|Streaks on Horowitz (impact) Crater, Mars|
The dark features are dubbed "recurring slope lineae" (RSL)
The tails of the arrows are about 20 meters in length.
An excellent animation of these features can be found in Richard Kerr's ScienceNow blurb "Is Mars Weeping Salty Tears?"
The features of interest, the dark lines in the photo, are narrow (0.5-5 meters) marks on steep (25-40 degree) slopes. Repeated images taken by the Mars Reconnaissance Orbiter HiRise instrument shows that these features appear when the warm season starts, grow downslope (toward the XXX in this photo), attaining lengths of 100 m. When the warm season (250-300 K) passes, the marks fade; the orange streaks in the photo near the arrows may be older faded RSL's.
The RSL's only occur in the southern hemisphere, and between latitudes 32-48 S. They tend to form on equator-facing slopes. There are seven confirmed locations, and a couple dozen candidate sites. They extend downslope from rocky areas (toward the upper right in the adjacent photo), attaining lengths of hundreds of meters. More than 1000 lineae may be present in a particular site.
Individual lineae may split or merge. They terminate on steep slopes, leading to the conclusion that the volume of material to form them is limited. Growth rates vary from 0-20 m/day on average. There can be no activity on some for weeks.
McEwen et al. consider a number of mechanisms, including CO2 sublimation, and conclude that the seasonal behavior strongly suggests surface expression of briny water. The association with bedrock outcrops could indicate control of the subsurface migration of the fluids by the bedrock structure, or that the bedrock contains hygroscopic salt-rich lenses to provide the salts to lower the melting temperature of ice. The text of the article examines other hypotheses in detail.
**McEwen et al., Seasonal Flows on Warm Martian Slopes, Science, 333(6043) 740-743, 2011. There may be hundreds to a thousand at one site.