The Ways of the Planets

by Martha Evans Martin

With so dense an atmosphere as Jupiter most likely has, it is sometimes doubtful whether his surface can be seen by us at all. But it is certain that we see something apparently much more dense and stable than an atmosphere is supposed to be; and hence it is thought that, in spite of its thickness, the atmosphere may be only partially opaque, and that it may be in some places even more or less transparent.

It does not seem probable that the markings on Jupiter are wholly atmospheric. Some of them indicate that the substance we see has considerably more consistency than a mere gas. The whole surface of the planet is covered with belts and spots of various colors and varying shapes. The belted appearance is particularly marked. It has been noticed for more than two hundred years, and can be seen with a comparatively small telescope. Sometimes as many as twenty or thirty belts have been seen at one time. All of them are parallel with the equator.

Two broad red belts on each side of the equator, called the tropical belts, are very distinct, and sometimes retain the same shape and color for months at a time, though sometimes they change rapidly in both color and outline. Between them is the equatorial belt, which is also a semi-permanent feature, remaining often for a considerable period unchanged. These belts, and the spots that sometimes appear on and near them, have been closely watched, because about the equator, and especially just south of it, is the region of greatest activity on Jupiter’s surface.

One feature that more nearly suggests solidity and permanency than anything else on Jupiter is the famous great red spot which lies in the southern hemisphere just below the southern tropical belt. It appeared about thirty-five years ago, in July, 1878, as a pale pink spot, grew brighter for two or three years, and then faded, until, at the end of two or three more years, it was almost invisible. In another year it came again, and increased in brightness for five or six years. Then it grew a little fainter, and has since remained a rather faint red spot, but plainly visible.

In shape the great red spot is an immense oval as much as thirty thousand miles from east to west and seven thousand miles from north to south, which gives it a surface four or five times as large as the land area on the entire earth, and larger even than the whole surface of the earth including the oceans. Although retaining its own shape, it seems to drift about among its surroundings, showing that it is not attached to any solid surface; and yet it has a suggestion of solidity in itself, which was shown when it and another smaller spot were seen to be drifting toward each other, and then finally to meet. Instead of colliding or going over or under, they calmly drifted to one side and went around each other.

Appearances such as this have suggested the idea that the great spot might be a continent in process of formation. Such an idea is at best a speculation; but it would be interesting if it should prove that we are witnessing on Jupiter the process through which our own earth must at one time have passed when its crust began to solidify in patches, as one of the steps in the long period of evolution which has prepared it for our uses. It is not at all certain that Jupiter will ever be just like the earth. The differences between its atmosphere and ours may have some influence in its development that we have little knowledge of at present, and there are some other fundamental differences between the two planets which may in some way effect a difference in development. But in a general way we know that the planet will in time become more condensed than it now is and will finally solidify. Whether the processes will be carried on in just the same way in which they have been here on the earth is not so certain.

JUPITER’S SYSTEM OF SATELLITES

Jupiter is the center of a superb system of satellites, eight in number. Four of them were first seen in 1610, and have the honor to be the first heavenly bodies discovered by means of the telescope. The fifth one was not discovered until 1892. The sixth was first seen in 1904, and the seventh in 1905. After three years an eighth was discovered (in 1908).

When the first four satellites were discovered they were named respectively, in the order of their distances from Jupiter, Io, Europa, Ganymede, and Callisto. Ganymede is not only the largest of the four, but is also the largest satellite in the solar system. It is larger than Mercury, and not much smaller than Mars. Callisto is next to Ganymede in size, and is about the size of Mercury. Io is about the size of our moon, and Europa is not much smaller. Under very favorable conditions Ganymede and Callisto can be seen by the naked eye; but a good many persons think they see the moons of Jupiter when they see only some small stars in that region. They are invisible to most people, but probably could be seen oftener if it were not for the glaring light of the planet, which more or less obscures anything so near it.