As you know, Bob, a ragged pattern of kilometer-deep canyons spiral through the ice cap around the north pole of Mars. A neat short paper by geologist Jon Pelletier ("How Do Spiral Troughs Form on Mars?", Geology 32 (2004): 365--367; link) works through the basic physics, which has to do with the sublimation of ice into vapor on the sunnier side of an incipient crack, and its condensation on the darker, and shows that it leads, in reduced form, to one of the classic models of excitable media (the FitzHugh-Nagumo equations, for those who like to keep score). Simulations show a pretty convincing set of imperfect spiral waves. Those who don't want to (or can't) read the paper should still check out Pelletier's page on the spirals, which has full-scale versions of the tiny figures in the paper (plus errata). Clearly, I need to go to Mars to further my research. (Via Apothecary's Drawer, who gives many links to other examples of excitable media.)