Figure 6-21 RIVUXG Signs of Ancient Martian Water (a) This Viking Orbiter image shows a network of dry riverbeds extending across the cratered southern highlands. Liquid water would evaporate in the sparse, present-day Martian atmosphere, so these must date from an earlier era when the atmosphere was thicker. (b) These teardrop-shaped islands rise above the floor of Ares Valles. They were carved out by a torrent of water that flowed from the bottom of the image toward the top. Similar flood-carved islands are found on Earth in eastern Washington State.