"Each spring, they report, "gray fuzzy spots" appear in the bottom of the ice cover. By the middle of the first half of spring, these spots become darker, are bounded, and grow in size. By early summer defrosting, the naked dark soil of the dune is visible, and surrounded by a lighter ring.

Year by year, the dark dune spots "renew" on the same place with almost the same configuration, or "constellation" of patches. This repeat action, the team asserts, strengthens their suggestion of fixed, biological causes of spot formation."

Some Mars Researchers See Life in Planet's Dunes

I don't believe for a second it's life, but what geological (areological?) processes could cause these formations?