Lunar and Planetary Science XXXIII (2002) 2019.pdf

THE CENTRAL MESAS OF HEBES, GANGES, AND EAST CANDOR CHASMAE. J. A. Jernsletten, Rice University Dept. of Geology and Geophysics, 1917 Florida Dr., Seabrook, TX 77586-2985, [email protected].

Introduction: This paper presents a comparative Discussion: Figure 1 shows the DTM of Hebes study of the topographies of the central mesas in Hebes Chasma, Figure 2 shows the Ganges Chasma DTM, Chasma, Ganges Chasma, and East Candor Chasma. It and Figure 3 is the DTM of East Candor Chasma. is proposed that among the main factors in the post- These canyons are similar in width (~ 125 km), and all formation evolution of these central mesas are; fault- have central mesas located within them. The figures ing, erosion, mass wasting [8], tectonic processes [2, 4- have scale bars for reference, and the vertical shaded 7], geological structure and stratigraphy [3, 4-7], and bars indicate where the profiles in the following figures weathering and surface temperature [4-7]. All three of are drawn. Note that each north-south profile is these canyons are generally east-west trending [1]. The smoothed by averaging several profiles located at a data in this study is Digital Terrain Models (DTMs) range of longitudes together (the width of the vertical derived from Viking Orbiter 1 images used as stereo shaded bars show the range of longitude over which pairs. The mesas have roughly similar cross-sectional each summed profile is averaged). topographies and surface textures, indicating similar formation or post-formation evolution processes.

DTM Left Img. Right Img. Lat. Long. Hebes f645a60 f682a27 0.0° 77.5° Ganges f649a59 f610a13 -8.4° 49.4° E. Candor f647a55 f608a75 -9.5° 65.5° Table 1. Center Locations of the Canyons.

Method: We study the DTMs by drawing profiles across a portion of each DTM that contains the mesa with which we are concerned, and comparing those profiles, as well as comparing the DTMs themselves. Table 1 shows the latitude and longitude of the center Figure 2. Ganges Chasma DTM. of each canyon. For reference, the Viking Orbiter 1 image numbers for the images comprising the stereo Figures 4, 5, and 6 show the profiles drawn across pair for each canyon are also shown. Hebes Chasma is Hebes, Ganges, and East Candor Chasma, respectively. located to the north of Candor Chasma (central Valles We see that Hebes to the north of the mesa is nearly 7 Marineris), East Candor Chasma is located to the north km deep, while the south side is about 5 km deep. of Melas Chasma and Coprates Chasma, and Ganges Ganges Chasma is somewhat shallower than Hebes (~ Chasma is located north of Capri Chasma, at the far 4 km in the north, ~ 4.5 km in the south), while East eastern end of Coprates Chasma and Valles Marineris. Candor Chasma is roughly a kilometer deeper than Hebes (~ 8 km in the north, ~ 4 km in the south).

Figure 1. Hebes Chasma DTM. Figure 3. East Candor Chasma DTM. Lunar and Planetary Science XXXIII (2002) 2019.pdf

CENTRAL MESAS OF HEBES, GANGES, AND EAST CANDOR: J. A. Jernsletten

The textures and structures of the slopes also seem to indicate faulting, erosion, and mass wasting as good candidate processes at work in the post-formation evo- lution of these central mesas. Figure 7 shows the stereo image pair for Hebes Chasma, and gives a view of the texture and surficial structure of the canyon and the central mesa within it. Its top is fairly flat, suggesting that the layered deposits were there first, and the sur- rounding material was eroded away to from the mesas, like mesas observed on Earth. Conclusions: The central mesas in Hebes Chasma, Ganges Chasma, and East Candor Chasma show strong similarities, and likely have similar processes at work Figure 4. Profile of Hebes Chasma. in post-formation evolution of their topographies. This

paper suggests that faulting, erosion, and mass wasting Processes such as aeolian, volcanic, mass wasting, are the most probable processes active in the post- and lacustrine deposits have been proposed for the formation evolution of the mesas. initial formation of these interior layers [8]. It seems

likely that faulting, erosion, and mass wasting are the most probable processes for post-formation evolution of the slopes of these mesas. The profiles of these can- yons with their central mesas shown in this paper seem to fit that hypothesis.

Figure 7. Hebes Chasma Stereo Pair.

References: [1] Lucchitta B. K. et al. (1992) Figure 5. Profile of Ganges Chasma. Mars, 453-492. [2] Schubert G. et al. (1992) Mars, 147-183. [3] Tanaka K. L. et al. (1992) Mars, 345-382. [4] Abramson L. W. et al. (2002) Slope Stability and Stabilization Methods. [5] Andersland O. B. and Ladanyi B. (1994) An Introduction to Frozen Ground Engineering. [6] Davis N. (2001) Permafrost - A Guide to Frozen Ground in Transition. [7] Kliche C. A. (1999) Rock Slope Stability. [8] Weitz C. M. (1999) LPS XXX, 1344–1345

Figure 6. Profile of East Candor Chasma.