The Polar Caps

General Description of the Polar Caps

The main aspects of the martian surface were described briefly in chapter I. They will now be considered in greater detail, starting with the polar caps. During the course of the local autumn and winter on Mars, a brilliant white cap, steadily increasing in size, develops over each of the poles in turn. Most of the time the polar cap appears to be covered by a white cloud or hood, but toward the end of the winter, just before the local spring equinox, the cloud disappears and the sharp outlines of the cap itself become clearly visible. Subsequently, during the spring, the size of the cap gradually decreases and reaches a minimum shortly after the summer solstice, around the beginning of the local summer.

Since winter in the northern hemisphere of Mars corresponds to summer in the southern hemisphere, the northern polar cap grows at the same time as the area covered by the southern cap diminishes. Half a Martian year later, the situation is reversed; the cap around the north pole gets gradually smaller, during the local summer, while the southern cap grows in size. This seasonal waxing and waning, in turn, of the polar caps was reported by William Herschel at the end of the 18th century, and it has been observed repeatedly since that time. It is only at certain Martian apparitions that both caps can be seen simultaneously, of course, and these conclusions have been drawn mainly from observations on each cap made at different times.

When it reaches its maximum size, the southern polar cap covers a larger area than does the northern cap. The reason is that the autumn and winter, when the cap is being formed, is longer (and colder) in the southern hemisphere (381 Earth days) of Mars than in the northern hemisphere (306 days). The northern polar cap does not often extend farther south than a latitude of 60° N, but the southern cap generally reaches as far as 50° S (fig. 6.1), and sometimes to 45° (or less) S latitude. It is of interest that the extent of the polar caps on Mars, relative to the size of the planet, is much the same as on Earth. The terrestrial polar caps, as indicated by the snow cover at high altitudes, extend to about 50° latitude by the end of winter.

Polar Cap irfißate:

pected to be the coldest places, are commonly attributed to differences in altitude of the surface in the polar regions. It has been assumed that there are elevated regions centered at distances of 400 kilometers from the south pole and 65 kilometers from the north pole. By analogy with conditions on Earth, it is thought that these elevated regions would warm up more slowly than the lower areas around the poles themselves. As will be seen shortly, however, some scientists dispute this conclusion.

Recession of the Polar Caps

FIGURE 6.1. Approximate representation of the maximum extent of the south polar cap of Mars.

In spite of its larger area at the end of the local winter, the south polar cap on Mars decreases in size much faster than does the cap in the northern hemisphere. This is so because the summer, although shorter than in the north, is much hotter in the southern hemisphere, since the planet is then nearer to the Sun. In some years, the cap around the south pole vanishes completely. The cap in the northern hemisphere, on the other hand, may become quite small, about 320 kilometers (200 miles) across, but it has never been known to disappear entirely.

The remnants of the polar caps are no I centered over the actual north and south poles; that is, the points through which ihe axis of rotation of Mars passes. The locations are, however, always the same. Tli<-center of what remains of the south polar cap at the end of the local summer is some 400 kilometers (250 miles) from the south pole, whereas in the northern hemisphere the distance from the center of the cap to the pole is only about 65 kilometers (40 miles).

The deviations of the center of the polar caps from the poles themselves, which are ex-

Apart from relatively small variations from year to year, such as might occur on Earth, the rate at which the polar caps recede is remarkably regular. E. C. Slipher of the Lowell Observatory, Ariz., has compiled the results of observations of the south polar cap made over a period of several months at the times of a number of appropriate oppositions in 1798 and from 1877 through 1924. Furthermore, he checked the measurements with various drawings made between 1781 and 1862, and found them to be in general agreement. In figure 6.2, the dimensions of the cap, as indicated by its width and by the

0 60 120 147 180 240. Days from spring equinox

FIGURE 6.2. Variation of dimensions of the south polar cap during the Martian spring and summer. (After E. C. Slipher.)

0 60 120 147 180 240. Days from spring equinox

FIGURE 6.2. Variation of dimensions of the south polar cap during the Martian spring and summer. (After E. C. Slipher.)

latitude to which it extends, are plotted against the number of Earth days after the Martian spring equinox in the southern hemisphere. The breadth of the curve is a measure of the deviations in size of the polar cap from year to year and of the results obtained by different observers in any given year. Moreover, because of local differences in the surface elevations, the extent of the cap is not the same at all longitudes.

On the whole, Slipher concluded that the "study revealed no evidence of any irregularity in the . . . [recession] of the south polar cap . . . during this long period of observations." Although such detailed measurements are not available for the north polar cap, the indications are that it decreases in size at about the same rate every year.

It is probable that the polar caps also grow in size in a regular manner during the local autumn and winter, but it has not been possible to verify this by actual measurements. While the caps are increasing in size they are generally covered by white clouds, as noted earlier, and so the caps themselves are rarely visible. It is only just before the spring equinox that the cloud, or hood, begins to lift. The sharp outlines of the polar cap then appear and the cap itself is seen to be somewhat smaller than the hood. Thus, the actual size of the cap is not known during its period of growth. It is only from a few days before the local spring equinox until about 90 days after the summer solstice (fig. 6.2) that measurements can be made of the extent of the polar cap.

When a polar cap starts to recede, its outer rim becomes ragged. Some parts are seen to extend much farther than others, leaving projections which may later become detached from the main polar cap. About the middle of spring, dark rifts appear at various locations in the cap. The rifts increase in size and cause the cap to break up into a number of isolated regions which gradually decrease in area (fig. 6.3). Conversely, during the winter, these regions are the ones that form first as the polar cap develops.

"Each Martian year," says A. Dollfus, "the same details are seen in the same locations during the same part of the Martian season . . . although some delays or variations may

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