greenish areas that appear to spread over the planet. American astronomer Percival Lowell (1855­1915) believed these to be artifacts of an irrigation system used by an intelligent race to channel water from the planet’s polar caps (another of the planet’s prominent features). The Viking missions have shown that the canals are in fact large areas of exposed rock, where winds have blown away the ubiquitous layers of dust. 
    The streaks across the central portion of the image belong to an immense system of canyons called the Valles Marineris. These canyons, which cover some 4,000 kilometers—about the distance from the Atlantic to Pacific coasts of the United States—were caused