Earlier observers of Mars had suggested that the dark, greenish areas might be vegetation of some sort. Sagan proposed that the dark areas were hills, which the Martian wind stripped of the finer, lighter-colored dust particles that collected in the valleys. Sagans theory was confirmed by the Mariner 9 spacecrafts visit to Mars. During the 1970s Sagan studied the present atmosphere of Earth. He studied the way that winds circulated dust through the atmosphere and how large amounts of dust, such as that from volcanic explosions, might affect Earths climate.
His study of Earths atmosphere led him to formulate the idea of nuclear winter with American scientists Paul and Anne Ehrlich in the 1980s. Sagan and the Ehrlichs theorized that the dust and ash thrown into the atmosphere by the explosions of a nuclear war and the ensuing fires might be so thick and widespread that it would block the Suns light for months or years. The damage that a nuclear winter would cause to crops and Earths ecosystems would be at least as devastating as the nuclear explosions. The idea of nuclear winter was met with much controversy, and scientists have continued debating the theory.