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Local season

Southern hemisphere

FIGURE 6.13. Seasonal variation in polarization difference between light and dark areas. (After A. Dollfus.)

Local season

FIGURE 6.13. Seasonal variation in polarization difference between light and dark areas. (After A. Dollfus.)

planet Mars, to keep an open mind on this subject.

Elevations of Bright and Dark Areas

On the Moon, the darker regions are undoubtedly at a lower elevation than the bright, highly cratered areas. For about a hundred years, the general opinion among astronomers had been that an analogous situation existed on Mars. The dark areas were considered to be low-lying regions, whereas the bright ones were thought to be uplands. This view was based to a great extent on observations of the formation of clouds, the deposition of frost, and the disappearance of the polar caps. The word "frost" is used here in a general sense to include both water frost (hoarfrost) and solid carbon dioxide.

In the first place, it is reported that white clouds on Mars seem to be formed preferentially, and seem to remain, over certain bright areas (ch. VII). Furthermore, frost deposits, which are seen to be present on the western limb of the planet as it emerges from night into daylight, tend to be localized in the bright regions. Finally, the south polar cap is often extended locally into such bright areas as Hellas and Argyre. In addition, the remains of this polar cap are located over a bright region, whereas the south pole itself, from which the cap disappears in the summer, is dark. The bright areas are assumed to be colder, at a higher altitude than the dark regions.

If the frost deposits and the polar caps consist mainly of solid carbon dioxide, then at least some of the foregoing arguments may be invalid. (They may also be invalid even if the polar caps consist of condensed water, such as hoarfrost, and not carbon dioxide.) It is probable that the Martian atmosphere is mostly carbon dioxide gas, so the tendency for carbon dioxide frost to deposit on the ground or to cause a solid carbon dioxide polar cap to form would be determined by the atmospheric (barometric) pressure, rather than by the temperature. In these circumstances, the bright areas would be those where the atmospheric pressure is highest, and they would be at low rather than at high altitude.

Regardless of the nature of the Martian frost and polar caps, it is by no means certain that the higher elevations on Mars will be significantly colder than adjacent low-lying areas. There are three main reasons why this situation exists on Earth. First, the "greenhouse" effect, whereby significant quantities of heat radiation from the ground are trapped by the atmosphere, decreases with increasing altitude. Second, because of the up-and-down slopes in a mountainous region, the radiation from the Sun is spread more thinly over a larger area than on flat, low-lying ground. Third, the air is cooled as a result of expansion when it rises to higher elevations where the atmospheric pressure is lower.

C. Sagan and J. B. Pollack (1966) have argued, however, that none of the circumstances described above as existing on Earth is of great significance on Mars. On the whole, it appeared that the highlands on the latter planet might be no more than a few degrees cooler than the nearby lowlands. The common belief that the higher elevations on Mars must be substantially colder than low-lying areas might be quite unjustifiable, and conclusions based on this belief may consequently be incorrect.

The foregoing considerations indicate that the bright areas on Mars are not necessarily uplands. Arguments will now be examined which suggest that they are actually at lower elevations than the dark areas. In the first place, D. G. Rea pointed out in 1964 that a variation in size of the surface particles combined with a range of elevations could result in a fractionation (or separation) by the winds over a period of time. If this occurred, there might be a tendency for the smaller particles to collect on large, flat, low-lying areas, and these would be bright in appearance. On high, flat regions and gentle slopes, the particle-size distribution would depend on the atmospheric circulation, but there would be a preference in favor of the larger particles. The elevated areas would then be darker in color. Seasonal exchange of the smaller particles by the wind, between the higher (bright) and lower (dark) areas, would account for the observed changes in the dark regions.

Another argument was put forward in 1965 by R. A. Wells, University of London, based on the observations reported by A. Doll-fus and J. H. Focas that certain isolated white (condensation) clouds in the Martian atmosphere tend to form and remain stationary over bright areas. Such clouds seem to be adjacent to, and alined with, the boundary between a bright area and a dark region. They occur, for example, at the boundaries of the dark Sabaeus Sinus with the bright Deuca-lionis Regio (to the south) and Edom (to the north).

On Earth, similar stationary clouds are known to form on the lee side of a high ridge, the side protected from the wind. It is by no means certain that the conditions for cloud formation on Mars are the same as those on Earth; but if they are similar, then the bright areas over which the clouds are observed are at a lower elevation than the adjoining dark areas.

C. Sagan and J. B. Pollack have presented other meteorological evidence that appears to favor the idea that the dark areas on Mars are elevated regions. It will be seen in the next chapter that the yellow dust clouds, which form occasionally in the Martian atmosphere, develop over the bright areas and then are

FIGURE 6.14. Apparent deflection of dust clouds by dark areas. (Drawings by A. Dollfus, Annales d'Astrophysique, Vol. 28, p. 722 ( 1965).)

carried, presumably by winds, across the planet. There are indications that, although the yellow clouds sometimes obscure dark areas adjacent to the bright region where they originated, the darkest of such areas are rarely crossed by these clouds. In fact, the dust clouds often appear to be deflected by dark areas (fig. 6.14). The major dark areas thus seem to constrain the possible paths of a dust-storm. It is to be expected that the dust particles would be carried by the winds mainly along valleys and across lowlands, in general.

If this is the case, then the dark areas, which the clouds seem to avoid, are uplands.

During the oppositions of 1963 and 1965, R. M. Goldstein, using the facilities of the Jet Propulsion Laboratory at Goldstone, Calif., studied radar signals at a wavelength of 12.5 centimeters, which were reflected from Mars as it rotated. In this manner, the radar tracked a swath around the planet between latitudes of approximately 10° N and 30° N, cutting across several prominent dark areas on the surface as well as many bright regions.

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