UA geologist Jon Pelletier expected his team's analysis of HiRise images of Mars would verify reports that flowing water produced the bright features seen in some gullies. Instead, they found that avalanches of dry granular debris such as sand and gravel match the Martian features. The viscous fingers prove that water was not the agent at work here.[right: HiRise photo and corresponding models, from Jon Pelletier, UA]