Sapping in Nirgal Vallis

Kurzfassung

The topographic information provided by the Mars Orbiter Laser Altimeter has been used in combination with Viking and Mars Observer Camera imagery to estimate the three-dimensional structure of the Nirgal Vallis drainage system in order to constrain the formation process. Based on precisely correlated Viking, Mars Orbiter Laser Altimeter (MOLA) and Mars Orbiter Camera (MOC) cartographic data, we have measured morphometric and topologic parameters of the valley network. Although there is no single parameter, which unambiguously distinguishes between run-off and sapping, the combination of parameters such as surface dip angle, longitudinal profile, width to depth ratio, drainage density, structural control, valley terminations and bifurcation ratio will constrain the formation process. All topographic based valley network parameters indicate an origin by groundwater sapping processes and headward erosion for Nirgal Vallis and confirms former geomorphologic analyses. The extremely low drainage density of Nirgal Vallis may either be caused by very slow erosion, due to low groundwater supply or arid conditions, or by sequential interruptions of the erosion process due to probable climate changes.