6. Water Channels on Earth
It is difficult to understand Mars without having been there. For most of the martian surface, we have only spacecraft images that don't show objects smaller than ~ 250 meters (800 feet) across, about the size of a city block. For a few areas of Mars, we now have Mars Global Surveyor images tht can show objects as small as 3 meters (10 feet) across, but geologists and biologists usually need to see better than that. To help understand what these spacecraft images show, scientists compare the 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.