and Its Canals (1906), and Mars As the Abode of Life (1908). With these writings, Lowell more than anyone else popularized the long-held belief that these markings showed that Mars sustained intelligent life forms. His works include a detailed description of what he termed the "non-natural features" of the planet's surface, including especially a full account of the "canals," single and double; the "oases," as he termed the dark spots at their intersections; and the varying visibility of both, depending partly on the Martian seasons. He theorized that an advanced but desperate culture had built the canals to tap Mars' polar ice caps, the last source of water on an inexorably drying planet. While this idea excited the public, the astronomical community was skeptical. Many astronomers could not see these markings, and few believed that they were as extensive as Lowell claimed. As a result, Lowell and his observatory were largely ostracized. Although the consensus was that some actual features did exist which would account for these markings, in 1909 the sixty-inch Mount Wilson Observatory telescope in Southern California allowed closer observation of the structures Lowell had interpreted as canals, and revealed irregular geological features, probably the result of natural erosion. The existence of canal-like features was definitively disproved in the 1960s by NASA's Mariner missions. Mariner 4, 6 and 7, and the Mariner 9 orbiter (1972), did not capture images of canals but instead showed a cratered Martian surface. Today, the surface markings taken to be canals are regarded as an optical illusion. Psychologist Matthew J. Sharps has argued that perception of the canals by Lowell and others could have been the result of a combination of psychological factors, including individual differences, Gestalt reconfiguration, and sociocognitive factors. Venus spokes Although Lowell was better known for his observations of Mars, he also drew maps of the planet Venus. He began observing Venus in detail in mid-1896 soon after the Alvan Clark & Sons refracting telescope was installed at his new Flagstaff, Arizona observatory. Lowell observed the planet high in the daytime sky with the telescope's lens stopped down to 3 inches in diameter to reduce the effect of the turbulent daytime atmosphere. Lowell observed spoke-like surface features including a central dark spot, contrary to what was suspected then (and known now): that Venus has no surface features visible from Earth, being covered in an atmosphere that is opaque. It has