Astronomy: Moisture on Mars

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To early astronomers, it seemed obvious that there was water on Mars. What else could form the white polar caps that shrank so noticeably every spring and began to grow again in the fall? And what could possibly produce the springtime darkening of the Martian surface other than rapid vegetation growth stimulated by water released from the melting caps?

Contemporary scientists have been far less convinced that Mars has water. They believe that the caps consist not of snow or ice but largely of frozen carbon dioxide, the principal constituent of the atmosphere of Mars. Some have suggested that the color changes are due to unknown chemical reactions, or to seasonal winds that alternately deposit sand from bright lowlands onto the dark highland surfaces and then blow it away. Their views have not been contradicted by spectrographic studies of the Martian atmosphere. Although some of these studies have detected traces of water vapor, the evidence has not been distinct enough to be completely convincing to many scientists. Now all doubts have apparently been erased. A team of astronomers working at the McDonald Observatory in Texas announced last week that they had obtained the first "absolutely conclusive proof" of water vapor on Mars.

Better Focus. To obtain their proof, Stephen Little of the University of Texas and Astronomer Ronald Schorn of the Jet Propulsion Laboratory turned to the same kind of tools that they and other scientists had previously used in attempts to detect Martian water: the telescope and the spectrograph, which breaks light into its rainbow (spectrum) of colors and records it on a photographic plate. With the aid of a $100,000 NASA grant, mirrors had just been refinished on the 82-in. McDonald telescope, bettering its focusing by a factor of three. The optics of the spectrograph had also been improved to enable astronomers to distinguish finer details of the dark absorption lines that would be produced in the spectrum by any moisture in the Martian atmosphere. On two of the nights the astronomers were viewing Mars, the air above them was uncommonly dry, minimizing the obscuring effects of the earth's atmospheric moisture on their results.

Furthermore, their spectrograms came when the planets were nearing conjunction (closest approach) and the earth was rapidly approaching Mars; this motion shortened the wave lengths of light being reflected from Mars. On the resulting spectrograms, the characteristic lines of Martian light were thus shifted away from the spectral lines produced by the earth's atmosphere, making them easily distinguishable.

After the spectrograms were developed, Schorn saw what he had been looking for. "There was the water—pow!" he says. The dark absorption lines, which stood out "as bold as fence posts," revealed that all the water vapor in the Martian atmosphere equals about a cubic mile of water, less than in a large lake on earth. Spread over the planet's surface, it would be only a thousandth of an inch deep. There was about twice as much water vapor in the Northern Hemisphere (where it is now late summer) than in the southern half (where it is late winter). This suggests that the excess vapor in the north came from the melting ice cap.

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