obliquely towards the east, any flow exiting the canyon would be forced to turn southwest, thereby leading to the initial building of the western fan. As the western fan continued to build, the flow was eventually forced to switch to the east. This shift may also be driven by changes in effective drainage area and sediment supply. As fan building continued on the eastern fan, the older western fan experienced deflation. Relevant observations from Curiosity’s cameras will be discussed.