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We describe in detail an annual seasonal process that occurs on the surface of the Russell Crater megadune on Mars. We give these features the name ‘perennial rills’, because their surface topographical expression persists from year-to-year and they form a distinctive, downstream-branching network of small channels, or rills. We used time-series images, elevation data from stereophotogrammetry and spectral data to characterize the evolution of these features over 6 Mars years. Growth and modification of these networks occurs abruptly in spring (at a solar longitude of c. 200°) after most of the seasonal CO2 ice has sublimated. We find that the peculiar morphology of perennial rills seems to be the only aspect that sets them apart from active linear dune gullies. By comparison to terrestrial analogues, we identified two conditions favouring the production of such a network: (a) the presence of an impermeable layer; and (b) the repeated formation of obstacles in front of propagating channels. We find that the most plausible formation mechanisms that can explain the formation of both the perennial rills and the active linear dune gullies are levitating CO2 blocks or liquid debris flows of water/brine, but neither can completely satisfy all the observational evidence.

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