ON THE EXISTENCE OR NON-EXISTENCE OF A LUNAR ATMOSPHERE.
At the close of the preceding chapter we stated that any force acting in
opposition to that of gravity would be six times more effective on the
moon than on the earth. But, in fact, it would in many cases be still
more so; at all events, so far as projectile forces are concerned; for
the reason that “the powerful coercer of projectile range,” as the
earth’s atmosphere has been termed, has no counterpart, or at most a
very disproportionate one, upon the moon.
The existence of an atmosphere surrounding the moon has been the subject
of considerable controversy, and a great deal of evidence on both sides
of the question has been offered from time to time, and is to be found
scattered through the records of various classes of observations. Some
of the more important items of this evidence it is our purpose to set
forth in the course of the present chapter.
With the phenomena of the terrestrial atmosphere, with the effects that
are attributable to it, we are all well familiar, and our best course
therefore is to examine, as far as we are able, whether counterparts of
any of these effects are manifested upon the moon. For instance, the
clouds that are generated in and float through our air would, to an
observer on the moon, appear as ever changing bright or dusky spots,
obliterating certain of the permanent details of the earth’s surface,
and probably skirting the terrestrial disc, like the changing belts we
perceive on the planet Jupiter, or diversifying its features with less
regularity, after the manner exhibited by the planet Mars. If such
clouds existed on the moon it is evident that the details of its surface
must be, from time to time, similarly obscured; but no trace of such
obscuration has ever been detected. When the moon is observed with high
telescopic powers, all its details come out sharp and clear, without the
least appearance of change or the slightest symptoms of cloudiness other
than the occasional want of general definition, which may be proved to
be the result of unsteadiness or want of homogeneity in our own
atmosphere; for we must tell the uninitiated that nights of pure, good
definition, such as give the astronomer opportunity of examining with
high powers the minute details of planetary features, are very few and
far between. Out of the three hundred and sixty-five nights of a year
there are probably not a dozen that an astronomer can call really fine:
usually, even on nights that are to all common appearance superbly
brilliant, some strata of air of different densities or temperatures, or
in rapid motion, intervene between the observer and the object of his
observation, and through these, owing to the ever-changing refractions
which the rays of light coming from the object suffer in their course,
observation of the delicate markings of a planet is impossible: all is
blurred and confused, and nothing but bolder features can be recognized.
It has in consequence sometimes happened that a slight indistinctness of
some minute detail of the moon has been attributed to clouds or mists at
the lunar surface, whereas the real cause has been only a bad condition
of our own atmosphere. It may be confidently asserted that when all
indistinctness due to terrestrial causes is taken account of or
eliminated, there remain no traces whatever of any clouds or mists upon
the surface of the moon.
This is but one proof against the existence of a lunar atmosphere, and,
it may be argued, not a very conclusive one; because there may still be
an atmosphere, though it be not sufficiently aqueous to condense into
clouds and not sufficiently dense to obscure the lunar details. The
probable existence of an atmosphere of such a character used to be
inferred from a phenomenon seen during total eclipses of the sun. On
these occasions the black body of the moon is invariably surrounded by a
luminous halo, or glory, to which the name “corona” has been applied;
and, further, besides this corona, apparently floating in it and
sometimes seemingly attached to the black edge of the moon, are seen
masses of cloud-like matter of a bright red colour, which, from the form
in which they were first seen and from their flame-like tinge, have
become universally known as the “red-flames.” It used to be said that
this corona could only be the consequence of a lunar atmosphere lit up
as it were by the sun’s rays shining through it, after the manner of a
sunbeam lighting up the atmosphere of a dusty chamber; and the red
flames were held by those who first observed them to be clouds of denser
matter floating in the said atmosphere, and refracting the red rays of
solar light as our own clouds are seen to do at sunrise and sunset. But
the evidence obtained, both by simple telescopic observation and by the
spectroscope, from recent extensively observed eclipses of the sun has
set this question quite at rest; for it has been settled finally and
indisputably that both the above appearances pertain to the sun, and
have nothing whatever to do with the moon.
[Illustration: Fig. 11.]
The occurrence of a solar eclipse offers other means in addition to the
foregoing whereby a lunar atmosphere would be detected. We know that all
gases and vapours absorb some portion of any light which may shine
through them. If then our satellite had an atmosphere, its black nucleus
when seen projected against the bright sun in an eclipse would be
surrounded by a sort of penumbra, or zone of shadow, in contact with its
edge, somewhat like that we have shown in an exaggerated degree in the
annexed cut (Fig. 11), and the passage of this penumbra over solar spots
and other features of the solar photosphere would to some extent obscure
the more minute details of such features. No such dusky band has however
been at any time observed. On the contrary, a band somewhat brighter
than the general surface of the sun has frequently been seen in contact
with the black edge of the moon: this in its turn was held to indicate
an atmosphere about the moon; but Sir George Airy has shown that a lunar
atmosphere, if it really did exist, could not produce such an
appearance, and that the cause of it must be sought in other directions.
If this effect were really due to the passage of the solar rays through
a lunar atmosphere a similar effect ought to be produced by the passage
of the sun’s rays through the terrestrial atmosphere: and we might hence
expect to see the shadow of the earth projected on the moon during a
lunar eclipse surrounded by a sort of bright zone or halo: we need
hardly say such an appearance has never manifested itself. Similarly as
we stated that the delicate details of solar spots would be obscured by
a lunar atmosphere, small stars passing behind the moon would suffer
some diminution in brightness as they approached apparent contact with
the moon’s edge: this fading has been watched for on many occasions, and
in a few cases such an appearance has been suspected, but in by far the
majority of instances nothing like a diminution of brightness or change
of colour of the stars has been seen; stars of the smallest magnitude
visible under such circumstances retain their feeble lustre unimpaired
up to the moment of their disappearance behind the moon’s limb.
Again, in a solar eclipse, even if there were an atmosphere about the
moon not sufficiently dense to form a hazy outline or impair the
distinctness of the details of a solar spot, it would still manifest its
existence in another way. As the moon advances upon the sun’s disc the
latter assumes, of course, a crescent form. Now if air or vapour
enveloped the moon, the exceedingly delicate cusps of this crescent
would be distorted or turned out of shape. Instead of remaining
symmetrical, like the lower one in the annexed drawing (Fig. 12), they
would be bent or deformed after the manner we have shown in the upper
one. The slightest symptom of a distortion like this could not fail to
obtrude itself upon an observer’s eye; but in no instance has anything
of the kind been seen.
Reverting to the consequences of the terrestrial atmosphere: one of the
most striking of these is the phenomenon of diffused daylight, which we
need hardly remind the reader is produced by the scattering or diffusion
of the sun’s rays among the minute particles of vapour composing or
contained in that atmosphere. Were it not for this reflexion and
diffusion of the sun’s light, those parts of our earth not exposed to
direct sunshine would be hidden in darkness, receiving no illumination
beyond the feeble amount that might be reflected from proximate
terrestrial objects actually illuminated by direct sunlight. Twilight is
a consequence of this reflexion of light by the atmosphere when the sun
is below the horizon. If, then, an atmosphere enveloped the moon, we
should see by diffused light those parts of the lunar details that are
not receiving the direct solar beams; and before the sun rose and after
it had set upon any region of the moon, that region would still be
partially illuminated by a twilight. But, on the contrary, the shadowed
portions of a lunar landscape are pitchy black, without a trace of
diffused-light illumination, and the effects that a twilight would
produce are entirely absent from the moon. Once, indeed, one observer,
Schroeter, noticed something which he suspected was due to an effect of
this kind: when the moon exhibited itself as a very slender crescent, he
discovered a faint crepuscular light, extending from each of the cusps
along the circumference of the unenlightened part of the disc, and he
inferred from estimates of the length and breadth of the line of light
that there was an atmosphere about the moon of 5376 feet in height. This
is the only instance on record, we believe, of such an appearance being
seen.
[Illustration: Fig. 12.]
Spectrum analysis would also betray the existence of a lunar atmosphere.
The solar rays falling on the moon are reflected from its surface to the
earth. If, then, an atmosphere existed, it is plain that the solar rays
must first pass through such atmosphere to reach the reflecting surface,
and returning from thence, again pass through it on their way to the
earth; so that they must in reality pass through virtually twice the
thickness of any atmosphere that may cover the moon. And if there be any
such atmosphere, the spectrum formed by the moon’s light, that is, by
the sun’s light reflected from the moon, would be modified in such a
manner as to exhibit absorption-lines different from those found in the
spectrum of the direct solar rays, just as the absorption-lines vary
according as the sun’s rays have to pass through a thinner or a denser
stratum of the terrestrial atmosphere. Guided by this reasoning, Drs.
Huggins and Miller made numerous observations upon the spectrum of the
moon’s light, which are detailed in the “Philosophical Transactions” for
the year 1864; and their result, quoting the words of the report, was
“that the spectrum analysis of the light reflected from the moon is
wholly negative as to the existence of any considerable lunar
atmosphere.”
Upon another occasion, Dr. Huggins made an analogous observation of the
spectrum of a star at the moment of its occultation, which observation
he records in the following words:—“When an observation is made of the
spectrum of a star a little before, or at the moment of its occultation
by the dark limb of the moon, several phenomena characteristic of the
passage of the star’s light through an atmosphere might possibly present
themselves to the observer. If a lunar atmosphere exist, which either by
the substances of which it is composed, or by the vapours diffused
through it, can exert a selective absorption upon the star’s light, this
absorption would be indicated to us by the appearance in the spectrum of
new dark lines immediately before the star is occulted by the moon.”
“If finely divided matter, aqueous or otherwise, were present about the
moon, the red rays of the star’s light would be enfeebled in a smaller
degree than the rays of higher refrangibilities.”
“If there be about the moon an atmosphere free from vapour, and
possessing no absorptive power, but of some density, then the spectrum
would not be extinguished by the moon’s limb at the same instant
throughout its length. The violent and blue rays would lay behind the
red rays.”
“I carefully observed the disappearance of the spectrum of η Piscium at
its occultation of January 4, 1865, for these phenomena; but no signs of
a lunar atmosphere were detected.”
But perhaps the strongest evidence of the non-existence of any
appreciable lunar atmosphere is afforded by the non-refraction of the
light of a star passing behind the edge of the lunar disc. Refraction,
we know, is a bending of the rays of light coming from any object,
caused by their passage through strata of transparent matter of
different densities; we have a familiar example in the apparent bending
of a stick when half plunged into water. There is a simple schoolboy’s
experiment which illustrates refraction in a very cogent manner, but
which we should, from its very simplicity, hesitate to recall to the
reader’s mind did it not very aptly represent the actual case we wish to
exemplify. A coin is placed on the bottom of an empty basin, and the eye
is brought into such a position that the coin is just hidden behind the
basin’s rim. Water is then poured into the basin and, without the eye
being moved from its former place, as the depth of water increases, the
coin is brought by degrees fully into view; the water refracting or
turning out of their course the rays of light coming from the coin, and
lifting them, as it were, over the edge of the basin. Now a perfectly
similar phenomenon takes place at every sunrise and sunset on the earth.
When the sun is really below the horizon, it is nevertheless still
visible to us because it is _brought up_ by the refraction of its light
by the dense stratum of atmosphere through which the rays have to pass.
The sun is, therefore, exactly represented by the coin at the bottom of
the basin in the boy’s experiment, the atmosphere answers to the water,
and the horizon to the rim or edge of the basin. If there were no
atmosphere about the earth, the sun would not be so brought up above the
horizon, and, as a consequence, it would set earlier and rise later by
about a minute than it really does. This, of course, applies not merely
to the sun, but to all celestial bodies that rise and set. Every planet
and every star remains a shorter time below the horizon than it would if
there were no atmosphere surrounding the earth.
To apply this to the point we are discussing. The moon in her orbital
course across the heavens is continually passing before, or occulting,
some of the stars that so thickly stud her apparent path. And when we
see a star thus pass behind the lunar disc on one side and come out
again on the other side, we are virtually observing the setting and
rising of that star upon the moon. If, then, the moon had an atmosphere,
it is clear, from analogy to the case of the earth, that the star must
disappear later and reappear sooner than if it has no atmosphere: just
as a star remains too short a time below the earth’s horizon, or behind
the earth, in consequence of the terrestrial atmosphere, so would a star
remain too short a time behind the moon if an atmosphere surrounded that
body. The point is settled in this way:—The moon’s apparent diameter has
been measured over and over again and is known with great accuracy; the
rate of her motion across the sky is also known with perfect accuracy:
hence it is easy to calculate how long the moon will take to travel
across a part of the sky exactly equal in length to her own diameter.
Supposing, then, that we observe a star pass behind the moon and out
again, it is clear that, if there be no atmosphere, the interval of time
during which it remains occulted ought to be exactly equal to the
computed time which the moon would take to pass over the star. If,
however, from the existence of a lunar atmosphere, the star disappears
too late and reappears too soon, as we have seen it would, these two
intervals will not agree; the computed time will be greater than the
observed time, and the difference, if any there be, will represent the
amount of refraction the star’s light has sustained or suffered, and
hence the extent of atmosphere it has had to pass through.
Comparisons of these two intervals of time have been repeatedly made,
the most recent and most extensive was executed under the direction of
the Astronomer-Royal several years ago, and it was based upon no less
than 296 occultation observations. In this determination the measured or
telescopic semidiameter of the moon was compared with the semidiameter
deduced from the occultations, upon the above principle, and it was
found that the telescopic semidiameter was greater than the occultation
semidiameter by two seconds of angular measurement or by about a
thousandth part of the whole diameter of the moon. Sir George Airy,
commenting on this result, says that it appears to him that the origin
of this difference is to be sought in one of two causes. “Either it is
due to irradiation[3] of the telescopic semidiameter, and I do not doubt
that a part at least of the two seconds is to be ascribed to that cause;
or it may be due to refraction by the moon’s atmosphere. If the whole
two seconds were caused by atmospheric refraction this would imply a
horizontal refraction of one second, which is only 1/2000 part of the
earth’s horizontal refraction. It is possible that an atmosphere
competent to produce this refraction would not make itself visible in
any other way.” This result accords well, considering the relative
accuracy of the means employed, with that obtained a century ago by the
French astronomer Du Séjour, who made a rigorous examination of the
subject founded on observations of the solar eclipse of 1764. He
concluded that the horizontal refraction produced by a possible lunar
atmosphere amounted to 1″·5—a second and a half—or about 1/1400 of that
produced by the earth’s atmosphere. The greater weight is of course to
be allowed to the more recent determination in consideration of the
large number of accurate observations upon which it was based.
But an atmosphere 2,000 times rarer than our air can scarcely be
regarded as an atmosphere at all. The contents of an air-pump receiver
can seldom be rarefied to a greater extent than to about 1/1000 of the
density of air at the earth’s surface, with the best of pneumatic
machines; and the lunar atmosphere, if it exist at all, is thus proved
to be twice as attenuated as what we are accustomed to recognise as a
vacuum. In discussing the physical phenomena of the lunar surface, we
are, therefore, perfectly justified in omitting all considerations of an
atmosphere, and adapting our arguments to the non-existence of such an
appendage.
And if there be no air upon the moon, we are almost forced to conclude
that there can be no water; for if water covered any part of the lunar
globe it must be vapourised under the influence of the long period of
uninterrupted sunshine (upwards of 300 hours) that constitutes the lunar
day, and would manifest itself in the form of clouds or mists obscuring
certain parts of the surface. But, as we have already said, no such
obliteration of details ever takes place; and, as we have further seen,
no evidence of aqueous vapour is manifested upon the occasion of
spectrum observations. Since, then, the effects of watery vapour are
absent, we are forced to conclude that the cause is absent also.
Those parts of the moon which the ancient astronomers assumed, from
their comparatively smooth and dusky appearance, to be seas, have long
since been discovered to be merely extensive regions of less reflective
surface material; for the telescope reveals to us irregularities and
asperities covering well nigh the whole of them, which asperities could
not be seen if they were covered with water; unless, indeed, we admit
the possibility of seeing to the bottom of the water, not only
perpendicularly, but obliquely. Some observers have noticed features
that have led them to suppose that water was at one time present upon
the moon, and has left its traces in the form of appearances of erosive
action in some parts. But if water ever existed, where is it now? One
writer, it is true, has suggested as possible, that whatever air, and we
presume he would include whatever water also, the moon may possess, is
hidden away in sublunarean caves and hollows; but even if water existed
in these places it must sometimes assume the vapoury form, and thus make
its presence known.
[Illustration: Fig. 13.]
[Illustration: Fig. 14.]
Sir John Herschel pointed out that if any moisture exists upon the moon,
it must be in a continual state of migration from the illuminated or
hot, to the unilluminated or cold side of the lunar globe. The
alternations of temperature, from the heat produced by the unmitigated
sunshine of 14 days’ duration, to the intensity of cold resulting from
the absence of any sunshine whatever for an equal period, must, he
argued, produce an action similar to that of the _cryophorus_ in
transporting the lunar moisture from one hemisphere to the other. The
cryophorus is a little instrument invented by the late Dr. Wollaston; it
consists of two bulbs of glass connected by a bent tube, in the manner
shown in the annexed illustration, fig. 13. One of the bulbs, A, is
half-filled with water, and, all air being exhausted, the instrument is
hermetically sealed, leaving nothing within but the water and the
aqueous vapour which rises therefrom in the absence of atmospheric
pressure. When the empty bulb, B, is placed in a freezing mixture, a
rapid condensation of this vapour takes place within it, and as a
consequence the water in the bulb A gives off more vapour. The
abstraction of heat from the water, which is a natural consequence of
this evaporation, causes it to freeze into a solid mass of ice. Now upon
the moon the same phenomenon would occur did the material exist there to
supply it. In the accompanying diagram let A represent the illuminated
or heated hemisphere of the moon, and B the dark or cold hemisphere; the
former being probably at a temperature of 300° above, and the latter
200° below Fahrenheit’s zero. Upon the above principle, if moisture
existed upon A it would become vapourised, and the vapour would migrate
over to B, and deposit itself there as hoarfrost; it would, therefore,
manifest itself to us while in the act of migrating by clouding or
dimming the details about the boundary of the illuminated hemisphere.
The sun, rising upon any point upon the margin of the dark hemisphere,
would have to shine through a bed of moisture, and we may justly
suppose, if this were the case, that the tops of mountains catching the
first beams of sunlight would be tinged with colour, or be lit up at
first with but a faint illumination, just as we see in the case of
terrestrial mountains whose summits catch the first, or receive the last
beams of the rising or setting sun. Nothing of this kind is, however,
perceptible: when the solar rays tip the lofty peaks of lunar mountains,
these shine at once with brilliant light, quite as vivid as any of those
parts that receive less horizontal illumination, or upon which the sun
is almost perpendicularly shining.
All the evidence, then, that we have the means of obtaining, goes to
prove that neither air nor water exist upon the moon. Two complicating
elements affecting all questions relating to the geology of the
terraqueous globe we inhabit may thus be dismissed from our minds while
considering the physical features of the lunar surface. Fire on the one
hand and water or the other, are the agents to which the configurations
of the earth’s surface are referrable: the first of these produced the
igneous rocks that form the veritable foundations of the earth, the
second has given rise to the superstructure of deposits that constitute
the secondary and tertiary formations: were these last removed from the
surface of our planet, so as to lay bare its original igneous crust,
that crust, so far as reasoning can picture it to us, would probably not
differ essentially from the visible surface of the moon. In considering
the causes that have given birth to the diversified features of that
surface, we may, therefore, ignore the influence of air and water action
and confine our reasoning to igneous phenomena alone: our task in this
matter, it is hardly necessary to remark, is materially simplified
thereby.