FADING OF SLOPE STREAKS ON MARS OVER THREE DECADES: A COMPARISON OF CTX AND VIKING IMAGES

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1 FADING OF SLOPE STREAKS ON MARS OVER THREE DECADES: A COMPARISON OF CTX AND VIKING IMAGES Kimberly M. Rottas Department of Geology and Geophysics University of Hawai i at Mānoa Honolulu, HI ABSTRACT Slope streaks are down-slope mass movements on the surface of Mars that are among the few known examples of contemporary geologic activity on Mars. Slope streaks are seen in rare high-resolution Viking images that were obtained in the 1970s by the Viking Orbiters. A previous space-grant project (with Lisa Tatsumi) studied changes of these streaks in Mars Orbiter Camera (MOC) images and led to the discovery that dark slope streaks fade with time and that they do so at much slower rates than they form. This project studies the fading of slope streaks between overlapping images taken by the Context Camera (CTX) on board Mars Reconnaissance Orbiter and Viking images. This study reveals that the fading rate of slope streaks is nearly equivalent to the rate of formation and provides strong evidence that streaks fade gradually over time on a slope by slope basis rather than suddenly in rare, large scale events. Slope streaks are relatively small areas of darker (or in rare cases, brighter) contrast than the surrounding slopes, typically less than 3km in length and less than 200 m wide (Sullivan et al. 2001, Aharonson 2003, Schörghofer et al. 2007). They often exhibit a pointsource morphology with a tail which widens downslope (Fig. 1) and are generally believed to be erosional features created by dust avalanches (Sullivan et al. 2001). Dark streaks are believed to be young and form in sudden events while bright streaks are interpreted as being relatively old, brightening and fading with time as dust settles INTRODUCTION Fig. 1: A portion of CTX image B19_016926_2006 showing slope streaks with point-source morphology. North is up and illumination is from upper left. from the atmosphere or gradually moves downslope (Schörghofer et al. 2007). There is no evidence of bright streaks forming suddenly, although some MOC images reveal dark streaks overlying bright streaks (Schörghofer et al. 2007). All slope streaks are located in the areas of

2 high albedo and low thermal inertia. Also, they are concentrated in relatively low latitudinal dust-covered areas (Sullivan et al. 2001). A previous space-grant project studied changes of slope streaks in targeted Mars Orbiter Camera (MOC) images and led to the discovery of the formation of tens of new dark slope streaks on the Martian surface and a few instances of dark streaks that had faded (brightened) with time (Schörghofer et al. 2007). This comparison between MOC and Viking images reports a rate of streak formation of ~3%/Mars year and a fading rate an order of magnitude less than that of formation, indicating that the number of slope streaks on the surface of Mars is increasing with time (Schörghofer et al. 2007). However, there was a deficit of streaks that had disappeared (~5% of the total number of streaks reported over 14 Mars years). This project provides a larger dataset to place more accurate constraints on the time required for a streak to fade by comparing the fading of slope streaks in overlapping CTX and Viking images. This comparison has allowed us to observe morphological changes and to obtain more accurate rates of formation and fading of the slope streaks. Additionally, CTX-Viking comparisons of images allow us to make observations of slope streaks over a longer time period than comparisons of MOC images with Viking images or other MOC images. VIKING/CTX OVERLAPS Seeking unambiguous instances of faded slope streaks, 41 Viking Orbiter images with a spatial resolution of finer than 60 meters per pixel where slope streaks were known to occur were initially surveyed. Twenty-eight of these Viking images were excluded from the final comparison as their resolutions were not high enough to allow for accurate comparison to CTX images. The 13 remaining images covered an area of ~275 km 2 of the Lycus Sulci region near Olympus Mons, from latitudes between 27.5 S and 28.1 N and longitudes between E and E. This site was chosen to be studied in detail due to the very high resolution (<10 m per pixel) of the Viking images and the abundance of dark slope streaks. The selected Viking images were compared to CTX images as shown in Table 1. Both Viking and CTX images were map projected to allow for easy comparison and Viking images were compiled to create a mosaic using Adobe Photoshop. Individual streaks were revisited multiple times in between each Viking and CTX image as well as between the composite Viking image and corresponding CTX images. Only those streaks that are unambiguous are included in the final results. Additionally, Viking images 441B02, 441B04, and 441B06 were identified as having a slope streak that appears to have partially faded over the course of 30 Earth years, or 16 Mars years. Table I: List of study area images and corresponding CTX images Viking Orbiter Image ID 441B01 441B09 441B10 441B13 CTX Image ID B19_016926_2066, P05_003160_2078 B02_010346_2072 Steaks were compared between Viking images (provided they were present in multiple images) as well as with multiple CTX images covering the same area. This was necessary to reduce ambiguity as the images were taken at various incidence angles at different times of day and year. This resulted in a range of apparent contrasts between streaks and the surrounding

3 slopes, even among Viking image overlaps. For this reason, only streaks that were completely gone in the corresponding CTX image were counted as having faded. If any part of the streak remained it was counted as persisting, regardless of how much lighter it appeared in the CTX image than in the Viking image(s). In addition, new streaks formed in close proximity, or perhaps even on top of previous, persisting streaks, thus making an accurate count difficult to achieve. Figure 2 shows a streak that appears to have faded from both the head and the tail while it persisted in the middle. This observation supports the theory that steaks fade gradually over time due to dust settling out from the atmosphere or moving gradually down-slope rather than in rare, large-scale events such as a dust storm. Fig. 2: a) Viking image 441B06. Points A F mark the heads of faded (disappeared) streaks. P indicates head of persisting streak. Black box shows partially faded streak. b) CTX image P05_003160_2078. N indicates head of newly formed streak. Point P and black box same as Fig. 2a. Illumination is from upper left. CHANGES OVER 30 YEARS OF SLOPE STREAK ACTIVITY The Viking-CTX comparison yielded a total of 77 unambiguous streaks, of which 20 persisted, 34 faded, and 23 were newly formed (Fig. 3). By dividing the number of new streaks (Δn) by the average number of slope streaks multiplied by the time elapsed between images in Mars years, a formation rate of ~3.0%/Mars year was calculated. Similarly, a fading rate of ~4.4%/Mars year was calculated by replacing Δn with the total number of faded streaks. When slightly ambiguous cases were included, the total number of streaks jumped to 185, 62 of which persisted, 75 faded, and 48 were newly formed. These numbers yield a formation rate of ~2.4%/Mars year and a fading rate of ~3.8%/Mars year. In both cases, the fading rate is consistent with previous studies, although the rate of fading is approximately an order of magnitude greater than that reported by Schörghofer et al. (2007). The most noteworthy result of this study is that, for the first time, the rate of formation

4 and rate of fading are nearly equal, revealing that the number of slope streaks on the surface of Mars is relatively constant rather than increasing with time, as previous studies had indicated. Fig 3: Map projected composite of Viking images 441B02 441B05 and 441B09. Illumination is from the upper left.

5 LARGE-SCALE OBSERVATIONS In addition to observing changes in individual slope streaks between Viking and CTX images, several large-scale observations were made. First, streaks may be isolated or form in groups. Larger regions of fading or persisting streaks were outlined by marking the heads of faded or persisting streaks in a map projected composite of Viking images and shading areas containing only one or the other type of streak (Fig. 4). Shaded regions are well constrained vertically, between topographic highs and topographic lows, though the lateral distance covered is difficult to determine. Regions with very few slop streaks or areas with both faded and persisting streaks were not shaded as there was insufficient data to draw concrete conclusions. From looking at the distribution of these shaded regions, it is apparent that fading in groups occurs on a slope-by-slope basis rather than over large regions. This provides further evidence that fading occurs gradually instead of sudden disappearance during large-scale events. Furthermore, regions of fading generally appear to be larger than regions of persistence. There does not seem to be preferential fading or persistence depending on whether the slope is facing the sun or is mostly shaded. COMPARISONS WITH PAST STUDIES As mentioned previously, the formation rate of slope streaks reported in this study is consistent with those reported in Fig 4: Map projected composite of Viking images 441B01 previous studies (Schörghofer et al. 441B05 and 441B09. Illumination is from the upper left. 2007). However, this comparison Green: regions of persisting streaks, red: regions of faded shows a much greater number of faded streaks, white lines: topographic lows, black lines: streaks, 55% and 63% for topographic highs. unambiguous counts and those including slightly ambiguous cases, respectively, as opposed to a previously reported 5% (Schörghofer et al. 2007). As a result, the rate of fading calculated here is much higher than previously reported. For the first time we see an approximate balance between fading and persisting streaks, indicating the number of streaks on Mars is relatively constant.

6 This study may reveal a greater number of slope streaks than previously described in studies between Viking and MOC images because CTX has a much larger field of view than MOC, thus allowing for a larger study site. Additionally, there is a greater time difference between Viking and CTX images than there is between Viking and MOC images: 30 Earth years, or 16 Mars years, rather than 25 Earth years (13 Mars years). The greater time elapsed between images may make a significant difference in number of faded streaks, particularly if the typical lifespan of a slope streak is only slightly greater than the time between Viking and CTX images. CONCLUSION While further study of multiple sites would be beneficial and provide a better overall picture, we are limited by the lack of older images with fine enough resolution to draw accurate comparisons with more recent high resolution images. However, with the results of this Viking- CTX comparison, we can say with confidence that slope streaks fade faster than previously thought. We may also conclude that, as formation rates are approximately equivalent to fading rates, that the number of streaks on Mars is constant rather than increasing with time. Furthermore, the discovery of a partially faded slope streak, in conjunction with supporting evidence that streaks fade on a slope-by-slope basis rather than over large areas, substantiates the theory that streaks fade gradually with time as opposed to suddenly during rare, large-scale events. REFERENCES Aharonson O, Schörghofer N, and Gerstell MF (2003), Slope-streak formation and dust deposition rates on Mars. J. Geophys. Res. E 108, Schörghofer N, Aharonson O, Gerstell MF, and Tatsumi L (2007), Three decades of slope streak activity on Mars. Icarus 191(1), Sullivan R, Thomas P, Veverka J, Malin M, and Edgett KS (2001), Mass movement slope streaks imaged by the Mars Orbiter Camera, J. Geophys. Res. E 106,