TOPOGRAPHIC ANALYSIS OF FLUVIAL LANDFORMS WEST OF NEWCOMB CRATER (MARS) FROM HRSC-MARS EXPRESS STEREO IMAGERY

English Abstract:

Based on Mars Express HRSC stereo imagery (resolution of ~10m/pixel) and DEMs (resolution of ~75m/pixel), well-developed fluvial valleys are mapped in highly cratered terrain dated of Noachian (t>3.6 Gyr), especially in Noachis Terra, west of Newcomb crater. The 2D organization of valleys is very similar to that observed in terrestrial fluvial basins. The exponent n of Hack’s law is a well-known parameter for determining the distribution of valleys or stream inside their watershed. This scaling law is an empirical power law relationship between the drainage basin area and the length of the stream, measured from the mouth of the basin to the crest of the drainage divide along the stream channel. Whatever the area of watersheds, the mean exponent n is ~0.7, which is very close to values for terrestrial valley networks. The valley depth, which is relief between the valley bottom and the interfluve summits, is <500 m, with a mean value of 100 m from HRSC DEMs. There is a spatial evolution of valley depth between heads and outlet at each given Strahler order, with a systematic increase in depth with a higher Strahler order for valleys debouching into plains like on Earth. The volume of valleys reaches a few 10s of km3, which is relatively significative even in arid conditions and implies a large volume of liquid water in a river sustained during a minimum period of time of 100 kyr. The quantitative assessment of fluvial valleys provides new items for their formation processes. During the Noachian period, the main erosive process is a sustained fluvial activity due to rivers fed by precipitation, whatever the precipitation comes from rainfall or snowfall and subsequent melting.