Did you know that in the 19th century it was widely believed that the other planets in our solar system were likely inhabited? At the same time, the quality of telescopes was rapidly increasing, making it possible for astronomers to see and sketch the incredible details of the geography of Mars. As we'll see the initial excitement fades as science begins to cast doubt on the possibility of life on our planetary neighbor.

As astronomers charted the features of the red planet, many believed they were seeing oceans that confirmed ideas about the likelihood of life on Mars. Later, observations of canals and apparent straight lines on the surface of Mars seemed to offer evidence of intelligent life and civilizations on the planet.

We know now that there is no intelligent life on Mars. Understanding the history of the arguments around that possibility offers insight into how evidence and authority work in scientific reasoning and argumentation.

Mars Presumed Inhabited

An 1831 astronomy textbook, The Young Astronomer, explained, "To the people who live on Mars, this earth probably appears larger than Mars does to us" (27). This was not simply a colorful turn of phrase. In the 1830s many astronomers believed Mars, the other planets, and even the sun, were inhabited.

Our image of Mars developed along with advancements in the quality of our instruments to study it. As that picture came into focus, similarities between Mars and Earth, and in particular the belief that dark portions of the planet were oceans, lead many to speculate about the possibility of life on the planet.

For example, in Celestial scenery or, The wonders of the planetary system displayed (1838) astronomer Thomas Dick suggested that "There is land and water analogues to those on our globe" and further that, "from the size of the dark spots, about one third or one fourth of the surface of that planet is covered with water." Based on this evidence he suggests, "respecting the physical and moral state of the beings that inhabit it, we might be apt to conclude that they are in a condition not altogether very different from that of inhabitants of our globe" (p.139). Much of this work was grounded in the notion of teleology, that everything in nature serves a purpose in a divine design. If the planets were places like earth, this line of reasoning suggested that a creator would have put life there like he did here.

The Plurality of Inhabited Worlds

By the 1860's the idea of life on other worlds had become particularly popular in the works of French author and astronomer Camille Flammarion. His book La pluralité des mondes habités (The Plurality of Inhabited Worlds, 1862) brought considerable attention to the idea.

His ideas about life on Mars also made their way into his popular astronomy books. English translations of books, like The wonders of the heavens (1871) and Astronomy for amateurs (1904) helped to disseminate his ideas. Throughout these works Flammarion devotes considerable discussion to the amount of water and ice on Mars. Between these books, how early maps of Mars were drawn and various interpretations of the light coming off the planet many scientists and astronomer's believed that Mars had oceans or seas.

Dawes Charts of the Martian Oceans

The decisions that go into how to draw a map confer considerable authority onto the ideas presented in the map. When astronomer and popular author Richard Proctor presented a map of Mars illustrating its oceans and continents he wanted readers to think of Earth's continents and oceans.

Looking at a map like this suggests to a reader that the oceans of Mars are a fact. Presentations of the geography of Mars made it seem all the more likely that it was an inhabited world.

Light Spectra and the Martian Oceans

After explaining the spectrum of light from Mars in Flowers in the Sky (1874) astronomer Richard Proctor suggested "There must, therefore, be seas on Mars External" (175). Alongside the arguments embedded in how Mars was mapped came arguments for Martian oceans tied to studies of the light reflected off the planet.

Maps of Martian oceans were not just based on telescope observations. One of the key arguments for oceans on Mars came from the developing technique of spectroscopy. In 1859 Gustav Kirchhoff and Robert Bunsen had made significant advances in studying dark spots in the spectrum of light that came off burning different chemicals. This work led to the ability to read the light spectrum coming from objects in the heavens as a kind of chemical fingerprint, identifying the makeup of objects in space.

Spectroscopy played an important role in our understanding of the composition of the heavens. Kirchhoff and Bunsen discovered that every element has a different signature of dark lines. The fact that one could study light coming from an object to understand what it was made of was very powerful for astronomy. It made possible something that many thought we would never know -- the physical composition of objects in the heavens.

In Star Studies: What we Know of The Universe outside the Earth(1871) Elias Colbert, emeritus assistant director of the Dearborn observatory explained that examination of the light spectrum from Mars indicated that there was a considerable amount of water on the planet. As a result he suggests "Mars is, therefore, adapted as a residence for rational beings, like ourselves" and, on the basis of teleology, that the Martians had likely "attained to a higher state of mental development than we have" (p. 78).

Spectroscopy is an invaluable tool for understanding the composition of objects in the heavens. However the inferences that were made about the quantity of water on Mars in the 1870s were problematic as there simply are not oceans and seas on Mars as Colbert and Proctor had suggested. This isn't a problem with spectroscopy as a method; it remains a critical mode for collecting data about the heavens. Instead, like other data collection techniques, the inferences one makes from the analysis of light spectra are only as good as the quality of the instruments and the underlying assumptions one brings to the analysis.

Schiaparelli and the Martian Canals

In contrast to existing maps (like the Dawes Map) Italian astronomer Schiaparelli's 1877 map showed a network of hard-edged straight lines crisscrossing the planet. Other astronomers couldn't produce such precise looking maps. With that look of precision came a form of authority. Without photographs these kinds of drawings were a powerful form of evidence.

While some astronomers argued Schiaparelli's observations were in error, many concurred. Schiparelli allegedly had higher quality of instruments and he asserted that his location was particularly well suited for observing Mars. In 1886 several astronomers confirmed Schiaparelli's study of Mars and interpreting these canals became an important issue. Unlike the oceans, the canals seemed to suggest clear evidence of the work of extraterrestrials on the landscape of Mars. Where the oceans of Mars had suggested the possibility of life on the planet, the canals seemed to offer hard evidence of intelligent life on the red planet.

Nonetheless, skeptics remained. For example, in 1892 astronomer George Davidson of the Lick Observatory reportedly told the San Francisco Call "Mars inhabited? Pshaw! Do you know that every true astronomer has been very sorry to see all this haberdash that some of the newspapers have been printing about Mars being inhabited and our seeing canals on the surface, and all that other rot and nonsense."

Percival Lowell and the Martian Canal Phenomena

In 1894 astronomer Percival Lowell, writing for popular magazines like The Atlantic Monthly, Scientific American and Century Magazine captured public attention with his arguments that the Martian canals meant there had to be intelligent life on planet.

Lowell went on to publish his ideas in three books: Mars (1895), Mars and it's Canals (1906), and Mars as the Abode of Life (1908). However, by 1903 many astronomers had turned against the idea of the canals. These astronomers, still unable to observe the canals themselves, suggested that they were optical illusions. But Lowell's images of the planet remained powerful and iconic representations.

On Natural and Artificial Lines

Percival Lowell's face appears prominently in a range of full-page illustrated stories about life on Mars. While there had been stories about the possibility of life on Mars in American newspapers before Lowell, they hadn't been nearly this dramatic and extensive. By this time scientific opinion was already turning against belief in intelligent life on Mars, but the public was excited to hear these stories.

One of Lowell's central arguments was that the structure of the canals he and other astronomers had seen on Mars was such that it clearly meant that they were something only intelligent life forms could produce. In comparing natural lines (cracks in mud, cracks on the earth, and cracks on the moon) to artificial lines (Streets in Montreal, Railroads in Illinois, irrigation cannels in Arizona and his and Schiaparelli's maps of Mars) he claimed that the Martian Canals, becoming an increasingly complex network of interconnected lines on his maps of the planet, clearly illustrated that Mars was a world inhabited by intelligent beings.

Who Believed in the Intelligent Martian Canal Builders?

Many Americans, like Alexander Graham Bell, found Lowell's line of reasoning and his vivid visual presentations of evidence convincing. In 1909 Bell wrote an 8 page letter to his wife about the matter. After reviewing Lowell's arguments he suggests, "there is no escape from the conviction that Mars is inhabited by a highly civilized and intelligent race of beings carrying on a process of agriculture, and wringing subsistence." Lowell's arguments carried considerable weight, and advanced the idea of intelligent life on Mars even further into the public imagination.

In 1907 Lowell invested a sizable sum of money in taking photographs of the Martian Canals. In his opinion these images removed all doubt about the existence of canals on Mars. However, by opening up the interpretation of the observations behind the various maps of the planet the images ended up contributing to a loss of credibility in his work. The precise looking maps, like the one from Schiaparelli, carried a kind of weight, clarity and authority in justifying the idea of canals on Mars, but with photography came a new representation which made the seemingly objective maps of the previous era look deeply subjective. While the canals appeared obvious to Schiaparelli and Lowell, the photographs were not convincing to others. As evidence mounted public option began to sway although the possibility of extraterrestrial life still captivated the imagination of many.

Mariner IV Finds No Canals

After 1907, astronomers largely became convinced that there were no canals on Mars. Instead of this being a real feature of Mars that needed to be explained, it became broadly accepted that the canals were instead some kind of optical illusion. Still debate continued about interpretations of the canals, including an argument from Carl Sagan and Jim Pollack that they could be the result of shifting dust on the planet's surface.

Discussion was eventually completely settled when NASA's Mariner IV photographed the planet and found that there were no canals at all. There was not even something that could be misinterpreted as canals. On closer observation the canals of Mars had been an optical illusion all along. In hindsight this story suggests the importance of skeptics in the face of bold claims and generally assumed ideas. Further, it suggests the important role that personal beliefs and ideas can play in gathering and interpreting data. While it was broadly believed that there was intelligent life across the solar system in the beginning of the 19th century, by the middle of the 20th the idea had become untenable.