Mars

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Life on Mars
In 1877, Giovanni Schiaparelli produced the first "modern" map of Mars, on which he showed a system of what he called canali. Although canali in Italian means "channel", without the implication of being an artificial feature, the word was commonly translated into English as "canal".


Schiaparelli's map of Mars (1888)

In 1910, Percival Lowell captured the imagination of the public with his book Mars As the Abode of Life. Based on his extensive visual observations (and as we know today, an active imagination) Lowell painted a compelling portrait of a dying planet, whose inhabitants had constructed a vast irrigation system to distribute water from the polar regions to the population centers nearer the equator.

Despite its appeal to the public, the astronomical community never gave serious credence to the details of Lowell's theory. The failure of many observers to confirm the existence of the canals eventually led scientists to suspect that their colleagues had been fooled into seeing the canals, by the difficulty in resolving fine detail from Earth and their own desire to believe. (This map, constructed from Viking orbiter images in the same format as Schiaparelli's -- south is up -- shows no sign of the canals, though a few features may have been interpreted as such.)


Mars from Viking

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...