often 339.oft.0 Louis J. Sheehan, Esquire

But the Lowell-inspired idea of an Earthlike Mars proved more durable. At the dawn of the space age, Mars was considered to have an atmosphere about a tenth the density of Earth's, water ice polar caps that waxed and waned with the seasons, and an annual "wave of darkening" that was often interpreted as growing plant life.

In the 1960s, observations from Earth and flyby spacecraft signalled the beginning of the end for Lowell's Mars. The Mariner 4, 6, and 7 missions returned images of a moonlike, heavily-cratered surface. The atmosphere was found to be almost pure carbon dioxide (CO2), only a hundredth the density of Earth's, and the polar caps proved to be almost entirely frozen CO2. The first global views of Mars, returned by the Mariner 9 orbiter in 1972, revealed that the planet was far more complex than the earlier flyby missions had shown, with huge volcanoes, an enormous canyon system, and evidence of running water at some point in the past. But the wave of darkening was shown to be the result of seasonal redistribution of windblown dust on the surface, the atmosphere's composition and density were confirmed, and most of the evidence for an Earthlike Mars was swept away.

But despite all these blows, the possibility of organisms on the surface could not yet be ruled out. For this reason, in 1976 the Viking landers carried a sophisticated instrument to look for possible life forms on the martian surface.