Granular flows-grains of sand and dust slipping downhill-may have caused the “dark steaks” on Mars, which had been considered evidence for flowing water on the red planet.The findings argue against the presence of enough liquid water for microbial life to thrive at these sites.Continuing examination of these still-perplexing seasonal dark streaks with the University of Arizona-led High Resolution Imaging Science Experiment, or HiRISE, camera on NASA's Mars Reconnaissance Orbiter, or MRO, shows they exist only on slopes steep enough for dry grains to descend the way they do on faces of active dunes.