The images show a network of meandering valleys across the landscape, all of which follow a dendritic, ramified or branched pattern. In hydrology, the term 'dendritic' is derived from 'dendron' (Greek for tree) and describes a valley into which, as one moves upstream, ever-smaller side valleys open, which in turn are fed by even smaller tributaries. This results in a pattern similar to the structure of a tree, with a trunk, branches and twigs. On Earth, this erosion pattern is found in most rivers and is the result of a water cycle with precipitation, runoff, evaporation and re-precipitation. In contrast, there are very few dendritic river valleys on Mars, and they have long since dried up. Most Martian valleys exhibit a different, rather straight structure with few tributaries, and they have a different origin – because they were formed by flowing groundwater.