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FIGURE 6.3. Changes in the polar cap as it recedes in the spring. (Drawings by A. Dollfus; courtesy University of Chicago Press.)

occur, as is characteristic of natural phenomena." The irregular breakup of the polar caps, which follows the same pattern from year to year, must be attributed to permanent local conditions, possibly variations in altitude.

One of the best known of the polar-cap irregularities, which has been observed repeatedly during the recession of the southern cap, consists of some exceptionally bright isolated spots. They were discovered by O. M. Mitchel, at the Cincinnati Observatory, in 1845, and were named the Mitchel Mountains (or Mountains of Mitchel) by N. Green in 1877. These "mountains" appear regularly at the same place (latitude 73° S, longitude around 290° W) and they are always seen as isolated spots on about the same date in the Martian year (fig. 6.4). In general, the bright spots vanish after a few days and only rarely can they be seen for as long as 2 weeks.

In 1894, Percival Lowell reported observing the Mountains of Mitchel some days earlier than usual, while they were still surrounded by parts of the polar cap. "As I was watching the planet," he wrote, "I saw suddenly two points flash out in the midst of the polar cap. Dazzlingly bright upon the duller white background . . . these stars shone for a moment and then slowly disappeared." The estimated latitude and longitude of the bright spots indicated that they were two of the Mountains of Mitchel.

It has usually been assumed that the isolated parts of the polar caps, such as the Mountains of Mitchel, which remain after the surrounding areas of the cap have disappeared, are elevated regions. It is true that this would generally be the case on Earth, where snow remains on mountain peaks long after it has gone from adjacent areas at lower altitude. There is, however, no evidence that this is so on Mars. It will be seen in due course that under certain circumstances the

FIGURE 6.4. The Mountains of Mitchel in 1909, 1924, and 1941. (Lowell Observatory photographs. )

material forming the polar caps may possibly disappear first from the higher altitudes.

As each polar cap recedes, a dark band, or fringe, appears to form at the edge. The band follows—"tightly hugs" in the words of E. C. Slipher—the polar cap during its recession. At first, the band is fairly wide, but it shrinks as the polar cap decreases in size. After the summer solstice, when the cap is quite small (fig. 6.2), it is described as being a "barely discernible thread" around the white cap. On the occasions when the south polar cap disappears completely, so also does the dark band.

A dark band was evidently seen by W. Beer and J. H. von Madler around the north polar cap in 1830, but it attracted little or no interest until P. Lowell observed the formation of such a band in the southern hemisphere of Mars in 1894. Since that time, the dark bands around the polar caps have been reported on several occasions. Some astronomers have claimed that the apparent band is an optical illusion, resulting from the contrast between the bright polar cap and the surrounding darker areas, but the evidence, on the whole, seems to point to its reality as a Martian phenomenon.

Writing in 1906, Lowell described the band as being "deep blue" in color and referred to it as a "badge of blue ribbon about the melting cap." There is some doubt, however, about the blue color, and most later observers have said that under good seeing conditions it appears to be dark brown or black. According to A. Dollfus, small telescopes show a continuous dark belt surrounding the polar cap, but at greater magnification the band is seen to consist of many separate dark spots.

Composition of the Polar Caps

Until recent years, most astronomers accepted the assumption made by William

Herschel that the Martian polar caps, like those of Earth, consisted of some form of solidified water. In the early 1890's, however, A. C. Ranyard and G. J. Stoney, apparently independently, had suggested that the polar caps might be made up of solid carbon dioxide, a white crystalline substance similar to snow in its general appearance.1 At the time there was no evidence that the Martian atmosphere contained substantial quantities of carbon dioxide gas and the possibility was not taken seriously. In 1954, G. de Vaucouleurs wrote: "The polar caps are without the slightest doubt layers of crystallized water—probably more like white frost than solid ice or snow."

In 1895, P. Lowell had suggested that the water in the Martian polar caps is "probably deposited as hoarfrost"; that is to say, the material is deposited directly from the atmospheric water vapor as a solid without the intermediate formation of liquid water. If the polar caps are actually a form of solidified water, then it is certain that they must be hoarfrost deposits. If the amount of water vapor in any atmosphere, either of Earth or Mars, contributes a pressure, i.e., it has a partial pressure, of less than about 6 millibars, hoarfrost, but not liquid water, will form when the temperature falls sufficiently below 0° C.

The average vapor pressure of water in the Martian atmosphere is much less than 6 millibars. In fact, the total pressure of all the atmospheric constituents, of which water vapor is a very minor one, may not be greatly in excess of 6 millibars. If solid water is condensed from atmospheric water vapor on a cold area of Mars, it must be in the form of hoarfrost. Conversely, when the planet warms up, it is expected that the solid hoarfrost will be converted directly into vapor, a process called sublimation.

1 The commercial material known as "Dry Ice" is made by compressing solid carbon dioxide "snow."

The first report concerning an experimental study of the composition of the Martian polar caps was made by G. P. Kuiper in 1948. He compared the infrared reflection spectrum from the north polar cap with the spectrum of ordinary snow, on the one hand, and with carbon dioxide snow, on the other. Snow, like hoarfrost, is produced by direct condensation from the vapor to the solid state. The difference between hoarfrost and snow is that the former is deposited on a cold surface, whereas the latter is produced in the atmosphere at high altitude and then falls to the ground.

Kuiper found that the spectrum of ordinary (water) snow was similar to that of the Martian polar cap, but the spectrum of carbon dioxide snow was quite different. He stated, therefore, "that the Martian polar caps are not composed of carbon dioxide and are certainly composed of water frost at low temperature [much below 0° C]." From a study of the infrared reflection spectra, published in 1966, V. I. Moroz in the U.S.S.R. also concluded that the polar caps consist of solid water, rather than of carbon dioxide.

Apparent confirmation of this view was reported by A. Dollfus in 1950 on the basis of the polarization (p. 85) of the scattered light reflected from the polar caps. He showed that, when hoarfrost is heated by an electric arc, it partly sublimes into vapor, without the intermediate formation of liquid water. "The remainder," wrote Dollfus, "takes on the appearance of opal glass, full of small holes and cavities . . . and the polarization becomes very similar to that of the Martian polar caps . . . It thus seems probable that the white spots [caps] at the poles are deposits of hoarfrost." Unfortunately, Dollfus did not perform analogous experiments with solid carbon dioxide for comparison.

The albedo (p. 65) of a thick layer of fresh snow is 0.8, but the value found for the polar caps of Mars is only about 0.5. In the experiment referred to above, Dollfus noted that the porous material remaining after partial sublimation of hoarfrost had a lower albedo than originally. Thus, the surface reflecting power of the polar cap might well depend on its condition, such as size of crystals, presence of small cavities, etc. This dependence of reflectivity on particle size is well known, too, for materials other than hoarfrost.

By comparing the rate of recession of the polar caps during the spring with the heat absorbed from the Sun, G. de Vaucouleurs concluded that the average thickness of the solid layer of hoarfrost is "of the order of a few centimeters (about 1 or 2 inches)." He went on to say: "Owing to the thinness of the layer, it seems likely that it does not cover completely and uniformly . . . the roughest parts of the polar areas. This lack of continuity easily accounts for the small value of the apparent albedo." The thinness of the caps would also account for their relatively rapid disappearances during the local spring. On Earth, the polar caps are much thicker. In some regions near the poles, they are measured in miles rather than in inches.

If the polar caps consist of a form of solidified water, then their growth and regression may be supposed to occur in the following manner. When the surface temperature in the vicinity of one of the Martian poles falls sufficiently below 0° G (probably to -80° C) in the local autumn, hoarfrost condenses out from the water vapor in the atmosphere. The polar cap then starts to develop and to increase in size. As the autumn proceeds and passes into winter, the area where the temperature is always considerably below 0° C, during both day and night, extends farther and farther from the pole to successively lower latitudes. Thus, the polar cap grows steadily in size.

Toward the end of winter, the edge of the cap has reached a latitude where either the temperature is not low enough for hoarfrost to form or where the hoarfrost deposited during the Martian night disappears as the surface is warmed by the Sun in the daytime. The polar cap has then attained its maximum size.

As the winter ends and spring begins, the surface of the planet gets warmer and the hoarfrost starts to sublime (turn directly into water vapor). The temperature will first increase at low latitudes, in the regions farthest from the pole, and so the polar cap will start to recede. Apart from variations resulting from special local conditions, possibly differences in elevation, the size of the cap will decrease throughout the spring and early summer. Apparently, in some years the temperatures near the south pole rise sufficiently to permit the cap to disappear. This is not the case, however, in the northern hemisphere, with its cooler summer. The north polar cap diminishes in size, but it has never been observed to disappear completely.

One polar cap on Mars is always growing while the other is receding. In view of the small total quantity of water vapor in the atmosphere, it is evident that, assuming the caps to consist of hoarfrost, there is a continuous transfer of water vapor back and forth across the planet from one hemisphere, where the polar cap is receding in the local spring and summer, to the other hemisphere, where the cap is advancing in the autumn and winter. There is evidence from the seasonal changes in abundance of water vapor in the atmosphere of the two hemispheres, mentioned in chapter V, that such a transfer does take place.

The dark band (or collar) that surrounds each polar cap as it recedes has been ascribed to moist ground. This would imply the formation of liquid water at the edge of the cap. If equilibrium exists between the hoarfrost of the polar cap and the water vapor in the atmosphere, the hoarfrost would not be con verted into liquid water. But it is not impossible that where the temperature of the ground at the periphery of the polar cap exceeds 0° C, local conditions may permit the formation of some liquid water. As the atmosphere warms up during the summer, the water would evaporate and the dark collar would disappear, as is actually observed.

According to A. Dollfus, the polarization of the dark band does not agree with that of moist soil, but because polarization depends on many factors, it is not in this instance a compelling argument, one way or the other. There is, however, no reasonably satisfactory alternative for the not too probable explanation that the dark collar to the receding polar cap is caused by moist soil.

Carbon Dioxide in the Polar Caps

In spite of the evidence which appears to indicate that the polar caps of Mars consist of solidified water, several scientists during the early 1960's have revived the view that carbon dioxide is the sole (or chief) constituent. It is now known that the Martian atmosphere is composed mainly of carbon dioxide gas, and that its abundance is much larger than that of water vapor. In fact, the total mass of carbon dioxide in the atmosphere of Mars is probably some 10 000 times as great as that of water vapor. The problem of the transfer of carbon dioxide back and forth from one hemisphere to the other, as the polar caps wax and wane in turn, would thus be much less severe than for water vapor. The idea that the polar caps may be made up of solid carbon dioxide, rather than of solidified water, appears to be less improbable than it did to P. Lowell, who in 1895 referred to its proponents as having "that class of mind which likes to make of molehills of questions, mountains of doubts."

If the Martian polar caps consist largely of solid carbon dioxide, then the latter would be deposited, like hoarfrost, directly from the gas present in the atmosphere, without the intermediate formation of liquid, when the surface of the planet becomes cold enough. Similarly, upon warming up, during the spring, the solid carbon dioxide would sublime and be reconverted into gas. The transfer of the gas from one hemisphere to the other would probably be accompanied by a change in the carbon dioxide abundance in the atmosphere. By 1968, however, no measurements had been made whereby such a change might be observed.

One of the arguments against the view that the polar caps are solid carbon dioxide is that the temperatures attained are not low enough for the carbon dioxide gas in the atmosphere to condense out as a solid. The relatively limited, and not too accurate, estimates of surface temperature, based on measurement of the infrared radiation from Mars indicate that the average daily (day and night) temperature in the polar regions never gets below about -100° G (173° K). This would certainly not be low enough to permit the deposition of solid carbon dioxide unless the pressure of the gas in the Martian atmosphere were much higher than it is thought to be. The partial pressure of carbon dioxide in the atmosphere of Mars is about 5 to 7 millibars (see p. 90), and it is known from laboratory measurements that, in these circumstances, the temperature would have to be less than approximately — 125° G (148° K) for condensation of solid carbon dioxide to occur.

In 1966, R. B. Leigh ton and B. G. Murray of the California Institute of Technology reported results of calculations concerning the surface temperature at various latitudes during the course of a Martian year. These calculations, which were admittedly oversimplified, took into consideration the heat absorbed by the surface from solar radiation, the heat emitted from the surface, and the heat exchanged between the surface and the underlying layers of the ground by thermal conduction. Two important results were obtained in favor of the view that carbon dioxide could be condensed in solid form from the atmosphere of Mars.

First, at latitudes above about 50° N and 45° S, respectively, the computations indicated that the temperature should fall low enough to permit solid carbon dioxide to be formed. If allowance is made for the requirement that the temperature must remain below — 125° C (or so) during the day as well as the night, these calculated latitudes are in good agreement with the average latitudes (about 60° N and 50° S) to which the polar caps are observed to extend.

Second, by taking into consideration the latent heat of sublimation, which must be provided when either solid carbon dioxide or solidified water is converted into vapor, the theoretical rate at which the polar cap should recede during the spring and early summer can be determined. It was found that if the caps are assumed to be solid carbon dioxide, the calculated rate agrees well with that observed (fig. 6.2). On the other hand, if water is the main component of the polar caps, the calculated and observed recession rates are quite different.

The maximum amount of solid carbon dioxide which might be expected to condense on the polar caps is estimated to be from 100 to 150 g/cm2. The density of solid carbon dioxide in closely packed form is 1.56 g/cm3. Such a deposit would represent a minimum thickness of from 58 to 96 centimeters (roughly 2 to 3 feet). Since it is unlikely that the carbon dioxide crystals will be tightly packed, the average thickness of the polar caps might be 3 to 5 feet.

Another approach to the subject was made independently in 1966 by C. Leovy of the

National Center for Atmospheric Research, Boulder, Colo. During the period of winter darkness in the polar regions, essentially no heat is received from the Sun and conduction in the ground can be ignored. There is then a balance between the heat radiated from the Martian surface into the atmosphere and that returned to the surface from the atmosphere by radiation and by turbulent motion. This balance involves the temperatures of both the atmosphere and the ground, as well as certain characteristic properties of the atmosphere.

By combining available experimental data with the heat balance equation, Leovy calculated the minimum atmospheric temperature that would prevent the Martian surface from reaching — 125° C when solid carbon dioxide can be deposited. The results showed that, for a clear atmosphere, meaning one free from clouds, it is probable that the condensation of solid carbon dioxide will occur in the winter polar region. On the other hand, if the local atmosphere contains a cloud of microscopic ice crystals, with a total mass of 0.001 g/cm2, then the deposition of solid carbon dioxide on the ground might be partially inhibited but not prevented. Because such a cloud would require almost all the water present in the Martian atmosphere, it is very unlikely that it could ever form over the winter pole. The conclusion drawn was "that carbon dioxide condensation in the Mars polar caps is quite likely. However, the deposition rate may be strongly dependent on the formation of extremely tenuous water ice clouds in the winter polar regions."

Further evidence that the polar caps might consist of carbon dioxide was reported in 1968 by D. M. Morrison and C. Sagan. They used infrared measurements, made by W. H. Sinton and J. Strong in connection with the temperature measurements described on page 133, to calculate the temperature gradient on the

Martian surface between the equator and 45° latitude. A relatively short extrapolation to the latitude of the edge of the polar cap gave a predicted surface temperature that is consistent with the formation of solid carbon dioxide.

If the polar caps of Mars are made up mainly of solid carbon dioxide, then what of the infrared reflection spectra and the polarization measurements which seem to indicate that the polar cap material is a form of solidified water? In answer to this question, Leigh-ton and Murray have stated: "We are not aware of a sufficiently thorough . . . study of the reflection spectra of both solid water and carbon dioxide under simulated Martian conditions to justify the identification." Furthermore, they point out that Dollfus "found difficulty in producing a form of [solidified] water . . . that would exhibit the polarization . . . [of the Martian polar caps] . . . and he did not study the properties of carbon dioxide frost." It would appear, therefore, that the available evidence from reflection spectra and polarization is not conclusive. More extensive experimental studies in this area are clearly desirable.

Even if the polar caps are largely carbon dioxide, it does not mean that they do not also contain some solidified water. Because hoarfrost will condense from water vapor in the Martian atmosphere around —80° C, before the temperature gets low enough for solid carbon dioxide to form, there must inevitably be some solid water in the polar caps. The observed variations in the water-vapor content of the atmosphere according to the local season provides support for this view. In the local spring, as the surface warms up, the carbon dioxide would sublime more readily than the hoarfrost, and there would be a tendency for solidified water to remain.

A consideration of the atmospheric temperatures derived from the Mariner IV occultation experiment described in chapter V has led G. Fjeldbo, W. G. Fjeldbo, and V. R. Eshleman of the Stanford University Center for Radar Astronomy to the following conclusions :

The low daytime temperature in the lower atmosphere over Electris tends to support the carbon dioxide polar-cap theory. . . . The winter polar cap may be predominantly dry ice [i.e., solid carfbon dioxide], while the bottom part of the thicker central region, some of which persists through the summer, may be largely water ice.

The formation of liquid carbon dioxide from the solid is extremely improbable under any conditions likely to exist on Mars. Ground moistened by liquid carbon dioxide could not possibly account for the dark band which hugs the polar cap as it recedes in the local spring. It might be produced by liquid water if the polar cap contains a sufficient quantity of solidified water in addition to carbon dioxide. On the other hand, it is quite possible that the dark collar around the polar cap is not caused by moist soil but by some other, but still unknown, circumstances.

The greater the partial pressure of carbon dioxide gas in the Martian atmosphere, the higher would be the temperature at which solid carbon dioxide would condense out. This is a general conclusion that is known from laboratory measurements and is applicable under all circumstances. Because the atmosphere of Mars appears to consist mainly of carbon dioxide, the higher the atmospheric pressure, the higher the temperature at which solid carbon dioxide will deposit on the surface when its temperature drops. In other words, solid carbon dioxide will condense more readily in regions where the atmospheric pressure is high. Conversely, as the ground warms up, solid carbon dioxide will tend to vaporize most readily from areas where the atmospheric pressure is low, and least readily where the pressure is high.

The surprising conclusion to be drawn from the foregoing arguments is that solid carbon dioxide may be expected to form earliest in the autumn, and to disappear latest in the summer, at low altitudes where the barometric (and carbon dioxide) pressure is highest (fig. 6.5). This is quite opposite to the long-accepted view based on the terrestrial analogy, that the parts of the polar caps at the highest altitudes remain while the surroundings at lower altitudes disappear.

These considerations led B. T. O'Leary and D. G. Rea, University of California, Berkeley, in 1967, to suggest that the temporary isolated bright patches in the polar caps, observed repeatedly at the same locations, are actually depressions, such as craters or valleys, rather than elevated areas. A similar conclusion was reached independently by J. B. Pollack and C. Sagan on somewhat different grounds (p. 122). If the bright areas are actually at lower elevations than their surroundings then the supposed Mountains of Mitchel should more properly be named the Depressions of Mitchel. The permanent north polar cap, centered some 65 kilometers from the pole, has been attributed to the presence of a plateau at an elevation of about 1000 meters (3300 feet). If the views ex-

Barometric pressure low (solid carbon dioxide disappears first and deposits last)

Barometric pressure high (solid carbon dioxide deposits first and disappears last)

Barometric pressure high (solid carbon dioxide deposits first and disappears last)

FIGURE 6.5. Deposition and disappearance of solid carbon dioxide at different elevations.

pressed above are correct, this region would be a large, flat valley rather than a plateau.

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