JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi:10.1029/2005je002454, 2005 Cross-sectional and longitudinal profiles of valleys and channels in Xanthe Terra on Mars A. Kereszturi Institute for Advanced Study, Collegium Budapest, Budapest, Hungary Department of Physical Geography, Eotvos Lorand University of Sciences, Budapest, Hungary Received 15 April 2005; revised 23 September 2005; accepted 5 October 2005; published 2 December 2005. [1] On the basis of Mars Orbiter Laser Altimeter data and Viking Orbiter and Mars Global Surveyor images, cross-sectional and longitudinal profiles of channels and valleys were analyzed in Xanthe Terra on Mars. Concave and convex longitudinal profiles were observed at Bahram and Tyras Valles, respectively, while no general trend was visible at other valleys. The longitudinal profiles also showed signs of knickpoints and some short reaches with increasing elevation in the flow direction. The cross-sectional profiles of Tyras Valles show more V-shaped of upper and U-shaped of lower reaches, but the great variety of profiles of other channels suggests a complex relationship with the regional lithology/topography. Also at Tyras Valles the longitudinal profiles could be divided into upper and lower sections that probably formed at different periods under different conditions. The largest observed valleys contain small, previously unnoticed inner channels. There is no correlation between the shape of cross-sectional profiles and the direction of curvature of the valley s long axis (left/right or straight), as is found in riverbeds on the Earth, suggesting that the analyzed depressions are probably valleys and not dry riverbeds, but substantial wind-blown sedimentary infill could have modified their shape too. Citation: Kereszturi, A. (2005), Cross-sectional and longitudinal profiles of valleys and channels in Xanthe Terra on Mars, J. Geophys. Res., 110,, doi:10.1029/2005je002454. 1. Introduction [2] The origin of Martian valley networks has been attributed dominantly to surface runoff [Craddock and Howard, 2002; Irwin and Howard, 2002], groundwater sapping [Carr, 1981; Carr and Malin, 2000; Goldspiel and Squyres, 2000; Grant, 2000] or a combination of runoff and sapping (first suggested by Milton [1973]) is controversial. Sharp and Malin [1975] also suggested some channels with dendritic tributaries could be formed by runoff fed seepage and headward growth by sapping. The origin of Martian valley networks is still controversial today, and paleodischarge values [Irwin et al., 2005] and other characteristics are poorly known. Most fluvial structures are old, although some small gullies and outflow channels have probably formed in recent Martian history [Mellon and Phillips, 2001; Plescia, 2002] It is also uncertain which structures are valleys (elongated depressions that were presumably carved over time by water, but which were never full of water) and which are river channels (once filled with water), thus the fluvial structures analyzed here were termed valleys, and inner channels is used to distinguish smaller troughs within in the valleys. In this article cross-sectional and longitudinal profiles were Copyright 2005 by the American Geophysical Union. 0148-0227/05/2005JE002454 analyzed to find out their general characteristics and to search for similarities/differences between these landforms and those formed on the Earth. The analogies could help evaluate the origin of Martian valleys in some cases or at least give information that can be used together with other results (e.g., connection with the surrounding topography, with morphological or directional changes of valleys) to answer basic questions. The use of individual Mars Orbiter Laser Altimeter (MOLA) tracks in the analysis of these fluvial landforms was also tested. [3] Altogether 216 cross-sections of 14 valleys and valley segments were analyzed in Xante Terra (Figure 1), including Bahram, Ochus, and Tyras Valles and several deeply incised, confined branches of the Meumee and Vedra Valles outflow channels. This cratered highland terrain includes small valley networks, several larger valleys with immature tributary development, and the branches of Maja Valles (which was the latest channel to be formed of the outflow channels in this region [Crumpler, 1997]). Most valleys are considered to be older than 3.5 billion years [Mars Channel Working Group, 1983], or Late Noachian and Early Hesperian [Tanaka, 1986; Carr, 1995]. There is probably a connection between the regional geology and the orientation of valleys. Nanedi Valles is arranged between small clusters of massifs and shows a change in orientation at a topographic boundary [Crumpler, 1997] between the higher southern and lower northern part of the terrain. In addition 1of7
break between the wall and the nearly horizontal plain outside the valley); (2) width (the distance between the top of the opposite steep walls); and (3) the concave/convex shape and topography of the longitudinal profile. [5] The measured parameters were compared to similar parameters from the Earth to find possible analogies and differences between the characteristics of the valleys on the two planets. The various morphological and morphometrical results were also compared to each other to find connections that can help to evaluate possible reasons for certain characteristics. Figure 1. The analyzed valleys in Xanthe Terra indicated with boxes: (a) Bahram Valles; (b) a tributary of Bahram Valles; (c) Ochus Valles; (d) a tributary of Ochus Valles; (e) Tyras Valles; (f j) unnamed valleys at 13 N, 53 W; (k m) branches of Meumee Valles; and (n) Vedra Valles. (Image of the area is bounded between 23 N, 59 W and 5 N, 44 W.) Sabrina and Hypanis Valles show deltaic-like sediments at this abrupt change of slope [Hauber et al., 2005]. The valleys in this region, mainly Bahram Valles, and the physiography of the surrounding terrain were investigated by several authors [Greeley et al., 1977; Rice and De Hon, 1996; De Hon and Washington, 1999; Hauber et al., 2005; Neukum and the HRSC Team, 2005]. 2. Methods [4] Images from the Viking Orbiter and Mars Global Surveyor Mars Orbiter Camera (MOC) were used to identify valleys and channels and to locate individual MOLA tracks. Instead of gridded MOLA data, Precision Experiment Data Records (PEDR) data were used with processing version L [Smith et al., 1999]. These data uses IAU2000 planetocentric coordinates, incorporate crossover analysis of individual profiles, and are referenced to the latest Mars gravity model. In every case a small section of each MOLA track was matched to the images manually to ensure that it represents the analyzed valley. After defining the location of the profile and its angle with respect to the valley s long axis, a corrected cross-sectional profile was computed from the apparent profile as the best possible approximation of the valley s real shape (Figure 2). Because of the uncertainty of this method, several cross-sectional profiles close to each other were used for general conclusions. The following parameters were used in the analysis: (1) depth (the vertical distance between the bottom of the depression and the top of the steep wall, fixed manually at the prominent slope 3. Discussion [6] The longitudinal profiles of the analyzed valleys and channels can be compared in Figure 3. Theoretically the shape of a longitudinal profile is the result of several factors including discharge; sediment supply, size, and transport rates; flow resistance; change over time of relief and base level; and bedrock resistance where outcrops occur. The interaction between fluvial erosion and the lithology/ erodibility of the terrain results in variable profile shapes [Gregory and Walling, 1973]. The analyzed longitudinal profiles differed from each other substantially. The valleys could be classified into three groups on the basis of the visual appearance of the profiles, although more data is necessary for representative classification. On the basis of the difference in elevation between the surrounding terrain and the valley s floor, (1) a concave-up bed and valley wall profile were observed at Bahram Valles, (2) a convex-up profile was observed at lower Tyras Valles (while in the same valley a linear profile was observed in upper reaches) (Figure 4), and (3) no similar characteristic was observed in the other valleys. In the third group the topography of the floor roughly followed the topography of the surrounding terrain. There can be various reasons for the formation of concave and convex profiles. One possibility is that the Figure 2. Example of the method for transforming crosssectional profiles: (left) MOLA track, (top right) original cross-sectional profile, and (bottom right) profile transformed to a corrected one that would have been perpendicular to the valley s long axis with the measured parameters indicated. (Image center: 21.3 N, 58.4 W.) 2of7
Figure 3. (top) Some longitudinal profiles and (bottom) example images with small parts of the observed valleys. The black lines are for the floor, and gray is for the top of the valley walls with 10x vertical exaggeration. Dots are locations of topographic measurements; where two lines converge (a + b, c + d, f + g + h), tributaries connect to main valleys. At the bottom of the figure, small parts of the analyzed channels are indicated, and their locations and names are visible in Figure 1. efficiency of erosion (flow velocity, discharge, duration, terrain s erodibility) was higher in the case of Bahram Valles, so the flow was able to remove larger volume of regolith, creating a concave-up profile resembling equilibrium profiles observed at old rivers on the Earth [Scheidegger, 1970], while the original surrounding terrain shows a concave-up profile. [7] At Tyras Valles the longitudinal profile can be divided into two parts: the nearly linear upper reaches and the convex-up lower reaches (Figure 5). The linear segment is probably the older part of the system with shallower incision, while the convex-up part at lower reaches with deeply incised valleys formed later. On the basis of the width, the depth and the shape of cross-sectional profiles, upper Tyras Valles is more similar to the unnamed valleys at 13 N, 53 W than it does lower Tyras Valles. This suggests that upper and lower reaches formed under different conditions and probably at different periods. The convex-up longitudinal shape of lower Tyras Valles could be the result of the decline of base level or some other process. In the lower reaches, the slope of the longitudinal profile is steeper, coincident with the deeper incision. At the top of the valley walls, sharper slope breaks in the cross-sectional profiles can also be found. It is possible that substantially more significant mass movements took place on the walls of the deeper lower reaches, causing this sharper slope break at the top of the walls. At the other analyzed valleys, no concave or convex longitudinal profiles were observed (see two examples in Figure 6) possibly because of the lack of adequate topographic data or the absence of these shapes. 3of7
Figure 4. Longitudinal profiles of (left) Tyras and (right) Bahram Valles with 50 vertical exaggeration. Black dots mark the bottom of valley, and gray dots mark the top of wall. In the case of upper Tyras linear, lower Tyras convex profile is present both at the floor and at the top of the valley wall. Both the valley thalweg and the surrounding surface have linear gradients in upper Tyras Vallis and convex-up gradients in lower Tyras Vallis. Dashed lines connecting the first and last points of the channels floors are to visualize the convex and concave shapes. [8] Vertical undulations were also observed beside the large-scale shape of the longitudinal profiles. About 10 50 m vertical undulations were observed at 2 5 km distances, which could be the result of depressions formed later on the valley floors or may have formed originally during the fluvial process. At the observed reaches of outflow channels the undulations were higher, about 100 200 m at 2 5 km distances, although more data are necessary for representative values. It is also evident that the floors of the outflow systems are the most undulating among the observed valleys and channels. This can arise from differences in the sedimentary infill or in the original depth of bed scour, which could be larger at the outflow channels because of the likely higher discharge. [9] In some cases small reverse slope segments were observed with increasing altitude in flow direction. These changes are usually not larger than 100 m over 1 5 km distances. On Earth such phenomena are usually interpreted as pools, the result of differential erosion of the channel s bottom [Scheidegger, 1970] or differences in rock erodibility. On Mars these could be the signs of pools or later massmovements as well. In the Martian case, both the channel floor and the surrounding terrain often showed this tendency. Unfortunately the density of topographic data is too low to give statistical information on lithology or youth/maturity of fluvial erosion. In several cases it was difficult to define the top of the valleys walls and it was also not evident which parts of the system represent the original valley and which the water-filled channel. [10] Altogether 216 cross-sectional profiles were analyzed in Xanthe Terra. Some examples are visible in Figure 7. The shape of a cross-sectional profile is the result of fluvial erosion and mass movements together. In terrestrial literature, V-shaped profiles of fluvial valleys are said Figure 5. (top) Longitudinal and (bottom left and right) cross-sectional profiles and (bottom middle) their locations of Tyras Valles. Note (top left) the shallow upper reaches with nearly straight longitudinal profile with (bottom left) the shallow valleys, and (top right) the lower reaches with convex-up longitudinal profile with (bottom right) deeply incised valleys. (Image center: 9 N, 50.3 W.) 4of7
Figure 6. (top) Longitudinal and cross-sectional profiles and (bottom) their locations of Ochus Valles (left a, b, image center 7.5 N, 45.4 W) and Vedra Valles (right c, d, image center: 19.6 N, 55.5 W). to be younger than U-shaped ones [Scheidegger, 1970], implying more recent downcutting in the former case. On Mars, Baker and Partridge [1986] have found V-shaped profiles more often in the downstream reaches, which they attributed to a late stage of sapping along lower reaches, while Williams and Phillips [2001] found the opposite situation: V-shaped cross sections in upstream reaches. In the analysis of Tyras Valles presented here, V-shaped cross-sectional profiles are evident at upper and U-shaped profiles are found in lower reaches more frequently (Figure 5). At the Vedra Valles outflow channel, no such trend is visible (Figure 6). In the other analyzed channels, a weak tendency or none at all was observed, possibly because of the lack of ideally located cross-sectional profiles. On the basis of these results, in certain systems the shape can change substantially, and V- or U-shaped cross-sectional profiles can be found at several locations at the same system. These results are in agreement with Hynek and Phillips [2003], suggesting a strong dependence on the local lithological environment. [11] Smaller segments of cross-sectional profiles were also analyzed to get information on small-scale processes. Undercut banks or small inner channels were observed on the floors of several valleys (Figure 8). They were observed most frequently at Bahram and lower Tyras Valles, where 56% and 19% of the cross-sectional profiles showed depressions of small inner channels, respectively. At Bahram, some of them could be interpreted on the images of the Mars Global Surveyor Mars Orbiter Camera (MOC), Mars Odyssey Thermal Emission Imaging System (THEMIS) and even on Viking images in the western part of the system. Along these two valleys there were sections with and without signs of the small inner channels, suggesting that the small inner channels cannot be easily observed and that MOLA topography may not show them at every location where they occur. The lack of small inner channels at the observed outflow systems at the same terrain can be the result of differences in eolian infilling or differences between their formation. For example these small channels may have formed during the last part of the system s activity under long-term low discharge that was missing at outflow channels, possibly because of the negative slope gradient [Williams and Phillips, 1999] at some locations, or even changes in flow direction [Ori and Mosangini, 1998] inhibiting the flow of water during the last stage. [12] Correlations were considered between the crosssectional and longitudinal profiles and other characteristics of the valleys. At the Vedra Valles outflow channel (Figure 6) a possible connection can be interpreted between cross-sectional and longitudinal profiles. At the sections with a high slope angle in the longitudinal profile (e, n), the crosssectional profile showed deeper incision that is concordant to 5of7
Figure 7. Examples of cross-sectional profiles of various valleys with 5 vertical exaggeration. The profiles from left to right are ordered in the flow direction, and the numbers of MOLA tracks are also indicated. Note the differently shaped profiles at upper and lower Tyras Valles. not enough to reach a mature condition or the later sedimentary infill was smaller. At Tyras Valles, the upper and lower reaches show a different shape of longitudinal and cross-sectional profiles, suggesting a different origin probably at different period in Martian history. At upper reaches the cross-sectional profiles resemble the unnamed valleys at 13 N, 53 W suggesting they could form under similar conditions. At lower reaches the longitudinal profile showed a convex shape with deeply incised cross-sectional profiles relative to the upper reaches. At the other observed valleys, a variety of longitudinal profiles were observed, suggesting different evolution. [14] The shape of cross-sectional profiles shows variety within the same fluvial systems probably because of selective erosional and depositional processes in areas with different lithology and/or geomorphology. V- and U-shaped cross-sectional profiles were observed within the same system suggesting strong dependence of cross-sectional profiles on local environment. No general rule could be found in the analyzed cases regarding the location of U- and V-shaped sections along the valleys. The observed small inner channels suggest low-discharge flow episodes during the last part of several systems activity, excluding the outflow channels. Along Bahram and lower Tyras Valles there were sections with and without signs of the small inner channels, suggesting that MOLA topography may not show them at every location where they occur or that eolian infilling modified them. There is no correlation between the shape of cross-sectional profiles and the direction of curvature of the valleys, suggesting that the analyzed depressions are probably valleys and not dry riverbeds. [15] In addition to the geomorphic analysis, this work considered and tested the utility of detailed analysis of individual cross-sectional profiles. This technique requires much time and effort but gives good examples for the comparison of the geomorphology between valleys and channels that likely have different origin. As new data from the theory of fluvial erosion. A similar correlation could be observed only at a few locations in other valleys while deeply incised cross-profiles are present at many locations without substantial increase of observed slope angle. There is no correlation between the shape of cross-sectional profiles and the direction of curvature of the valley s long axis (left/right or straight), as is found in riverbeds on the Earth, suggesting that the analyzed depressions are probably valleys and not dry riverbeds, or substantial amount of eolian material has modified the cross-sectional profile. 4. Conclusions [13] The observed valleys and channels in Xanthe Terra provide insight into the great variety of fluvial and later geomorphic processes. Bahram Valles longitudinal profile shows a concave shape like that observed at mature rivers on Earth. The vertical undulations of the longitudinal profile of the valley floor were smaller than in the case of the other observed valleys, also suggesting longer and more effective erosion or stronger sedimentary infill by wind-blown sand. In the other systems, undulating profiles were observed suggesting the time and/or effectiveness of the erosion was Figure 8. Examples of small inner channels on the floor of some analyzed systems with 4x vertical exaggeration. Dots mark the positions of MOLA shots. Each profile was transformed to a corrected one perpendicular to the direction of flow. 6of7
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