THE ORIGIN AND EVOLUTION OF THE PEACE VALLIS ALLUVIAL FAN SYSTEM

On August 6, 2012, Curiosity rover landed near the Peace Vallis (PV) fan. Topographic maps reveal that the eastern part of the fan, near Bradbury landing, entered an enclosed basin suggesting that the fan’s distal end could be the location of fine-grained, possibly lacustrine, deposits. These deposits and the proximity to exposures of three distinct geologic units contributed to the decision to drill in the distal region. HIRISE images and topographic data were used to explore fan form and infer formation processes.

PV fan (~80 km²) has a low gradient (~1%) and exits an ~730 km² catchment with a network of incised sharp-edged valleys. The more erosion-resistant upper fan (~49 km²) is characterized by a smooth, mottled surface, while the distal more eroded end of the fan (~31 km²) is characterized by light to heavy fracturing. As the fan spreads from its apex, it crosses a complex valley floor with three lower boundaries: 1) an elevated flat area that extends from Mt. Sharp to the base of the western side of the fan, 2) a central rise (Bradbury Rise), and 3) a depression (Yellowknife Bay) on the eastern side of the fan. These boundaries are bordered by nearly flat-lying, cratered surfaces (perhaps underlain by preserved lake sediments). The western, central, and eastern portions of the fan all exhibit longitudinal topographic profile transitions from exponential to linear in shape; these transitions do not correspond to changes in the upper to lower fan deposits. There are 43 inverted ridges on the fan, aligned downslope, interpreted to be inverted channels. Boulders (~1 m) within the channels on the upper fan and western distal end of the fan likely record transport conditions, not cratering or outcrop decay. A systematic shift in fan morphology suggests that the western fan deposited first and the fan sediment may have fined as it shifted eastward. The catchment has ~107 km of incised channels, with exposed boulders in some walls. Estimated channel erosion volumes are similar to estimates of total fan volume. Despite the modest size of the fan, its fluvial origin, the coarseness of the sediment, and the extent and form of the valley network all suggest an enduring hydrologic cycle, perhaps involving snowmelt, as suggested for fans elsewhere on Mars. Geomorphic evidence suggests the fan is one of the youngest hydro-geomorphic features in Gale.