Two pulses of seasonal activity in martian southern mid-latitude recurring slope lineae (RSL)

Recurring slope lineae (RSL) are narrow dark features that incrementally lengthen down steep low-albedo slopes when temperatures are warm, subsequently fade, and reoccur annually. RSL could be caused by wet, wet-triggered debris, or dry granular flow mechanisms. We have classified 190 candidate and confirmed southern mid-latitude (SML) RSL sites using six Mars years of high-resolution imagery to provide new constraints on RSL flow mechanisms. We demonstrate that at least three confirmed SML RSL sites have at least two pulses of active lengthening. We have also confirmed that RSL start much earlier than previously thought. The first SML RSL pulse occurs from solar longitude (L-s) 187 degrees +/- 6 degrees to 226 degrees +/- 14 degrees or 62 +/- 32 sols, while the second pulse occurs from L-s 260 degrees +/- 26 degrees to 329 degrees +/- 29 degrees or 113 degrees +/- 89 degrees sols. Even with the newly observed early pulse, the total SML RSL active duration is still shorter than RSL found elsewhere on the planet. No significant seasonal variations exist between the dominant (N, NW, W) slope-face orientations. The central peak of Hale crater appears to be anomalous in that it has a third pulse that rejuvenates RSL late in the southern summer (L-s 330 degrees +/- 1 degrees to 348 degrees +/- 19 or 48 degrees +/- 26 degrees sols). These additional RSL pulses of activity add increased complexity to RSL behavior. Follow-up missions may be needed to unambiguously discriminate between RSL mechanisms. (C) 2017 Elsevier Inc. All rights reserved.