Mars' mysterious dark streaks continue to intrigue and puzzle scientists. Over the past several years, Mars-orbiting spacecraft have spotted dark, fingerlike features that scientists call "recurring slope lineae," or RSL for short. These are dark flows, possibly caused by liquid water, found at many locations throughout low and middle latitudes on Mars. More than 120 candidate and confirmed RSL sites have been discovered, with some sites containing 1,000 or more individual RSL.
Yet another eye-catching manifestation on Mars is the "slope streak," which can propagate down steep slopes on the Red Planet. Slope streaks could be caused by dust avalanches, scientists say, and they differ from RSL in morphology, setting and mode of occurrence.