I have a new story up on the main Nature News site about the mystery of the polygons on Mars, which are analogous to the polygons found in the Arctic and Antarctic on Earth. Mike Mellon thought he had them all figured out — he had a model that perfectly explained the five meter polygons he was seeing from Mars Reconnaissance Orbiter’s HiRise camera (pictured here). Then Phoenix landed and they were too small. Strangely, Mellon also sees the faint imprint of really large polygons, maybe more than 20 meters across. One idea is that each polygon size reflects the freezing mechanics of a different climatic epoch. The most prominent polygons reflect the active climate, while the receding ones are the half-erased remnants of climates of yestersol (a Mars-mad neologism that I heard a JPL engineer use yesterday). The surface of Mars would therefore be a palimpsest (perhaps my favorite word in the English language).