The orange and yellowish-brown areas which constitute the major part—about 70 percent—of the surface of Mars, mainly in the northern hemisphere, are responsible for the reddish color of the planet as seen by the unaided eye. In 1809, H. Flaugergues thought that the reddish areas were large clouds, but their permanent nature indicated to others that this was not so. Later, they were regarded as dry land, by contrast with the dark areas which were considered to be bodies of water, and the brighter areas were commonly known as continents. Because of their appearance, W. H. Pickering in 1886 referred to the orange and brown regions as deserts, and this name has been commonly used, although the areas probably bear little, if any, resemblance to terrestrial deserts.
Since the words "continent" and "desert" have certain implications, which are undoubtedly incorrect, it is preferable to avoid their use entirely. The regions under consideration are best described by the noncommittal term bright areas. They do not all have exactly the same color, but reds and yellows predominate in all cases. The adjective "bright" is used only to indicate brightness relative to the dark areas. Thus, the albedo in red light, for which the value is largest in the visible spectrum, is only 0.28 for the bright areas. This is relatively high in comparison with 0.16 for the dark regions. As noted earlier, the albedo for the polar caps is about 0.5.
In 1934, the German-born astronomer Rupert Wildt suggested that the bright areas on Mars were composed "of strongly oxidized sandy formations, with iron almost completely in the form of the oxide Fe203 [ferric oxide]." This oxide exists in different forms in several terrestrial minerals, but they are all characterized by their red, brown, and yellow colors.
An attempt at a more precise characterization of the oxide was reported by A. Dollfus in 1951 and confirmed in 1957 on the basis of polarization measurements made during the four Martian apparitions from 1948 through 1954. He compared the curve showing the polarization of the bright areas on Mars as a function of phase angle (p. 43) with that of various terrestrial minerals, and concluded that "the polarization of the desert regions on Mars is especially well reproduced by limonite, a hydrated iron [ferric] oxide . . . in a finely pulverized condition." A comparison of the polarization of the bright areas on Mars (full curve) with that of powdered limonite (dots) is given in figure 6.6.
Observations of the color and albedo of Mars, reported by V. V. Sharonov in the
U.S.S.R. in 1961, and of the infrared reflection spectra, by G. Sagan and his collaborators in 1965, provided support for the view that the bright areas are covered by the mineral limonite, which is so named because of its lemon-yellow color. It has been pointed out, however, that limonite is not a well-defined material and that the word is used in geology to include several ferric oxide minerals of different crystalline forms and containing different amounts of water. Disregarding impurities, the general composition may be represented by Fe203 • xH20, where x is a variable quantity. Thus, in 1964, A. L. Draper and his coworkers were able to reproduce the infrared reflection spectrum of Mars in the wavelength region of 1 to 2 ¡x (p. 76) by means of a finely powdered mixture of the minerals goethite (Fe203-H20) and hematite (Fe203). The latter is so named for its blood-red color.
A detailed review of the infrared spectra and polarization measurements on Mars and on various ferric oxide minerals has led J. B. Pollack and C. Sagan to conclude in 1967 that the bright areas of Mars are covered with significant quantities of powdered limonite, consisting mainly of goethite, with a minor proportion, if any, of hematite. There must consequently be at least one molecule of water to each molecule of ferric oxide. The fact that the molecule of water in goethite is apparently water of hydration, which can be driven off fairly readily by heat, is important in relation to the question of the availability of water on Mars (ch. X). The average diameter of the limonite particles was estimated to be about 25 microns.
Regardless of the exact significance of the term "limonite," it seems to be generally agreed that the color of the bright areas on Mars is attributable to the presence of hydra ted ferric oxide (or oxides). There are differences of opinion, however, concerning the relative abundance of this material. Some scientists claim that it is a major component of the bright areas, at least of the surface layers, whereas others argue that this is not so.
The suggestion made in 1964 by A. B. Binder and D. P. Cruikshank is that the limonite is merely a fairly thin surface stain formed by oxidation and hydration, the action of oxygen and water, respectively, on iron-bearing minerals in igneous rocks formed in the interior of the planet. This view is based on the observation that the infrared reflection spectrum of Mars can be well matched by rocks of this kind found in deserts on Earth. If the Martian surface consists of such a material, then there must have been a time when the atmosphere of the planet contained substantial quantities of oxygen and water vapor, incidentally, surface temperature measurements, to be described later in this chapter, show that the bright areas of Mars must be covered mainly by a finely powdered material rather than by rocks of substantial size.
Another possibility, based on geological considerations, has been proposed by R. A. Van Tassel and J. W. Salisbury in 1964. They suggested that powdered silicate minerals, formed from pulverized crustal rock, comprise the major portion of the surface of Mars, just as they do on Earth and evidently also on the Moon, as indicated by data from the Surveyor spacecraft. Furthermore, the composition of stony meteorites called chondrites indicates that there should be relatively more silicon than iron in the outer layers of Mars.
An added complication is provided by W. M. Sinton's contention that the strong emission band in the spectrum of Mars at a wavelength of 3.1 /i indicates the presence of hydrated minerals on the surface of the planet. Sinton and others, however, have called attention to the weakness of the band at about 0.88 ¡x normally exhibited by limonite. This implies that limonite cannot be the major hydrated mineral on the Martian surface. It is presumably not a silicate because silicates in nature are formed at high temperatures and do not contain water of hydration.
If the proportion of iron on the Martian surface is actually high, there are two factors which could be responsible. First, if there has been little or no differentiation in the interior of Mars, as suggested in chapter IV, the density of the outer layers, including the surface, will be approximately the same as the average value for the planet, about 4 g/cm3. It is to be expected that the surface material on Mars will contain a larger proportion of heavy elements, such as iron, than is present on Earth, in which differentiation has occurred. The density of the surface rocks on Earth ranges from about 1.6 to 3.5 g/cm3. It is probably a coincidence that the density of limonite is close to that of the average density of Mars as a whole.
Mars is close to the asteroidal belt (ch. Ill). For this reason, meteorites, many of which contain considerable proportions of iron, may have fallen on Mars over the ages in greater numbers than on Earth or the Moon. This possible way of accounting for the large proportion of iron on the Martian surface was apparently first proposed by the famous Swedish chemist Svante Arrhenius in 1910, and it has been revived in recent years. The oxygen required to produce the highly oxidized ferric state, in which the iron now appears to exist on the surface of Mars, would presumably have been derived from water by the action of ultraviolet radiation from the Sun.
The Dark Areas
For many years astronomers, including
G. V. Schiaparelli, thought that the darker areas on Mars were actually bodies of water, and they were called maria (plural of the Latin mare, meaning sea). In 1892, W.
H. Pickering claimed to have detected some canals crossing the mare, and this observation was confirmed in 1894 by A. E. Douglass. The idea which then came into prominence, based on a suggestion originally made by A. Liais in 1860, was that the darker areas on Mars were covered with vegetation. Some students of the planet still favor this point of view, but there are other possible explanations for these areas. In any event, they are certainly not seas, and they will be referred to here simply as the dark areas, because of their contrast with the brighter areas considered above.
The dark areas cover about one-fourth of the planet, mostly in the southern hemisphere. The degree of darkness is not the same for all the dark areas, and even for a given area it varies during the course of a Martian year. The albedo is also variable, but the average value for visible light is ap proximately 0.16, as stated earlier. Thus, the dark regions on Mars have a little more than half the brightness of the brighter areas.
The color of the dark areas has been the subject of much discussion. On the basis of early visual observations, these areas have been described as being gray (or blue gray) in the local winter, changing to green (or blue green) in the spring and summer. There is a possibility that these reported colors may be attributed to psychophysiological effects. When a neutral-colored area is adjacent to one that has a definite color, the former often appears to have the complementary color. This could cause regions adjoining the orange and reddish areas on Mars to appear greenish or bluish. The change in apparent color of the dark areas with the season is attributed to changes in the albedo.
During the opposition of 1956, G. P. Kui-per made a careful study of Mars, including the colors of the dark regions, with the 82-inch telescope of the McDonald Observatory, Texas. He concluded that in the spring the dark areas had a neutral-gray color, with a touch of moss green in some of the equatorial regions. The greenish or blue-green colors reported by other observers, he thought, were caused by poor seeing conditions.
In order to avoid possible subjective effects, C. F. Capen, of the Jet Propulsion Laboratory, and his associates, used color filters in their observations of Mars during the apparition of 1964-65. In an extensive account of this work, Capen says: "The southern maria [i.e., dark areas] were medium to dark contrasts of dark purple and brown. The Syrtis Major was changing from a blue-green to a green-blue hue. The Trivium Charontis showed only a moderate-contrast brown coloration."
In another section, Capen reports: "The Mare Acidalium changed from its winter shades of variegated gray and brown to its spring coloration of dark gray and blue-gray shades with gray-green oases. ... In . . . late spring and early summer, the Acidalium . . . became a very dark gray general shade with a black-green central area and large dark gray-green oases." There is obviously no simple answer to the question: What is the color of the dark areas on Mars?
Another approach to the color of these dark areas can be made on the basis of the variations of the surface reflectivity with the wavelength of the light. The quantity called the reflectivity here is a measure of the sunlight reflected back to Earth by the Martian surface when the directions of both the incident sunlight and the reflected light are almost at right angles to the surface. The results, derived from the data of A. Dollfus and J. H. Focas, are shown in figure 6.7 for both bright and dark areas of Mars. At the time the measurements were made, the dark areas were not at their darkest.
The marked increase in the reflectivity of the bright areas with increasing wavelength of the light, that is, in going from blue to red, accounts for the orange and reddish colors of
: : 45O0Y Y 75000 Y Y 5500 ; 6000 6500 '.Y-, Wavelength, .A' :•"• •FIGURE 6.7. Reflectivity of bright and dark areas of Mars at different wavelengths. (After
these regions. But the dark areas also exhibit an increase in reflectivity, although a smaller one, with wavelength. Hence the dominant color of these areas must be toward the red end of the spectrum. Thus, as J. B. Pollack and C. Sagan have stated: "The dark areas are red, although not as red as the bright areas." The neutral gray appearance can be accounted for, as will be seen in due course, but it is difficult to understand the blues and greens observed with color filters.
Although the bright areas differ somewhat from one another in color, each region seems to have a uniform appearance. But this is not true for the dark areas. In 1909, E. M. Antoniadi had reported, and later confirmed, what he called the "leopard-skin" structure of these areas; they consisted of-darker spots on a somewhat lighter background. According to the description given by J. H. Focas in 1962, the dark areas on Mars consist of "a dusty background of a rather granular appearance patched by dark blocks, groups of spots, or isolated spots of various sizes, constituting the fundamental nuclei of dark matter in these areas." Part of a map of Mars prepared by Focas, reproduced in figure 6.8, indicates how he saw the dark areas.
An important characteristic of the dark areas on Mars is the changes they display in the degree of darkness and often in their size. Three general types of changes in appearance have been described. There are, first, widespread seasonal (or periodic) changes which occur during the course of each Martian year. Then there are localized seasonal changes which may vary from year to year. Finally, there are irregular secular changes which may persist for several years.
In the 1880's, G. V. Schiaparelli had noticed that some dark areas did not appear equally dark at successive oppositions, and he suggested that this might be a seasonal effect. Similar changes were reported by the French astronomer E. L. Trouvelot in 1884. Then in
1894, P. Lowell stated that "during the summer of the Martian southern hemisphere, a wave of seasonal change swept down from the poles over the face of the planet." A more complete description of the change in the dark areas was given by Lowell in 1896. "The first marked sign of change," he wrote, "was the reappearance of Hesperia [240° W, 20° S], whereas in June  it had been practically nonexistent, by August it had become perfectly visible . . . but it returned in October to a midposition of visibility."
The period Ju,ne through October 1895 corresponded to late spring and early summer in the southern hemisphere of Mars. The area known as Hesperia, which had been very faint in the local winter, became much darker during the late spring. But in the early summer the intensity had decreased again. This temporary darkening effect was observed by W. H. Pickering on other dark areas of Mars, and in 1924, E. M. Antoniadi reported its general nature. Just before the local spring equinox each year, the dark areas nearest the pole become darker. The darkening spreads toward lower latitudes, later and later in the spring, but the already darkened areas nearer the poles become somewhat lighter again, as they were in the winter (fig. 6.9).
An attempt at a quantitative study of this wave of darkening, as it is now called, was made by G. de Vaucouleurs in 1939. He recorded visual estimates of the changes in intensity during the spring in the southern hemisphere of Mars of various dark areas, ranging from Depressio Hellespontia, at a latitude of approximately 60° S, at the edge of the south polar cap, to Niliacus Lacus, at about 40° N, in the northern hemisphere. "The darkening," said de Vaucouleurs, "starts near the end of the southern winter at about latitude 60° S; it then spreads and crosses the equator before mid-spring, reaching latitude 40° N before the end of the southern spring."
In 1962, J. H. Focas reported the results of observations on the seasonal darkening made with instruments (photometers), during several Martian apparitions, at the Pic du Midi Observatory in France. A general indication of the conclusions, to which there are some exceptions, is represented in figure 6.10. The dashed arrows for each hemisphere indicate the approximate seasonal time limits of the darkening period at various latitudes, whereas the continuous arrows show when the maximum darkening is attained. The phenomena are less clearly evident in the northern than in the southern hemisphere, but they are probably similar in both hemispheres.
The wave of darkening is seen to start from each polar cap in turn, at intervals of half a Martian year, toward the end of the local winter and to spread into the other hemisphere by late spring. According to Focas, "a darkening wave can be traced . . . traveling at a rate of 30 kilometers [19 miles] per day from the circumpolar area toward the equator and extending across the equator to latitudes 22° [approximately] in the opposite hemisphere."
An interesting point which is apparent from figure 6.10 is that in the vicinity of the equator, the darkening is essentially continuous. The wave from one pole commences just about the time that the wave from the other pole is ending. The equatorial regions thus remain fairly dark essentially the whole year around. "Temperate and circumpolar areas," says Focas, "may attain a maximum intensity equal to that of the equatorial areas, but they weaken or vanish as soon as the wave has passed."
A statistical study, published in 1967 by J. B. Pollack, E. H. Greenberg, and C. Sagan, shows that, although there is a significant correlation between the latitude and the time of maximum darkening, there are nevertheless some exceptions. An outstanding example is
Winter Spring Summer
Winter Spring Summer
Winter Spring Summer
FIGURE 6.10. Wave of darkening on Mars.
Winter Spring Summer
FIGURE 6.10. Wave of darkening on Mars.
provided by Tithonius Lacus, which is close to the equator (5° S latitude). When the darkening spreads down from the south pole of Mars, this area begins to darken "earlier than any other dark area and reaches its maximum darkening at a time similar to those for the highest latitudes [i.e., nearest the poles]." There are either local surface variations or atmospheric conditions (or both) in the Tithonius Lacus area that are responsible for the exceptionally early darkening.
Before discussing the possible cause of the wave of darkening, the two other types of changes in the dark areas will be described because all these changes may well be related. In addition to the regular seasonal variations, there are often erratic or irregular local changes associated with the regular ones. For example, certain dark areas appear different each spring and the boundaries vary from one Martian year to another and so also do the structural details. An outstanding illustration is provided by the region called Solis
1877 1907 1909
1924 1926 1939
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