The Planet Mars: A History of Observation and Discovery
Chapter 10: The Lingering Romance
<<G. P. Kuiper... who had hitherto embraced the vegetation hypothesis, changed his mind within the year, in large part because of the impression made on him by the great dust storm of 1956. The obvious ability of winds to move dust on Mars led him to propose that the dark areas might be dust-covered lava fields. As McLaughlin had in his original theory, Kuiper invoked seasonal removal of the dust by wind currents to explain the "wave of darkening." In 1958, a Russian astronomer named V. V. Sharanov arrived independently at the same explanation. The air currents on Mars, he wrote, "vary from season to season, depositing dust at some times of the year and blowing it away at other times. Thus, for instance, the inherently dark surface . . . may brighten at a definite time of the year as a result of settling of light-colored dust blown over from the desert areas."
Lowell had argued that the dark areas were dry sea bottoms, but radar studies in the early 1960s indicated that at least some of them were elevated rather than low-lying areas. This, incidentally, destroyed yet another argument once regarded as compelling support of the vegetation theory. In 1950, E. J. Opik had pointed out that if the dark areas were low, they ought to be quickly covered by yellowish dust unless they had some means of regenerating themselves. At the time this was thought to prove that they were tracts of vegetation. If, however, the dark expanses were actually elevated areas, there was no need to believe this---the dust might just as well be removed by wind scouring.
The century-long quest for Martian water vapor finally came to fruition at the mid-winter opposition of 1963. The detection of very minute amounts of water vapor in the Martian atmosphere was announced by Audouin Dollfus, who set up a special telescope at the Jungfraujoch high in the Swiss Alps, and by H. Spinrad, G. Munch, and L. D. Kaplan, who used a photographic emulsion especially sensitive to infrared radiation to record the spectrum of Mars near quadrature with the 100-inch reflector at Mount Wilson. The latter team found that the average amount of precipitable water on Mars (that is, the equivalent thickness of liquid water if all the atmospheric water were to be condensed onto the surface) was only about 14 micrometers, compared with 1,000 micrometers of precipitable water in even the driest desert areas of Earth. In addition, they estimated that the partial pressure of carbon dioxide on Mars was 4.2 millibars and that the total atmospheric pressure at the surface could not be more than about 25 millibars
. This was a drastic downward revision from the previously accepted figure of 85 millibars. Thus, Mars was drier and had an even thinner atmosphere than anyone had realized.
Parenthetically, the reason so many of the earlier investigators were so far wrong about the thickness of the Martian atmosphere was that most based their estimates on albedo studies. They had assumed the Martian atmosphere to be usually clear and transparent, but this is not the case; often there is at least some dust present, which makes the atmosphere more reflective than they had supposed.
By the time Spinrad, Munch, and Kaplan published the results of their monumental study, a very different view of Mars was beginning to come into focus. In many ways this new Mars resembled the modern view of the planet: a bone-dry world with an extremely rarefied atmosphere, a surface with perhaps considerable relief, and changes in its markings that were the result of windblown dust rather than vegetation. However, the paradigm shift was not yet complete by 1965 when the first spacecraft reconnaissance of Mars took place, and most astronomers and the lay public were shocked, even depressed, by what it revealed.>>