The possibility of life on Mars has been debated almost since the invention of the telescope.

Annual growth and shrinkage of the martian ice caps and seasonal changes in color were observed by astronomers such as Herschel and Whewell in the 18th and 19th centuries. Late in the 19th century, Schiaparelli reported the existence of linear features he called canali, and this led to speculation by Lowell (among others) that the Red Planet might host a civilization more advanced than our own.

The arrival of instrumented spacecraft, beginning with Mariner IV in 1964, showed a cratered, dry, cold planet that looked almost as inhospitable as our Moon; but later missions provided evidence of liquid water on Mars in the recent past. This has led to speculation about whether simple forms of life, like bacteria, might exist on Mars. The thin atmosphere of Mars does little to block out damaging radiation from the sun, and the surface of Mars seems to be sterilized by caustic chemicals like hydrogen peroxide, but scientists still hold out hope that life on Mars could survive protected below the surface.

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