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).]
Mars: Drainage Channels
It is difficult to understand Mars without having been there, and even more difficult because our Viking and Mariner images of most of Mars don't show features smaller that 200 meters across. To help understand, scientists compare features on Mars with similar- looking features on Earth; this is an example of comparative planetology. In fact, all interpretations of features on other planetary bodies are extrapolations or inferences from what we see on Earth.
The pattern of stream channels in this space shuttle photo are similar to the martian channels in slide #5. These stream channels are in the Republic of South Yemen, on the Arabian Peninsula, at the southern edge of the vast sand desert of the Rubh-al-Khali. After Earth's last Ice Age, the Rubh-al-Khali was an open grassland, with a temperate climate and much rainfall. Rain water collected in Yemen's coastal mountains, and carved these stream channels as it flowed to the ocean. But no water flows today. In this way, Yemen is a bit like Mars, wet long ago, but a desert today. (Drainage pattern, Yemen; NASA shuttle Earth observation photograph STS-41G, #17-36-039.)
