Examination of Martian outflow channel and valley network morphology using Mars Orbiter Laser Altimeter (MOLA) data.

摘要

Mars Orbiter Laser Altimeter (MOLA) topographic data is utilized to study outflow channels and valley networks to unravel details of the history of water on Mars. Martian ancient valley networks have mean valley width values of 2040 m and depth values of 109 m valley shapes, the later result suggests that liquid water must have been near-surface during valley network formation. The interdependence of top width and depth suggests that the depth of valley incisement governs valley width. Valley networks have decreasing width-to-depth ratios and increasing slopes as valley depth increases. Average U-shape valleys are slightly deeper (18 m) and significantly wider (1313 m) than V-shape valleys. Both shapes are commonly found within the same valley network system. These observations are consistent with a two-phase valley network formation: initial formation by surface runoff and subsequent reactivation by headward extending sapping processes. Mean valley depth decreases ∼50 m from equatorial to higher latitudes (∼50°), indicating that sapping depth is not governed by the ice-water boundary. Deeper equatorial valley networks may result from latitudinal variations in the availability of water or formation efficiency. Typical gradients of the circum-Chryse outflow channels range from -0.3 to 4 m/km, similar to the range of gradients for terrestrial alluvial rivers and catastrophic flood channels. Longitudinal profiles of these martian outflow channels appear to be largely influenced by changes in lithology and show the majority of the outflow channels approaching baselevel in the Acidalia/Chryse Planitia. Steep sections of the channel are characterized by relatively narrow widths, suggesting concentrated erosion occurred in constricted reaches. Maximum discharge estimates for the largest outflow channel, Kasei Valles, are 2 to 4 orders of magnitude lower (8 x 104--2 x 107 m3s-1) than estimated previously. Detailed examination reveals that Northern Kasei Vallis and Kasei Vallis are stratigraphically separated, suggesting the system involved several separate flood events over a significant period of time and that a martian ocean fed by outflow channels would require a long fill time (>25 years) and a warmer climate.