In the late nineteenth century, the Italian astronomer Giovanni Schiaparelli published sketches of linear features on Mars, which he called canali, meaning "channels." In the English press, these features came to be known as canals and were taken as evidence of a Martian civilization. Subsequently, astronomers determined that the canali were simply artifacts of color variations on Mar’s surface, caused by seasonal winds. While no canals or civilizations exist on Mars, close-up images of the planet taken by spacecraft in the last few decades have revealed well-defined braided stream channels. Astronomers have little doubt that liquid water flowed on the surface of Mars at time in the past. They speculate that the water formed when heat from the planet’s interior, caused by volcanic activity, melted the ice in near-surface permafrost. The water then flooded across the vegetation-free surface and quickly carved out river channels. And this all must have happened a long time ago, for meteor craters pockmark the channels. Nevertheless, the channels retain their fluvial character because of the lack of erosion on the planet.

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