In the late 19th century, through spectrometry it was generally believed the atmosphere of Mars was similar to Earth; only at the close of the 19th century and early 20th century was this contested more heavily, though it would not be until the 1960s that measurements were taken with Mariner 4. (Though by then the atmospheric composition was broadly known; in the 1920s the oxygen and atmospheric water levels were calculated to only exist in trace quantities. The atmospheric pressure had been approximated in the 1920s as well; later, Kuiper revisited the spectrometry of the 19th century in the 1940s to show that Mars had a much larger proportion of carbon dioxide than Earth). So we will want to focus on the latter half of the 19th century up to the early 20th century.

What else was known of Mars? Mars has a very similar--only slightly longer--day to Earth, though of course a longer year. Martian ice caps had long been recognized and it was known that they shrink and grow seasonally, just like Earth, and it was heavily speculated that Mars had liquid water or even seasonal plant growth visible through telescopes. Though there are many astronomers and geologists who contributed to this research, let us start with Giovanni Schiaparelli, who in 1877 produced produced a surface map of Mars, marked with landmasses, oceans, and, crucially, canali. These 'channels'--erroneously perceived by some to be canals--occupy a significant place in fin-de-siecle science fiction as potential evidence of advanced extraterrestrial life. While Schiaperelli drew attention worldwide for his research, the astronomer Camille Flammarion pursued these channels further, coming to the conclusion that they were artificial constructs from an intelligent race to distribute water across a world undergoing desertification. Flammarion is notable as an early science fiction novelist, envisioning what life on Mars might be like.

Probably the most influential figure for Martian research, Percival Lowell was a Boston businessman who became enraptured with the possibility of Martian canals. Lowell founded Lowell Observatory in Flagstaff in 1894 with the goal to uncover more information on the canals; Lowell also popularized these images in Mars As the Abode of Life, published 1908. Lowell, like Flammarion, believed these canals were intended to distribute water across the dying planet, and this is the setting for works like War of the Worlds, where Earth becomes the stage for a Martian invasion; in Arnould Galopin's Doctor Omega, Mars is the scene of internal strife in the spirit of The Time Machine. Mars as a promising abode finds a home in Alexander Bogdanov's Red Star, imagined as a socialist utopia; and Flammarion's Uranie, where Mars offers the setting for reincarnation.

By the 1920s, the notion that you could just walk around Mars was lost, as was the belief that Mars had man-made canals (though Lowell was largely on the margins of academic science for the fervor of his beliefs anyway). Certainly, into the 1930s and 1940s the idea that Mars had an environment habitable for human life was shed, and cemented in the 1960s with the Mariner missions. But the image of extraterrestrial life remains alluring, and the proximity of Mars makes it a ready setting for even contemporary science fiction.