Jupiter Introduction

A short introduction to the planet Jupiter.

JUPITER is the "giant planet " of our system, its mass largely exceeding that of all the other planets combined.  Jupiters mean diameter is about 85,000 miles ; but owing to its rapid rotation on his axis, his equatorial exceeds its polar diameter by 5000 miles.  In volume he exceeds our earth about 1300 times, while in mass he exceeds it about 213 times.  Its specific gravity is, therefore, far less than that of the earth, and even less than that of water.  His mean distance from the sun is 480 millions of miles, but, owing to the eccentricity of his orbit, his actual distance ranges between 457 arid 503 mill ions.  His time of revolution is fifty days less than twelve years.

The Surface of Jupiter

Except the sun and moon, there is no object of our system which has during the last few years been the subject of more careful examination than this planet.  Unlike Mars, there are no really permanent markings on his surface, and a map of Jupiter is therefore impossible.  But this surface always presents a very diversified appearance.  The earlier telescopic observers described light and dark belts as extending across it.  Until a quite recent period, it has been customary to describe these belts as two in number, one north of the equator, and the other south of it.  Commonly, they are seen as dark bands on the bright disk of the planet ; but it is curious that Huyghens represents them as brighter than the rest of the surface.  As telescopic power was in creased, it was seen that these so-called bands were of a far more complex structure than had been supposed, and consisted of great numbers of stratified, cloud-like appearances of the most variegated forms.  These forms change so rapidly that the face of the planet hardly ever presents the same appear ance on two successive nights.  They are most strongly marked at some distance on each side of the Jovian equator, and thus give rise to the appearance of two belts when a very small or imperfect telescope is used.


Both the outlines 'of these belts and the color of some parts of the planet, seem subject to considerable changes.  The equatorial regions, and indeed the spaces between the belts generally, are often of a rosy tinge.  This coloring is some times so strongly marked as to be evident to the most super ficial observer, while at other times hardly a trace of it can be seen.

Spots which are much more permanent than the ordinary markings on the belt are sometimes visible.  By watching these spots from day to day, and measuring their distance from the apparent disk, the time of rotation of Jupiter on his axis has been determined.  Commonly the spots are dark; but on some rather rare occasions the planet is seen with a number of small, round, bright spots like satellites.  Of these bright spots no explanation has been given.

From the changeability of the belts, and indeed of nearly all the visible features on the surface of Jupiter, it is clear that what we see on that planet is not the surface of a solid nu cleus, but vaporous or cloud-like formations which cover the entire surface and extend to a great depth below.  To all ap pearance, the planet is covered with a deep and dense atmosphere, through which light cannot penetrate on account of thick masses of clouds and vapor.  In the arrangements of these clouds in streaks parallel to the equator, and in the change of their forms with the latitude, there may be some thing analogous to the zones of clouds and rain on the earth.  But of late years it has been noticed that the physical consti tution of Jupiter seems to offer more analogies to that of the sun than to that of the earth.  Like the sun, he is brighter in the centre than near the edges.  This is shown in the most striking manner in the transits of his satellites over his disk.  When the satellite first enters on the disk, it commonly seems like a bright spot on a dark background ; but as it approaches the centre, it appears like a dark spot on the bright back ground of the planet.  The brightness of the centre is prob ably two or three times greater than that of the limb.  This diminution of light towards the edge may arise, as in the case of the sun, from the light near the edge passing through a greater depth of atmosphere, and thus becoming fainter by absorption.

A still more remarkable resemblance to the sun has some times been suspected nothing less, in fact, than that Jupiter shines partly by his own light.  It was at one time supposed that he actually emitted more light than fell upon him from the sun ; and if this were proved, it would show conclusive ly that he was self-luminous.  If all the light which the sun shed upon the planet were equally reflected in every direction, we might speak with some certainty on this question ; but in the actual state of our knowledge we cannot.  Zollner has found that the brightness of Jupiter may be accounted for by supposing him to reflect 62 per cent, of the sunlight which he receives.  But if this is his average reflecting power, the re flecting power of his brighter portions must be much greater; in fact, they are so bright that they must shine partly by their own light, unless they reflect a disproportionate share of the sunlight back in the direction of the earth and sun. Clouds would not be likely to do this.  On the other hand, if we as sume that the planet emits any great amount of light, we are met by the fact that, if this were the case, the satellites would shine by this light when they were in the shadow of the planet.  As these bodies totally disappear in this position, the quantity of light emitted by Jupiter must be quite small.  On the whole, there is a small probability that the brighter spots of this planet are from time to time slightly self-luminous.

Again, the interior of Jupiter seems to be the seat of an activity so enormous that we can attribute it only to a very high temperature, like that of the sun. This is shown by the rapid movements always going on in his visible surface, which frequently changes its aspect in a few hours.  Such a power ful effect could hardly be produced by the rays of the sun, because, owing to the great distance of the planet, he receives only between one-twenty-fifth and one-thirtieth of the light and heat which we do.  It is therefore probable that Jupiter is not yet covered by a solid crust, as our earth is, but that his white-hot interior, whether liquid or gaseous, has nothing to cover it but the dense vapors to which that heat gives rise.  In this case the vapors may be self-luminous when they have freshly arisen from the interior, and may rapidly cool off after reaching the upper limit to which they ascend.

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