Sustained wet-dry cycling on early Mars.

The presence of perennially wet surface environments on early Mars is well documented 1,2 , but little is known about short-term episodicity in the early hydroclimate 3 . Post-depositional processes driven by such short-term fluctuations may produce distinct structures, yet these are rarely preserved in the sedimentary record 4 . Incomplete geological constraints have led global models of the early Mars water cycle and climate to produce diverging results 5,6 . Here we report observations by the Curiosity rover at Gale Crater indicating that high-frequency wet-dry cycling occurred in early Martian surface environments. We observe exhumed centimetric polygonal ridges with sulfate enrichments, joined at Y-junctions, that record cracks formed in fresh mud owing to repeated wet-dry cycles of regular intensity. Instead of sporadic hydrological activity induced by impacts or volcanoes 5 , our findings point to a sustained, cyclic, possibly seasonal, climate on early Mars. Furthermore, as wet-dry cycling can promote prebiotic polymerization 7,8 , the Gale evaporitic basin may have been particularly conducive to these processes. The observed polygonal patterns are physically and temporally associated with the transition from smectite clays to sulfate-bearing strata, a globally distributed mineral transition 1 . This indicates that the Noachian-Hesperian transition (3.8-3.6 billion years ago) may have sustained an Earth-like climate regime and surface environments favourable to prebiotic evolution.