The Planet Mars:
A History of Observation and Discovery

William Sheehan

Chapter 15
Observing Mars

Mars is only half the diameter of the Earth, yet it never approaches closer than 55.7 million kilometers, or 140 times the distance of the Moon from the Earth. This makes it a difficult object to observe. A 2- or 3-inch (5- or 6.5-cm) telescope will show whatever polar cap is tilted toward the Earth (assuming the cap is large enough at the time) and a few of the main dark areas, such as Syrtis Major. On the whole, however, I consider a telescope of at least 5 inches (7.5 cm) the minimum necessary for a refractor, and 9 inches (22.5 cm) for a reflector---in the latter case, the mirror must possess a perfect figure, and preferably should be of long focus, say f/9 or f/10.

Most observations tend to be made near the oppositions, which occur at intervals of two years and two months. Since some of the most interesting questions about Mars involve time-dependent changes, however, useful studies can be made, and are strongly encouraged, several months before and after opposition, when the disk is as small as 6" or 7" of arc. At the best oppositions, which occur close to the time that Mars passes the perihelion of its orbit, the apparent diameter reaches 25.1". Unfortunately for Northern Hemisphere observers, the planet is then always low in the sky.1

The aphelic oppositions occur in February and March. The disk is much smaller then, of course, only about 14" of arc, but Mars is higher in the sky---a distinct advantage, since it can be observed through less of the Earth's atmosphere. Also, the Martian atmosphere is then generally clearer.

Indeed, as Schiaparelli pointed out long ago, the size of the disk is less important than the transparency of the Martian atmosphere in determining the visibility of minor features. The clarity of the atmosphere, in turn, depends on the season. The Martian dust storm season generally begins around late spring or early summer in the Martian southern hemisphere. At the perihelic oppositions there may be considerable amounts of dust suspended in the air above Mars, which tends to make the markings appear washed out; at the aphelic oppositions the atmosphere is nearly dust-free (though cirrus clouds are frequent), and in general the contrasts of the markings are much stronger.

Anyone seriously interested in observing the red planet will sooner or later wish to make a permanent record of what is seen. Most observers still carry out their work visually, and this means sketching the planet. A standard scale of 50 mm to the diameter of the planet has been adopted by the British Astronomical Association Mars Section; the American Association of Lunar and Planetary Observers Mars recorders, for some reason, use a scale of only 42 mm. These scales should be adopted in all drawings submitted for the section reports, but in terms of actual work at the telescope, this tends (except near opposition) to make for drawings on the large side. Harold Hill has pointed out that "for a disk of 18 arcseconds, a scale of 50mm corresponds to some 210 inches (5.3m) to the Moon's diameter for lunar drawings, and for a disk of 9 arcseconds, double that amount! No one, but no one, would consider the feasibility of adopting such a scale for lunar work."2 Hill uses a sliding scale of 3 mm to the arc-second to give a more realistic idea of how the planet looks on a good night. Although the phase of Mars can be ignored close to opposition, at other times it can be quite considerable---at maximum phase, Mars is only 89 percent illuminated and appears as gibbous as the Moon some three or four days from full. Last, I must emphasize that there is no point to drawing the planet unless the seeing is at least reasonably good.

In drawing the planet, it is generally best to begin with a line sketch showing the main reference points---the polar caps and hoods if present, and the outlines of the most prominent features. Once this is finished, one can fill in the finer features in more leisurely fashion. Clouds are conveniently indicated by dashed lines. Because of the planet's rotation, the positions of features are, of course, constantly changing---in general it is best to finish a sketch in fifteen or twenty minutes. One should do one's best to give a realistic portrayal of the markings---too many published drawings show them with a hardness and boldness that is quite misleading; it is difficult to estimate the amount of mischief that has been done by such misrepresentations over the years!

Once mastery of representing Mars in pencil tones has been achieved, one may wish to tackle the colors of the planet. To some extent, the apparent colors are illusory---most notably the bluish tones that sometimes appear in the dark areas, which are produced by simultaneous contrast with the salmon pink deserts. Richard Baum suggests the following technique:
Take your prepared disk. First lay down a background color, in this case orange-red. Do this by simply scraping off from a pastel (not the oil kind) a certain amount of dust directly on to the center of the disk. Smooth this in with cottonwool (not with finger because of its grease content) and work outwards towards the limb. You will at this stage have a reddish hued disk brighter at the limb, giving a good representation of the limb haze. Lay down the outlines of the markings to be sketched in (very gently as to leave no indentations, but don't use a soft B, rather HB). Then very gently shade in the dark areas, again as to leave no heavy marks. Smooth this detail out by rubbing with the reddish-coated cottonwool, and then gradually work up the shadings into what you require, all the time working gently but consistently---don't hurry the job. The insertion of cloud detail is easily accomplished by the use of a kneadable rubber. Also this type of rubber is very good in that when kneaded to a point, stippling effects on a dusky background are very easy. . . . I originally learned this technique at the age of nine from a superb marine artist, who did his work on clouds and waves and indeed ships in this way with truly superlative results.3
I have one of Baum's beautiful and artistic representations of the planet framed on the wall of my study. He gives a rather romantic representation of the planet using a warm orange-red color for the disk. Hill, by contrast, describes his impression of the Martian background as always a pale pink occasionally tinged with ruddiness, adding that "sometimes the warmth can be quite absent." He notes that "the dark markings at times show distinctly blue as shown by Lowell---improbable though such a color may seem. . . . The old adage applies that `everyone sees in his own way' and especially when confronted with a telescopic image displaying such exquisite delicacy of coloring and shading as does Mars under the best conditions."4 This is undoubtedly true. The color effects are sensitive to seeing conditions, the telescope's aperture size, the disk size, and Martian seasonal effects. During the southern hemisphere summer (Ls = 270--360°), when there is often a great deal of dust in the atmosphere, the contrast of the markings is, as noted earlier, more subdued; the desert areas are then apt to appear more yellowish or even lemon, while the dark areas appear neutral gray or brownish. At the aphelic oppositions, when dust is generally absent, the apparent bluish tints can be rather striking. Various subjective effects also play a role in what one sees, including differences in the response curves of pigment-sensing proteins, or visual pigments, from one individual to the next; the extreme case is complete insensitivity of one or more pigments, also known as color-blindness---remember Schiaparelli!

The discussion of the Martian colors brings us to the next topic---the use of colored filters. Undoubtedly there is no other planet for which their use is more indispensable, and the serious Martian observer simply cannot afford to do without them. A yellow filter (Wratten 12 or 15) increases the contrast of the dark areas with the background---this is what Schiaparelli used, and it always brought the markings out "like spots of India ink." Orange (W21 and W23A) and red (W25) also will increase the contrast of details and assist in the identification of dust clouds, of which more presently. On the other hand, green (W58), blue (W44A), and blue-violet (W47) filters will bring out limb hazes and terminator clouds and frost patches. Since some of these filters are quite dense, they require telescopes with fairly large light grasp---the blue-violet filter (W47) needs at least 9 inches (22.5 cm). Generally speaking, surface details are invisible in the blue and blue-violet filters, but one should be on the lookout for the so-called blue clearings.

Photographically, Mars has always been a difficult subject, and detailed knowledge of photographic emulsions and techniques is required to do it justice. Even state-of-the-art high-resolution film requires exposures of several seconds, which is long enough to hopelessly blur details. In any case, photography has now virtually been supplanted by the charge-coupled device, or CCD, so I will say no more about it here.

Still-frame CCD cameras are expensive and require a computer, but in the hands of such pioneers as Jean Dragesco, Isao Miyazaki, and Donald C. Parker they have yielded awe-inspiring images of Mars (fig. 21). Parker's best results to date were obtained in February--March 1995, at an aphelic opposition when the apparent diameter of Mars was never more than 13.8". He uses a 16-inch (41-cm) Newtonian at his observatory at Coral Gables, Florida, and enjoys extraordinary seeing owing to the generally laminar airflow off the ocean---on many nights, planetary images resemble steel engravings, rippled only now and then by an atmospheric tremor.

In addition to still-frame CCD cameras, video cameras using CCDs sensitive enough for planetary imaging have become commercially available and are quite affordable. As with ordinary photography, one projects the image through an eyepiece, but because of the remarkable resolution of the CCD (as measured by the number of picture elements, or pixels), very high magnifications can be used. Typically, video cameras have exposure times of 1/60 second and recording rates of thirty frames per second, and by playing back the videotape frame by frame, one can follow the moment-to-moment changes in seeing. Though the individual frames are generally far from good, every now and then a few frames will appear sharp and stationary.

Seeing is not the only cause of distortion; another is granularity due to electronic noise. This can be handled in various ways. For example, one can use a "frame grabber" device with image-processing software to construct a final image with an improved signal-to-noise ratio, though such equipment is expensive. A cheaper alternative is simply to composite an image by taking pictures off the monitor using a 35-mm single-reflex camera. Since the signal-to-noise ratio varies as the square root of the number of images, by combining four to eight consecutive frames when the seeing is good, one can produce an image in which the granularity due to electronic noise is reduced by a factor of two or three.5 However, even the best still frames fall short of capturing what Thomas Dobbins has aptly described as the "ineffable sense of reality" of viewing a videotape.6

Many amateurs are now producing CCD images that show detail beyond the reach of even the best visual observers using much larger instruments. Though the trained human eye was never seriously challenged by photography using silver-grain emulsions (requiring exposures of one second or more), its long reign in glimpsing fine planetary details is finally over, surpassed by the CCD.

The larger surface features of Mars are generally stable, and an observer equipped with one of the maps made by Schiaparelli or Flammarion, or even by Beer and Mädler, will easily recognize the main features of the planet. But the maps, even allowing for inevitable errors (and ignoring the canals!), are not identical with modern ones.

This is hardly surprising; the fact of time-dependent changes on Mars is well established. First of all, there are seasonal cycles, which affect the intensity and visibility of the various markings, the most obvious effect being the alternate waxing and waning of the two polar caps. The solid north polar cap is hidden during its deposition phase by a cloudy hood that covers it during much of the northern hemisphere autumn and winter. The cap emerges from the polar hood at the start of spring, at which time it extends to approximately 65° N. In general, the retreat of the north cap is quite symmetrical. In late spring, the cap becomes fissured into two portions by a dark rift, Rima Tenuis. Around the time of the summer solstice (Ls = 90°), the bright mass known as Olympia breaks off, separated from the main cap by the dark rift Rima Borealis. In most years the seasonal carbon dioxide frost cap evaporates off completely, leaving a residual water ice remnant. As it retreats, the north polar cap appears to be surrounded with a dark collar, sometimes known as the Lowell band, which was once regarded as a shallow sea but coincides in position with a wide swath of sand dunes.

The south polar cap is tilted toward the Earth at the perihelic oppositions, and thus is well presented as it rapidly shrinks during the southern spring and summer. It begins to break up by southern hemisphere mid-spring; the most notable remnant is the Mountains of Mitchel, also known as Novissima Thyle, located near the cap's retreating edge (and between Martian longitudes 300° and 330° W). It begins to detach from the polar cap at around Ls = 215°, and is fully separated by Ls = 230°. There are also various dark rifts in the cap, such as the Rima Australis and Rima Angusta. The seasonal carbon dioxide frost cap generally fails to disappear completely---thus, unlike the northern cap, the residual cap consists mostly of carbon dioxide frost rather than water, and is always much smaller than its northern counterpart.

During the northern hemisphere spring and summer (or, equivalently, the southern hemisphere fall and winter), there is generally little dust in the Martian atmosphere, although whitish clouds are frequent at the limb and terminator. The great basins of Hellas, Argyre, and Elysium are usually frost covered and often appear brilliant white, and there are also many smaller patches of frost.

The largest dust storms occur in the southern hemisphere spring and summer. However, the period in which they are known to occur is not confined to a narrow band, and planet-encircling and global storms have begun at points along Mars's orbit ranging from Ls = 204° to 310°. The sites most often associated with initiation of dust storm activity are, in the northern hemisphere, Chryse-Acidalia, Isidis--Syrtis Major, and Cerberus; in the southern hemisphere, Hellas, Noachis-Hellespontus, Argyre, and the Solis, Sinai, and Syria Plani regions.

There have been no planet-encircling or global storms since 1982. The much-anticipated global storm of 1988 never developed, though regional storms were observed, and there were also regional storms in 1990 and 1992. It now appears that planet-encircling storms (as in 1956, 1971, 1973, and 1977) are the exception, not the rule, but further observations are needed to establish their frequency.

The Martian winds scatter dust around, and the changing dust distribution produces changes in albedo features. E. M. Antoniadi claimed that Syrtis Major underwent regular seasonal changes; its eastern side became streaked and narrow in the spring, then widened in autumn. Nepenthes-Thoth, a conspicuous marking earlier in the century, has now all but disappeared, though Alcyonius Nodus remains conspicuous. There have also been marked changes in Solis Lacus; it was large and complex in the 1970s, but in the 1990s has become smaller and more circular, its classic form up until the 1920s.

The observer who wishes to become familiar with the main aspects of the planet as seen in a modest telescope may find the following guide of some use (also, see fig. 22). In addition to the main telescopic markings (albedo features), I highlight aspects of the topography of each Martian region, which, though not directly visible, are convenient to keep in mind when orienting oneself to the disk.

The Martian longitudes begin at 0° in the 0.5-kilometer-wide crater Airy-0 in Sinus Meridiani, and increase progressively to the west; thus Solis Lacus lies at about 90° W longitude; Mare Sirenum is at 180° W; and Isidis, the bright area just east of Syrtis Major, is at 270° W. Syrtis Major itself is centered on about 290° W longitude. The sense of rotation is the same as that of increasing longitude; that is, toward the west (right, on the inverted telescope image) at a rate of 14.62°/hour. Because the Martian day is about forty minutes longer than ours, as one watches from night to night the markings appear to fall gradually backward on the disk by some nine degrees each night. The planet thus appears to complete an illusory backward rotation over a period of forty days, during which the entire circumference passes in review before the observer.

It is convenient to begin our tour with the Sinus Sabaeus (Terra Sabaea), since this is the region through which the 0° meridian passes. The serpentine ribbon of Sinus Sabaeus runs just south of the equator and terminates in the Sinus Meridiani (Terra Meridiani), whose two northward-pointing forks are still sometimes referred to as Dawes' forked bay. The point between the forks, christened Fastigium Aryn by Schiaparelli, is the zero of Martian longitudes---or, more precisely, the small crater in this position known as Airy is. The forked appearance is not always apparent, but at times it can be distinct in only a 6-inch (15-cm) telescope.

South of Sinus Sabaeus are the moderately bright regions of Deucalion and Noachis. Their ancient and heavily cratered terrain dates back to the middle Noachian period of heavy bombardment, four billion years ago. The brighter equatorial continent to the north is also heavily cratered and is known as Arabia; among its leading craters are Schiaparelli, which lies just on the border between Sinus Sabaeus and Arabia---it was often seen as a circular brightish patch by E. M. Antoniadi---and Cassini. Again, though, none of these features can be seen with modest telescopes.

To the west of Sinus Sabaeus lies Margaritifer Sinus (Margaritifer Terra), whose beaklike extension sometimes appears broken off at the end. The Ares Valles, one of the largest Martian outflow channels, courses through the region on its way to Xanthe Terra to the northwest; Xanthe Terra is also the site of the great Tiu, Simud, and Shalbatana channels, in which spacecraft photographs have shown lemniscate islands and alluvial plains suggestive of massive flooding. These features originate in western Margaritifer Sinus in the rough-and-tumble region known as "chaotic terrain." The flooding that took place here was on a catastrophic scale---much greater than anything ever seen on Earth. With good seeing, one can see even in modest telescopes that this is a region of complex formation.

The dusky region south of Margaritifer Sinus is occupied by Mare Erythraeum, whose boundary is rather ill-defined; however, there is one notable feature: the large circular formation of Argyre, which lies at about latitude 50° S. It is a splendid feature, 1,500 kilometers across, and was formed by a huge impact late in the era of heavy bombardment. The impact was so violent that the debris fell in several concentric rings; the innermost ring is very rugged and forms the basin's rim, of which the northern part is known as the Nereidum Montes; the southern is known as the Charitum Montes. The basin's floor tends to be covered with frost in the southern hemisphere winter, which causes it to appear brilliant at times.

North of Xanthe Terra is the plain of Chryse Terra, in the middle of which lies the Viking 1 landing site; still farther north is Acidalia Planitia (Mare Acidalium), one of the vast northern plains. Mare Acidalium is among the most prominent features during the aphelic oppositions, when the northern hemisphere is tilted toward the Earth. One sometimes forgets the scale of what is being unfolded in the telescope; Mars is a small world, but since it has no oceans, its land area is equal to that of the Earth, and Mare Acidalium covers an area equal to about a quarter of the continental United States. In recent years, the extension of Mare Acidalium, Nilokeras, has been prominent enough to be made out easily in small telescopes.

As Mars rotates, Sinus Meridiani passes off the disk and the Solis Lacus region comes into view. Solis Lacus is a variable albedo feature centered within the bright circular region that Schiaparelli called Thaumasia Felix---the Land of Wonders. It was clearly visible in the 1830s, when Beer and Mädler drew it as small and round; by the 1860s it had become noticeably elongated in an east-west direction. In 1877, Schiaparelli and Trouvelot found it nearly round but slightly elongated north and south. Its east-west elongation was again evident in 1879, and this was the way it generally was figured (though with minor changes) until 1926, when it underwent a radical transformation. In that year Antoniadi found that it curved toward the northwest, at a right angle to its usual direction. In 1939 it was found to be made up of a number of small spots, and its form remained large and complex through the 1970s. In the 1990s, however, it has become smaller and rounder again. Obviously Solis Lacus is one of the most variable regions on the planet, which is hardly surprising given that the plain located here, Solis Planum, is one of the areas long associated with the initiation of dust storm activity.

The great Valles Marineris canyon system runs through the region just south of the equator, its system of interconnected canyons running east and west from Margaritifer Chaos to the complex Noctis Labyrinthus. The Martian canyons are on a stupendous scale compared with the Grand Canyon of the Colorado River; at their widest point, in Melas canyon, the span reaches a width of 200 kilometers. Thus, because of the curvature of the planet, if one stood on the north rim of the canyon, the walls of the south canyon would be completely below the horizon! Even in a small telescope, the course of Valles Marineris can be followed as a curving, dark, threadlike line; this was known as Agathadaemon on the canal-filled maps of the classical era.

South and west of the Valles Marineris complex are the great volcanic plains associated with the huge Tharsis bulge. In a small telescope, this enormous region extending from the edge of Mare Sirenum to the north pole appears bland and featureless, but since the spacecraft explorations it has become one of the most famous regions on Mars, for here lie the great shield volcanoes. Arsia Mons, Pavonis Mons, and Ascraeus Mons run along a southwest-to-northeast line, and Olympus Mons, the tallest mountain in the solar system, is located at longitude 130° W and latitude 20° N. In modest telescopes, one can sometimes make out faint dusky patches in these locations, or, more commonly, whitish patches---the latter, of course, was the form Schiaparelli saw when he discovered Olympus Mons long ago and christened it Nix Olympica (Snows of Olympus). The volcanoes are often overhung by clouds---use blue filter! The shield of Olympus Mons, though it rises some 25 kilometers above the surrounding plains, is 800 kilometers across at the base, so that the slope is not very steep---only about 6°. Thus, immense as Olympus is, its shadow at the terminator is not within the reach of Earth-based observers.

We continue to follow the features that come into view as Mars rotates. What Schiaparelli called the "great diaphragm" of the southern hemisphere begins with the strip of Mare Sirenum, which projects eastward toward Solis Lacus; this swath of darkness broadens as it continues on through Mare Tyrrhenum and breaks into a complex of smaller patches---Antoniadi's "leopard skin." The broken or mottled appearance is partly controlled by the underlying relief and indicates the action of wind depositing and sweeping away materials of different colors. The landforms consist of the rough, cratered terrain that occupies so much of the southern hemisphere of Mars; but these topographical features can only be inferred by the telescopic observer---they are nowhere directly observable. Between Mare Sirenum and Mare Tyrrhenum is a lighter-albedo band, Hesperia Planum, which precedes the large, roughly rectangular darkish patch of Mare Tyrrhenum onto the disk. The latter ends in the northward-pointing wedge of Syrtis Minor. Schiaparelli, inspired by the maritime view of Mars, thought that Hesperia was a floodplain or marsh lying between the two adjacent seas, a reasonable supposition at the time; but the spacecraft photographs have shown that it is a geologically distinct unit, the Hesperian system, which consists of ridged plains overlying the older cratered terrain of Noachian age.

The northern hemisphere in this part of the planet is dominated by the bright plains of Amazonis Planitia and Elysium Planitia. The latter is the site of the great Elysium volcanoes---Albor Tholus, Elysium Mons, and Hecates Tholus, of which nothing, of course, can be made out in modest telescopes. Nevertheless, it is always worth looking with the "mind's eye" and recalling that the volcanoes here are inferior only to those of Tharsis itself! One can make out a dark patch, Trivium Charontis--Cerberus, which has been rather faint in recent years. The Elysium basin often appears as a brightish patch and is sometimes frost covered. Though the region has been largely inundated by volcanoes, part of the basin's rim still stands above the volcanic plains and forms a mountain range, the Phlegra Montes, which was identified in the spacecraft photographs.

At aphelic oppositions one can make out Utopia Planitia---landing site of the Viking 2 spacecraft---in the extreme north, and the dark plains of Vastitas Borealis. The region between 75° and 85° S is peppered with sand dunes.

We come finally to the most celebrated area of Mars: Syrtis Major Planitia. The prominent, dark Syrtis Major is clearly shown in a 1659 drawing by Christiaan Huygens. A low-relief shield volcano has been identified within Syrtis Major whose eruptions were the source of the dark materials covering the region; and indeed, the whole region is an elevated volcanic plateau. The southern part of Syrtis Major is streaked and mottled, owing to wind action, and there are large dune fields within its great expanse. In the southwest, close to where the Sinus Sabaeus branches off, lies the great crater Huygens---it is partly filled by dark material and can sometimes be glimpsed from Earth. The peculiar Deltoton Sinus consists of three arcuate, semicircular "bays," or such Antoniadi thought them when he first saw them with the great Meudon refractor in 1909.

There are two huge basins in the Syrtis Major region of the planet: Isidis Planitia, which encroaches on Syrtis Major from the northeast, and Hellas, which lies directly to the south and is by far the most prominent basin on Mars. Hellas is 2,100 kilometers across and is enclosed on the east by the darkish strip of Mare Hadriaticum (Hadriaca Patera), and on the west by that of Hellespontus. In winter the Hellas basin is often partly or wholly filled with frost. There are some elusive and evidently variable albedo markings; Schiaparelli sketched crisscrossing canals there, the Peneus and Alpheus; and in 1892, J. M. Schaeberle and Stanley Williams figured a prominent dark patch near the basin's center, which Antoniadi later named Zea Lacus. This has returned to prominence in recent years---it was very marked at the 1988 opposition.

We have now followed Mars through a complete rotation, and we return once more to our starting point, Sinus Sabaeus. The Martian features have been described in the order in which they appear in the true rotation of the planet. In fact, however, since it is not possible to follow Mars through a complete rotation in a single night, and since the early evening hours are often the most convenient for viewing, in practical terms the observer tends to pursue the slow apparent drift of the markings in backward order from night to night---thus the Sinus Sabaeus gives way to Syrtis Major, followed successively by Mare Tyrrhenum, Mare Sirenum, Solis Lacus, and Margaritifer Sinus.

I must emphasize that what I have given here is only a first sketch of Mars. The novice observer will see little; but with experience, more and more comes into view. Mars is a difficult object to observe, but there is none more rewarding; and the observer's interest is always piqued by the ever-changing panorama of polar caps, dark markings, and clouds and dust storms.

© 1996 The Arizona Board of Regents

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The University of Arizona Press, 2/2/97 2:16PM