Where does the dust on Mars come from?

The movie “The Martian” begins with the hero being separated from his crew by a dust storm and then being left behind, presumed dead, alone on Mars. In fact, because of its very thin atmosphere, a storm on Mars would feel like a light breeze on Earth and would definitely not have the force to knock over a spaceship. But Martian storms could certainly produce problems, because they would darken the sky, and this would make generating energy from sunlight no longer possible.

But where does the dust come from that is currently covering almost all of Mars?

Weathering that grinds stone to dust on Earth has not occurred on Mars for billions of years and meteorite impacts would not produce such fine grains of dust from the soil. In search of a source for this dust, Mars scientists looked at the composition of the dust and found it has a very specific ratio of sulfur and chlorine. This ratio pointed to only a single region as a possible source: the Medusae Fossae Formation, which extends approximately 5000 kilometers (3100 miles) along the Mars equator. On radar images it can be seen that the formation consists primarily from volcanic dust – the largest known volcanic deposit in the Solar System. Over time, the wind has carried off and distributed some of the dust – so much that the dust could form a layer two to twelve meters thick over the entire planet.

Part of the Medusae Fossae, which has been sculpted by billions of years of eolian (wind) erosion into its current appearance (picture: NASA/JPL/University of Arizona)
The sky darkens during a dust storm (picture from the Curiosity Rover, picture: NASA/JPL-Caltech/MSSS)

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