life on Mercury
The proximity of Mercury to the Sun did not deter some pluralists, prior to the twentieth century, from advocating the existence of life on the innermost planet. However, speculations of a more scientific nature followed Schiaparelli's attempts, in the 1880s, to map the Mercurian surface and his conclusion that Mercury always kept the same face toward the Sun (see Mercury, rotation). This implied extreme permanent differences in temperature between the perpetually sunlit side of the planet, which would be unbearably hot, and the endlessly dark side, which would be almost unimaginably frigid. It also, crucially, implied the existence of a "twilight zone" where the temperature would be permanently moderate. Here, in this narrow margin between ever-lasting day and ever-lasting night, some astronomers speculated, there might exist primitive forms of Mercurian life. Such hopes disappeared, however, as it became clear that Mercury lacks any substantial atmosphere.