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FIGURE 6.11. Appearance of the Solis Lacus region near six Martian oppositions. (After E. M. Antoniadi, "La Planète Mars", and V. V. Sharonov, "Mars.")

Lacus (90° W, 28° S). Its appearance near six Martian oppositions from 1877 through 1939 is shown in figure 6.11. The changes are evidently not permanent, and E. C. Slipher reported that in 1958 and 1960 the Solis Lacus area closely resembled its appearance in 1907.

Among other regions showing irregular behavior are the Nepenthes-Thoth area, Utopia, Trivium Charontis, and Nilokeras. In 1909, Nepenthes-Thoth was almost invisible, but by 1939 it had developed into a conspicuous dark band. Although it was described as a slight dusty streak in 1952, at the next apparition in 1954 it had become a large dark area. In later years Nepenthes-Thoth was much less marked again. According to A. Dollfus, "it seems that all Martian [dark] features are, sooner or later, subject to such changes."

The third type of change observed in the dark areas is represented by the long-term

(or secular) changes. These variations often arise quite suddenly, that is, between two successive oppositions, and may last for several years. In 1941, A. Dollfus observed a broad and dark area in the Boreosyrtis (290° W, 55° N) and Dioscuria (320° W, 50° N) regions which had not been recorded on any earlier maps. It has been observed at subsequent Martian apparitions, although its intensity has varied from one opposition to the next. Another example is provided by the dark Mare Cimmerium which extended its northern edge by almost 1000 kilometers (620 miles) in 1939, and has remained so since. The southern boundary, along the bright area Zephyria, changed at the same time, but it returned to normal in 1948.

Occasionally, a large new dark area of considerable size develops within a bright region. E. C. Slipher reported in 1954 that a previously unobserved dark area of nearly 2.5 million square kilometers (600 000 square miles) had developed east of Thoth and remained essentially unaltered in later years (fig. 6.12). Many other cases of secular changes of the dark areas on Mars have been

FIGURE 6.12. Large dark area (bottom left) first observed in 1954. (Lowell Observatory photograph.)

reported, but the illustrations given above are sufficient to indicate the nature of the phenomenon.

In spite of the changes in the Martian surface, both seasonal and secular, the main features of the planet are permanent in nature. It is true that the same details cannot always be seen but, in general, the prominent named areas appear in the same locations from one apparition to another. Were this not so, it would not be possible to prepare maps of Mars, such as the one in figure 2.21.

Some regions have remained the same, apart from seasonal darkening, for over 100 years. An outstanding example is Syrtis Major which Huygens represented in his crude drawing of 1659. Another is the dark Sinus Sabaeus which stretches across the planet for some 3000 kilometers (1860 miles) almost parallel to and south of the equator. It is clearly seen in the map of W. Beer and J. H. von Madler of 1840 (fig. 2.15) and in R. A. Proctor's map of 1867, as well as in modern maps of Mars. There are sometimes minor differences in size and degree of darkness, but the reason is usually that the seasons are not always the same on Mars at successive oppositions.

Explanation of the Wave of Darkening

Interpretation of the changes in the dark areas of Mars is one of the most intriguing and controversial problems relating to this planet. An examination of figure 6.10, where the changes in the polar caps throughout most of the year are indicated at the top and bottom, shows that the wave of darkening commences at high latitudes, nearest the pole, at about the time the polar cap begins to recede. The wave then continues to move to lower and lower latitudes, while the polar cap gets smaller and smaller in size during the local spring.

It would seem, therefore, that there may be a direct connection between the disappearance of the polar cap and the steady advance of the wave of darkening toward and across the equator. If the polar cap consists (or contains a significant amount) of water, then it might be expected that the gradual transfer of water vapor from higher to lower latitudes is responsible in some way for the darkening. Although different ideas were expressed concerning the function of the water, the general point of view was accepted almost universally until 1957.

Perhaps the most popular interpretation of the wave of darkening, because it appears to be so simple and straightforward, is that the darkening is related to the presence of a primitive form of vegetation. This possibility was first indicated by E. L. Trouvelot when he wrote in 1884 that "one could believe . . . these changing grayish areas are due to Martian vegetation undergoing seasonal changes." During the Martian winter, this vegetation would be dormant because of the low temperature and the lack of water.

As the solidified water from the polar cap started to vaporize (and possibly to liquefy) at the end of the local winter, the water vapor would be carried to lower latitudes by the prevailing winds. The arrival of water in some form, combined with the simultaneous increase in temperature as winter turned to spring, would cause the vegetation to revive. The resulting change in the character of the surface would then appear as a darkening. Later in the year, as the amount of water vapor in the atmosphere declined, the vegetation would become dormant again and the surface would appear less dark.

It was pointed out by P. Lowell in 1895 that the foregoing hypothesis would imply a situation on Mars quite different from that on Earth. On Earth, the spring revival of vegetation commences at low latitudes, near the equator, where it is warmest, and extends to higher latitudes later and later in the year. On Mars, however, the reverse would be true. Revival would start near the pole and proceed toward the equator. The reason for this difference is that on Earth there is generally an ample supply of moisture and the revival of plant life is usually dependent on the temperature. But on Mars, water is the limiting factor, and this would become available first in regions nearest the pole where the main supply is concentrated.

What was at one time regarded as a cogent argument in favor of the vegetation hypothesis was put forward in 1950 by the Estonian-born astronomer E. J. Opik, then in Northern Ireland. Mars is subject to occasional severe duststorms which may cover extensive areas of the planet and last for several weeks (ch. VII). Opik pointed out that unless there was some means of regeneration, the dark areas would have become completely covered by the yellowish-orange dust during the course of time. Dark regions do appear lighter after a duststorm, but they recover their original appearance within a few weeks. It seemed that the growth of a form of plant life would be the most obvious way to explain this recovery. There is, however, an alternative possibility which will be examined shortly.

The idea that vegetation is responsible for the seasonal wave of darkening can also be applied to account for the other types of changes observed in the dark areas. The fading away of an existing dark region can be explained by the rapid extermination of the vegetation. It is much more difficult to understand how a new dark area, sometimes covering several hundred thousand square miles, could form within a single Martian year.

in contrast to the vegetation (or organic) hypothesis, some inorganic theories have been proposed to account for the role played by water in the darkening phenomenon. In 1912, S. Arrhenius suggested that the water vapor interacts with substances present in the Martian soil, and that this is responsible for the darkening. Somewhat similar effects are known to occur in certain terrestrial deserts, but the quantity of moisture required is probably much more than would be available in the Martian atmosphere. The views of Arrhenius have received some support, but on the whole they have not been found to be satisfactory.

Another inorganic mechanism, also involving water to account for the wave of darkening, was proposed by J. Otterman and F. E. Bronner of the General Electric Co.'s Missile and Space Division in 1966. "It is suggested that freezing [of moisture in the soil] during the Martian afternoon and evening produces one or more types of surface microrelief features." Such features, referred to as microhills, have been observed to form on Earth as a result of the freezing of moist soils. By increasing the complexity, that is, the porosity and roughness, of the surface, the microhills cause the reflectivity to diminish and the optical darkening to increase.

During the daytime, as the surface warms up, the frozen water would be vaporized and gradually transported to lower latitudes where it would be used again to produce darkening. The microhills would retain their structure for some time, even when dry, but would be eventually destroyed by wind erosion, and the surface would return to its original state and lighter color.

The theory outlined above requires that water be available in liquid form, at least for a short time each day. At the low partial pressure in the Martian atmosphere, the water would normally deposit as solid hoarfrost on cooling. Such deposition would occur in the evening or at night. The next day, when warmed by the Sun, part of the hoarfrost would vaporize by sublimation, but part might turn into liquid water and be absorbed and held by capillaries in the soil. Upon freezing, the microhills would then be produced.

The great drawback to the microhill hypothesis, and in fact to any explanation of the wave of darkening involving water in some way, is that the reported average content of water vapor in the Martian atmosphere, about 14 microns of precipitable water, is so low. According to J. Otterman (1967), the minimum quantity of precipitable water required for the microhill theory is 200 microns. He suggested that spectroscopic observations be made at various locations on Mars to see if the quantity of water in the atmosphere is indeed as low as has been reported. If subsequent investigations establish that the amount of precipitable water is everywhere substantially less than 200 microns, the microhill explanation of the wave of darkening would have to be abandoned.

Another difficulty, which is independent of the question of the water content of the Martian atmosphere, has been pointed out by C. Sagan. He has called attention to the fact that the microhills would be larger than the particle sizes derived from photometric and polarimetric observations.

Because the amount of water vapor in the Martian atmosphere appears to be quite small, theories of darkening which are independent of the presence of water are now attracting interest. The basis of these theories is that light-colored dust from the bright regions is responsible for the changes, both seasonal and secular, of the dark area of Mars. This idea was suggested in 1957 by G. P. Kuiper, who thought that the dark areas might be lava fields covered with dust. Removal of the dust by the wind, such as commonly occurs on terrestrial lava flows, would then result in a darkening of the surface. "The Martian maria [i.e., dark areas] could change their visibility with the seasons depending on the atmospheric circulation."

A similar view was expressed independently by V. V. Sharonov of the U.S.S.R. in 1958. "The air currents in the [Martian] atmosphere," he wrote, "vary from season to season, depositing dust at some time of the year and blowing it away at other times. Thus, for instance, the inherently dark surface . . . may brighten at a definite time of the year as a result of settling of light-colored dust blown over from the desert areas."

The interpretation of the changes in appearance of the dark areas on Mars in terms of particle size and winds has been developed by D. G. Rea in 1964 and by J. B. Pollack and C. Sagan in subsequent years. Only the smaller, light-colored particles would be carried by the winds; the larger particles, on the dark areas, would merely saltate (bounce) along the surface. The marked changes in appearance, therefore, would be expected only on the dark regions. There is, however, some evidence that the same bright areas brighten while the dark areas are darkening. Both of these effects would be caused by the wind blowing the finer and lighter colored particles from the dark to the bright regions.

"Photometry and, particularly, . . . po-larimetry [studies]," say Pollack and Sagan, "indicate that the principal event of the wave of darkening is a change in the mean particle size, . . . with no substantial change in composition."

One question may well be asked in connection with the theory of windblown dust. Is the regular nature of the wave of darkening, from one year to another, consistent with the behavior expected from a wind system? It would probably not be on Earth, but it may be so on Mars where the meteorological structure is less complex.

If the removal of dust from the dark areas and its subsequent replacement by the wind is responsible for the wave of darkening, then the correspondence in time of this effect with the recession of the polar caps is purely coincidental. Both phenomena, although independent of each other, might nevertheless be caused by seasonal temperature changes in the atmosphere and of the surface. Local wind conditions could perhaps explain why Tithon-ius Lacus, near the equator, starts to darken before areas that are much closer to the south pole. Certainly the darkening mechanisms involving water are inadequate in this situation.

The irregular seasonal variations in the dark areas would seem to find a ready interpretation in the hypothesis that darkening is a result of the removal of light-colored dust. Irregularities in the surface, local differences in elevation, and associated variations in the wind pattern could produce a different appearance from year to year. The formation of large, new dark areas, in the secular changes, could result from the removal of a thin layer of dust by the wind. That such changes take place within a single Martian year would, then, not be surprising.

Finally, when a dark area is covered with yellow dust as the result of a Martian dust-storm, removal of the dust by the wind would account for the relatively rapid restoration of such an area to its original dark appearance. The removal would be facilitated if the dark areas, as suggested later, are elevated regions with gentle slopes. Thus, what was regarded as one of the strongest arguments in favor of the vegetation theory of darkening appears to lose much of its force.

Nature of the Dark Areas

There seems little doubt that the surface of the bright areas of Mars consists to a significant extent of some form of hydrated ferric oxide, with probably some silicate minerals. The situation with regard to the dark areas, however, is less definite. For many years there was not much speculation concerning the composition of the darker surface material, but since about 1964 there has been growing evidence for the view that essentially the same substances are present on the surfaces of both bright and dark areas. The difference in appearance is ascribed to the particles being larger on the dark surfaces. Such particles would reflect less light than the smaller particles on the bright areas and so they would appear darker. It is well known that many materials which are dark red, almost black, in color, because they have very poor reflectivity, appear orange and yellow in powdered form.

It will be recalled from figure 6.7 that the reflectivities of both bright and dark areas on Mars increase as the wavelength of the light increases from blue to red. The smaller increase exhibited by the dark regions is just what would be expected if they consisted of larger particles of the same material as covered the bright areas. The larger particles will absorb more, and reflect less, of the sunlight at the red end of the spectrum than will the smaller particles in the bright areas. Because they reflect about the same in the blue region, the larger particles will tend to give the surface a bluish-gray appearance.

It is of interest, too,, that polarization measurements made by A. Dollfus show relatively little difference between light and dark areas. The dots in figure 6.13 represent the differences between the polarization of dark areas at different times of the year and the average for bright areas measured at a phase angle of 25°. It will be noted that, although there appear to be definite seasonal changes in the polarizations of the dark areas, the values differ by not more than ±0.002 from the polarization of a bright area. In fact, for most of the local spring and summer, the polarization of the dark regions at a phase angle of 25° is essentially identical with that of the bright areas.

In a detailed review of the photometric (reflection) and polarization properties of the surface of Mars completed in 1967, J. B. Pollack and C. Sagan have concluded:

[Hydrated] ferric oxides are a major constituent of the bright areas . . . [and] also ... of the dark areas. [This] . . . follows from the near identity of the refractive indices of bright and dark areas [as calculated from reflectivity data. The] . . . agreement . . . [arises] from the very low contrast in the blue, violet, and ultraviolet, and from the fact that the polarization ... of the dark areas can be derived from that of the bright areas merely by increasing the mean particle size. Even during the seasonal darkening of the dark areas, their index of refraction remains almost the same as for the bright areas.

It should be emphasized that the idea developed above, that the bright and dark areas of Mars have the same composition but differ in particle size, is only a theory, although admittedly an attractive one. It is thus necessary, as with so many other aspects of the

Local season

Southern hemisphere

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