Summary

The outflow channels that emptied into Chryse Planitia provide the best evidence that great quantities of water once flowed on the Martian surface. Some channels were created when the cryosphere ruptured and ground-water discharged from chaos or from cavi along major fault zones. Some chaos formed on channel floors when fluvial erosion thinned the cryosphere, leading to catastrophic breakout of confined groundwater. These chaos can be used to estimate the cryosphere thickness, crustal heat flux and climate trends. At Iamuna Chaos the cryosphere was 700–1000 m thick when Ravi Vallis formed, indicating a cold, long-term climate similar to present-day Mars. The discovery of outflow channels at elevations >2500 m in Ophir Planum shows that Hesperian recharge likely occurred in upslope areas to the west (e.g. Sinai Planum, Tharsis highlands, Syria Planum). The larger circum-Chryse channels were carved by floods that issued directly from the ancestral canyons, which likely were smaller and less interconnected than today. A plausible mechanism for water release was catastrophic drainage of chasm lakes caused by the collapse of topographic barriers or ice-debris dams. We report evidence that a megaflood filled Capri Chasma and overtopped its eastern rim, carving two crossover channels and spectacular dry falls cataracts. This flooding may represent an initial outpouring of canyon lakes via a gateway in eastern Coprates Chasma. The recent discovery of hematite and abundant hydrated sulphates in the Valles Marineris canyons provides compelling evidence of a water-rich history.