Figure 6-67 Rivers on Mars and Earth (a) Winding canyons on Mars, such as the one in this Viking 1 orbiter image, appear to have been formed by sustained water flow. This theory is supported by the terraces seen on the canyon walls in high-resolution Mars Global Surveyor images. Long periods of water flow require that the planet’s atmosphere was once thicker and its climate more Earthlike. (b) The Yangtze River near Chongqing, China. Typical of rivers on Earth, it shows the same snakelike curve as the river channels on Mars.