The Erosional History of Athabasca

As one of the candidate Mars Exploration Rover (MER) landing sites a landing ellipse in the Athabasca Valles was under consideration. The erosional history of the proposed area is complex and diverse. Geology. The valley system is located in an area between 200 and 220 W longitude and extends from the equator to 15 northern latitude to an area called Cerberus Planitia, part of Elysium Planitia. The plains are flooded by lava from the Elysium volcano group in the northeast. The main valley cuts the plain in northeast–southwest direction, but spreads out as gullies almost perpendicular to it following the overall topography in southeast direction. The possible origin of the valley correlates with the Cerberus Fossae, a set of sub-parallel grabens, which might have emanated during the rising of the Elysium volcano bulge. To the south young plains extend. Age Determination. In order to get an impression of the stratigraphic relationships and the origin of the valley we have measured the ages in terms of crater frequencies superimposed on certain units and have applied the Hartmann/Neukum chronology model [1] for the different geological units in the region of Athabasca Valles. For this task Athabasca Valles were mapped on Viking and MOC imagery accompanied by topographic information from the MOLA data base. For the absolute age determination the craters on the different geological units have been measured and their size–frequency distributions have been used to calculate the absolute age. This method is based on the assumption, that the terrestrial planets are bombarded by a single projectile source, i. e. bodies from the asteroid main belt [2,3,1]. Absolute ages have been determined from these crater frequency measurements by applying the Martian cratering chronology model by Hartmann and Neukum [1] and making use of the analytical expression derived by Ivanov [4]: N(1) = 2.68 10 (exp(6.93T) – 1) + 4.13 10 T, where N(1) is the cumulative number per km for craters with diameter 1 km; T is to be given in Gyr. Here we used the polynomial expression by Neukum and Ivanov [2,1] to represent the isochrons in form of the cumulative crater size frequency distribution (so-called crater production function) given for a certain age. Maximum uncertainties in absolute ages of young surfaces (ages less than 2 Gyrs) as discussed here mainly are roughly a factor of two. Stratigraphic Relationships. As has been discussed by Berman and Hartmann [5], the Cerberus plains have been discussed by various authors to be of Middle or Late Amazonian age (around 0.5 Gyrs) and show signs of even younger resurfacing episodes. Our evaluation of the cratering statistics for the areas mapped in low-resolution (Viking and MOC-WA) imaging data resulted in similar ages (0.9 Gyrs) but also in very old ages for the same area. We have been able to separate out at least three episodes. The overall plains age determined on MOC-WA imagery is 3.4 Gyrs, i. e. Late Hesperian. The plains underwent a resurfacing process at about 1.6 Gyrs ago, i. e. in Early Amazonian. The oldest areas are remnants embedded in an area whose age is 3.9 Gyrs, i. e. Noachian. Areas surrounding the origin of the valley and along the valley rims especially to the south are about 3.6 Gyrs old. For the valley itself we could determine two even younger ages of about 2.6 and 0.9 Gyrs. The youngest age of 0.9 Gyrs in the low-resolution images we were able to constrain with the results of the most densely cratered units in both MOC-NA images. Another age of 0.5 Gyrs was found in both low and high resolution images. This area does not seem to be connected to the valley, despite the fact that areas of age 0.5 Gyrs can be seen in spots in both MOC-NA images used. From the high-resolution MOC-NA images we obtained almost ten different ages between 1.3 and 0.003 Gyrs, which imply that especially the valley was geologically active over the last billion years until very recently. The oldest ages belong to the stratigraphically highest terraces and the younger ones to the bottom of the valley. This implies that the valley originated longer ago than commonly believed and was subsequently modified by frequent resurfacing events. From the analysis of our imagery data these resurfacing processes appear to have been volcanic or aeolian rather than fluvial.