A 2.5d Hydraulic Model for Floods in Athabasca Valles, Mars

Introduction: The role of past and present water is the current focus of the Mars Exploration Program. Athabasca Valles is widely considered to be the site of the most recent major aqueous floods on Mars [1-5]. However, the previous models used to investigate the hydraulics of this flood had significant limitations. The goal of this project is to modify and apply a cutting-edge hydraulic model to Mars. Athabasca Valles: Athabasca Valles is located in north-central Elysium Planitia [1]. It is a 10 to 30-km-wide, 300-km-long valley that follows the northwestern side of a wrinkle ridge. The source of the channel is a pair of shallow depressions that straddle a branch of the Cerberus Fossae [2-4]. The average down-flow slope is only 0.05 %. While there is some debate about the precise age of the valley carving, the surrounding plains are reliably dated as no more than a few hundred Ma, firmly placing the last major flooding in the very latest Amazonian Period. Model ages from impact crater size frequency distributions allow ages as young as a few Ma [4,6,7] but an age of several tens of Ma is considered more likely [5,8,9]. Of particular note is that fact that Athabasca Valles debouches into a nearly fully enclosed topog-raphic basin [9]. This basin is filled with " platy-ridged " material that has been variously interpreted as mudflows, lava flows, glacio-fluvial deposits, casts from icebergs, and an extant frozen sea [1-13]. A better understanding of the fluvial activity within Athabasca Valles can only help to resolve this debate. The evidence for floods within Athabasca Valles is irrefutable. A series of streamlined forms surround surviving impact craters and knobs of older highlands terrain. Distributary channels cut across the wrinkle ridge in several locations. Other flood-related landforms that have been reported include dunes, longitudinal grooves, and layered sediments [4]. Previous Models: While there is general consensus that water-rich fluids carved Athabasca Valles, the nature of this flow is less well understood. The most quantitative work to date uses the HEC-RAS model from U.S. Army Corp of Engineers. Assuming bank-full flow and a floor roughness with a Manning coefficient of n=0.04, a peak water flux of 1-2 x 10 6 m 3 s-1 was estimated [13]. There are a number of issues with this estimate. First, the use of the Manning coefficient derived from empirical terrestrial data has been shown to be inappropriate for Mars …