Life on Mars
Early speculation
Mars's polar ice caps were discovered in the mid-17th century. In the late 18th century, William Herschel proved they grow and shrink alternately, in the summer and winter of each hemisphere. By the mid-19th century, astronomers knew that Mars had certain other similarities to Earth, for example that the length of a day on Mars was almost the same as a day on Earth. They also knew that its axial tilt was similar to Earth's, which meant it experienced seasons just as Earth does—but of nearly double the length owing to its much longer year. These observations led to increasing speculation that the darker albedo features were water and the brighter ones were land, whence followed speculation on whether Mars may be inhabited by some form of life.
Early speculation
In 1854, William Whewell, a fellow of Trinity College, Cambridge, theorized that Mars had seas, land and possibly life forms. Speculation about life on Mars exploded in the late 19th century, following telescopic observation by some observers of apparent Martian canals—which were later found to be optical illusions. Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906, proposing that the canals were the work of a long-gone civilization. This idea led British writer H. G. Wells to write The War of the Worlds) in 1897, telling of an invasion by aliens from Mars who were fleeing the planet's desiccation.
Early speculation
The 1907 book Is Mars Habitable? by British naturalist Alfred Russel Wallace was a reply to, and refutation of, Lowell's Mars and Its Canals. Wallace's book concluded that Mars "is not only uninhabited by intelligent beings such as Mr. Lowell postulates, but is absolutely uninhabitable." Historian Charles H. Smith) refers to Wallace's book as one of the first works in the field of astrobiology.
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