Lunar and Planetary Science XXXIII (2002) 1579.pdf Nirgal Vallis: Evidence for Extensive Sapping. R. Jaumann and D. Reiss, DLR, German Aerospace Center, Institute of Space Sensor Technology and Planetary Exploration, D-12489 Berlin, Rutherfordstr. 2, Germany, [email protected] Abstract: The topographic information provided Nirgal Vallis is located in the southern highlands by the Mars Orbiter Laser Altimeter has been used in at about 29°S ranging from 47°W to 38°W and span- combination with Viking and Mars Orbiter Camera ning a height difference of about 1000 m. The dip an- imagery to estimate the three-dimensional structure of gle of the surface dissected by the valley, is extremely low (0.01°). The longitudinal profile of the valley is the Nirgal Vallis drainage system in order to constrain linear (Fig. 2). The width to depth ratio ranges from 9 the formation process. The analysis of morphometric upstream to 8 downstream (Fig. 3) which is nearly and topologic network parameters indicates sapping as constant compared to run-off values on earth (ranging the major valley network forming process. from 2 to > 40). The heads of all tributaries are steep Introduction: Since the discovery of valley net- walled amphitheater-like boxes of well-developed al- works in Mariner 9 images the relative roles of surface coves [see also 11]. run-off versus groundwater processes in valley net- work formation have been debated [e.g 1,2,3]. Due to features such as U-shaped cross sections, alcove-like terminations of tributaries and the structural control of the networks, most of the highland valleys are attrib- uted to an origin by groundwater sapping. However, most of the arguments voting for sapping are based on planimetric information derived from two-dimensional images. Now Mars Global Surveyor provides topog- raphic information and highly resolved images which complement the Viking imagery. This enables a more quantitative view on the valley network formation. Nirgal Vallis with a length of 665 km and a down- stream width of 7 km is a valley network large enough to be resolved in MOLA-tracks. Therefore, Nirgal Vallis is a target for a three-dimensional examination of its drainage system. Method: Based on cartographic precisely corre- Figure 1. Digital Terrain Model of Nirgal Vallis (Overlay lated Viking, Mars Orbiter Laser Altimeter (MOLA) with MDIM2). and Mars Orbiter Camera (MOC) data [4,5,6], we measured morphometric and topologic parameters of Nirgal Vallis. Although there is no single parameter, which unambiguously distinguishes between run-off and sapping, the combination of parameters which drainage basin control the drainage system of a valley network, con- Height (m) strain the formation process. The most important pa- 600 rameters that describe different valley morphologies 400 50 km are listed in table 1 [7, 8, 9, 10]. 200 0 Groundwater Flow Overland Flow -200 Surface dip angle Low (terrestrial < 4°) high -400 Longitudinal profile linear concave -600 Width/depth ratio constant increasing -800 Valley terminations amphitheater-like, steep tapered, gradual 0 100 200 300 400 500 600 700 Structural control strong influenced by topography Length (km) Bifurcation ratio)1 high Low (terrestrial 3-4) Drainage density)2 low high Figure 2. Longitudinal Profile and Drainage Basin of Nirgal Table 1. Parameters to describe valley morphologies. )1 ratio Vallis. of the stream frequencies of subsequent orders; )2 ratio of total valley length and drainage area. Lunar and Planetary Science XXXIII (2002) 1579.pdf NIRGAL VALLIS: EVIDENCE FOR EXTENSIVE SAPPING: R. Jaumann and D. Reiss B T Conclusion: All topographic based valley network 15 parameters indicate for Nirgal Vallis an origin by groundwater sapping processes and headward erosion and confirms former geomorphologic analyses. The 10 extremely low drainage density and immature valley development of Nirgal Vallis may either be caused by very slow erosion, due to low groundwater supply or arid conditions, or by sequential interruptions of the 5 erosion process due to probable climate changes. References: [1] Baker et al., 1992, In: Mars, Univ. of Arizona Press. [2] Carr, 1995, JGR 100, 7479. [3] 0 Malin and Carr, 1999, Icarus, 397, 589. [4] Zeitler and 0 100 200 300 400 500 600 700 Oberst, 1999; JGR, 104, 14051. [5] Hauber et al., Length (km) 2000, Int. Arch. Photogram. Rem. Sens. XXXIII, 360. Figure 3. Width to depth ratio of Nirgal Vallis. [5] Heller et al., 2001, ISPRS-ET WS Planet. Map.. [6] Horton, 1945, Geol. Sic. Amer. 56. 275. [7] Strahler, 1964, In: Handbook Appl. Hydrogeol. McGraw Hill. N N [8] Leopold et al, 1964, In: Fluvial Proc. Geomorph. Freeman [9] Summerfield, 1991, In: Global Geo- morph. Longman, Burnt Mill.[10] Ritter et al. 1995, In: Processes Geomorph. Wm.C.Brown Publ.. [11] Reiss W E W E and Jaumann, 2002, LPSC, this issue. [12] Schumm et al., 1995, Geomorph., 12, 281. [13] Baker 1990, Groundw. Geomorph., SP, 235. [14] German, 1963, Ber. Dt. Landesk. 12, 13. S S Figure 4. Orientation of the main valley (left) and the 1. or- der tributaries (right). The main orientation of Nirgal Vallis follows the E-W direction of the major graben systems north (Valles Marineris) and south of the valley. First order tributar- ies, however, are orientated almost perpendicular to the main valley (Fig. 4). Both directions indicate a strong structural control by the circum-tharsis tectonical pat- tern. The stream frequency versus the stream order of Nirgal Vallis is concave-shaped similar to typical sap- ping valley in Florida [12]. The bifurcation ratio for the 1. and 2. order tributaries is 7 and reflects the ex- treme high number (98%) of tributaries of low order which is comparable with terrestrial sapping valleys in Utah and Florida [12, 13]. Based on the MOLA- derived digital terrain model (Fig. 1) we estimated the drainage area to amount 255000 km2. Therefore, with a total length of the valley network of 1879 km the drainage density of Nirgal Vallis is 0.0074 km-1 (typi- cal terrestrial values are in the range of 0.3 km-1 – 2 km-1 [14]). As precipitation and overland flow will intensively dissect and erode the surface, drainage den- sities are much higher for surface run-off as in the case of infiltration and subsurface flow [6]. Low drainage densities are also indicative for the immature develop- ment of a valley network..