The evolution of Nirgal Vallis (Mars) based on morphometric results: does tectonics play a role?

One of the unique valleys in the southern hemisphere of Mars is the 700 km long Nirgal Vallis. The catchment area of the river-like valley, presumably formed in the Hesperian period (3000–3700 Ma ago), has been shaped over time by many influences. The goal of this study is to explore and separate these potential influences including changes in the water level of the Uzboi Vallis, its erosion base. The other aim is to detect and describe the effects of tectonic processes for we have found potential evidence. Our working hypothesis is that Nirgal Vallis was once a river valley: our study analyses this possibility. The climate of Mars has undergone drastic changes in the past, and liquid water is currently not stable on the surface. The formation of former river valleys after the active wet period continued, including Nirgal Vallis. Morphometric techniques were applied based on MOLA, THEMIS, and HiRISE data. The topography of the Nirgal Vallis have been analyzed by swath analysis, calculation of various geomorphometric parameters, and runoff model. HiRISE digital terrain models having decimeter-scale resolution derived from the HiRISE stereopairs have been used for the detailed analysis.

A subdivision of the Nirgal Vallis according to its tectonic geomorphological structures was established. The swath analysis and the runoff model calculation show that, although the valley does not have a strict thalweg today, i.e., water currently cannot flow down it, the valley does have a trend of slope along the whole length and even intermittent slope changes can be observed along the whole valley. Several small-scale morphological features were also identified using MOLA data.

We conclude that the tectonic processes have transformed the morphology of the riverbed: it is divided into small sections (closed basins) so that currently water would not be able to flow down in it. Extraterrestrial (i.e. effects from outside Mars) causes on Mars also influence this, i.e. the formation of larger and smaller craters has also left its mark on the topography. We interpret the subperpendicular elongated morphological features on the plateau surrounding Nirgal Vallis as wrinkle ridges: their size, shape and spacing is similar to those identified by previous authors elsewhere. We also found evidence that the valley bottom has also been modified by the processes that formed the wrinkle ridges. Our observations imply younger tectonic influence of the area, presumably, postdating the fluvial erosional regime.

The Luki Crater, located near the confluence of Nirgal Vallis and Uzboi Vallis, has also been analyzed, and evidence has been found to be tectonically modified. This modification is in congruence with the results of other analyses indicating tectonic influence of Nirgal Vallis. Our results may also have implications for formation of the Uzboi Vallis, though this will require further studies.

These results suggest that the area of the Nirgal Vallis once formed a real valley with terrestrial-like hydrological properties, but since the liquid water disappeared from the area, the thalweg has been modified and the vallis was exposed to presumably longer-term or repeated tectonic influences.