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Straight Dope Classic from Cecil's storehouse of human knowledge28-Jun-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 to the effect that we'd have to wait for further exploration before
we could know 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? --Tom L., Madison, Wisconsin
Cecil replies:
It's important to distinguish between two kinds of silence, Tom--suspicious and
embarrassed. We have here a premier example of the latter. The controversy surrounding the
Martian canals, probably the most famous episode in the history of astronomy, was not
cleared up until the early 1970s, almost a century after it began. 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," which suggested 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 could not be seen very
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
some sort of comprehensible whole. Confirming this idea was the fact that on those few
occasions when the view of Mars was exceptionally clear, the canals could not be seen.
The matter wasn't settled until the Mariner flights of the '60s and '70s, and even then
there was some 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 fairly 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. Scientists thereupon
concluded that the whole thing had simply been an optical illusion. Curiously, the Mariner
photographs did reveal what appear to be dry riverbeds and alluvial channels that seem to
have been formed by water (or at least liquid), although there is little water to be found
on the planet now. How these channels came to be is not known, but they appear to be of
purely 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, I'd rather not talk about it.
--CECIL ADAMS
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