4.1 Factors Contributing to Widespread RSL

The study region contains vastly more RSL sites than the other documented locations on Mars. For example, 13 confirmed sites were reported for the southern midlatitudes [Ojha et al., 2014], 4 in the northern midlatitudes [Stillman et al., 2016a], and 5 equatorial locations not in Valles Marineris [McEwen et al., 2014], as compared with 23 described herein. An independent assessment of the global inventory of detections estimates ~50% of RSL sites to be located in Valles Marineris [Stillman et al., 2016a].

The RSL within Coprates and Melas are also detected within diverse and widespread geologic environments. Craters, landslides, canyon walls, and, possibly, duneforms all host these RSL, including zones thought to be source areas, although it is unclear if they all form from identical mechanisms. The 41 sites identified by HiRISE are very densely populated with active RSL commonly with CTX candidates in the adjoining regions (Figure 2). These locations occur over a broad elevation distribution (−5.1 km to +3.7 km) spanning a greater range than most other RSL localities on Mars. For example, southern midlatitude sites range from −1.5 km to +2 km in elevation [Ojha et al., 2014]. One partially confirmed site is located on the floor of Hellas at −6.4 km (see ESP_040443_1345), so this may simply reflect the relatively uniform high elevation of the highlands. The one clearly relevant factor is the abundance of steep slopes in Valles Marineris (see below). This broad‐scale spatial and vertical distribution may suggest that environmental conditions and/or other surface properties (e.g., albedo, composition, and presence of salts) are also more conducive to RSL formation and recurrence than other global locations. Regardless, the elevation distribution of RSL within eastern Coprates falls in the range of wall stratigraphy earlier spectral analysis has described as diverse, with erosion‐resistant, high‐calcium pyroxene‐dominated, basaltic material (+2 km to −1.5 km), weakly layered chlorite‐ or iron‐smectite‐bearing material (−1.5 km to −2.8 km), overlaid on massive, low‐calcium pyroxene‐dominated mafic units (<−2.8 km) [Murchie et al., 2009; Ehlmann et al., 2011].

Why conditions in Coprates and Melas Chasmata are so conducive to RSL formation is unclear. These canyon systems contain large expanses of steep slopes and roughness values detected at multiple scales, which are greater than most locations on Mars [Kreslavsky and Head, 1999; Smith et al., 2001; Chojnacki et al., 2014b; Jaumann et al., 2015]. RSL are known to be located only on steep slopes [McEwen et al., 2011; Ojha et al., 2014; Dundas et al., 2016], yet many regions on Mars, including western Valles Marineris, have steep slopes but are higher albedo (dust mantled) and largely lack detected RSL. Slope streaks are present, and we have monitored several areas but have not seen the temporal behavior of RSL. Valles Marineris, particularly the eastern chasmata, are also known to show some of the lowest albedo locations on Mars [Smith et al., 2001; Putzig and Mellon, 2007; Chojnacki et al., 2014b]. This low albedo is partly due to numerous dark dune fields, sand sheets, and fans populating the canyon floors and walls [Chojnacki et al., 2010, 2014c]. These extensive “blankets” of dark sand that frequent the midwall locations of Coprates and Melas may play a role in RSL formation. For example, these relatively low thermal inertia sand materials will rapidly respond to early morning sunlight and heat up faster and to greater absolute temperatures [Putzig and Mellon, 2007; Putzig et al., 2014] than talus or bedrock surfaces and would provide a thermal dichotomy with those contacting surfaces. The frequent detection of RSL at these sand‐bedrock or sand‐talus contacts may imply some sort of thermal control. Another contributing factor may be the favorable geometry of parallel rows of spurs and ridges providing multiple slope aspects, some of which will be ideally suited for RSL formation in a relatively small region (e.g., Figures 2 and 3).

This region is also prone to a number of atmospheric phenomena. Both the 2001 and 2008 global dust storms crossed the Valles Marineris system [Smith et al., 2002; Cantor, 2007; Wang and Richardson, 2014]. This is relevant as RSL activity was more extensive and prolonged during the 2007 (MY28) global dust storm [McEwen et al., 2014]. There is also a yearly Acidalia track dust storm system that can enter the canyons [Cantor et al., 2001, 2014] and has been correlated with notable darkening of RSL fans (e.g., MY31 [McEwen et al., 2014]). During these storms, dust storm tracks intersect with Valles Marineris (e.g., Syria‐Claritas and Acidalia) and variable‐altitude dust clouds can obscure troughs entirely for a short period [Cantor, 2007]. Large‐scale, annual recurring cloud trails emerging out of Valles Marineris have also been detected and appear to be unique to the region [Clancy et al., 2009]. These distinctive features are interpreted as high‐velocity (up to 40 m/s) thermally driven updrafts that formed along canyon walls composed of dust and water ice particles [Clancy et al., 2009].