The Planet Most Like Earth

When you look at Mars in your telescope, you will often find it disappointing. For many months at a time it appears as a tiny orange dot or speck, without the slightest detail. Its apparent diameter compared to that of other bright planets when they are farthest is only about one-fourth the width of Venus and Mercury, one-fifth the width of Saturn, and one-tenth the width of Jupiter. But just wait until Earth starts catching up to this slower, outside neighbor of ours. Mars can then grow almost a hundred times brighter and up to 7 or even 8 times wider in our telescopes. Even at distant oppositions where it grows to only about 5 times its minimum apparent size, we cross a threshold of visibility for surface features that makes all the difference.

All the difference? This featureless speck is revealed to be an orange globe rotating in almost the same amount of time as Earth, with almost the same amount of axis tilt as Earth, sporting polar ice caps that look dramatically like those of Earth and one thing else: seemingly green patches that grow more prominent when spring comes to the Martian hemisphere they are in. This is what astronomers started seeing of Mars as telescopes improved in the eighteenth and nineteenth centuries. They could see for themselves that Mars was the planet most like our own and suspected that the green areas were large regions of vegetation that were invigorated by water running down from the melting ice cap in spring.

In the nineteenth and early twentieth centuries, scientists came to realize that conditions for life anything like we know it were borderline at best on Mars. That is not only because Mars is much farther from the Sun than Earth and is therefore very cold, but also because Mars is only half the size of Earth and its gravity can support only a very thin atmosphere. But a thicker atmosphere in past eras of the planet could have retained more heat. So speculations began that life could have evolved on Mars long ago and then found ways to adapt to the deepening cold and thinning air.

Such speculations were on the mind of the American millionaire socialite-cum-astronomer Percival Lowell when he began to think he was seeing long thin lines connecting the green areas. The Italian astronomer Giovanni Schiaparelli had seen such lines (most of them later proved to be a kind of optical illusion) and called them canali—a word that should have been translated into English as "channels." Instead, it looked like the English word "canals"—artificial waterways. Lowell fell prey to the optical illusion of seeing these features when Mars came close and believed that they were indeed canals—canals built by an ancient, still-surviving race of Martians to conserve and direct the planet's dwindling water supplies. What followed Lowell's publicity of this scenario was H. G. Wells's book War of the Worlds (dramatized on radio so believably in 1938 by Orson Welles that it set off a panic that Martians really were invading New Jersey). In the science-fiction stories and movies of the first two-thirds of the twentieth century, by far the most common origin for imagined extraterrestrial invasion was Mars. The saying "little green men from Mars" became well known.

Our unmanned spacecraft visits to Mars in the past forty years have shown us just how harsh conditions on the planet are. The Martian atmosphere was shown to be about 95 percent carbon dioxide because several things that converted that gas to other substances in the history of Earth didn't happen on Mars. One that didn't occur, apparently, was the development of plant life that could change much of the carbon dioxide into oxygen. Most— though not all—of the ice in the polar caps and frost detected by the Viking spacecraft was "dry ice"—that is, frozen carbon dioxide rather than frozen water. A problem for life not fully appreciated before spacecraft visited Mars was the planet's lack of an ozone layer and resultant high doses of sterilizing ultraviolet radiation. But it's certainly not time to give up on the idea of Martian life. Discoveries made in the past few years have reopened the possibility that life once existed, just maybe even still exists, on Mars.

This long—and excitingly continuing—history of humankind's search for life on Mars percolates through us when the Red Planet comes close. We begin to glimpse many of its surface features in our backyard telescopes and perceive for ourselves the ways in which Mars resembles Earth.

Telescopes Mastery

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