How Did Lines on Mars Become Canals?

A canal is an artificial channel made to carry water or boats. Lowell’s Martian canals were more than a name for dark marks. He thought they formed a system that intelligent beings had built to manage water.

That claim involved two steps. There had to be continuous features on Mars, and there had to be evidence that someone had constructed them rather than nature forming them. A straight line on a map does not establish both steps at once.

Schiaparelli and Lowell left drawings and maps of their observations. These are valuable records of what they reported seeing. A hand-drawn map organizing the shapes an observer recognized, however, is a different kind of record from an image captured by a camera.

Deciding where a faint mark ends, or whether separate marks connect, involves interpretation. Clear lines on a finished map can make the original view seem equally clear. The clarity of a representation and the detail available in an observation need to be considered separately.

Lowell began observing Mars in 1894. He emphasized the lines’ straightness, uniform width and arrangement around particular points. He interpreted their regularity as evidence of planned construction rather than naturally occurring marks.

The story cannot be reduced to one mistranslated word. The rendering of Italian canali as English canals played a part, but so did the meaning observers attached to the lines. Schiaparelli also imagined a Martian civilization in an 1895 article, while distinguishing that speculation from a scientific result.

Some astronomers supported the canals; others questioned whether they existed at all. Important questions included whether better observations revealed continuous structures and whether other observers could identify the same forms.

One explanation was that indistinct markings could be interpreted as line-like shapes. That does not establish that every observer experienced an identical illusion. Here, we examine what people recorded and whether further observations supported their interpretation.

In 1965, Mariner 4 photographed a cratered surface at close range. It found no such canals in areas where earlier maps had drawn them. But the flyby photographed only part of Mars. It did not survey the entire planet at once.

Mariner 6 and 7 followed in 1969. Mariner 9 entered Mars orbit in 1971 and, after a dust storm subsided, recorded much more terrain in detail in 1972. These observations revealed enormous volcanoes, canyons and other real landscapes. They did not confirm the old artificial canal network.

This story becomes more useful when we follow changes in the evidence, rather than dismissing people in the past as simply gullible. An observation, a line on a map and an explanation involving artificial construction are related, but they are not the same fact.

The diagram and connected lab use an original board of dark marks. Explore a loss of detail separately from the addition of connecting lines. This example does not recreate historical telescopes or people’s vision. It is a small exercise in separating what a record shows from the markings we add.