Our Changing views of Mars, or What Happened to the Canals?

Not all that long ago, our view of Mars was completely different from what we know today. The generation that grew up with The Beatles and Elvis Presley in the second half of the 20th century will have read popular articles about Mars that regarded it as quite possible that the planet was covered with vegetation, and maybe even with animals scuttling about between the waving fronds. There might have been the remains of an ancient civilisation that built great canals to bring water from the poles to the parched deserts of a world that was dying from thirst. So what changed?

This year’s National Astronomy Week is all about the close approach, or opposition, of Mars. These oppositions happen every two years, and for much of the rest of the time the Red Planet is just a tiny, enigmatic dot. We get a few months of reasonable viewing time through Earth’s turbulent atmosphere to see our neighbour in space in close-up. Before the first spacecraft reached Mars in 1965, our understanding of conditions there was limited to what astronomers could see and measure through telescopes.

The first canals are seen

It was the close approach of 1877 that created some of the most enduring myths about Mars. Not only was Mars particularly close that year, but telescopes had improved in quality so astronomers were keen to make the most of the opportunity. One key observer was Italian astronomer Giovanni Schiaparelli of Milan Observatory, using a 220 mm (8¾-inch) refracting telescope. According to historian Agnes Clerke, he ‘detected a novel and curious feature. What has been taken for Martian continents were found to be, in point of fact, agglomerations of islands, separated from each other by a network of “canals” (more properly channels). These are obviously extensions of the “seas”, originating and terminating in them, and sharing their grey-green hue, but running sometimes to a length of three or four thousand miles in a straight line, and preserving throughout a nearly uniform breadth of about sixty miles.’ Moreover, at a subsequent good opposition, in 1881, many canals were seen to be double. 

Soon, other people began to see the canals as well. It was widely believed that the dark areas on Mars really were seas, so channels running between them seemed at least feasible. American businessman Percival Lowell even built an observatory and giant refracting telescope to enable him better to study the canals, which he saw in profusion. A rational explanation at the time was that they ran along the line of great rifts in the planet, like the Rift Valley in Africa. But to some, including Lowell, the explanation that they were created by intelligent beings, trying to irrigate a drying planet, was a powerful incentive.

This view was strengthened by the occasional curious observation, such as bright areas, which were interpreted as signals from the Martians to Earth. And one such observation triggered in the mind of British author H G Wells the notion that a bright flash might be the launch of an army to invade Earth, with its clearly more beneficial climate and vegetation.

However, not everyone saw canals, and they were never seen in photographs. By the mid 20th century it was decided that their appearance was so fleeting, and restricted to visual observers only, that their reality was in question.

But you can’t keep a good myth down, and belief in the canals continued much longer than it had any right to. Throughout the first half of the 20th century the notion of canals persisted in popular works, and even as late as 1965 NASA released diagrams showing the track of its Mariner 4 spacecraft superimposed on maps covered with a web of canals.

But Mariner 4 itself was the first to photograph the planet in close-up and show that there were no canals. Instead, Mars was covered with craters, although much more worn down than those of the Moon. These had hardly been suspected up to this point, although a few experts had suggested that they might exist.

And curiously, the subsequent Mariner 9 spacecraft showed that there really are giant rifts in the Martian surface – such as the colossal Valles Marineris – but they don’t tie in with any of the old canals! 

So what were the canals? Tests show that the eye, or rather the eye–brain combination, has a tendency to join dots and blobs into lines. Rarely when observing Mars visually do you get a bright, rock-steady view, even with a large telescope, and if you believe that there are straight lines you are likely to glimpse them. In general, and not just on Mars, people see what they expect to see!

Cabbages on Mars?

Another belief that persisted well into the middle of the 20th century was that of vegetation on Mars. Until the very end of the 20th century, photographs of Mars were of poor quality and were mostly in black and white. The tiny planet, seen through Earth’s turbulent atmosphere, challenged the sensitivity of film to its limit. Visual observers could actually see more than could be photographed, even from mountain-top observatories and using state-of-the-art cameras. And the eye saw a red planet covered with dark markings which were widely thought of as being grey–green in colour, as mentioned in the above quote from Agnes Clerke, although we now regard them as simply darker shades of red. The eye is notoriously bad at seeing accurate colour, and uses comparisons rather than absolute measurement of tint. 

And what would green be, if not vegetation? The view was endorsed by the ever-changing appearance of Mars. Unlike the Moon, whose features are absolutely constant not just from one month to the next, but over thousands of years, Mars changes its appearance noticeably over a matter of months. What’s more, a so-called ’wave of darkening’ apparently spread from the poles towards the equator each spring – clear evidence that the melting snows were causing the vegetation to flourish. Earth-based beasurements of the conditions on Mars suggested a thin but still appreciable atmosphere of carbon dioxide.

It was acknowledged that the vegetation was probably little more advanced than algae, but there was still the possibility of something more developed. And such authors such as Arthur C Clarke and Patrick Moore made the most of this by writing fiction in which the vegetation turned out to be far more exciting, pushing the boundaries of belief to the limit, with Clarke actually inventing advanced kangaroo-like herbivores in The Sands of Mars (1951). 

Writing in the 1962 Yearbook of Astronomy, Patrick Moore was able to say that ‘It is probably true that most astronomers now support the idea that the dark areas are due to living organisms.’

Within just a few years, he was proved wrong. Mariner 4 showed that the atmospheric pressure on Mars was much less than had been thought, making conditions much more hostile to life, and the later probes revealed nothing more than wind-blown sand, covering and uncovering the rocks and giving the appearance of real changes. 

The reality

Today we know Mars to be a world where no definite signs of life have yet been detected despite many studies by remotely controlled craft. Yet a few oddities have given cause for hope. In 1976, one experiment yielded a positive sign of life – but NASA decided that this was probably a freak chemical reaction, given the lack of photographic evidence. And in 1996, headlines proclaimed that there was indeed life on Mars following the remarkable discovery of what seemed to be fossilised micsoscopic biological organisms in a meteorite whose properties showed it to have originated on Mars. Such meteorites have been thrown off Mars by impacts of space debris in the distant past. But scientific opinion is now mostly that the structures can be explained by non-biological effects that are more likely than the presence of life.

More recently, methane has been detected in the atmosphere, with the amounts varying in a curious fashion. On Earth, methane is predominantly produced by life forms or biological processes, although it can also be produced by interaction between rocks and water.

Even so, enthusiasts still pore over the latest detailed pictures sent back from martian rovers, looking for any sign of life. Their claims often require the eye of faith to see, for example, a rabbit’s ears poking above the soil, or a sculpture that most others would agree is simply the result of aeons of wind-blown erosion. 

But even if life does not exist on the surface, it could be thriving deep inside Mars, having been formed during the undoubted wet and warm period in the past. It is more likely to be similar to the extremophile bacteria that we find in Antarctic subterranean deposits than anything that can even wriggle.

But we have been wrong before about Mars….