Icarus 191 (2007) 132–140 www.elsevier.com/locate/icarus Three decades of slope streak activity on Mars N. Schorghofer a,∗ , O. Aharonson b , M.F. Gerstell b , L. Tatsumi c a Institute for Astronomy and NASA Astrobiology Institute, 2680 Woodlawn Drive, University of Hawaii, Honolulu, HI 96822, USA b Division of Geological and Planetary Sciences, MC 150-21, California Institute of Technology, Pasadena, CA 91125, USA c Department of Geology and Geophysics, 1680 East-West Road, University of Hawaii, Honolulu, HI 96822, USA Received 7 January 2007; revised 19 March 2007 Available online 5 May 2007 Abstract Slope streaks are surficial mass movements that are abundant in the dust-covered regions of Mars. Targeting of slope streaks seen in Viking images with the Mars Orbiter Camera provides observations of slope streak dust activity over two to three decades. In all study areas, new and persisting dark slope streaks are observed. Slope streaks disappeared in one area, with persisting streaks nearby. New slope streaks are found to be systematically darker than persisting streaks, which indicates gradual fading. Far more slope streaks formed at the study sites than have faded from visibility. The rate of formation at the study sites was 0.03 new slope streaks per existing streak per Mars year. Bright slope streaks do not presently form in sudden events as dark slope streaks do. Instead, bright streaks might form from old dark slope streaks, perhaps transitioning through a partially faded stage.  2007 Elsevier Inc. All rights reserved. Keywords: Mars, surface; Mars, atmosphere 1. Introduction Slope streaks are dark narrow fan-shaped features extend- ing downslope, seen in equatorial, high albedo, dust covered regions of Mars (Sullivan et al., 2001; Schorghofer et al., 2002). They were first found in images taken by the Viking Orbiters as early as 1977 (Morris, 1982; Ferguson and Luc- chitta, 1984), but at the time it was unknown they were ac- tively forming. New slope streaks were recognized in overlap- ping images acquired by the Mars Orbiter Camera (MOC) on- board the Mars Global Surveyor spacecraft (Edgett et al., 2000; Sullivan et al., 2001). Since then, overlapping MOC/MOC im- age pairs have been extensively studied to determine the rate of formation (Aharonson et al., 2003). To date, over 200 new slope streaks have been detected and documented. Fig. 1 shows a par- ticularly dramatic example of a new slope streak. While many new streaks have been documented, no disappearing streaks had ever been observed. * Corresponding author. Fax: +1 808 956 7264. E-mail address: norbert@hawaii.edu (N. Schorghofer). The nature of slope streaks remains uncertain, but they are commonly interpreted as dust avalanches (Sullivan et al., 2001). Recently discovered terrestrial analogs in the Dry Valleys of Antarctica contain moisture (Head et al., 2007). Miyamoto et al. (2004) inferred the bulk viscosity and bulk yield strength of anastomosing slope streaks must be extremely low. Baratoux et al. (2006) present evidence that slope streaks form where the dust cover is thick, and the thickness of the dust cover is in turn related to wind direction. The present work pertains mainly to streak fading over the past two and a half decades since the Viking observations. 2. Viking/MOC overlaps: Three decades of change 2.1. Study sites Seeking slope streaks, we surveyed 7887 Viking Orbiter im- ages with a spatial resolution finer than 100 m per pixel. Images from latitudes between 25 ◦ S and 45 ◦ N and all longitudes where slope streaks are known to occur were included in the survey. Images flagged for likely slope streaks were revisited several times. The final result of this survey was a list of Viking images with unambiguous dark slope streaks, shown in Table 1. 0019-1035/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2007.04.026