MERCURY, A WORLD OF TWO FACES AND MANY CONTRASTS
Mercury, the first of the other worlds that we are going to consider,
fascinates by its grotesqueness, like a piece of Chinese ivory carving,
so small is it for its kind and so finished in its eccentric details. In
a little while we shall see how singular Mercury is in many of the
particulars of planetary existence, but first of all let us endeavor to
obtain a clear idea of the actual size and mass of this strange little
planet. Compared with the earth it is so diminutive that it looks as if
it had been cut out on the pattern of a satellite rather than that of an
independent planet. Its diameter, 3,000 miles, only exceeds the moon's
by less than one half, while both Jupiter and Saturn, among their
remarkable collections of moons, have each at least one that is
considerably larger than the planet Mercury. But, insignificant though
it be in size, it holds the place of honor, nearest to the sun.
It was formerly thought that Mercury possessed a mass greatly in excess
of that which its size would seem to imply, and some estimates, based
upon the apparent effect of its attraction on comets, made it equal in
mean density to lead, or even to the metal mercury. This led to curious
speculations concerning its probable metallic composition, and the
possible existence of vast quantities of such heavy elements as gold in
the frame of the planet. But more recent, and probably more correct,
computations place Mercury third in the order of density among the
members of the solar system, the earth ranking as first and Venus as
second. Mercury's density is now believed to be less than the earth's in
the ratio of 85 to 100. Accepting this estimate, we find that the force
of gravity upon the surface of Mercury is only one third as great as
upon the surface of the earth--i.e., a body weighing 300 pounds on the
earth would weigh only 100 pounds on Mercury.
This is an important matter, because not only the weight of bodies, but
the density of the atmosphere and even the nature of its gaseous
constituents, are affected by the force of gravity, and if we could
journey from world to world, in our bodily form, it would make a great
difference to us to find gravity considerably greater or less upon other
planets than it is upon our own. This alone might suffice to render some
of the planets impossible places of abode for us, unless a decided
change were effected in our present physical organization.
One of the first questions that we should ask about a foreign world to
which we proposed to pay a visit, would relate to its atmosphere. We
should wish to know in advance if it had air and water, and in what
proportions and quantities. However its own peculiar inhabitants might
be supposed able to dispense with these things, to _us_ their presence
would be essential, and if we did not find them, even a planet that
blazed with gold and diamonds only waiting to be seized would remain
perfectly safe from our invasion. Now, in the case of Mercury, some
doubt on this point exists.
Messrs. Huggins, Vogel, and others have believed that they found
spectroscopic proof of the existence of both air and the vapor of water
on Mercury. But the necessary observations are of a very delicate
nature, and difficult to make, and some astronomers doubt whether we
possess sufficient proof that Mercury has an atmosphere. At any rate,
its atmosphere is very rare as compared with the earth's, but we need
not, on that account, conclude that Mercury is lifeless. Possibly, in
view of certain other peculiarities soon to be explained, a rare
atmosphere would be decidedly advantageous.
Being much nearer the sun than the earth is, Mercury can be seen by us
only in the same quarter of the sky where the sun itself appears. As it
revolves in its orbit about the sun it is visible, alternately, in the
evening for a short time after sunset and in the morning for a short
time before sunrise, but it can never be seen, as the outer planets are
seen, in the mid-heaven or late at night. When seen low in the twilight,
at evening or morning, it glows with the brilliance of a bright
first-magnitude star, and is a beautiful object, though few casual
watchers of the stars ever catch sight of it. When it is nearest the
earth and is about to pass between the earth and the sun, it temporarily
disappears in the glare of the sunlight; and likewise, when it it is
farthest from the earth and passing around in its orbit on the opposite
side of the sun, it is concealed by the blinding solar rays.
Consequently, except with the instruments of an observatory, which are
able to show it in broad day, Mercury is never visible save during the
comparatively brief periods of time when it is near its greatest
apparent distance east or west from the sun.
The nearer a planet is to the sun the more rapidly it is compelled to
move in its orbit, and Mercury, being the nearest to the sun of all the
planets, is by far the swiftest footed among them. But its velocity is
subject to remarkable variation, owing to the peculiar form of the orbit
in which the planet travels. This is more eccentric than the orbit of
any other planet, except some of the asteroids. The sun being situated
in one focus of the elliptical orbit, when Mercury is at perihelion, or
nearest to the sun, its distance from that body is 28,500,000 miles, but
when it is at aphelion, or farthest from the sun, its distance is
43,500,000 miles. The difference is no less than 14,000,000 miles! When
nearest the sun Mercury darts forward in its orbit at the rate of
twenty-nine miles in a second, while when farthest from the sun the
speed is reduced to twenty-three miles.
Now, let us return for a moment to the consideration of the wonderful
variations in Mercury's distance from the sun, for we shall find that
their effects are absolutely startling, and that they alone suffice to
mark a wide difference between Mercury and the earth, considered as the
abodes of sentient creatures. The total change of distance amounts, as
already remarked, to 14,000,000 miles, which is almost half the entire
distance separating the planet from the sun at perihelion. This immense
variation of distance is emphasized by the rapidity with which it takes
place. Mercury's periodic time, i.e., the period required for it to make
a single revolution about the sun--or, in other words, the length of its
year--is eighty-eight of our days. In just one half of that time, or in
about six weeks, it passes from aphelion to perihelion; that is to say,
in six weeks the whole change in its distance from the sun takes place.
In six weeks Mercury falls 14,000,000 miles--for it _is_ a fall, though
in a curve instead of a straight line--falls 14,000,000 miles toward the
sun! And, as it falls, like any other falling body it gains in speed,
until, having reached the perihelion point, its terrific velocity
counteracts its approach and it begins to recede. At the end of the next
six weeks it once more attains its greatest distance, and turns again to
plunge sunward.
Of course it may be said of every planet having an elliptical orbit
that between aphelion and perihelion it is falling toward the sun, but
no other planet than Mercury travels in an orbit sufficiently eccentric,
and approaches sufficiently near to the sun, to give to the mind so
vivid an impression of an actual, stupendous fall!
Next let us consider the effects of this rapid fall, or approach, toward
the sun, which is so foreign to our terrestrial experience, and so
appalling to the imagination.
First, we must remember that the nearer a planet is to the sun the
greater is the amount of heat and light that it receives, the variation
being proportional to the inverse square of the distance. The earth's
distance from the sun being 93,000,000 miles, while Mercury's is only
36,000,000, it follows, to begin with, that Mercury gets, on the
average, more than six and a half times as much heat from the sun as the
earth does. That alone is enough to make it seem impossible that Mercury
can be the home of living forms resembling those of the earth, for
imagine the heat of the sun in the middle of a summer's day increased
six or seven fold! If there were no mitigating influences, the face of
the earth would shrivel as in the blast of a furnace, the very stones
would become incandescent, and the oceans would turn into steam.
Still, notwithstanding the tremendous heat poured upon Mercury as
compared with that which our planet receives, we can possibly, and for
the sake of a clearer understanding of the effects of the varying
distance, which is the object of our present inquiry, find a loophole to
admit the chance that yet there may be living beings there. We might,
for instance, suppose that, owing to the rarity of its atmosphere, the
excessive heat was quickly radiated away, or that there was something in
the constitution of the atmosphere that greatly modified the effective
temperature of the sun's rays. But, having satisfied our imagination on
this point, and placed our supposititious inhabitants in the hot world
of Mercury, how are we going to meet the conditions imposed by the
rapid changes of distance--the swift fall of the planet toward the sun,
followed by the equally swift rush away from it? For change of distance
implies change of heat and temperature.
It is true that we have a slight effect of this kind on the earth.
Between midsummer (of the northern hemisphere) and midwinter our planet
draws 3,000,000 miles nearer the sun, but the change occupies six
months, and, at the earth's great average distance, the effect of this
change is too slight to be ordinarily observable, and only the
astronomer is aware of the consequent increase in the apparent size of
the sun. It is not to this variation of the sun's distance, but rather
to the changes of the seasons, depending on the inclination of the
earth's axis, that we owe the differences of temperature that we
experience. In other words, the total supply of heat from the sun is not
far from uniform at all times of the year, and the variations of
temperature depend upon the distribution of that supply between the
northern and southern hemispheres, which are alternately inclined
sunward.
But on Mercury the supply of solar heat is itself variable to an
enormous extent. In six weeks, as we have seen, Mercury diminishes its
distance from the sun about one third, which is proportionally ten times
as great a change of distance as the earth experiences in six months.
The inhabitants of Mercury in those six pregnant weeks see the sun
expand in the sky to more than two and a half times its former
magnitude, while the solar heat poured upon them swiftly augments from
something more than four and a half times to above eleven times the
amount received upon the earth! Then, immediately, the retreat of the
planet begins, the sun visibly shrinks, as a receding balloon becomes
smaller in the eyes of its watchers, the heat falls off as rapidly as it
had previously increased, until, the aphelion point being reached, the
process is again reversed. And thus it goes on unceasingly, the sun
growing and diminishing in the sky, and the heat increasing and
decreasing by enormous amounts with astonishing rapidity. It is
difficult to imagine any way in which atmospheric influences could
equalize the effects of such violent changes, or any adjustments in the
physical organization of living beings that could make such changes
endurable.
But we have only just begun the story of Mercury's peculiarities. We
come next to an even more remarkable contrast between that planet and
our own. During the Paris Exposition of 1889 a little company of
astronomers was assembled at the Juvisy observatory of M. Flammarion,
near the French capital, listening to one of the most surprising
disclosures of a secret of nature that any _savant_ ever confided to a
few trustworthy friends while awaiting a suitable time to make it
public. It was a secret as full of significance as that which Galileo
concealed for a time in his celebrated anagram, which, when at length he
furnished the key, still remained a riddle, for then it read: "The
Mother of the Loves imitates the Shapes of Cynthia," meaning that the
planet Venus, when viewed with a telescope, shows phases like those of
the moon. The secret imparted in confidence to the knot of astronomers
at Juvisy came from a countryman of Galileo's, Signor G. V.
Schiaparelli, the Director of the Observatory of Milan, and its purport
was that the planet Mercury always keeps the same face directed toward
the sun. Schiaparelli had satisfied himself, by a careful series of
observations, of the truth of his strange announcement, but before
giving it to the world he determined to make doubly sure. Early in 1890
he withdrew the pledge of secrecy from his friends and published his
discovery.
No one can wonder that the statement was generally received with
incredulity, for it was in direct contradiction to the conclusions of
other astronomers, who had long believed that Mercury rotated on its
axis in a period closely corresponding with that of the earth's
rotation--that is to say, once every twenty-four hours. Schiaparelli's
discovery, if it were received as correct, would put Mercury, as a
planet, in a class by itself, and would distinguish it by a peculiarity
which had always been recognized as a special feature of the moon, viz.,
that of rotating on its axis in the same period of time required to
perform a revolution in its orbit, and, while this seemed natural enough
for a satellite, almost nobody was prepared for the ascription of such
eccentric conduct to a planet.
The Italian astronomer based his discovery upon the observation that
certain markings visible on the disk of Mercury remained in such a
position with reference to the direction of the sun as to prove that the
planet's rotation was extremely slow, and he finally satisfied himself
that there was but one rotation in the course of a revolution about the
sun. That, of course, means that one side of Mercury always faces toward
the sun while the opposite side always faces away from it, and neither
side experiences the alternation of day and night, one having perpetual
day and the other perpetual night. The older observations, from which
had been deduced the long accepted opinion that Mercury rotated, like
the earth, once in about twenty-four hours, had also been made upon the
markings on the planet's disk, but these are not easily seen, and their
appearances had evidently been misinterpreted.
The very fact of the difficulty of seeing any details on Mercury tended
to prevent or delay corroboration of Schiaparelli's discovery. But there
were two circumstances that contributed to the final acceptance of his
results. One of these was his well-known experience as an observer and
the high reputation that he enjoyed among astronomers, and the other was
the development by Prof. George Darwin of the theory of tidal friction
in its application to planetary evolution, for this furnished a
satisfactory explanation of the manner in which a body, situated as near
the sun as Mercury is, could have its axial rotation gradually reduced
by the tidal attraction of the sun until it coincided in period with its
orbital revolution.
Accepting the accuracy of Schiaparelli's discovery, which was
corroborated in every particular in 1896 by Percival Lowell in a special
series of observations on Mercury made with his 24-inch telescope at
Flagstaff, Arizona, and which has also been corroborated by others, we
see at once how important is its bearing on the habitability of the
planet. It adds another difficulty to that offered by the remarkable
changes of distance from the sun, and consequent variations of heat,
which we have already discussed. In order to bring the situation home to
our experience, let us, for a moment, imagine the earth fallen into
Mercury's dilemma. There would then be no succession of day and night,
such as we at present enjoy, and upon which not alone our comfort but
perhaps our very existence depends, but, instead, one side of our
globe--it might be the Asiatic or the American half--would be
continually in the sunlight, and the other side would lie buried in
endless night. And this condition, so suggestive of the play of pure
imagination, this plight of being a two-faced world, like the god
Janus, one face light and the other face dark, must be the actual state
of things on Mercury.
There is one interesting qualification. In the case just imagined for
the earth, supposing it to retain the present inclination of its axis
while parting with its differential rotation, there would be an
interchange of day and night once a year in the polar regions. On
Mercury, whose axis appears to be perpendicular, a similar phenomenon,
affecting not the polar regions but the eastern and western sides of the
planet, is produced by the extraordinary eccentricity of its orbit. As
the planet alternately approaches and recedes from the sun its orbital
velocity, as we have already remarked, varies between the limits of
twenty-three and thirty-five miles per second, being most rapid at the
point nearest the sun. But this variation in the speed of its revolution
about the sun does not, in any manner, affect the rate of rotation on
its axis. The latter is perfectly uniform and just fast enough to
complete one axial turn in the course of a single revolution about the
sun. The accompanying figure may assist the explanation.
[Illustration: DIAGRAM SHOWING THAT, OWING TO THE ECCENTRICITY OF ITS
ORBIT, AND ITS VARYING VELOCITY, MERCURY, ALTHOUGH MAKING BUT ONE TURN
ON ITS AXIS IN THE COURSE OF A REVOLUTION ABOUT THE SUN, NEVERTHELESS
EXPERIENCES ON PARTS OF ITS SURFACE THE ALTERNATION OF DAY AND NIGHT.]
Let us start with Mercury in perihelion at the point _A_. The little
cross on the planet stands exactly under the sun and in the center of
the illuminated hemisphere. The large arrows show the direction in which
the planet travels in its revolution about the sun, and the small curved
arrows the direction in which it rotates on its axis. Now, in moving
along its orbit from _A_ to _B_ the planet, partly because of its
swifter motion when near the sun, and partly because of the elliptical
nature of the orbit, traverses a greater angular interval with reference
to the sun than the cross, moving with the uniform rotation of the
planet on its axis, is able to traverse in the same time. As drawn in
the diagram, the cross has moved through exactly ninety degrees, or one
right angle, while the planet in its orbit has moved through
considerably more than a right angle. In consequence of this gain of the
angle of revolution upon the angle of rotation, the cross at _B_ is no
longer exactly under the sun, nor in the center of the illuminated
hemisphere. It appears to have shifted its position toward the west,
while the hemispherical cap of sunshine has slipped eastward over the
globe of the planet.
In the next following section of the orbit the planet rotates through
another right angle, but, owing to increased distance from the sun, the
motion in the orbit now becomes slower until, when the planet arrives at
aphelion, _C_, the angular difference disappears and the cross is once
more just under the sun. On returning from aphelion to perihelion the
same phenomena recur in reverse order and the line between day and night
on the planet first shifts westward, attaining its limit in that respect
at _D_, and then, at perihelion, returns to its original position.
Now, if we could stand on the sunward hemisphere of Mercury what, to our
eyes, would be the effect of this shifting of the sun's position with
regard to a fixed point on the planet's surface? Manifestly it would
cause the sun to describe a great arc in the sky, swinging to and fro,
in an east and west line, like a pendulum bob, the angular extent of the
swing being a little more than forty-seven degrees, and the time
required for the sun to pass from its extreme eastern to its extreme
western position and back again being eighty-eight days. But, owing to
the eccentricity of the orbit, the sun swings much faster toward the
east than toward the west, the eastward motion occupying about
thirty-seven days and the westward motion about fifty-one days.
[Illustration: THE REGIONS OF PERPETUAL DAY, PERPETUAL NIGHT, AND
ALTERNATE DAY AND NIGHT ON MERCURY. IN THE LEFT-HAND VIEW THE OBSERVER
LOOKS AT THE PLANET IN THE PLANE OF ITS EQUATOR; IN THE RIGHT-HAND VIEW
HE LOOKS DOWN ON ITS NORTH POLE.]
Another effect of the libratory motion of the sun as seen from Mercury
is represented in the next figure, where we have a view of the planet
showing both the day and the night hemisphere, and where we see that
between the two there is a region upon which the sun rises and sets once
every eighty-eight days. There are, in reality, two of these lune-shaped
regions, one at the east and the other at the west, each between 1,200
and 1,300 miles broad at the equator. At the sunward edge of these
regions, once in eighty-eight days, or once in a Mercurial year, the sun
rises to an elevation of forty-seven degrees, and then descends again
straight to the horizon from which it rose; at the nightward edge, once
in eighty-eight days, the sun peeps above the horizon and quickly sinks
from sight again. The result is that, neglecting the effects of
atmospheric refraction, which would tend to expand the borders of the
domain of sunlight, about one quarter of the entire surface of Mercury
is, with regard to day and night, in a condition resembling that of our
polar regions, where there is but one day and one night in the course of
a year--and on Mercury a year is eighty-eight days. One half of the
remaining three quarters of the planet's surface is bathed in perpetual
sunshine and the other half is a region of eternal night.
And now again, what of life in such a world as that? On the night side,
where no sunshine ever penetrates, the temperature must be extremely
low, hardly greater than the fearful cold of open space, unless
modifying influences beyond our ken exist. It is certain that if life
flourishes there, it must be in such forms as can endure continual
darkness and excessive cold. Some heat would be carried around by
atmospheric circulation from the sunward side, but not enough, it would
seem, to keep water from being perpetually frozen, or the ground from
being baked with unrelaxing frost. It is for the imagination to picture
underground dwellings, artificial sources of heat, and living forms
suited to unearthlike environment.
What would be the mental effects of perpetual night upon a race of
intelligent creatures doomed to that condition? Perhaps not quite so
grievous as we are apt to think. The constellations in all their
splendor would circle before their eyes with the revolution of their
planet about the sun, and with the exception of the sun itself--which
they could see by making a journey to the opposite hemisphere--all the
members of the solar system would pass in succession through their
mid-heaven, and two of them would present themselves with a magnificence
of planetary display unknown on the earth. Venus, when in opposition
under the most favorable circumstances, is scarcely more than 24,000,000
miles from Mercury, and, showing herself at such times with a fully
illuminated disk--as, owing to her position within the orbit of the
earth, she never can do when at her least distance from us--she must be
a phenomenon of unparalleled beauty, at least four times brighter than
we ever see her, and capable, of course, of casting a strong shadow.
The earth, also, is a splendid star in the midnight sky of Mercury, and
the moon may be visible to the naked eye as a little attendant circling
about its brilliant master. The outer planets are slightly less
conspicuous than they are to us, owing to increase of distance.
The revolution of the heavens as seen from the night side of Mercury is
quite different in period from that which we are accustomed to, although
the apparent motion is in the same direction, viz., from east to west.
The same constellations remain above the horizon for weeks at a time,
slowly moving westward, with the planets drifting yet more slowly, but
at different rates, among them; the nearer planets, Venus and the earth,
showing the most decided tendency to loiter behind the stars.
On the side where eternal sunlight shines the sky of Mercury contains no
stars. Forever the pitiless blaze of day; forever,
"All in a hot and copper sky
The bloody sun at noon."
As it is difficult to understand how water can exist on the night
hemisphere, except in the shape of perpetual snow and ice, so it is
hard to imagine that on the day hemisphere water can ever be
precipitated from the vaporous form. In truth, there can be very little
water on Mercury even in the form of vapor, else the spectroscope would
have given unquestionable evidence of its presence. Those who think that
Mercury is entirely waterless and almost, if not quite, airless may be
right. In these respects it would then resemble the moon, and, according
to some observers, it possesses another characteristic lunar feature in
the roughening of its surface by what seem to be innumerable volcanic
craters.
But if we suppose Mercury to possess an atmosphere much rarer than that
of the earth, we may perceive therein a possible provision against the
excessive solar heat to which it is subjected, since, as we see on high
mountains, a light air permits a ready radiation of heat, which does not
become stored up as in a denser atmosphere.
As the sun pours its heat without cessation upon the day hemisphere the
warmed air must rise and flow off on all sides into the night
hemisphere, while cold air rushes in below, to take its place, from the
region of frost and darkness. The intermediate areas, which see the sun
part of the time, as explained above, are perhaps the scene of
contending winds and tempests, where the moisture, if there be any, is
precipitated, through the rapid cooling of the air, in whelming floods
and wild snow-storms driven by hurrying blasts from the realm of endless
night.
Enough seems now to have been said to indicate clearly the hopelessness
of looking for any analogies between Mercury and the earth which would
warrant the conclusion that the former planet is capable of supporting
inhabitants or forms of life resembling those that swarm upon the
latter. If we would still believe that Mercury is a habitable globe we
must depend entirely upon the imagination for pictures of creatures able
to endure its extremes of heat and cold, of light and darkness, of
instability, swift vicissitude, and violent contrast.
In the next chapter we shall study a more peaceful and even-going world,
yet one of great brilliancy, which possesses some remarkable
resemblances to the earth, as well as some surprising divergences from
it.