Topography and morphology of the Argyre Basin, Mars: implications for its geologic and hydrologic history
Argyre, located in the southern highlands southeast of Tharsis, is one of the largest impact basins on Mars and formed in Early Noachian time. We use Mars Global Surveyor (MGS) data to characterize the basin and its geologic features and units. It has been proposed that meltback of a south polar ice cap during the Noachian completely filled the basin with water, that the outflow channel in the north drained the basin, and that the water eventually entered the northern lowlands (Parker T.J., 1994.) If true, this would be the longest drainage system on either Mars or the Earth and would have immense implications for the hydrologic cycle and the evolution of the atmosphere on Mars. In order to address this question, we used topographic data from the Mars Orbiter Laser Altimeter (MOLA) and imaging data from the Mars Observer Camera (MOC). We also tested several alternative models proposed by previous workers (i.e., eolian, volcanic, mudflows, glaciers, fluvial/lacustrine) for the evolution of the Argyre basin. Based on our investigation we conclude that the Argyre basin went through a complex geologic history with several geologic processes contributing to its current appearance. Glacial and fluvial/lacustrine processes in conjunction with eolian modification were probably most important in the evolution of the interior of the Argyre basin. The Hesperian wrinkle ridged unit Hr was previously interpreted as volcanic in origin due to the occurrence of ridges. Based on our observations we conclude that ridges in Argyre Planitia are dissimilar to wrinkle ridges in other occurrences of unit Hr. The new data suggest that these are eskers and based on the occurrence of these esker-like features, we propose a model in which the floor of Argyre was covered by ice. There is evidence for areally significant amounts of water having ponded in the Argyre basin in its past history, but a complete fill to depths of ̃4 km and overflow remains questionable. On the basis of our findings it is unlikely that Uzboi Vallis drained the basin to the north, because the basin would have to be completely filled with at least 2.1×10 6 km3 of water and this is not consistent with current hydrologic models. Instead, new MOLA data show evidence for drainage into the basin from the north, south of crater Hale and Uzboi Vallis. We performed estimates of the available water and found that the amount of water that can be produced by meltback of a Hesperian ice cap appears insufficient to completely fill the Argyre basin. We propose that water that ponded in the Argyre basin would have sublimed, evaporated or migrated into the substrate rather than flowing through the northern outflow channel. In summary, a significant input of sediments and a partial fill of Argyre basin with water during the Hesperian is suggested by several channels emptying into the Argyre basin from the south and north, signs of fluvial erosion on the southern basin floor, the formation of small deltas at the mouths of Surius Vallis and the valley at the north rim, the amount of available water, and the smoothness of unit Hr. The formation of esker-like features indicates that this body of water very likely froze over. Finally MOC images reveal evidence that eolian activity, that is deflation of floor material and accumulation of dunes, modified the basin floor. On the basis of the MOLA and MOC data and our observations we outline a scenario for the evolution of the Argyre basin. In our model, water, produced by a Hesperian meltback of the south polar ice sheet, entered the Argyre basin, partly filling the floor of the basin to form a temporary ice covered lake. A downward freezing front propagated faster than the ice could sublime, resulting in an increasing ice thickness with time. As influx of water continued, in shallower regions of the lake (i.e., close to the incoming channels), the ice was grounded and incoming water formed subglacial channels in which esker-like ridges were deposited. After the influx ceased, continued sublimation and migration of water into the substrate reduced the amount of water/ice in the basin. Throughout the entire geologic history, eolian activity played an important role in the Argyre basin, mantling or exhuming morphologic features, influencing sublimation rates, and contributing to the present day morphology.