Dating martian climate change

1 Geological evidence indicates that low-latitude polygonally-patterned grounds on Mars, 2 generally thought to be the product of flood volcanism, are periglacial in nature and record a 3 complex signal of changing climate. By studying the martian surface stratigraphically (in terms 4 of the geometrical relations between surface landforms and the substrate) rather than geneti5 cally (by form analogy with Earth), we have identified dynamic surfaces across one fifth of mar6 tian longitude. New stratigraphical observations in the Elysium-Amazonis plains have revealed a 7 progressive surface polygonisation that is destructive of impact craters across the region. This 8 activity is comparable to the climatically-driven degradation of periglacial landscapes on Earth, 9 but because it affects impact craters – the martian chronometer – it can be dated. Here we show 10 that it is possible to directly date this activity based on the fraction of impact craters affected by 11 polygon formation. Nearly 100% of craters (of all diameters) are superposed by polygonal sculp12 ture: considering the few-100 Ma age of the substrate, this suggests that the process of polygon 13 formation was active within the last few million years. Surface polygonisation in this region, of14 ten considered to be one of the signs of young, 'plains-forming' volcanism on Mars, is instead 15 shown to postdate the majority of impact craters seen. We therefore conclude that it is post16 depositional in origin and an artifact of thermal cycling of near-surface ground ice. Stratigraphi17 cally-controlled crater counts present the first way of dating climate change on a planet other 18 than Earth: a record that may tell us something about climate change on our own planet. Paral19 lel climate change on these two worlds – an ice age Mars coincident with Earth’s glacial Quater20 nary period – might suggest a coupled system linking both. We have previously been unable to 21 generalise about the causes of long-term climate change based on a single terrestrial example – 22 with the beginnings of a chronology for climate change on our nearest planetary neighbour, we 23 can. 24