Mystery of the Martian Spirals

Mariner 9 image of the north polar cap of Mars. The image was taken on 12 October 1972, about one-half Martian month after summer solstice, at which time the cap had reached its minimal extent. The cap is about 1000 km across. The interior dark markings are frost-free sun-facing slopes. A smooth layered sedimentary deposit underlies the cap. The image is centered at 89 N, 200 W. (Mariner 9, MTVS 4297-47)