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For a time in the late 19th and early 20th centuries, it was erroneously believed that there were canals on Mars. These were a network of long straight lines in the equatorial regions from 60° N. to 60° S. Lat. on the planet Mars. They were first described by the Italian astronomer Giovanni Schiaparelli during the opposition of 1877, and confirmed by later observers. Schiaparelli called these canali, which was translated into English as "canals". The Irish astronomer Charles E. Burton made some of the earliest drawings of straight-line features on Mars, although his drawings did not match Schiaparelli's. By the early 20th century, improved astronomical observations revealed the "canals" to be an optical illusion, and modern high resolution mapping of the Martian surface by spacecraft shows no such features.
Some people went so far as to propose the idea that the canals were irrigation canals built by a supposed intelligent civilization on Mars. Percival Lowell was a strong proponent of this view, pushing the idea much further than Schiaparelli, who for his part considered much of the detail on Lowell's drawings to be imaginary. Some observers drew maps in which dozens if not hundreds of canals were shown with an elaborate nomenclature for all of them. Some observers saw a phenomenon they called "gemination", or doubling - two parallel canals.
Other observers disputed the notion of canals. The observer E. E. Barnard did not see them. In 1903, Joseph Edward Evans and Edward Maunder conducted visual experiments using schoolboy volunteers that demonstrated how the canals could arise as an optical illusion. This is because when a poor-quality telescope views many point-like features (e.g. sunspots or craters) they appear to join up to form lines. In 1907 the British naturalist Alfred Russel Wallace published the book Is Mars Habitable? that severely criticized Lowell's claims. Wallace's analysis showed that the surface of Mars was almost certainly much colder than Lowell had estimated, that the atmospheric pressure was too low for liquid water to exist on the surface, and he pointed out that several recent efforts to find evidence of water vapor in the Martian atmosphere with spectroscopic analysis had failed. He concluded that complex life was impossible, let alone the planet-girding irrigation system claimed by Lowell. The influential observer Eugène Antoniadi used the 83-cm (32.6 inch) aperture telescope at Meudon Observatory at the 1909 opposition of Mars and saw no canals, the outstanding photos of Mars taken at the new Baillaud dome at the Pic du Midi observatory also brought formal discredit to the Martian Canals theory in 1909, and the notion of canals began to fall out of favor. Around this time spectroscopic analysis also began to show that no water was present in the Martian atmosphere.
The arrival of the United States' Mariner 4 by NASA in 1965, which took pictures revealing impact craters and a generally barren landscape, was the final nail in the coffin of the idea that Mars could be inhabited by higher forms of life. A surface atmospheric pressure of 4.1 to 7.0 millibars (410 to 700 pascals) and daytime temperatures of −100 degrees Celsius were estimated. No magnetic field or Martian radiation belts was detected.
History of canals 
The Italian word canale (plural canali) can mean "canals" (including artificial canals or ducts) or "channels" or "gullies". The first person to use the word canale in connection with Mars was Angelo Secchi in 1858, although he did not see any straight lines and applied the term to large features —for example, he used the name "Atlantic Canale" for what later came to be called Syrtis Major Planum.
It is often stated that Schiaparelli intended the meaning "channels" and that "canals" was a misunderstanding or mistranslation into English. Nevertheless, the English term "canals" was used from the very earliest accounts in English, and as far as is known, Schiaparelli made no effort to correct the supposed misunderstanding if he was aware of it. As the word "canal" can also mean "channel" or "watercourse" – not necessarily artificial – in English, the charge of "mistranslation" is unwarranted.
It is not necessarily odd that the idea of Martian canals was so readily accepted by many. At this time in the late 19th century, astronomical observations were made without photography. Astronomers had to stare for hours through their telescopes, waiting for a moment of still air when the image was clear, and then draw a picture of what they had seen. They saw some lighter or darker albedo features (for instance Syrtis Major) and believed that they were seeing oceans and continents. They also believed that Mars had a relatively substantial atmosphere. They knew that the rotation period of Mars (the length of its day) was almost the same as Earth's, and they knew that Mars' axial tilt was also almost the same as Earth's, which meant it had seasons in the astronomical and meteorological sense. They could also see Mars' polar ice caps shrinking and growing with these changing seasons. It was only when they interpreted changes in surface features as being due to the seasonal growth of plants that life was hypothesized by them (in fact, Martian dust storms are responsible for some of this). By the late 1920s, however, it was known that Mars was very dry and had a very low atmospheric pressure.
In addition, the late 19th century was a time of great canal building on Earth. For instance, the Suez Canal was completed in 1869, and the abortive French attempt to build the Panama Canal began in 1880. It is natural that some thought similar projects were being undertaken on Mars. In 1889, astronomer Charles A. Young reported that Schiaparelli's canal discovery of 1877 had been confirmed in 1881, though new canals had appeared where there hadn't been any before, prompting "very important and perplexing" questions as to their origin.
During the favourable opposition of 1892, W. H. Pickering observed numerous small circular black spots occurring at every intersection or starting-point of the "canals". Many of these had been seen by Schiaparelli as larger dark patches, and were termed seas or lakes; but Pickering's observatory was at Arequipa, Peru, about 2400 meters above the sea, and with such atmospheric conditions as were, in his opinion, equal to a doubling of telescopic aperture. They were soon detected by other observers, especially by Lowell.
During the oppositions of 1892 and 1894, seasonal color changes were reported. As the polar snows melted the adjacent seas appeared to overflow and spread out as far as the tropics, and were often seen to assume a distinctly green colour. The idea that Schiaparelli's canali were really irrigation canals made by intelligent beings, was first hinted at, and then adopted as the only intelligible explanation, by Lowell and a few others. Newspaper and magazine articles about Martian canals captured the public imagination.
At this time (1894) it began to be doubted whether there were any seas at all on Mars. Under the best conditions, these supposed 'seas' were seen to lose all trace of uniformity, their appearance being that of a mountainous country, broken by ridges, rifts, and canyons, seen from a great elevation. These doubts soon became certainties, and it is now universally agreed that Mars possesses no permanent bodies of surface water.
In popular culture 
Although the concept of the canals had been available since Schiaparelli's 1877 description of them, early fictional descriptions of Mars omitted these features. They receive no mention, for instance, in H. G. Wells' The War of the Worlds (1897), which describes a slowly drying Mars, covetous of Earth's resources, but one which still has dwindling oceans such as are depicted on Schiaparelli's maps. Later works of fiction, influenced by the works of Lowell, described an ever-more arid Mars, and the canals became a more prominent feature, though how they were explained varied widely from author to author.
- Garrett P. Serviss' Edison's Conquest of Mars (1898) repeatedly mentions Schiaparellian canals (which play a key part in the denouement of the story), but does not describe them in detail, apparently considering them simply irrigation canals comparable to those on Earth — ignoring the fact that, in that case, they could hardly be visible from Earth. Serviss' Mars also has lakes and oceans.
- George Griffith's A Honeymoon in Space (1900) describes the canals as the remnants of gulfs and straits "widened and deepened and lengthened by... Martian labour".
- Edgar Rice Burroughs' influential A Princess of Mars (1912) describes an almost entirely desert Mars, with only one small body of liquid water on the surface (though swamps and forests appear in the sequels). The canals, or waterways as Burroughs calls them, are still irrigation works, but these are surrounded by wide cultivated tracts of farmland which make their visibility somewhat credible.
- Alexander Bogdanov's Engineer Menni (1913) details the social, scientific, and political history of the construction of the Martian canals and the socio-economic ramifications the construction had on Martian society.
- Otis Adelbert Kline's Outlaws of Mars (1933) has multiple parallel canals, surrounded by walls and terraces, and describes the construction of the canals by Martian machines.
- In C. S. Lewis' Out of the Silent Planet (1938), the "canals" (handramit in Martian) are actually vast rifts in the surface of an almost airless, desert Mars, in which the only breathable atmosphere and water have collected where life is possible, with the rest of Mars being entirely dead.
- In Robert A. Heinlein's Red Planet (1949), colonists use the frozen canals for travel and a seasonal migration (by iceboat during winter when the canals are frozen and by boat when the ice melts during the Martian summer). Teenagers Jim Marlowe and Frank Sutton set out to skate the thousands of miles to their homes on the frozen Martian canals when escaping the Lowell Academy boarding school.
- In Ray Bradbury's The Martian Chronicles (1950), the canals are artificial waterways stretching between stone banks, filled with blue water, or sometimes poetically described as full of "green liquors" or "lavender wine". Bradbury revisited the martian canals in 1967 in his short story "The Lost City of Mars".
- In the BBC radio production Journey into Space: The Red Planet (1954–1955), the canals are valleys filled with a plant life resembling giant rhubarbs.
- In Robinson Crusoe on Mars (1964) Kit Draper and Friday flee from the enemy aliens through the underground canals on their way to the polar ice cap.
- The Mars of the steampunk role-playing game Space: 1889 (1988) is crisscrossed by artificial canals which support cities inhabited by the ancient civilization of the Canal Martians.
- The 1991 computer game Ultima: Martian Dreams features a plot based around Victorian expeditions to Mars. The Martian canals play a very prominent role as the main characters have to find a way to refill them using ice from the polar caps.
- Kim Stanley Robinson's science fiction chronicling of the terraforming of Mars in the Mars trilogy (1993-1999) and 2312 (2012) features the creation of canals on Mars ("burned" into the land with magnified sunlight) with the Lowell maps as inspiration. "Thus a nineteenth-century fantasy forms the basis for the actual landscape."
List of canals 
The canals were named, by Schiaparelli and others, after real and legendary rivers of various places on Earth or the mythological underworld.
See also 
- Classical albedo features on Mars
- Face on Mars
- History of Mars observation
- Life on Mars
- Outflow channel
- Solis Lacus
- Valley networks (Mars)
- Evans, J. E. and Maunder, E. W. (1903) "Experiments as to the Actuality of the 'Canals' observed on Mars", MNRAS, 63 (1903) 488
- Chambers P. (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 0-7137-2747-0
- Wallace, Alfred. "Is Mars Habitable (S730: 1907)". The Alfred Russel Wallace Page hosted by Western Kentucky University. Retrieved 2007-05-13.
- story of the Pic du Midi observatory
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- Robots On Mars Search And Catalog Red Planet. Audio recording, supporting statement is approx. 34:00 after start.
- Young, Charles A. "A Textbook of General Astronomy. 1889. Ginn and Co. Boston.
- Robinson, Kim Stanley (2012). 2312. New York: Orbit. pp. 554–555. ISBN 978-0-316-19280-4.
- Wallace, A. R. (1907) Is Mars habitable? A critical examination of Professor Percival Lowell's book "Mars and its canals", with an alternative explanation, by Alfred Russel Wallace, F.R.S., etc. London, Macmillan and co.
- Antoniadi, E. M. (1910) "Sur la nature des »canaux« de Mars", AN 183 (1910) 221/222 (in French)