A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Whatever happened to the canals of Mars?

June 28, 1985

Dear Cecil:

Whatever happened to the Martian canals? When I was little I distinctly recall seeing pictures of Mars that showed an elaborate network of lines that some thought were the remnants of an irrigation system built by a lost race of intelligent beings. But there was always some disclaimer that we'd have to wait for further exploration till we could say for sure. Well, NASA has since sent any number of satellites flying by Mars equipped with cameras — and yet I don't remember hearing a peep about the canals. What's the story? Is there something they're not telling us, or was the whole thing a con intended to drum up support for the space program?

Cecil replies:

Eh, don't be surprised you haven't heard much about this. The controversy surrounding the Martian canals, probably the most famous episode in the history of astronomy, wasn't cleared up until the early 1970s, almost a century after it began. With a buildup like that, you're not going to get a lot of astronomers sending out notices saying, "Dear World: We're sorry, but it was all a big mistake."

It's not clear who initially spotted the canals, but they were first publicized in 1877 by Giovanni Schiaparelli, an Italian astronomer. He called the dim network of lines he saw "canali," Italian for "channels." But the English-speaking world translated "canali" as "canals," suggesting that someone — e.g., a superior civilization — had built them.

This idea was taken up a few years later by the amateur American astronomer Percival Lowell, who had built an observatory near Flagstaff, Arizona. Lowell made detailed observations of Mars and published several popular books about the planet, notably Mars and Its Canals (1903), which included elaborate maps of the canals and outlined the theory that they were waterways used for irrigation. This notion caught the public fancy, but it was never widely accepted by other astronomers. Some thought the canals were natural features, such as volcanic rift valleys, earthquake cracks, and so on; others doubted they existed at all.

The problem was that the even during the best of times the canals couldn't be seen distinctly, and many astronomers were never able to see them at all. (Indeed, it was nine years before somebody managed to confirm Schiaparelli's 1877 sighting.) A few wondered whether the whole thing wasn't simply a matter of wishful thinking. In one famous if somewhat casual experiment, a diagram of Mars featuring all of its generally agreed-upon features, but excluding the canals, was tacked up in front of a roomful of schoolchildren, who were asked to copy it. The kids in front, who could see the map clearly, reproduced it accurately. But the kids in the back, to whom many of the fine details on the map were simply a blur, tended to come up with maps that had canal-like lines connecting the smaller landmarks. From this some concluded that the Martian canals were an optical illusion, the result of the psychological tendency to connect indistinct features into a comprehensible whole. Confirming this idea was the fact that on those few occasions when the view of Mars was exceptionally clear, the canals couldn't be seen.

The matter wasn't settled until the Mariner flights of the '60s and '70s, and even then there was controversy. One photo from an early fly-by seemed to show an unnaturally straight feature that one writer claimed might be a canal. The Mariner 9 flight in 1971, however, photographed almost the entire surface of Mars at close range, and it became clear that not only did no canals exist, the old canal maps provided by Lowell et al corresponded only rarely with the planet's actual features. Obvious conclusion: the whole thing had been an illusion. Curiously, the Mariner photographs did reveal what look like dry riverbeds and alluvial channels formed by water (or at least liquid), although little water is to be found on the planet now. How these channels came to exist isn't known, but they appear to be of natural origin.

Oh, while I think of it — the old theory that one of the Martian moons is an artificial satellite is out the window, too. As for that business in recent years about a "face" on Mars … please, let's say no more.

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