Popular Science Monthly/Volume 75/December 1909/The Planet Venus
|THE PLANET VENUS|
By Dr. PERCIVAL LOWELL
THE special object of the observatory which I have the honor to represent is the study of planets of our solar system, beginning with their present state and passing thence to their evolutionary history. So extended to-day is the astronomic field that to do good work one must specialize his endeavor, restricting himself to one particular branch of it and incidentally refraining, we may add, from discussing that of which he has not expert knowledge. Now research on the planets constitutes one such division, making as it were, an entity in itself. For diverse as the planets are to-day, they are all the result of one particular evolutionary process and knowledge of each member throws light upon the development, past or present, of the others.
It is popularly imagined that our gaze is concentrated on Mars, to the exclusion of much else, and that we are particularly concerned with its habitability. That this is a popular fallacy I shall show you tonight. For we shall contemplate together another planet in the light that study of the past thirteen years at the Lowell Observatory casts upon it, and we shall see not only that such a study has indicated it not to be habitable, but that the question of habitability has not in the least affected our research. In short, to us habitation by organic life or non-habitation is merely an incident in the study of a planet's history, which we view with as strict scientific impartiality as we do the presence or absence of water-vapor in its air. We are concerned solely with the facts, a romantic enough revelation in themselves.
Venus, I need not remind you, is the planet which stands orbitally next inside the earth in the solar family. To us she is by far the most brilliant star in the firmament, excelling Sirius some sixteenfold and paling Jupiter when seen near him by almost the like amount. Her incomparable splendor is partly the result of propinquity; nearness to ourselves and nearness to the sun. Relatively so close is she to both bodies that to show she does not need the abetting background of the night, but without waiting for the sun's withdrawal may nearly always be seen in the daytime in clear air if one knows where in the sky to look. Situate about seven tenths of our own distance from our common giver of light and heat, she gets about double the amount of solar radiation that falls to our lot, so that her surface is proportionately brilliantly illuminated. Being also relatively near us, she displays a correspondingly large disk.
Nevertheless, until recently astronomic inquiry regarding her has proved singularly baffling. The beauty of her face was equaled only by the blankness of her expression. Hers proved one of those countenances that dazzle on a first glance, to tell you nothing on a second. From the time when Galileo first saw her phases, little was learned of her for two centuries and a half, and even the little she seemed to show proved misleading. The few traits thought to be discerned there were so faint and fugitive that, while some observers deemed them substantial, others ascribed them in whole or part to cloud. A cloud-enshrouded planet Venus in consequence was considered to be; a covering not so much for her own sins of commission as for the sins of omission of observers to see.
It was not until Schiaparelli attacked the subject that any real light was shed on her who reflected so much. Through a new departure, by choosing daylight for his observation time, that most eminent observer first solved one riddle she had read astronomers so long, the length of her day. Hitherto she had been scanned chiefly for a few moments at twilight and the recurrent aspect her disk presented on successive days had been for much in imputing to her a rotation not differing substantially from the earth's in length. Schiaparelli's method allowed of repeated scanning during several hours at a stretch and in this manner he learned that the periodic punctuality of the same features night after night was not because they managed so nearly to keep pace with our own, but because they failed to move at all in the meantime. In other words, her day must be immensely long. He then critically examined the older observations and found that they could all be thus explained. Six years later he repeated his observations and with the like result.
In 1896 the subject was taken up at Flagstaff. Very soon it became evident that markings existed on the disk, most noticeable as fingerlike streaks pointing in from the terminator, faint but unmistakable from the positional identity of their successive presentation. A projection near the south cusp was also clearly discernible, as well as two others, one in mid-terminator, one near the northern cusp. Other dark markings also came out, developing into a sort of collar round the southern pole. Spots, too, small, not large, stood blotched upon the disk. In this investigation not only was care taken to guard against illusion, but no regard at the time was paid to any previous observations.
The configurations thus disclosed proved permanent in place. By watching them assiduously it was possible to note that no change in position occurred in them upon the face the planet showed, first through an interval of five hours, then through one of days, then of weeks. It thus became evident that they bore always the same relation to the illuminated portion of the disk. This illuminated part, then, never changed. In other words, the planet turned always the same face to the sun. The fact lay beyond a doubt, though of course not beyond a doubter. The fundamental importance of this primary fact upon the world of Venus we shall note as we proceed.
In character these markings were peculiar and distinctive. In addition to some of more ordinary form were a set of spoke-like dark rays which started from the planet's periphery and ran inwards to a point not very distant from the center. The spokes began well-defined and broad at the edge, dwindling and growing fainter as they proceeded, requiring the best of definition for their following to their central hub. They were most noticeable on the edge of the disk which marked the boundary of light and shade, the sunrise line the terminator, as it is called; less so on that which the sky cut off, called the limb.
From so much of the planet as was then presented earthwards, it was possible to make a map giving the chief features of Venus's globe. In addition to demonstrating the durational identity of the axial rotation and the orbital revolution, the markings showed the planet's poles to be practically perpendicular to the orbital plane. Thus the central equatorial longitude lay in perpetuity directly under the sun.
The difficulty in seeing these tell-tale marks lies in their faintness. A very good reason for such astronomic concealment on the part of Venus will appear as we go on. In consequence, the contrast must be intensified as much as possible and to secure it a small aperture and low powers are best. It is only in very good air that these helps to definition can be disregarded.
Corroboration of these markings has been obtained at Flagstaff in the years that have since elapsed. In 1903 a bulletin giving drawings of that year was published, showing confirmation, and in 1907 and 1909 other drawings afforded like witness to their actuality. Markings have been seen by almost every member of the staff and independent observations made on identical dates show remarkable agreement.
But the most telling testimony is the concordance in results between different methods of investigation. The first of these we shall mention is the spectroscope.The spectroscope is primarily an instrument for analyzing light.
The sketches at the top were made on November 1, 1907, by Dr. Lowell (on the left), and on November 2, 1007, by V. M. Slipher (on the right). The sketches In the middle were made by V. M. Slipher and E. C. Slipher, on February 27, 1909. The sketches at the bottom by Dr. Lowell and E. C. Slipher, on April 12, 1909.
Light is due to wave-motion of the ether, and ordinary light consists of a mixture of light of various wave-lengths. By means of a prism or grating these are refracted differently and so sifted into a colored ribbon or band, the longer waves lying at the red end of the spectrum, as the ribbon is called, the shorter at the violet. Now the spectroscope is such a prism or grating placed between the image and the observer, by means of which a series of colored images of the object are produced. In order that these may not overlap and so confuse one another, the light is allowed to enter the prism only through a narrow slit placed across the telescopic image of the object to be examined. Thus successive images of what is contained by the slit are presented arranged according to their wave-lengths. In practise the rays of light from the slit enter a small telescope called the collimator, and are there rendered
Sketches of different observers showing agreement, made at Tacubaya, Mexico, on February 7, 1897.
each according to its kind, into a spectral image band which may then be viewed by the eye or caught upon a photographic plate.
One of the interesting applications of the spectroscope lies in its ability to detect motion in the line of sight, or in just the direction in which the eye can not.
It was reasoned by Doppler in 1842 that if an object be coming toward the observer emitting light as it does so, each wave-length of its spectrum should be shortened in proportion to the relative speed of its approach as compared with the speed of light, because each new wave is given out nearer the observer than would otherwise be the case and its wave-length thus seemingly decreased. Reversely it will be lengthened if the object be receding from the observer or he from it. This would change the color of each wave-length and so of the object, were it not that while each hue moves into the place of the next, like the guests at Alice's tea-party in Wonderland, some red rays pass off the visible spectrum, but new violet rays come up from the infra violet and the spectrum is as complete as before. This unfortunate infecundity of his principle in Doppler's own hands was remedied in 1848 by Fizeau, who pointed out that the dark lines in the spectra can be used as measures of the shift. In all spectra are gaps where individual wavelengths are absorbed or omitted and these, the lines in the spectrum, tell the tale.
This principle is applicable not only to a body moving as a whole, but to differing motions of its parts if the body be large enough to show a disk. Now, if a body be rotating, one side of it will be approaching the observer, while the opposite side is receding from him, and if the slit be placed perpendicular to the axis about which the spin takes place, each spectral line will appear not straight across the spectrum of the object, but skewed, the approaching side being tilted to the violet end, the receding side to the red.
This principle was put in practise by one of the observatory staff, Dr. Slipher, to determine spectrographically the rotation of Venus. By placing the slit parallel to the ecliptic or, more properly, to the orbit of Venus, which is practically the same thing, it would find itself along what we have reason to suppose the equator of the planet and thus by its tilt give evidence of the rotation period capable of measurement. Even a considerable error in the position of the equator would make little difference in the rotational result. In order that there might be no question of illusion or personal bias, photographs instead of eye observations of the spectrum were made.
Dr. Slipher began by considering the take off before he jumped. His sagacity greatly influenced the result. It might seem as if the best time to examine the planet for rotation were when it is farthest from the sun and is best seen. This indeed was the time selected by Belopolsky, who examined Venus spectrographically between the time of the inception of the investigation at Flagstaff and its execution and thought he had detected a short rotation for it.
Belopolski made his attempt at elongation in spite of knowing what I shall now explain. For elongation, although the time when the planet is easiest seen, is not that in which it is best examined spectrographically for rotation.
In the case of a body reflecting light, the shift varies from what it is if the body be emitting it, from twice as much in some positions to nothing at all in others. This is because the reflecting surface itself moves to or from the waves. Thus if a planet be on the side of the sun away from the earth, the rim of it which is approaching the earth advances to meet each new wave and so shortens it by just the amount
|October 15, 1896.||February 12, 1897.||March 26, 1897.|
of its own advance, thus doubling the shortage which would result if it emitted the waves itself. The receding side in the same manner doubles the recession. If the planet be at right angles to the sun the waves are affected as if they were emitted and we have a single shortening or lengthening, as the case may be. If the planet be between us and the sun, the rim is running from the sun at just the speed it is approaching us and the total effect is nil.Thus in the case of Venus, the evidence of tilt obtained depends entirely on where you take her. Superior conjunction or when she lies beyond the sun is the best time spectrographically and it was this that Dr. Slipher chose. He caught the planet just as she was coming out from' behind the sun as evening star. In this he was abetted by the clear and steady air of Flagstaff, which enabled him to got her while she was still not far from the sun himself.
plates were made in like manner on that planet. Now Mars is an object which by reason of its smaller size is twice as difficult a test for rotation in twenty-four hours as Venus. The plates too did not happen to be so good. Nevertheless, on measurement they yielded a result within a twenty-fourth part of what we know to be the Mars day. For we know this time to within the hundredth of a second. Now in consequence of the smaller quantity to be measured an error of 55 minutes in the case of Mars corresponds to one of 31 minutes on Venus. To this precision, then, the day of Venus would have been determined had it been of twenty-four hours' duration.
Another test of like character was forthcoming in Dr. Slipher's spectrograms of the rotation time of Jupiter. Inasmuch as Jupiter's day at the equator is 9 hours 50.4 minutes long, while Jupiter's diameter is some twelve times that of Venus, the precision possible is here thirty times as great. Thirty-one minutes' error on Venus would mean about one minute for Jupiter. The spectrograms did even better than this. The known speed of rotation at Jupiter's equator is 12.63 km.; Dr. Slipher's spectrograms gave 12.62 km., or within half a minute of
the true rotation period. This means that they would have shown Venus's to within 14 minutes if the conditions were as good. As Jupiter's spectrograms are easier to measure than Venus's, while Mars's are more difficult, we may take 25 minutes for the mean of the two criteria. I need perhaps not tell you that no previous spectrograms of Jupiter for rotation had come up to this precision.
From these two determinations on Jupiter and Mars we may determine the utmost period of rotation for Venus which the spectroscope could disclose. This would be the period for which the probable error was just equal to the quantity to be measured. From Mars we have for a 24-hour period on Venus a probable error of 31 minutes. This is one forty-eighth of the quantity measured on the supposition of a day's period. One of 48 days, therefore, would have its probable error equal to the quantity itself. From Jupiter we get in the same way 96 days. Thus from two to three months would be the limit of leisureliness the spectroscope could be got to note, and it was just this quantity that the investigations on Venus themselves expressed.
The spectroscope, therefore, definitely asserts that the rotation of Venus does not take place in anything approaching twenty-four hours, and by negativing any period up to two or three months long corroborates to the limit of its ability that shown by eye observation, one of 225 days.
The care at Flagstaff with which the possibility of error was sought to be excluded in this investigation of the length of Venus's day and the concordant precision in the results are worthy of notice. For it is by thus being particular and systematic that the accuracy of the determinations made there in other lines besides this has been secured.
Now a certain peculiarity of Venus's appearance of a totally different kind from those so far spoken of, here comes in to corroborate both of the previous determinations: the perfect roundness of her figure. For this very rondure has something to disclose. If Venus rotated in anything like twenty-four hours her disk should be perceptibly flattened at the poles, her figure becoming squat in consequence of her spin. For though as rigid as steel to sudden impulses she would be like putty at the hands of long-continued ones. We can calculate about how much that flattening would be. That of the earth is 1⁄293 that of Mars 1⁄190, and both planets rotate in approximately twenty-four hours. That of Venus for a like spin would lie between the two figure, because in mass and density she falls between the earth and Mars. Let us say 1⁄275 for it, which would be close to the truth.
Now Venus on occasion offers peculiar opportunity to measure any such flattening if it existed. For at times she passes in transit across the sun's face. At that moment she presents an absolute absence of phase and in consequence any correction due to asymmetrical illumination is self-eliminated. Furthermore, she then shows the largest of all planetary disks, one of 60 seconds in diameter. A flattening of 1⁄275 would amount therefore in her case to 0″.22. Such a quantity could not possibly miss of detection. For that of Mars, which is only half as much and is not so well displayed, has nevertheless been measured. Yet no divergence from perfect sphericity has ever been found in the globe of Venus, though diameters at all azimuths have been carefully taken when she is seen silhouetted in transit against the sun.
I may have seemed to dwell at unnecessary length upon the time that Venus takes to turn. But there is cause. The rotation time of Venus, the determination, that is, of the planet's day, is one of the fundamental astronomical acquisitions of recent years. It is not a question of academic accuracy merely, of a little more or a little less in actual duration, but one which carries in its train a completely new outlook on Venus and sheds a valuable sidelight upon the history of our whole planetary system. For upon it turns our whole knowledge of the planet's physical condition. More than this, it adds something which must be reckoned with in the framing of any cosmogony.
To this we shall now proceed and if the deductions and the phenomena which corroborate them appear almost romantically strange it is in the facts themselves that the romance exists.
In the first place such isochronism gives us a glimpse into the planet's past. That the day should coincide with the year means that it has been brought to this condition. For that it can always have been so is mechanically highly improbable. On the other hand, there is a cause continually tending to bring about such a result; tidal friction. Under the immense forces at work the planetary masses behave as if they were plastic. In consequence tides of the whole substance are set up in them if they rotate, and these tides act as a brake upon the rotation until they finally retard it to coincidence with the orbital revolution.
That Venus now turns the same face in perpetuity to the sun, lets us look down a long vista in her career, and gives us a very instant idea of a phase in a planet's history: that long slow change by which a day is lengthened to infinity. That Venus should have suffered such action is in keeping with theory, though it could not have been predicted in the absence of fact?. For Venus falls exactly on debatable ground, on the line as it were. Tidal action depends, for the time necessary to produce a given result, on the square of the radius of the body acted on and as the sixth power of its distance from the exciting cause. In consequence for solar action the nearer planets would show its effect first. Now Mercury already turns the same face always to the sun; the earth, as we know, does not. Venus comes between the two in distance and might therefore a priori do either, depending upon how long the action has been going on. That she agrees with Mercury in continuously staring at the sun thus affords valuable evidence on the general evolution of the solar system.
Interesting as this information is, it is second to what we learn in consequence about the body itself. To have the same hemisphere exposed everlastingly to sunlight while the other is in perpetuity turned away, must cause a state of things of which we can form but faint conception from what we know on earth. Baked for æons without let-up and still baking, the sunward face must, if unshielded, be a Tophet surpassing our powers adequately to portray. And unshielded it must be, as we shall presently see. Reversely, the other must be a hyperborean expanse to which our polar regions are temperate abodes. For upon one whole hemisphere of Venus the sun never shines, never so much as peeps above the star-studded horizon. Night eternal reigns over half of her globe! The thought would appall the most intrepid of our arctic explorers, and prevent at least everybody from going to the pole; or rather what here replaces it "through the dark continent."
Deduction from our known premises enables us to go further in sketching the picture of Venus's globe. Venus we know has an atmosphere. The effects of it are patent at the times when she passes between, or nearly between, us and the sun. She is then seen haloed by a rim of light due, as Wilson has shown, to reflection chiefly, not to refraction as was formerly supposed, from an atmosphere about her. Now the intense heating to which the center of her sunward side is exposed must necessarily expand the air there, causing it to rise funnel-wise up in a world-wide western cyclone. To fill the space thus depleted currents must set in toward the center from all points of the compass, continuing out to the lighted rim. Their place in turn would be occupied by surface indraughts from the dark side. Meanwhile the heated air would spread like an umbrella round into the cold hemisphere there to descend and replace the outgoing superficial current back to the sunlit face. A regular aerial round of travel is thus started, which is the same forces that began it must keep up. The course is surface-wise from the dark to the illuminated hemisphere; aloft from the sunlit to the night one.
Now this simple, regular and reliable meteorological service explains a feature of the visual observations which has deterred many timid souls from crediting their reality. One of the most striking features of Venus's disk are the tongues of shading that make in from all parts of the lighted rim toward the center. They are the beginnings of those spoke-like markings the methodical oddity of which makes their actuality so difficult of belief. They seem a thought too peripherally positioned to be other than optically evolved. Their recurrent showing in the same places marks them as facts, however, and as such we must regard them. Now when we consider them in the light of Venus's meteorology their cause at once suggests itself. And with this index-finger to guide us we perceive that far from being surprising they are just the phenomena we ought to expect. For consider the surface indraught along the bounding rim of constant sun-exposure. With the immense temperature gradient which exists between the day and the night side of Venus, the power of these winds must be enormous. Being essentially surface ones, they must sweep the face of the planet with irresistible force and, what is more, having once found a pathway of preference, must from the general unchangeableness of the conditions continue to follow it perpetually. For the only thing to alter their direction, the libration, is from the circularity of Venus's orbit negligibly small. Sweeping in originally through valleys or mountain passes, the points offering the easiest access, they must eventually have polished the surfaces over which they passed to a differentiation of appearance visible even across millions of miles of intervening space. Essentially surface currents at the rim, they would become less and less so as they neared the center of the lighted side and furthermore would converge as they approached it. They would seem to us to narrow and become at the same time less salient as they advanced. This is just what their spoke-like character shows. Thus the peculiar look of the Venusian markings proves to be in exact keeping with what the conditions demand, and by so doing bears testimony that those conditions actually exist.
Not less strange on its face and equally interesting for its disclosure is another phenomenon connected with the planet which also has been deemed incredible—the exceeding brightness of Venus's disk. Her great luster is, as we saw above, in part attributable to her proximity first to the sun and secondly to us. But this is not the sole cause of it. Though a part of her splendor is due to her position, a part is her own. Her intrinsic brightness, her albedo, as it is called, has been found by Müller, of Potsdam, who has made the last and most authoritative determination of it, to reach the excessive figure of.92 of absolute reflection. This figure has seemed to many impossible, but we shall see from consideration that it simply reflects the conditions.
The rising currents on the sunward side must from their great heat be capable of holding much water-vapor in suspension. This they would take over with them in large part to the night side and becoming chilled there deposit it as snow. Being cold on their return they would reenter the warm side relatively dry and thus be fitted to act again as water-carriers from that side to the other. This process of depletion on the one and accumulation on the opposite hemisphere must end in taking the whole supply, surface or aerial, from the day side to pile it up in perpetual ice upon the night one. Dry air more or less laden with dust must therefore constitute now the atmospheric covering of the sunward hemisphere. Now this is what gives Venus her excessive luster—an atmosphere devoid of cloud. It is precisely because she is not cloud-covered that her luster is so great. She "clothes herself with light as with a garment" in consequence of a physical fact of some interest. As becomes the Mother of Loves, this drapery is gauze of the most attenuated character, and yet on that very account is a great heightener of effect. For it is a well-known property of matter that a substance when comminuted reflects much more light than when massed as a solid or an opaque cloud. Now an atmosphere is itself such a comminuted affair and especially is made lucent by the dust of one sort and another which it holds in suspension. This would particularly be true of Venus for the reasons we have exposed and thus stands explained her albedo of .92 which were she cloud-covered could not exceed .72, the albedo of cloud. This brightening character of an atmosphere stands corroborated by what we perceive of the other planets. Mercury and the moon, which are airless bodies, have an albedo of only .17; Mars, which has some air but not much, one of .27; while Venus, whose sky is clear, one of .92.
Another phenomenon which has greatly puzzled astronomers stands accounted for by what we have learned latterly of the world of Venus. For years by one observer or another a sort of faint phosphorescent shine has been reported of the unilluminated part of her disk; the ashen light, it has been called. The side of her which should be dark has appeared ghostly lighted up. The phenomenon has seemed the weirder for the difficulty of explaining it. It is like what dimly reveals the old moon in the new moon's arms. With the moon this is earth-shine; the moon-shine the earth herself lends her satellite. But Venus has no neighbor to act as mirror near her, though such be her astronomic symbol. The earth is too far off and the stars inadequate to the occasion. But the state of things we have sketched furnishes an explanation. If the night side of Venus be a vast stretch of polar ice, here is just the surface to reflect the starlight with something approaching a phosphorescent shine. Nor would this necessarily be dimmed by the dust of ages because of a slow process of glacier rejuvenation constantly in progress, due partly to the winds, partly to a slow sinking of the débris to the bottom.
Such are some of the peculiar phenomena presented by the planet. When we thus reason about them—and even in science reasoning is not so much to be despised as some mechanical souls would have us believe—we see that they lose their oddity, becoming the very pattern and prototype of what we should expect.
Logical deductions from well-established fact has led us to explanation of what had seemed inexplicable or untrue. Our several results check one another. For, in conclusion, we may note how full of significance it is that the outcomes of such various investigation should fit into one another to an articulated whole. Their dove-tailing at times is indeed surprising, so diverse the character of the converging lines of research. Thus that the planet's albedo should have anything to say about the length of its day, should actually come forward in corroboration of the markings' own forthright showing, would hardly have been supposed. Or that the ashen light of the dark side should find interpretation in the same axial rotation through a long chain of concatenated circumstance was not to be anticipated. Still less would one have divined that the cycle would stand complete and that the very markings which enable us to determine the duration of the Venusian day should have had their peculiar features determined by it.
The force that such agreement to a common end imparts to the chain of argument needs no comment. It speaks for itself.
The picture of Venus thus presented to our gaze may seem forbidding—one hemisphere a torrid desert, the other deserted ice. Which side strikes us as the worse is matter of personal predilection. But the portrait has its grand features for all that; features which give us a new conception of what exists in the universe and lure our thought afield in space with all the greater insistence for being drawn not from fancy but from fact.
Not less of interest is the way in which our knowledge has been obtained; for it has been acquired by research along very different lines, and then by reasoning upon the results of that research to their necessary conclusions. That these conclusions lead to a consistent conception assures us of their truth. Two things are suggested to us by such procedure: first, the pregnancy of considering a subject from many points of view, and secondly, the importance of reasoning upon facts after they have been acquired.
The fact-gatherer has his uses, but they are not those of the highest class. It is not enough to have a thing on our plates, we must know that we have it there and interrogate it for meaning if we would extract from it the knowledge it is capable of yielding and so most truly add to the advance of our day.
In the case before us the result is of special interact because it exemplifies the eventual effects of a force in astronomical mechanics, the importance of which is only beginning to be appreciated: tidal friction. It has brought Venus as a world to the deathly pass we have contemplated together. Starting merely as a brake upon her rotation, it has ended by destroying all those physical conditions which enable our own world to be what it is. Night and day, summer and winter, heat and cold, are vital vicissitudes unknown now upon our sister orb. There nothing changes while the centuries pass. An eternity of deadly deathlessness is Venus's statuesque lot.
- An evening lecture at the vicennial of Clark University.