Scientists have been trying to figure out what forms dark streaks trailing down the slopes of some Martian sand dunes, and they think they now have the answer.
The Wall Street Journal says the created the same Martian conditions in a lab, simulating the thin atmosphere on Mars.
In a thin atmoshpere, water either freezes or boils.
The strongly suspect that the same phenomenon is occuring on Mars.
In the lab experiment, the water boils and forces the sand to bubble up and flow downward along the slope, forming the streaks.