The Ways of the Planets

by Martha Evans Martin

MERCURY’S SIZE AND THE CONSEQUENCES OF IT

Mercury is the smallest of all the major planets. His diameter is about three thousand miles. It is only about nine hundred miles greater than that of our moon. The surface of Mercury is only one-seventh that of the earth, and his volume only one-twentieth. Jupiter and Saturn each have a satellite that is considerably larger.

Mercury would make a splendid satellite or a giant asteroid, but as a planet seems hardly to have had a fair chance in life. For being a small planet means something more than being constructed on smaller lines than some others are. It means a difference in physical development. It means less power to hold the gases that compose an atmosphere, which is the cover that shields the planets from the too burning rays of the sun and keeps their internal heat from radiating too quickly into space. It means less power to resist the tidal friction that the parent body uses as a brake to retard rotation. It means a shorter time of activity in life, and a long, dull, monotonous old age.

The nucleus that was detached from the great spiral, or the portion of nebula that was separated in whatever way from the parent body, to form Mercury chanced to be a small one. Being small, it was unable to add materially to its mass by attracting other particles to it through the power of gravitation, as a larger planet might do, and thus Mercury was doomed to develop with the limitations that nature’s law has decreed as inevitable in the small bodies of our solar system, be they planets, satellites, or asteroids. Of these limitations the first and most far-reaching in its effect is the feebleness of its force of gravity, or power to attract other bodies.

Mercury’s force of gravity is small. It is smaller than that of any of the other planets. It is a little less than one-quarter that of the earth. The same weight of feathers that would compose a pillow here would make a whole feather bed on Mercury. Any object weighing one hundred pounds here would weigh only twenty-four there. The materials composing our earth and all the planets are held together only by the force of gravity. The air we breathe would dart off into space with almost incredible fleetness if the earth had not sufficient gravitative force to hold it. Its particles are struggling all the time to get beyond this power. The lightest of them do get beyond it and are lost, and the less power we have to hold them the sooner they leave us. The greater the mass of a body, the rarer the gases it can hold in its atmosphere, for this mysterious force which pulls everything toward the center of a planet depends upon its mass, or the quantity of material in it. The planet may be very large because it is very much expanded. It may be gaseous even, and its mass would then be very small in proportion to that of a solid body of the same size. As it condenses, the particles draw closer and closer together, the density increases; but the mass is the same. It is only the size that diminishes.

So a planet with a small mass starts out in life with a disadvantage. It not only has little power to grow by drawing in particles from its environment, but also has little power to hold such as by their nature are volatile and swift of motion, as the molecules of gases are. The mass of Mercury is not exactly known. The only way we have of measuring the masses of the planets is by their influence through gravitation on other bodies near them. When a planet has satellites, the movements of the satellites tell the story, and by mathematical calculation the amount of material in the planet can be determined. But Mercury has no satellite, and the only way to determine his mass is by observation of his influence on Venus, and on an occasional comet which passes near enough to be disturbed by the planet. The particular comet which has been useful in determining the mass of Mercury is Encke’s. On passing near the sun it comes sometimes near Mercury, and the pull it has repeatedly received from that little planet on such occasions is thought to be largely responsible for the comet’s having become a part of the solar system. The changes in its orbit caused by these encounters show the power of Mercury, and hence the mass.

In these ways the mass of Mercury has been found, with reasonable belief in its accuracy, to be about three one-hundredths that of the earth. Yet there are, indeed, considerable differences regarding it among astronomers. The exact figures are not important to any but the close student. It is certain that the mass of Mercury is very small—so small that the planet probably never had much atmosphere, and almost undoubtedly has none to speak of now. The planet could not hold any molecule moving faster than two and forty-five one-hundredths miles a second, and few gases move as slowly as this. The proportion of light that Mercury reflects to that which he receives also points to a probable scarcity of atmosphere. If he had an atmosphere, it would have clouds. Clouds have a very high reflecting power, giving out about seventy-two per cent. of the light that falls upon them. Mercury reflects only fourteen per cent. of the light he receives, which shows at least a lack of clouds, and something more. It indicates a hard, dark, almost metallic surface, and a very considerable density. Density, however, is the only quality in the possession of which Mercury seems to occupy a middle ground among the planets, being slightly less dense than either Venus, or Mars, or the earth. The earth is the densest of all the planets, and it is about one-third more dense than Mercury. Density is simply the closeness with which the particles composing a body are packed together. A piece of gold, for example, is denser than a piece of iron of the same size.