Morphometric measurements of Martian valley networks from Mars Orbiter Laser Altimeter (MOLA) data

摘要

Morphometric measurements of martian valley networks using Mars Orbiter Laser Altimeter (MOLA) topographic data yield mean valley width values of (x) over bar +/- sigma : 2040 +/- 1548 in and depth values of (x) over bar +/- sigma : 109 +/- 137 m. Our analysis of changes in valley shape with latitude and wall slope with depth indicate infilling and mass wasting processes have not greatly altered the original valley shape; thus valley network morphometry is dominantly due to channel formation mechanisms. Both U- and V-shaped profiles are observed, commonly within the same valley network system. The average U-shaped valley is slightly deeper (18 in) and significantly wider (1313 m) than its V-shaped counterpart. Valley networks have decreasing width-to-depth ratios and increasing wall slopes as valley depth increases. Our observations are consistent with a two-phase valley network formation model. (1) Valleys initially form via surface runoff, a process that creates V-shaped profiles and quasi-dendritic form. The observed linear correlation of top width and depth (below 1.25 m depth) suggests the depth of vertical incisement governs top width. (2) Reactivation of the same valleys by headward extending sapping processes widens the channel to form U-shaped cross sections in downstream reaches. The availability of liquid water within a few hundred meters of the surface appears to be a necessary requirement for valley network formation. Mean valley depth decreases by similar to 50 in from equatorial to higher latitudes (similar to 50 degrees), contrary to the relationship predicted if the sapping depth was governed by the ice-water boundary. Deeper equatorial valley networks may result from latitudinal variations in the availability of water or formation efficiency. [References: 64]