While the present-day Martian surface is generally dry and cold, its sedimentary rocks contain compelling evidence for the former presence of liquid water. According to a new analysis of orbital images of 3.7-billion-year-old sedimentary layers at Izola mensa, an outcrop in the northwestern rim of the Hellas impact crater on Mars, deep rivers were active in this region for over 100,000 years. This false-color map, produced by the Mars Orbiter Laser...