5. Mars: Drainage Channels
Liquid water is thought to be essential for life. Early in this century Percival Lowell mistakenly believed that he saw signs of canals on Mars and many people believed that intelligent civilizations had inhabited the Red Planet. By the early 1960s few people believed in martian civilizations — even then, Mars was known to be too cold and airless for advanced life such as that on Earth. The Mariner 4, 6, and 7 flybys of Mars finally revealed a cratered, dry, and apparently lifeless body, something like the Moon.
Mariner 9 and Viking Orbiter imagery, however, revealed networks of interconnecting dry river beds that exhibited dendritic (from the Greek, meaning “treelike”) drainage patterns characteristic of water-carved channels. This is the strongest evidence that Mars was once a warmer and wetter place with sufficient atmospheric pressure to retain liquid water on or near the surface.
Scene is 160 kilometers across, Viking Orbiter image 606A56, centered at 42.5°S 92.6°W. Image processing by Brian Fessler (Lunar and Planetary Institute).