Composition of the drainage network on early Mars

Among the 2208 martian fluvial features plotted in a previous study [Z. Geomorphol. 37-1 (1993) 57], the hydrological characteristics of 71 pristine fluvial valley networks, located between 18.0°N and 55.0°S, were surveyed using Viking Orbiter photomosaic bases at 1:2 million scale. The surveyed networks display: (1) systems organized mainly as first-order streams, 85% of which have lengths not exceeding 1000 km with a large amount of the statistical sample having lengths between 200 and 500 km (generally shorter than the degraded valleys studied by Baker and Partridge [J. Geophys. Res. 91 (1986) 3561]), (2) drainage basins with average areas of about 5000 km2, (3) a random distribution, and (4) headwater tributary drainage densities (kd) ranging systematically between 0.03 and 0.2 km−1, which are comparable to terrestrial drainage densities in arid climates. The compositions of the surveyed pristine valley networks are comparable to the degraded fluvial systems that dissect older martian surface materials. Thus, it appears that no dramatic changes occurred in martian hydrology since the earlier period of formation of the “degraded valleys”, such as described by Baker and Partridge [J. Geophys. Res. 91 (1986) 3561]. Subsurface aquifer drainage may have been a large contributor to drainage basin formation since early in Mars history.