Geomorphometric Analysis of the Martian Uzboi-Nirgal Region

The area on the eastern slopes of Tharsis (Thaumasia), northern from Argyre and southern from Margaritifer is dominated by the Uzboi Vallis and Nirgal Vallis. Their (at least partial) fluvial origin is accepted since the Mariner-Viking era. Uzboi Vallis thought to be part of the ancient Uzboi–Ladon–Morava River System (ULM). ULM is thought to be created while it served as the overflow channel of the southern Argyre crater. Nirgal Vallis is the largest tributary of Uzboi. Its source region is in the direction of the Thaumasia Mountains.

The area experienced numerous effects during its history. In this study our goal was to separate these effects in a chronological order as far as it is possible. Therefore comprehensive investigations (using MOLA and THEMIS) were carried out and a detailed analysis of these two valleys was made using HiRISE images and HiRISE-derived digital elevation models. HiRISE DEMs allow the surfaces to be studied and evaluated with a resolution of better than one meter.

Several geomorphometric methods were applied for the area: swath analysis, the distribution of the tributary valleys, sinuosity calculation, and runoff modeling. The vallis was divided into sections based on its main features. Section A, unlike other sections, is characterized by dendritic tributary system. Tributaries are found both on left and right sides. Section B is a kind of transitional zone, the tributaries are getting rare. Section C was defined as a sinuous segment. Tributaries here are very rare.  Section D is deeply incised, the thalweg is broken at several points and has a subhorizontal trend. According to the extremely low number of tributaries and the modifying effect of the neighboring impact craters the water divides are fuzzy. The path of this section is also sinuous. Section E is the deepest part of the whole Nirgal Vallis this is an unusual condition, because in the case of the terrestrial rivers the deepest part is regularly at the end of the confluence. Section F has to be separated from Section E because it shows influence of several effects. Section F has got only one (SW-NE) tributary which is significant in length and dominates the morphology of the area south from the Nirgal‒Uzboi confluence.

In our interpretation the sections detailed above seem to be at least partially related to wrinkle ridges noticeable from Thaumasia Mountains to Uzboi Vallis and are similar to those on Solis Planum. The effect of the wrinkle ridges is controlling the morphology of the plateau, and may be in connection with the tributaries.

Further geomorphological investigation may lead to the separation of the different effects formed Nirgal throughout its history.