Astronomy for Everybody/Part 4/Chapter 4

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More public interest has in recent years been concentrated on the planet Mars than on any other. Its resemblance to our earth, its supposed canals, oceans, climate, snowfall, etc., have all tended to interest us in its possible inhabitants. At the risk of disappointing those readers who would like to see certain proof that our neighbouring world is peopled with rational beings, I shall endeavour to set forth what is actually known on the subject, distinguishing it from the great mass of illusion and baseless speculation which has crept into popular journals during the past twenty years.

We begin with some particulars which will be useful in recognising the planet. Its period of revolution is six hundred and eighty-seven days, or forty-three days less than two years. If the period were exactly two years, it would make one revolution while the earth made two, and we should see the planet in opposition at regular intervals of two years. But, as it moves a little faster than this, it takes the earth from one to two months to catch up with it, so that the oppositions occur at intervals of two years and one or two months. This excess of one or two months makes up a whole year after eight oppositions; consequently, at the end of about seventeen years, Mars will again be in opposition at the same time of the ​year, and near the same point of its orbit, as before. In this period the earth will have made seventeen revolutions and Mars nine.

The difference of a month or so in the interval between oppositions is due to the great eccentricity of the orbit, which is larger than that of any other major planet except Mercury. Its value is 0.093, or nearly one tenth. Hence, when in perihelion, it is nearly one tenth nearer the sun than its mean distance, and when in aphelion nearly one tenth farther. Its distance from the earth at opposition will be different by the same amount, measured in miles, and hence in a much larger proportion to the distance itself. If opposition occurs when the planet is near perihelion, the distance from earth is about forty-three million miles; but if near the aphelion, about sixty million miles. The result of this is that, at a perihelion opposition, which can occur only in September, the planet will appear more than three times as bright as at an aphelion opposition, occuring in February or March. An opposition occurred near the end of March, 1903; the next following early in May, 1905. We shall then have oppositions near the end of June, 1907, and in August, 1909, which will be quite near to perihelion.

Mars, when near opposition, is easily recognised by its brilliancy, and by the reddish colour of its light, which is very different from that of most of the stars. It is curious that a telescopic view of the planet does not give so strong an impression of red light as does the naked eye view.

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The great Huygens, who flourished between 1650 and 1700, studying Mars with the telescope, was the first one to recognise the variegated character of its surface, and to make a drawing of the appearance which it presented. The features delineated by Huygens can be recognised and identified to this day. By watching them it was easy to see that the planet rotated on its axis in a little more than one of our days (24h. 37m.).

This time of rotation is the only definite and certain one among all the planets besides the earth. For two hundred years Mars has rotated at exactly this rate, and there is no reason to suppose that the time will change appreciably any more than the length of our day will. The close approach to one of our days, the excess being only thirty-seven minutes, leads to the result that, on successive nights, Mars will, at the same hour, present nearly the same face to the earth. But, owing to the excess in question, it will always be a little farther behind on any one night than on the night before, so that, at the end of forty days, we shall have seen every part of the planet that is presented to the earth.

All that was known of Mars up to a quite recent period could be embodied in a map of the planet, showing the bright and dark regions of its surface, and in the fact that a white cap would be generally seen to surround each of its poles. When a pole was inclined toward us, and therefore toward the sun, this cap gradually grew smaller, enlarging again when the pole was turned from ​the sun. In the latter case it would be invisible from the earth, so that the growth would be recognised only by its larger size when it again came into sight. These caps were naturally supposed to be snow and ice which formed around the poles during the Martian winter, and partly or wholly melted away during the summer.

In 1877 commenced Schiaparelli's celebrated observations on the surface of Mars, and his announcement of the so-called canals. The latter consisted of streaks passing from point to point on the planet, and slightly darker than the general surface. Seldom has more misapprehension been caused by a mistranslation than in the present case. Schiaparelli called these streaks canale, an Italian word meaning channels. He called them so because it was then supposed that the darker regions of the surface were oceans, and the streams connecting the oceans were therefore supposed to be water, and so were called channels. But the translation "canals" led to a widespread notion that these streaks were the works of inhabitants, as canals on the earth are the works of men.

Up to the present time there is some disagreement between observers and astronomical authorities on the subject of these channels. This arises from the fact that they are not well-defined features on an otherwise uniform surface. Everywhere on the planet are found variations of shade—light and dark patches, so faint and ill defined that it is generally difficult to assign exact ​

​form and outline to them, running into each other by insensible gradations. The extreme difficulty of making them out at all, and the variety of aspects they present under different illuminations and in different states of our atmosphere, has resulted in a great variety of inconsistent delineations of these objects. At one extreme we have the drawings made by the observers at the Lowell Observatory at Flagstaff, Ariz. These show the channels as fine dark lines, so numerous as to form a network covering the greater part of the surface of the planet. In Schiaparelli's map they are rather broad faint bands, not nearly so well defined as in Lowell's drawings. Lowell's channels are much more numerous than those seen by Schiaparelli. We might therefore suppose that all marked by the latter could be identified on Lowell's map. But such is far from being the case; there is only a general resemblance between the features seen at the two stations. One of the most curious features of Lowell's drawings is that the points where the channels cross each other are marked by dark round spots like circular lakes. No such spots as these are shown on Schiaparelli's map.

One of the best marked features of Mars is a large, dark, nearly circular spot, surrounded by white, which is called Lacus Solis, or the Lake of the Sun. All observers agree on this. They also agree in a considerable part as to certain faint streaks or channels extending from this lake. But when we go farther we find that they do not agree as to the number of these channels, nor is there an exact agreement as to the surrounding ​features. It will be interesting to study two drawings of this region made at the Lick Observatory, probably under the best possible conditions, by Campbell and Hussey, respectively.

It is not likely that any observatory is more favoured by its atmosphere for observations on this planet than the Lick on Mount Hamilton. Its telescope is the largest and finest in the world that has ever been especially

directed to Mars, and Barnard is one of the most cautious observers. It is therefore very noteworthy that on the face of Mars, as presented to Barnard in the Lick telescope, the features do not quite correspond to the channels of Schiaparelli and Lowell. When the air was exceptionally steady he could see a vast number of minute and very faint markings, which were not visible in the smaller telescopes used by the other observers. These ​were so intricate that it was impossible to represent them on a drawing. They were not confined to the brighter regions of the planet, or the supposed continents, but were found to be more numerous on the so-called seas. They showed no such regularity that they could be considered as channels running from one region to another. The eye could indeed trace darker streaks here and there, and some of these corresponded to the supposed channels, but they were far more irregular than the features on Schiaparelli's and Lowell's maps.

The matter was explained by Cerulli, a careful and industrious Italian observer, in a way which seems very plausible. He found that after he had been studying Mars for two years he was able, by looking at the moon through an opera glass, to see, or fancy he saw, lines and markings upon its surface similar to those of Mars. This phenomenon is not to be regarded as a pure illusion on the one hand, or an exact representation of objects on the other. It grows out of the spontaneous action of the eye in shaping slight and irregular combinations of light and shade, too minute to be separately made out, into regular forms.

The probable facts of the case may be summed up as follows:

1. The surface of Mars is extremely variegated by regions differing in shade, and having no very distinct outlines.

2. There are numerous dark streaks, generally some​what indefinite in outline, extending through considerable distances across the planet.

3. In many cases the dark portions appear as if chained together to a greater or less extent, and thus give rise to the appearance of long dark channels.

The appearance on which this third phenomenon, which we may regard as identical with that observed by Cerulli, is based, may be well illustrated by looking, with a magnifying glass, at a stippled portrait engraved on steel. Nothing will then be seen but dots, arranged in various lines and curves. But take away the magnifying glass and the eye connects these dots into a well-defined collection of features representing the outlines of the human face. As the eye makes an assemblage of dots into a face, so may it make the minute markings on the planet Mars into the form of long, unbroken channels.

The features which we have hitherto described do not belong to the two polar regions of the planet. Even when the snowcaps have melted away, these regions are seen so obliquely that it would be difficult to trace any well-defined features upon them. The interesting question is whether the caps which cover them are really snow which falls during the Martian winter and melts again when the sun once more shines on the polar regions. To throw light on this question we have to consider some recent results as to the atmosphere of the planet.

All recent observers are agreed that, if Mars has any atmosphere at all, it is much rarer than our own, and ​contains little or no aqueous vapour. This conclusion is reached from observations both with the telescope and the spectroscope. The most careful eye observations of the planet show that the features are rarely, if ever, obscured by anything which can be considered as clouds in the Martian atmosphere. It is true that the features are not always seen with the same distinctness; but the variations in the appearance are no greater than would be due to the changes in the steadiness and purity of our own atmosphere, through which the astronomer necessarily makes his observations. Although, near the edge of the apparent disk of the planet, the features appear to be softened, as if seen through a greater thickness of the atmosphere, this appearance is, at least in part, due to the obliquity of the line of sight, which prevents our getting so good a view of the edge of the disk as of its centre. Something of the same sort may be noticed when the moon is viewed with the naked eye or an opera glass. Yet it is quite possible that a certain amount of the softening may be due to a rare atmosphere on Mars.

The most careful spectroscopic examination of the planet was made by Campbell, who compared its spectrum with that of the moon. He could not detect the slightest difference between the two spectra. Now, if Mars had an atmosphere capable of exerting a strong selective absorption on light, we should see lines in the spectrum due to this absorption or, at least, some of the lines would be strengthened. Our general conclusion therefore must be that, while it is quite probable that Mars has an atmosphere, it is one of considerable rarity, ​and does not bear much aqueous vapour. Now snow can fall only through the condensation of aqueous vapour in the atmosphere. It does not therefore seem likely that much snow can fall on the polar regions of Mars.

Another consideration is that the power of the sun's rays to melt snow is necessarily limited by the amount of heat that they convey. In the polar regions of Mars the rays fall with a great obliquity, and even if all the heat conveyed by them were absorbed, only a few feet of snow could be melted in the course of the summer. But far the larger proportion of this heat must be reflected from the white snow, which is also kept cool by the intense radiation into perfectly cold space. We therefore conclude that the amount of snow that can fall and melt around the polar regions of Mars must be very small, being probably measured by inches at the outside.

As the thinnest fall of snow would suffice to produce a white surface, this does not prove that the caps are not snow. But it seems more likely that the appearance is produced by the simple condensation of aqueous vapour upon the intensely cold surface, producing an appearance similar to that of hoarfrost, which is only frozen dew. This seems to me the most plausible explanation of the polar caps. It has also been suggested that the caps may be due to the condensation of carbonic acid. We can only say of this, that the theory, while not impossible, seems to lack probability.

The reader will excuse me from saying anything in this chapter about the possible inhabitants of Mars. He ​knows just as much of the subject as I do, and that is nothing at all.

No discovery more surprised the whole world than that of two satellites of Mars by Professor Asaph Hall, at the Naval Observatory, in 1877. They had failed of previous detection owing to their extreme minuteness. It was not considered likely that a satellite could be so small as these were found to be, and so no one had taken the trouble to make a careful search with any great telescope. But, when once discovered, they were found to be by no means difficult objects. Of course the ease with which they can be seen depends on the position of Mars both in its orbit and with respect to the earth. They are never visible except when the planet is near its opposition. At each opposition they may be observed for a period of three, four, or even six months, according to circumstances. At an opposition near perihelion they may be seen with a telescope of less than twelve inches diameter; how small a one will show them depends on the skill of the observer, and the pains he takes to cut off the light of the planet from his eye. Generally a telescope ranging from twelve to eighteen inches in diameter is necessary. The difficulty in seeing them arises entirely from the glare of the planet. Could this be eliminated they could doubtless be seen with much smaller instruments. Owing to the glare, the outer one is much easier to see than the inner one, although the inner one is probably the brighter of the two.

​Professor Hall assigned the name Deimos to the outer and Phobos to the inner, these being the attendants of Mars in ancient mythology. Phobos has the remarkable peculiarity that it revolves around the planet in less than nine hours, making its period the shortest of any yet known in the solar system. This is little more than one third the time of the planet's rotation on its axis. The consequence of this is that, to the inhabitants of the planet, its nearest moon rises in the west and sets in the east.

Deimos performs its revolution in 30 hours 18 minutes. The result of this rapid motion is that some two days must elapse between its rising and setting.

Phobos is only 3,700 miles from the surface of the planet. It must therefore be an interesting object to the inhabitants of Mars, if they have telescopes.

In size these bodies are the smallest visible to us in the solar system, with the possible exception of Eros and possibly some others of the fainter asteroids. From Professor Pickering's photometric estimates their diameter was estimated to be not very different from seven miles. Their apparent size as we view them is therefore not very different from that of a small apple hanging over the city of Boston, and seen with a telescope from the city of New York. In this respect they form a singular contrast to nearly or quite all of the other satellites, which are generally a thousand miles or more in diameter. The one exception to this is the fifth satellite of Jupiter, to be described in the chapter on Jupiter and its satellites. Although this is much less than a thousand miles in diam​eter, it must considerably exceed the satellites of Mars in size.

The satellites have been most useful to the astronomer in enabling him to learn the exact mass of Mars. How this is done will be explained in a subsequent chapter, where the methods of weighing the planets are set forth.

The satellites also offer many curious and difficult problems in gravitation. Their orbits seem to have a slight eccentricity, and the position of the planes in which they revolve changes in consequence of the bulging of the planet at its equator, produced by its rotation. The calculation of these changes and their comparison with observations have opened up a field of research in which Professor Hermann Struve, now of the University of Koenigsberg, Germany, has taken a leading part.