What are the seasonal dark streaks observed on some Martian slopes called?

The seasonal dark streaks observed on some Martian slopes are called recurring slope lineae. Often abbreviated RSL, they appear on steep, warm-facing slopes, lengthen during the warmer season, and fade during colder periods.

RSL are narrow, downhill markings that can extend for hundreds of metres. Their seasonal behaviour originally made them an important possible sign of liquid water on present-day Mars. Researchers proposed that salty brines might briefly flow or seep near the surface, leaving darker tracks as they moved.

The water interpretation is not settled. Later observations found that dry granular flows, involving dust or sand destabilized by heating, can also explain important features of RSL. They should not be confused with ordinary dark slope streaks, which are generally broader, less strongly seasonal, and commonly interpreted as dry dust avalanches. The related term “recurrent slope lineae” is also used, but “recurring slope lineae” is a standard name.