Popular Science Monthly/Volume 16/April 1880/What Is Jupiter Doing?
| WHAT IS JUPITER DOING? |
By HENRY J. SLACK.
THE question, so often suggested by changes in the aspect of the planet Jupiter, "What is he doing?" is again forcibly put by the appearance of a remarkable spot of enormous dimensions, and of a reddish or orange-brown tint, which has occupied the attention of observers for several months, and which seems to be identified, so far as relates to position and form, though not in color, with what has been seen on former occasions.
Probably no celestial bodies reach a permanent condition: constant change seems a law of nature; but there may be great variations in the rates at which changes occur. If we assume as probable a modification of the nebular theory, suns and their attendant planets are formed by the condensation of matter in an extreme state of tenuity, and the mass of suns and planets may receive frequent additions in the shape of any smaller or less heavy bodies they are able to attract. Our sun is probably a great devourer of meteors; and, as our earth crosses the orbits of certain meteoric swarms, we have showers of shooting stars, fortunately so small that their bombardment is unnoticed.
Scarcely anything is known, or plausibly guessed, concerning the condition and properties of nebulous matter. If, for example, the
Fig. 1.—Spots on Jupiter observed and drawn by Mr. E. L. Trouvelot.—Observation of September 25, 1878. with the shadow of a satellite.
spectrum of a nebula indicates hydrogen, we may be pretty sure it is not in the state of the gas as it is known in our laboratories. The recent discoveries of Crookes concerning the properties of matter a million times more attenuated than common air lead to the hope that fresh light may be thrown upon many astronomical questions; but in the mean time it is impossible to form more than a vague idea of the condition of any star or planet that does not in its main features resemble our earth; and this can be said only of Mars, on whose globe we can discover what is probably land and what is water, and see white masses, which it is reasonable to believe are snow, form and melt away as the planet's winter and summer affect them in turns.
Our earth has long been in a state of slow, as distinguished from that of rapid, change. The geologist finds the oldest rocks he can discover affording indications that they were formed when the circumstances of the globe were sufficiently like what they are now for fair comparison. The earth's surface may have been warmer, its atmosphere more moist, and it may have contained more carbonic acid than we now find; storms may have been more frequent and more violent, but the assemblage of differences between what now is and what was at the time of any formation the geologists can reach would not noticeably approach the enormous difference that separates the condition of our earth from that of either Jupiter or Saturn. It is possible that they now represent stages which our earth passed through in remote times, and they may be undergoing changes that are approximating
Fig. 2.—Spots on Jupiter observed and drawn by Mr. E. L. Trouvelot.—Observation of December 23, 1878, with the shadow of the third satellite.
them to our present condition. It is, however, probable that, while there are analogies and resemblances in the life-histories of all the heavenly bodies, there are also individual peculiarities and diversities not less important or less striking.
Jupiter's diameter is about eleven times that of our earth, and his mean density is about a quarter that of the earth, or about a third more than water. Now, a bulky body may be composed of heavy materials, and still, as a whole, be light, like an iron ship or a lump of pumice-stone, that will float in water. The pumice-lump is light on account of its vesicular formation, so that the mass consists of heavy feldspathic material and the air it contains. Extract the air, and the pumice loses its floating power, though still far from heavy in proportion to its bulk. Most of the earth's crust is formed of solids much heavier than water. Granites are more than two and a half times heavier than water, slaty rocks much about the same, and so are ordinary limestones, the variations of all being from about 2∙5 to 2·9. The ironstone group contains denser minerals; red hematite has a specific gravity of 4·5; magnetic ironstone, 4·5 to 5·2, etc.; and many other ores are heavy.
At some remote period, when only part of the now solid earth had been condensed from gaseous and vapory matter, our planet might have had a mean density like that of Jupiter, as its rocky materials contain between forty and fifty per cent, of oxygen; and while condensations and chemical combinations were going on rapidly our globe must have been the scene of
"Thunders, lightnings, and prodigious storms."
And it is probable that certain stars which have suddenly blazed forth with passing splendor have exhibited to us the spectacle of conflagrations extending over millions and billions of square miles. Color changes in Jupiter—such as those noticed by Mr. Browning and the writer in 1869-'70—may have been caused by soda-flames, though not fierce enough or extensive enough to add materially to his ordinary luminosity, which is estimated as always exceeding, though not in a very high degree, what it would be by mere reflection of light received from the sun. A drawing after Mr. Browning was published in the fifth volume of the "Student and Intellectual Observer," showing a broad, full, yellow equatorial belt; also broad belts of purplish brown edged with narrower yellow bands above and below it, and curious white patches in the upper dark belt. The polar belts were purplish and olive. The appearance and disappearance of these remarkable belts indicated great physical changes, and it is to be regretted that spectroscopes could not afford so much information as was hoped for. The planet, though appearing much brighter than any star, gives, according to Mr. Browning, a spectrum fainter than that of a star of the second magnitude. It is the size of the planet and his nearness, as compared with the distance of any fixed star, that make him such a brilliant object. The size of a luminous body greatly affects our estimation of the intensity of its light. Mr. Huggins, at the time mentioned, discovered some dark lines in the Jovian spectrum not belonging to the solar spectrum, and probably resulting from the absorptive action of the planet's atmosphere. He also pointed out that the remarkable yellow color had been seen some years before. Quite recently Mr. Huggins has been employing his large reflector to take photographic spectra of the planet, and he informs the writer that "from G to O in the outer violet there is no sensible modification, either in addition or absence of lines, of the solar spectrum." This is curious, as Jupiter has exhibited a good deal of primrose tint, with orange-brown belts and a big orange-brown spot.
A telescopic view of Jupiter usually exhibits some dark belts, occupying a zone of considerable breadth, on either side of the planet's equator, with less conspicuous markings nearer the polar regions. It is also common to find various-shaped patches brighter than the rest. Sometimes the general pattern formed by these markings lasts for months with little visible alteration. At other times a few minutes are sufficient for changes of enormous magnitude. The first question that arises is, What do the dark bands or spots mean? Are they portions of the solid body of the planet, which have some fixity of shape, in any degree analogous to that of our mountain-chains or great continents? Or are they cloudy matter of less light-reflecting power than the bright and dense atmosphere by which the planet appears to be surrounded? Or are they merely more transparent parts of that atmosphere, through which no lower objects happen to reflect light enough to be visible? If the bright parts of the Jovian disk are light-reflecting clouds, and the dark belts the body of the planet, we should suppose it would be common to see a notched appearance of the edges; but this is not so. "Ordinarily," as Captain Noble says, "the belts fade perceptibly as they approach the actual edge of the disk; but," he adds, "I have seen the belts right up to it." The softening of the belts, as the planet's rotation brings them to the edges of the disk, probably arises from the dark parts being considerably below the boundary of the Jovian atmosphere, and thus seen through a greater thickness of it when near the edges. When the dark belts touch the edge without noticeable softening they must be higher up, and less likely to be any part of the solid body, if Jupiter has anything that can be so called. The great spot of this season has never been seen close to the edge. A very moderate magnification is sufficient to show that as the planet rotates it comes into view decidedly at some distance from the luminous margin, and disappears at a similar distance from the opposite margin.
Telescopes, under the most favorable conditions, and of the greatest power, only reveal very large features of the planet. It is impossible to see anything like details of structure, and this makes the identification of objects seen at different times more or less uncertain. If we had glimpses of great mountain-chains in Jupiter, it would be something like seeing the Andes or the Himalayas all in a lump, from some skyey perch, so far off as to prevent the separate peaks and valleys from being noticed. Jupiter is about five and one fifth times as far from the sun as we are—our mean distance, according to the last reckoning, being 92,620,000 miles. With the moon only 240,000 miles off, and very frequently bearing a much higher magnification than can be applied without confusion to Jupiter, telescopes bring no object near. A magnification of 1,000 linear—only usable under very favorable circumstances—makes lunar objects as big, but not as distinct, as a naked-eye vision of them would do if it could approach within 240 miles. With the enormously greater distance of Jupiter it must be evident how impossible it is for anything but huge masses to be seen.
Jupiter's atmosphere is much larger in proportion to any solid matter he may contain than that of our earth to its solid matter. It is also much denser, and from its greater distance only gets about one twenty-fifth as much solar influence as reaches us. For these and other reasons it is not unlikely that some of his cloud formations may be more lasting than ours. That his gaseous envelope is, however, at times subject to violent disturbances arising from a prodigious exertion of internal forces is proved by instances of sudden changes in the diameter of the disk. The Rev. T. W. Webb, in his "Celestial Objects" mentions as "inexplicable" an observation of Smyth, confirmed by similar observations of Maclear and Pearson, all being at different places, namely, that on June 26, 1828, Jupiter's second satellite, after fairly entering upon the disk, in the course of its revolution, was subsequently seen for four minutes outside it, and then suddenly vanished. More recently Secchi noticed a similar phenomenon; and the explanation can only be that Jupiter's atmosphere was suddenly blown out for some thousands of miles and retreated again. Secchi states that on April 2, 1874, he watched the first satellite as it was about to cross the planet's disk, which appeared "finely undulated." "When the satellite approached within its own diameter of the margin of the planet the latter sprang toward it, appeared to touch it, and immediately retired. This happened, backward and forward, until the satellite had plainly entered upon the planet; that is to say, for four or five minutes.[1]. . . The satellite appeared fixed, and all the movement seemed to belong to the disk of the planet."
In October, 1879, Mr. Kidd, of Bramley, Guildford, saw, as is described in the "Observatory" for November, the second satellite first touch the disk, then appear separated from it, and finally pass behind it, but remain for some time visible through it. The "Observatory" for November also quotes the "Chicago Tribune," to the effect that observations at the Dearborn Observatory indicate that changes in the outline of the planet take place from day to day. Two sets of measures at the interval of a week are stated to have shown a difference in the direction of the major axis amounting to 5°.
When extensive belts or bright portions change rapidly, the storm effects must be immensely greater than in any of our hurricanes. Jupiter's motion at the equator is at the rate of about 28,000 miles an hour; his daily rotation is completed in a few seconds less than ten hours; and objects in Jupiter weigh about two and a half times as much as on our earth. When our winds move with a hurricane-speed of 85 miles an hour, they exert a pressure upon whatever they strike equal to 36 pounds per square foot. What, then, must be the force of a Jovian storm, moving much heavier matter than our air, at the rate of 300 miles an hour, as was observed on one occasion by Herschels?
On another occasion South saw a spot 22,000 miles long, and before a friend who was present could commence a sketch it had nearly all changed. There may in such cases be violent chemical action, a terrific clashing together of atoms, and the precipitation of solid oxides of metals, like the fumes produced by the burning of magnesium wire.
In considering the persistence of spots or markings, it seems that the dark ones are more lasting than the light. A dark spot noticed by Cassini in 1665 was visible up to 1713, though obscured at intervals—at one time for eight years.
Some interesting white spots were noticed in 1878 by Niesten, of the Brussels Observatory, to change from a circular to an elongated form as they appeared in the center or nearer the sides of the disk. This would indicate something like a columnar form, looking round when seen vertically, and elongated when seen aslant.
Lately, as already mentioned, a very fine dark spot has been seen upon the south equatorial belt. It was found by Niesten to be 13" long, and 3" wide, the polar diameter of the planet being 48". When Captain Noble saw this spot, on August 22, 1879, he made a memorandum that "the remarkable spot sketched on November 19, 1858 (nearly twenty-one years ago), reappears—or one very similar indeed to it does—to-night."
M. Niesten kindly sent to the writer—who published a translation of it in the "Astronomical Register" for November—a list of observations of red spots more or less identical in aspect with this one, and probably of the same formation. It is not to be expected that in the revolutionary state of things existing in Jupiter there would be the same persistence of form that belongs to our islands and continents; and it is quite possible that there may be huge islands of vesicular formation, far bigger than all Australia, floating in viscous seas; so that if the figure of a spot remains the same, or spots seen at different times bear a strong resemblance to each other, they might be identical, even though there had been some change of place. Mathematicians tell us that the flattening at the poles noticeable in Jupiter and Saturn, and caused by their rapid rotation, would be greater than measurement shows, if such light bodies were homogeneous. There must, therefore, be some portions much denser than others, and these planets most probably contain matter in all intermediate stages, from the attenuated gaseous, through the viscid, to the solid. It must often happen, as Chacornac considered traceable in the sun, that condensation produces a great down-rush, and substances that have been solidified falling into hotter regions get melted up or vaporized again.
The great red spot lies like a continent some 24,000 miles long, surrounded by a rather narrow sea of light, and over it Niesten noticed two brilliant little spots which he appropriately named "pearls." There is a general concurrence of opinion that the big spot grew ruddier than when it first appeared, or rather richer in color; its "redness" has been chiefly caused by the want of achromatism in the telescopes employed. Glass mirrors silvered—which represent colors most correctly—show the tints to be orange-brown. The bright parts, as seen by the writer with a With-Browning silvered mirror and a fine prism, closely resembled the color of autumn beech-leaves in full sunlight. Some Merz telescopes add, from their defects, a purple tint; and an instrument of another maker gives the spot the color known as Venetian red. Dr. Pigott, who has a With-Browning silvered mirror instrument, and a fine refractor by Wray, finds the latter so unusually well corrected that its performance coincides closely with that of the former. Color-changes, both as regards time and intensity, may be caused by the greater or less translucency and refracting powers of the atmosphere through which any object is seen; but they may also very frequently arise from the greater or less heat and luminosity of solid or viscid matter below the cloudy strata, and from important modifications of chemical action. Between September 3d, at from 10.45 to 10.55 p. m., and October 4th, 10.40 p. m., Captain Noble's drawings, made at Maresfield, show a great change in the aspect of the planet, affecting the brightness and the tint of enormous spaces. Parts above the great spot which were brilliant on the former occasion had become cloudy, and, southeast of the spot, there came a round white spot, with very dark surroundings. These changes must have affected many millions of square miles.
On October 16th, at 10.5 p. m., he noticed the color of the red spot "more marked than ever." There were also extensive changes in the belts, and the polar regions were more cloudy. He made the following entry in his note-book: "It is a most noticeable feature; the red spot reposes like an island in the middle of a light space on the planet's disk, and the belts, north and south of it, seem in a great measure to conform to its curved outline. This would indicate a disturbance of a stupendous character, from the amount of the area involved."
On the whole, during the season for observation of 1879-'80 Jupiter has been more than usually interesting. From pole to pole changes of great magnitude have been produced with prodigality of violence rather than with economy of time. Perhaps the mighty planet is still in the stage of youth, with blazing and explosive energies that a few hundred thousands of years may be required to tame down to the soberness of our comparatively quiescent earth.—Belgravia.
[Note.—The red spot spoken of above was watched by an astronomer in this country, Mr. E. Leopold Trouvelot, of the observatory at Cambridge, during a part of 1878. He has published an account of his observations in "The Observatory," and has furnished to "La Nature" two views of the spot as seen at times three months apart, which we reproduce. In his published description, Mr. Trouvelot says that in looking at Jupiter on September 25, 1878, at six hours and fifty minutes, he noticed a remarkable red spot a little above the southern border of the equatorial belt, with its center situated a little to the east of the central meridian. It occupied apparently about one fifth of the diameter of the planet, and was quite distinct, its intense rose-color forming a striking contrast with the luminous white ground on which it was projected. It was of the same shade, uniform from one end to the other, without any obscure border. It appeared isolated and entirely independent of the equatorial belt, from which it was separated by a brilliant white band. In shade, its color was quite different from the pale rose-color of the equatorial belt, and from every other object which the observer had ever seen on Jupiter, and might be described as a blending of vermilion and blue. Fig. 1 is a copy of the original sketch made immediately after the observation, a a indicating the red spot. After this observation, the return of the spot was noticed, and it was drawn fifteen times. It was last seen on December 30, 1878, after which further observations were prevented by the proximity of the sun. The form of the spot changed somewhat during this time: at first, it was long and narrow (Fig. 1, a a); finally (Fig. 2, b b), it became shorter, considerably wider, and extended farther toward the south.—Editor.]
- ↑ "Comptes Rendus," 1874, vol. lxxviii., p. 1468.