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By Root 1099 0

of a second, and the fundamental changes in attitude and knowledge responsible for this remarkable transformation would fill only the last few seconds. The seventeenth century saw the first widespread application of simple lenses and mirrors for astronomical purposes. With the first astronomical telescope Galileo was astounded and delighted to see Venus as a crescent, and the mountains and the craters of the Moon. Johannes Kepler thought that the craters were constructions of intelligent beings inhabiting that world. But the seventeenth-century Dutch physicist Christianus Huygens disagreed. He suggested that the effort involved in constructing the lunar craters would be unreasonably great, and also thought that he could see alternative explanations for these circular depressions.

Huygens exemplified the synthesis of advancing technology, experimental skills, a reasonable, hard-nosed and skeptical mind, and an openness to new ideas. He was the first to suggest that we are looking at atmosphere and clouds on Venus; the first to understand something of the true nature of the rings of Saturn (which had seemed to Galileo as two “ears” enveloping the planet); the first to draw a picture of a recognizable marking on the Martian surface (Syrtis Major); and the second, after Robert Hooke, to draw the Great Red Spot of Jupiter. These last two observations are still of scientific importance because they establish the permanence at least for three centuries of these features. Huygens was of course not a thoroughly modern astronomer. He could not entirely escape the fashions of belief of his time. For example, he presented a curious argument from which we could deduce the presence of hemp on Jupiter: Galileo had observed that Jupiter has four moons. Huygens asked a question few modern planetary astronomers would ask: Why does Jupiter have four moons? An insight into this question, he thought, could be garnered by asking the same question of the Earth’s single moon, whose function, apart from giving a little light at night and raising the tides, was to provide a navigational aid to mariners. If Jupiter has four moons, there must be many mariners on that planet. But mariners imply boats; boats imply sails; sails imply ropes; and, I suppose, ropes imply hemp. I wonder how many of our present highly prized scientific arguments will seem equally suspect from the vantage point of three centuries.

A useful index of our knowledge about a planet is the number of bits of information necessary to characterize our understanding of its surface. We can think of this as the number of black and white dots in the equivalent of a newspaper wirephoto which, held at arm’s length, would summarize all existing imagery. Back in Huygens’ day, about ten bits of information, all obtained by brief glimpses through telescopes, would have covered our knowledge of the surface of Mars. By the time of the close approach of Mars to Earth in the year 1877, this number had risen to perhaps a few thousand, if we exclude a large amount of erroneous information—for example, drawings of the “canals,” which we now know to be entirely illusory. With further visual observations and the development of ground-based astronomical photography, the amount of information grew slowly until a dramatic upturn in the curve occurred, corresponding to the advent of space-vehicle exploration of the planet.

The twenty photographs obtained in 1965 by the Mariner 4 fly-by comprised five million bits of information, roughly comparable to all previous photographic knowledge about the planet. The coverage was still only a tiny fraction of the planet. The dual fly-by mission, Mariner 6 and 7 in 1969, increased this number by a factor of 100, and the Mariner 9 orbiter in 1971 and 1972 increased it by another factor of 100. The Mariner 9 photographic results from Mars correspond roughly to 10,000 times the total previous photographic knowledge of Mars obtained over the history of mankind. Comparable improvements apply to the infrared and ultraviolet spectroscopic data obtained by Mariner 9, compared