MARTIAN ATMOSPHERE
As on Venus, carbon dioxide is the most abundant constituent of the thin Martian atmosphere, amounting to about 95 percent. We know that the atmosphere also contains about 3 percent nitrogen, about 2 percent argon, lesser amounts of atomic and molecular oxygen, and traces of other constituents.
A small, daily, and seasonally variable amount of water vapor has been detected on Mars. The abundance of water in the atmosphere, however, is far too low for rain. Because the atmospheric pressure on Mars's surface is low (less than 1 percent of the earth's sea-level atmospheric pressure), water vapor cannot exist as a liquid on the open, flat ground, and rain could not fall even if water were more abundant.
Early-morning fog lying in craters and other low places is probably evidence of an exchange of water vapor between subsu rface or su rface ice and the atmosphere. The Martian atmosphere also possesses clouds, which are most probably condensations of water ice and carbon dioxide ice.
The warmest daytime temperature is around 30° C at the Martian equator, while the nighttime temperature drops to -130°C. Over the polar regions it is even colder. During summer the north polar ice cap gets up to only about -70°C-though very cold, not cold enough for the residual cap to be made of carbon dioxide ice. Thus it appears that it is water ice, which is consistent with finding more water vapor in the atmosphere at high latitudes near the poles.
One of the most exciting things to happen since the landing of the Vikings was the photographing of frost on the surface at the Viking 2 site. The frost occurred during the n~rthern winter, between May and November, 1977. The composition of the frost is not known; the air temperature was too warm for it to have been pure carbon dioxide ice, and the air was too dry for it to have been pure water ice. The best speculation is that it is some kind of mixture of carbon dioxide and water.
There is speculation that water ice may be a remnant of a denser atmosphere that Mars may have had in the first billion years or so of its existence. If that early atmosphere was a denser carbon dioxide (say 1 00 to 200 times more than at present) and water-vapor one, then it could have acted to trap infrared radiation in the greenhouse effect, and it would have been warm enough to contain substantial amounts of water vapor. This increased amount of carbon dioxide could easily have been provided by outgassing from the body of the planet early in its history. However, over time the formation of carbonate rocks removed carbon dioxide from the atmosphere and lowered both the temperature and pressure. In such conditions the
atmosphere could no longer retain much water, and water could not exist as a liquid on the surface. Thus the era of water erosion ended for Mars some time ago.
The skies at the locations of Viking 1 and 2 are yellowish brown in color and seem to remain that way over the course of the Martian year. This color is probably due to dust particles suspended in the atmosphere below about 50 kilometers. Surface winds can stir the atmosphere sufficiently to hold dust particles. From the Viking data we know that the prevailing winds are westerly, as on earth, with velocities up to 70 kilometers per hou r at the su rface and over 360 kilometers per hour at altitudes above 10 kilometers. The winds are strong enough to create major dust storms that can engulf almost the whole planet and last for months. Undoubtedly the winds come from unequal solar heating among different portions of the Martian surface, driving air from highpressure areas to low-pressure areas (as on the earth).
