Aganippe Fossa is a surface feature

Fossa (geology)

Fossa is a term used in planetary geology to describe a long, narrow depression on the surface of an extraterrestrial body, such as a planet or moon...

 on Mars

Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 which runs from 4.1° to 13° south latitude and 124.9° to 126.9° west longitude. It is named after a classical albedo feature

Albedo feature

An albedo feature is a large area on the surface of a planet which shows a contrast in brightness or darkness with adjacent areas....

. Its name was approved by the IAU in 1976.

Figure 1 shows dark streaks on the slopes of Aganippe Fossa. Such streaks are common on Mars. They occur on steep slopes of craters, troughs, and valleys. The streaks are dark at first. They get lighter with age. Sometimes they start in a tiny spot, then spread out and go for hundreds of meters. They have been seen to travel around obstacles, like boulders. It is believed that they are avalanches of bright dust that expose a darker underlying layer. However, several ideas have been advanced to explain them. Some involve water or even the growth of organisms. The streaks appear in areas covered with dust. Much of the Martian surface is covered with dust. Fine dust settles

Mars dust settling

Martian soil is the fine regolith found on the surface of Mars. Its properties can differ significantly from those of terrestrial soil.The term Martian soil typically refers to the finer fraction of regolith, that which is composed of grains one centimeter in diameter or less...

out of the atmosphere covering everything.