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Each spring, jets of CO2 gas erupt from the Martian polar ice caps, carrying dark sand and dust high aloft, which fall back to the surface creating dark patches.

Over winter, a layer of frozen CO2 forms over the permanent ice caps, with dust and sand trapped underneath. Spring sunlight warms and sublimates the ice near the dust layer, creating pockets of pressurized gas, which erupt thru weak spots in the layer.
The artist's impression below imagines the scene.
https://themis.asu.edu/news/gas-jets-spawn-dark-spiders-and-spots-mars-icecap
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