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.