MORPHOMETRY AND ORIGIN OF HESPERIAN TO AMAZONIAN-AGE VALLEY NETWORKS ON XANTHE TERRA, MARS

Located in the equatorial highlands of Mars, Xanthe Terra exhibits multiple amphitheater-headed valley networks. Age relationships suggest episodic formation spanning the Hesperian to Early Amazonian, requiring climatic conditions that supported erosion of (order of) 100-km-long, 100-m-wide, 10-m-deep bedrock canyons. The morphometry of the channels was analyzed to explore origins and to constrain climate conditions. Data analysis was conducted using ArcHydro to generate network maps, long-profiles, and to determine width, depth, contributing area, drainage density, and stream order. A High Resolution Stereo Camera (HRSC) DEM mosaic at 150 m was used. The HRSC DEM was mosaicked with a Mars Orbiter Laser Altimeter (MOLA) DEM at 463 m to fill gaps. The study includes Nanedi, Hypanis, Ochus, Sabrina, Tyras and eight smaller, unnamed valley networks. Nanedi and Hypanis are 5^th order streams with a drainage density of ~0.05 km^-1, the highest in this region. The smallest channels are 1^st or 2^nd order streams. The long-profiles of most channels lack any concavity and mimic trends of the regional landscape. Shorter, steeper channels that enter impact craters (e.g. Tyras Vallis) exhibit concave-down profiles. The width and depth data indicate broad fluctuations down valley, with a marginal increase in contributing area. This suggests that the cross-sectional morphometry of these channels is not related to contributing area. Rather, slope, lithology, and water losses are the most likely controlling factors. The channel morphometry and patterns indicate that sustained precipitation did not occur at this location on Mars at this time. However, observations also challenge groundwater sapping interpretations. CTX imagery reveal shallow channels that converge onto the amphitheater headwalls of several channels. The slope break here suggests that the headwalls are knickpoints. These observations extend the source region of the channels further south than previously considered. Possible origins for the water include: (1) low-volume spillover of Hesperian to Amazonian-age floodwaters, associated with circum-Chryse flooding, (2) snow/ice melt derived from the highlands north of Valles Marineris (icy highlands model), or (3) other upstream sources that released overland flow (e.g., groundwater, paleo-lakes).