Popular Science Monthly/Volume 37/May 1890/Scenes on the Planet Mercury

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NO one of the planets that were known in ancient times is so difficult to observe as Mercury, and none presents so many obstacles to the study of its orbit and physical constitution. As to its orbit, Mercury is the only planet the course of which seems even now to have partly cut loose from the laws of universal gravitation, and the theory of which, although well built up by the genius of Leverrier, is still in considerable disagreement with the observations. The little we know of its physical construction is derived from the observations made a hundred years ago by Schroeter at Lilienthal. A telescopic examination of this planet is really a difficult affair. Describing a small orbit around the sun, Mercury is never seen so far from it as to make it possible to observe it, in temperate latitudes, in the full darkness of night. It is rarely possible to observe it in the twilight before sunrise or after sunset; it being then so near the horizon and so affected by the agitations and unequal refractions of the lower strata of the atmosphere that it usually presents itself to the telescope with an uncertain and flaring aspect which appears to the naked eye as a strong scintillation. For this reason the ancients called it Στίλβων, or the scintillating star. No other resource is left than to essay observations in broad daylight, in the presence of the sun always near, and in an always illuminated atmosphere.

Some efforts I made in 1881 persuaded me that it was possible both to see the spots of Mercury and to get sufficiently connected and continuous observations of them in broad daylight, and I decided in the beginning of 1882 to make a regular study of this planet. During the eight years since then, I have had Mercury in the field of my telescope several hundred times; often, it is true, with little profit and at the expense of great loss of time, either because of the agitation of the atmosphere, which is often strong during the day—especially in the summer months—or on account of the insufficient transparency of the air. But by patience I have succeeded in seeing the spots on the planet one hundred and fifty times with more or less precision, and in making also fairly satisfactory drawings of them, employing at first, for the purpose, our eight-inch Merz equatorial, but afterward our great eighteen-inch Munich instrument.

I found the rotation of the planet quite different from what it has hitherto been supposed to be, on the basis of insufficient observations made with imperfect telescopes a hundred years ago. I may describe it in a few words by saying that Mercury revolves around the sun in the same manner as the moon revolves around the earth. As the moon's journey around the earth is performed in such a way that it always shows nearly the same face and the same spots, so Mercury, in traversing its orbit around the sun, constantly presents nearly the same hemisphere to that source of light. I say nearly—not exactly—the same hemisphere. For Mercury is subject, like the moon, to the phenomenon of libration. In observing the full moon, even with a small telescope, we remark that the same spots generally occupy the central regions of its disk; but, if we study them and their distances from the eastern and western borders more minutely, we shall soon perceive, as Galileo first did about two hundred years ago, that they oscillate to a considerable degree, now toward the right and now toward the left—exemplifying the phenomenon called libration in longitude. This arises from the moon's directing one of its diameters perpetually and almost exactly, not toward the center of the earth, and not toward the center of the elliptical lunar orbit, but toward the one of the two foci of its orbit which the earth does not occupy. To the observer occupying this point, the moon would consequently always present the same appearance. But to us, who are at a mean distance of forty-two thousand kilometres from that point, the moon presents somewhat different aspects according to the time when we look at it, sometimes showing us a little more of its eastern, sometimes a little more of its western, regions. Mercury presents itself to the sun in different phases of its cycle in a similar manner. It constantly directs one of its diameters, not toward the focus of its elliptical orbit which is occupied by the sun, but toward the second focus. These two foci being distant from one another not less than a fifth of the whole diameter of the orbit of Mercury, the libration of the planet is enormous. The point that receives the rays of the sun vertically changes its place on the surface of the planet, and performs an oscillatory movement along the equator forty-seven degrees in amplitude, or through more than one eighth of the equatorial circumference. The whole duration of this oscillation, including the going and returning, is equal to the time employed by Mercury in traversing its orbit, or about eighty-eight terrestrial days. Thus Mercury stands oriented toward the sun like a magnet toward a mass of iron; but this orientation is not constant to the point of excluding a movement of oscillation of the planet to the east and to the west, like that which the moon performs toward us.

This oscillation is of great importance for the physical condition of the planet. Suppose, for instance, that it did not exist, and that Mercury always turned the same hemisphere to the light and heat of the sun, the other hemisphere remaining plunged in perpetual night. The point of the surface situated at the central pole of the illuminated hemisphere would have the sun eternally in the zenith; the other points of the planet accessible to the solar rays would have the sun always at the same point in their horizon, at the same height, without any apparent movement, without any perceptible change; consequently, no alternation of night and day, no variety of season; the stars eternally invisible because of the perpetual presence of the sun; and, Mercury having no moon, we can hardly imagine how the inhabitants of those regions, condemned to an endless day, could find a means of regularly computing time.

Such are, in fact, nearly the conditions that prevail in Mercury, but only approximately. The oscillating movement of the Mercurial globe as toward the sun would be attributed by an observer on the surface of the planet to the sun, as we attribute to the sun the diurnal movement which really appertains to the earth. To us the sun seems to circle regularly from east to west, defining in twenty-four hours the period of day and night; to the observer on Mercury, the sun will describe a back-and-forth movement through an arc of forty-seven degrees in the celestial vault, while the position of the arc as toward the horizon will always be the same. The complete period of the double oscillation will comprise almost exactly eighty-four terrestrial days. According as the arc of solar oscillation is all above the horizon of the observer or all below it, or partly above and partly below it, there will be different appearances and a different distribution of light and heat. In the regions, covering three eighths of the planet, where the arc is all below the horizon, the sun will never be seen, and the darkness will be perpetual. Thick and eternal night will reign there, except perhaps from the accidental appearance of some light produced by refraction and atmospheric glows, or phenomena like the aurora borealis; together with the light emitted by the stars and planets.

Another part of Mercury, including also three eighths of its surface, will have the arc of oscillation all above its horizon, and will be continually exposed to the rays of the sun, without any other change than the variations in the obliquity of the rays through the different phases assumed during the period of eighty-eight days. Night is absolutely impossible. In other regions, covering a quarter of the planet, in which the arc of oscillation is partly above and partly below the horizon, there will be alternations of light and darkness. In these privileged regions the period of eighty-eight days will be divided into two intervals, one characterized by a continuous light, the other by darkness; the two intervals will be equal in some places, of different length in others, according to the position of the place on the surface of the planet, and the length of the part of the solar arc which appears above the horizon.

The possibility of organic life in a planet constituted after this manner depends on the existence of an atmosphere capable of distributing heat into different regions, in such a way as to diminish the extremes of heat and cold. Schroeter, a hundred years ago, suspected the existence of an atmosphere round Mercury; my observations afford more definite indications of it, and affirm its existence with a much greater probability. The spots of the planet are most clearly visible when they are in the central parts of the disk, and grow dimmer and ultimately disappear as they approach the border. I have been able to assure myself that this phenomenon is not merely due to the greater obliquity of the perspective, but is because some obstacle is really presented to the view of spots situated in such positions. That obstacle can hardly be anything else than the greater extent of atmosphere that the light-rays have to traverse in coming from the edges than from the center of the disk. We have, therefore, reasons for believing that the atmosphere of Mercury is less transparent than that of Mars, and more nearly like that of the earth. The circular contour of the planet, moreover, in which the spots become less visible, always appears more luminous than the rest, but often irregularly luminous, more so at some points than at others; and sometimes we can see on its borders bright white regions that remain in sight several days in succession, but are generally changeable, and show themselves sometimes in one place and sometimes in another. I attribute these phenomena to condensations going on in the atmosphere of Mercury, which reflects more light into space the more opaque it becomes. Similar white regions are also often seen in the interior of the disk, but they are not so brilliant there as on its border. Further, the dark spots of the planet, while they are permanent as to form and arrangement, are not always equally evident. They are sometimes more intense, at other times paler. Sometimes, also, one or another of them will become momentarily invisible. Such peculiarities can not be attributed to any other cause than atmospheric condensations similar to our clouds, which veil the ground of the planet in different degrees, sometimes in one region, sometimes in another. An observer, looking from the depths of space upon the countries of our earth covered with clouds, would perceive a like spectacle.

Very little can be said of the nature of the surface of Mercury. We must recollect that three eighths of it are inaccessible to the solar rays and to sight; on that side, therefore, we have but slight hopes of ever learning anything certainly. It will also be hard to gain a correct and sure knowledge of the part we can see. The dark spots, even when they are not clouded, usually appear under the form of extremely thin trails of shadow. In ordinary conditions they are distinguishable only at the expense of much attention and weariness. Under the best conditions they have a brown, warm tint, like that of sepia; of a tone very indistinct upon the general color of the planet, which is usually of a clear rose bordering on copper. Forms or bands so vague and diffuse, with indistinct borders, always leaving a place for arbitrary definition, are not easily represented in a satisfactory manner. Still, I believe the indeterminateness of outlines is, in the majority of cases, only apparent, and a result of the insufficient optical power of the instrument; for the more perfect the view and the finer the image we get of the shadows, the more do we find them disposed to break up into a multitude of smaller details. By employing more powerful telescopes, they could doubtless be resolved into more reduced forms.

While it is so hard to make a good study of the dark spots of Mercury, it is not easy to express a well-founded opinion upon their nature. They might be attributed to the different materials composing the solid surface of the planet or to its structure, as we know is the case with the moon. But if we are disposed to consider them as in some way resembling our seas, and to suppose the existence of an atmosphere around the planet, with condensations and precipitations, I do not know of any decisive arguments that can be opposed to the opinion. The spots are not gathered in large masses, but are disposed in areas and zones of small extent; are greatly ramified, and alternate with considerable uniformity with clear spaces. We may, therefore, conclude that no vast oceans or great continents exist on Mercury; but that land and sea interpenetrate one another and give rise to conditions very different from those which exist on the earth, but which may be more desirable.

Mercury is a world that differs from ours as much as Mars does. The sun lights it and warms it much more intensely than it does the earth, and in a very different way. If life exists in that world, it is doubtless under conditions so different from ours that we can hardly imagine them. The eternal presence of the sun, darting its rays almost vertically on some regions, and its perpetual absence in the opposite countries, would seem intolerable to us. And yet, if we reflect upon it, we shall remark that such a contrast would produce a more rapid, more powerful, and more regular atmospheric circulation than that which spreads the elements of life over the earth; and it possibly is brought about in this way that as complete and even perhaps more perfect equilibrium of temperature is produced on the whole planet than with us.

Mercury, by directing the same face toward the sun during its whole revolution, is peculiarly distinguished from the other planets, all of which the length of whose rotation has been determined, turn round their axes in a few hours. This mode of rotation, however, which would be unique among the planets, seems common enough among the satellites. All testimony is to the effect that our moon has always conformed to it. The first three satellites of Jupiter probably behave in the same way, and the observations of Auwers and Engelmann demonstrate that the fourth does so. Cassini verified the same fact for Japhet, the eighth satellite of Saturn. It may, therefore, be considered the rule among the satellites, while it is an exception among the planets.

The exception may probably be attributed to the proximity of Mercury to the sun, and perhaps also to the fact that it has no satellites; and depends, I think, on the way Mercury was formed when the solar system took its present shape. The peculiarity constitutes a new datum to be added to those which astronomers will have to take account of in studying solar and planetary cosmogony—Translated for the Popular Science Monthly from a French version by F. Terby in Ciel et Terre.

  1. Address before the Royal Academy Dei Lincei, December 8, 1889.