Absolute dune ages and implications for the time of formation of gullies in Nirgal Vallis, Mars

Kurzfassung

Transverse dunes cover the valley floor of Nirgal Vallis, a 670 km long valley network at 318°E and 29°S. The dunes are superposed by small impact craters. This shows that the dunes are inactive under present atmospheric conditions. The last active phase of dune movement (absolute age) can be determined by crater size frequency measurements. The dune surfaces (= 432km²) have been mapped in 69 high–resolution Mars Orbiter Camera (MOC) images for crater measurements in these areas. We have dated the dunes for four main image resolutions separately and also for all resolutions together, because of the different recognizable crater diameters due to the different spatial image resolutions. We determined a best fit age and in addition an upper-limit age for all classifications. The best fit absolute model ages range from 140,000 to 380,000 years, and the upper limit ages range from 390,000 years to 1.4 Ma, both with an error of a factor of 2. The last active dune phase might be a consequence of higher atmospheric pressures in recent geological times, possibly due to high obliquity phases of Mars. At higher atmospheric pressures winds of sufficient strength could be reached to move sand–sized grains via saltation. The absolute model ages are in agreement with obliquity models in which obliquities of more than 30° may have been reached in the last 3 Ma. The last phase of more than 30°–obliquity at around 300,000 years correlates well with the best fit absolute model ages for dune activity. The dunes are superposed by gully deposits (aprons) which implies a younger relative age of the gullies. Therefore the gullies must have been formed after the last active phase of the dunes and are younger than 3 Ma, possibly less than 300,000 years. This age determination of dunes by crater counts confirms the suggested young age of the gullies.