The canals have been the subject of more heated and prolonged discussion than any other aspect of the planet Mars. Many reputable astronomers, following G. V. Schiapa-relli, claim to have seen the long and narrow linear markings on the surface, whereas others, equally reputable, have been unable to confirm these observations. There are some differences in the views of those who affirm that the canals exist, indicating that there is a degree of subjectivity in their identification. Thus, in 1906, W. H. Pickering wrote: "Some observers like [A. E.l Douglass draw them [the canals] very wide; others, like [P.] Lowell . . . extremely narrow. The writer [W. H. Pickering] gives them an intermediate breadth . . . The long canals never appear to him to be as narrow as they are drawn by Lowell."
The failure of many astronomers to detect the Martian canals was attributed by P. Lowell to poor seeing conditions. He thought that only on relatively rare occasions, when the atmosphere is clear and free from turbulence, can the canals be observed. "When a fairly acute eyed observer sets himself to scan the telescopic disk of the planet in steady air," said Lowell, ". . . he will ... of a sudden become aware of a vision of a thread stretched across the orange areas. Gone as quickly as it came, he will instinctively doubt his own eyesight . . . [then] with the same startling abruptness, the thing stands before his eyes again."
The opposite point of view, that the apparently continuous linear features are the result of poor resolution, was put forward by E. M. Antoniadi in 1929. To illustrate this point, he presented the drawings reproduced in figure 6.15 of the Elysium region of Mars. The one at the left is by G. V. Schiaparelli based on observations made with telescopes of 21.8- and 49-centimeter (8.5- and 19-inch) aperture between 1877 and 1890, whereas the one on the right was made by Antoniadi, using an 83-centimeter (33-inch) instrument between 1909 and 1926.
In the same connection, two drawings made by A. Dollfus, with a 60-centimeter (24-inch) refracting telescope, of the Syrtis Major region in 1948 are of interest. The left-hand drawing in figure 6.16, made under conditions of average seeing, appears to show the double canals Thoth and Nilosyrtis. With perfect seeing, as in the drawing on the right, the continuous lines (and dark areas, in general) are broken up into light and dark
patches. In the words of Dollfus: "Canals may sometimes be noted across ocher-colored deserts; they appear only when the seeing is not very good, either as floating filaments or as soft bands. When the seeing becomes excellent, one sees at the same place only small spots or markings, more or less aligned."
At the present time, the majority of astronomers would probably agree that the ex
tensive geometric network of canals drawn by G. V. Schiaparelli and by P. Lowell (ch. II) does not exist. Nevertheless, there definitely appear to be a number of linear features on the surface of Mars. They are probably at least 100 kilometers (62 miles) in width and, in detail, largely discontinuous and irregular. The seasonal variations in the appearance of the canals are quite similar to the changes
FIGURE 6.16. Drawings made by Dollfus under conditions of average (left) and perfect ( right ) seeing. ( Courtesy University of Chicago Press. )
exhibited by the large dark areas during the wave of darkening.
Thus, Schiaparelli noted that in the local winter the canals are faint and often invisible, but during the spring they become darker. The darkening, as Lowell reported, begins near the poles and travels toward the equator, just as it does in the other dark regions. At some apparitions, often separated by several years, the canals are broad and distinct, whereas at other times they are almost invisible. G. de Vaucouleurs stated in 1966 that "the Nilosyrtis, the largest and darkest canal on Mars throughout the latter half of the Nineteenth Century, is now visible only with difficulty as a faint shadowy band, resolvable into small irregular spots." It seems probable, therefore, that the canals are relatively long and narrow, almost linear features, having the same nature as the dark areas, that run across the bright regions of Mars.
Whether the canals are depressions or elevations presumably depends on whether the Martian dark areas are at low or high altitudes, respectively. As already explained, both points of view have been propounded. According to C. W. Tombaugh (1959), for example, the canals may correspond to faults or fractures several miles wide in the Martian crust. The color changes at different seasons are attributed to vegetation growing in the low-lying regions.
On the other hand, C. Sagan and his associates claim that radar observations indicate that the canals rise above the bright areas which they cross, attaining the same maximum elevation as some of the larger dark areas, roughly 6 kilometers (almost 20 000 feet). The slopes of the canals, which are estimated to be greater than the usual 3 or 4 degrees, are steeper. If the theory is accepted that the color changes of the dark areas are caused by fine particles, deposited or removed by the wind, the marked secular variations shown by the canals can be readily accounted for by the proximity of bright areas.
A novel suggestion concerning the Martian canals was made in 1964 by F. A. Gifford of the U.S. Weather Bureau Research Station at Oak Ridge, Tenn. He contended that the deserts of Mars should contain long, straight, and narrow systems of sand dunes whose appearance would resemble the observed canals. Dune systems of this kind occur on Earth in the deserts of Africa and Arabia. One in Libya, for example, is reported to be about 650 kilometers (400 miles) long with an average width of roughly 8 kilometers (5 miles). On Mars, it is expected that the winds which produce the sand dunes have a much higher velocity than on Earth, because of the lower atmospheric pressure. The dune systems would thus probably be longer and wider on Mars. The changes in appearance of the canals might still be accounted for by the size of the particles on the surface.
It was hoped at one time that the pictures taken by the Mariner IV spacecraft when it was close to Mars in July 1965 would throw light on the nature of the canals. Unfortunately, this has not been the case for several reasons. In the first place, the spacecraft crossed the northern hemisphere, where most of the canals are located, during the local winter when the canals are very faint or invisible against the bright background. Second, each view taken by the Mariner IV cameras was of an area approximately 320 kilometers (200 miles) square. This is similar to the width of many canals, and so each picture is equivalent to little more than a very small area on a map and not much more than a point in a telescopic image of Mars. Finally, the position of the Sun when the Mariner IV pictures were taken was not too favorable for bringing out the contrast between different surface areas.
Nevertheless, examination of the Mariner IV pictures has revealed a number of what appear to be linear topographical features known to geologists as lineaments. These are, however, much narrower than the canals. They could not have been detected from Earth by the best telescopes even under ideal viewing conditions. There are similar lineaments on the Moon, as seen in pictures taken by the Orbiter spacecraft, and they are known to exist on Earth. It is generally considered that linear systems of this kind result from stresses in the crust produced by changes in the rotational velocity of the planet (or Moon) that have occurred over long periods of time. Whether or not the canals are related to the lineaments is not known.
A striking feature of the photographs of Mars taken by the Mariner IV spacecraft in July 1965 (ch. XIII) was the evidence of a highly cratered surface, surprisingly similar to that of the Moon. Although this discovery aroused considerable interest when it was reported, the possibility of craters on Mars had been predicted several years earlier. It was mentioned, for example, in 1944 by a science writer D. L. Gyr in a book entitled "Life on Mars." Later, around 1949 and 1950, three astronomers, R. B. Baldwin, C. L. Tombaugh, and E. J. Opik, independently, suggested that there might be craters on the surface of Mars. This view was based largely on the proximity of the planet to the asteroidal belt, so that formation of craters by impact with asteroidal and related fragments was to be expected.
Although there are some indications of craters in the second picture from Mariner IV, they are more clearly seen in picture 3 (fig. 13.12).2 The failure to observe craters a In all the Mariner IV photographs, north is at the top of the picture.
in some photographs is probably due to the local light and other conditions rather than to an actual absence of craters. The resemblance to the lunar surface is apparent in figure 6.17, which is Mariner IV picture 7. The area viewed is wholly within the southeastern part of the bright region Zephyria. The dimensions are about 290 by 290 kilometers (180 by 180 miles), so that the area is roughly 84 000 square kilometers (32 400 square miles). Approximately 50 possible craters have been counted in this area. Picture 9, reproduced in figure 6.18, shows a highly cratered region which is largely or wholly within the dark Mare Sirenum. As far as can be determined, there is no great difference in crater density on the light and dark areas of Mars viewed by Mariner IV.
Figure 6.19, which is picture 11, is principally within the Mare Sirenum, but the top right (northeast) corner is probably in the lighter Atlantis area between the Mare Si renum and Mare Cimmerium. This photograph is of special interest because it shows the largest, well-defined, although not entirely complete, crater observed. Its diameter is about 175 kilometers (109 miles). There is some possible evidence of larger craters, but it is not definite enough to permit positive identification. Incidentally, a marked lineament, starting at the bottom left corner and extending at least 200 kilometers (125 miles) in an almost northeasterly direction, is an important aspect of picture 11.
Craters were detected in at least 14 of the Mariner IV photographs, but only one more, picture 14, is reproduced here (fig. 6.20). The area shown is wholly within the bright region Phaethontis. The features of special interest are the light-colored rings which are thought to be the rims (or inner walls) of craters partially covered by frost or snow, either of water or carbon dioxide. Some of the rings are incomplete in the northwest
(upper left) quadrant, as would be expected if the morning Sun had evaporated the thin deposit of frost formed during the night. It was late midwinter in the locality where the picture was taken and the area was only some 10° to 15° latitude from the edge of the south polar cap.
A few craters as small as 3 kilometers (1.9 miles) in diameter have been observed in the Mariner IV pictures, but the great majority lie within the range of 5 to 175 kilometers (3.1 to 109 miles) diameter. In pictures 7 through 12, which are the most useful from the standpoint of clarity, 206 craters with diameters greater than 5 kilometers have been identified as definite or probable, and 126 more as possible. It is expected that at least half of the latter are actual craters. If the crater density in the areas covered by pictures 7 through 12 is taken to be typical of the whole planet, then there should be somewhere between 60 000 and 100 000 craters, with diameters exceeding 5 kilometers, on the surface of Mars.
In general, the Martian craters resemble those on the Moon in several respects. The size distribution, except perhaps for the smallest and largest, is similar. Some of the craters on Mars have well-defined polygonal, rather than circular, shapes and there are also a number, as on the Moon, with central peaks. On the whole, however, the Martian craters are smoother in outline and have lower walls and shallower interiors than lunar craters.
These differences, taken together with the relatively low frequency of the smaller craters on Mars, suggest that there has been a greater degree of erosion on this planet than on the Moon. The great majority of the smallest observable Martian craters have sharp outlines, whereas the larger ones are usually less distinct. This indicates that many smaller craters have been completely eroded away over the course of time and that most of those now existing were formed relatively recently. The largest craters, on the other hand, which are the most difficult to erode, are much older, on the average.
Possible causes of erosion on Mars are obliteration of smaller craters by larger ones produced subsequently, the transport of particles by the wind, which is known to be capable of attaining hgh velocities, and thermal creep. The latter phenomenon is familiar on Earth; pieces of rock will move down a slope as a result of alternate expansion and contraction arising from temperature changes. The large daily temperature fluctuations on Mars would make thermal creep especially effective.
On Earth, an important source of erosion is flowing water. Although it is highly improbable that there are now streams or bodies of water on Mars, there is no clear proof that they may not have existed in the distant past, during the first 1 or 2 billion years after the formation of the planet. Because of the erosion by other means that has undoubtedly occurred in more recent times, the signs of water flow would have been obliterated.
C. R. Chapman, J. B. Pollack, and C. Sagan wrote in 1968:
If substantial aqueous-erosion features—such as river valleys—were produced during earlier epochs on Mars, we should not expect any trace of them to be visible on the Mariner IV photographs unless they were of greater extent than typical comparable features on Earth. . . . Thus, any conclusions . . . that the apparent absence of clear signs of aqueous erosion excludes running water during the entire history of Mars . . . must certainly be regarded as fallacious.
The suggestion has been made that some of the Martian craters, and also some lunar craters, have a volcanic, or internal, origin. In 1968, for example, J. A. O'Keefe and P. D. Lowman indicated the possibility that circular areas on Mars may be similar to ring dikes on Earth. The latter are fissures, produced by volcanic action, that became filled with magma which subsequently solidified. On the whole, however, the general opinion seems to be that the majority of the Martian craters are impact craters. Like most of the craters on the Moon and a few terrestrial craters, they have been formed by the impact of meteoroidal, asteroidal, and cometary fragments during the past 4.6 billion years or so.
Soon after the Martian craters, less than 100 in number, had been identified in the Mariner IV pictures, a rough comparison was made with the average density of craters on the Moon. As a result, the conclusion was drawn by the Mariner IV experimenters, R. B. Leigh tori and his associates, that—
the heavily cratered surface of Mars must be very ancient—perhaps 2 to 5X109 years old . . . [and] it is difficult to believe that free water in quantities sufficient to form streams or to fill oceans could have existed anywhere on Mars since that time. The presence of such amounts of water . . . would have caused severe erosion over the entire surface.
The foregoing conclusion concerning the age of the craters on Mars was soon called into question by several scientists. They pointed out that, in view of the proximity of the planet to the asteroidal belt, the rate of formation of craters on Mars should have been significantly greater than on the Moon. In comparing the age of the surface of Mars with that of the lunar surface, allowance should be made for this difference. As a result of various calculations it appeared that the average age of the Martian craters was only from 300 to 900 million years. Since the age of the planet is some 4.6 billion years, it followed, in the words of E. Anders and J. R. Arnold, that "the crater density on Mars no longer precludes the possibility that liquid water and a denser atmosphere were present . . . during the first 3.5 billion years of its history."
In the course of recent years, it has become clear that any estimate of the age of the Martian surface based on crater densities involves so many uncertainties and requires so many assumptions that the results have little value. There is a general feeling, however, that many of the larger craters on Mars have an average age of more than 2 billion years, so some are almost as old as the planet itself. The smaller craters, however—those less than 30 kilometers (19 miles) in diameter—which constitute over 80 percent of those with diameters exceeding 5 kilometers, may have ages of only some tens or hundreds of million years. Thus, it appears justified to conclude that the Mariner IV photographs provide no evidence, one way or another, concerning the possible existence of bodies or streams of water in the early history of Mars.
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