COLOUR AND BRIGHTNESS OF LUNAR DETAILS: CHRONOLOGY OF FORMATIONS, AND
FINALITY OF EXISTING FEATURES.
Speaking generally, the details of the lunar surface seem to us to be
devoid of colour. To the naked eye of ordinary sensitiveness the moon
appears to possess a silvery whiteness: more critical judges of colour
would describe it as presenting a yellowish tinge. Sir John Herschel,
during his sojourn at the Cape of Good Hope, had frequent opportunities
of comparing the moon’s lustre with that of the weathered sandstone
surface of Table Mountain, when the moon was setting behind it, and both
were illuminated under the same direction of sunlight; and he remarked
that the moon was at such times “scarcely distinguishable from the rock
in apparent contact with it.” Although his observations had reference
chiefly to brightness, it can hardly be doubted that similarity of
colour is also implied; for any difference in the tint of the two
objects would have precluded the use of the words “scarcely
distinguishable;” a difference of colour interfering with a comparison
of lustre in such an observation, though it must be remembered that he
observed through a dense stratum of atmosphere. Viewed in the telescope,
the same general yellowish-white colour prevails over all the moon, with
a few exceptions offered by the so-called seas. The _Mare Crisium_,
_Mare Serenetatis_, and _Mare Humorum_ have somewhat of a greenish tint;
the _Palus Somnii_ and the circular area of Lichtenberg incline to
ruddiness. These tints are, however, extremely faint, and it has been
suggested by Arago that they may be mere effects of contrast rather than
actual colouration of the surface material. This, however, can hardly be
the case, since all the “seas” are not alike affected; those that are
slightly coloured are, as we have said, some green and some red, and
contrast could scarcely produce such variations. The supposition of
vegetation covering these great flats and giving them a local colour is
in our view still more untenable, in the face of the arguments that we
shall presently adduce against the possibility of vegetable life
existing upon the moon.
It appears to us more rational to consider the tints due to actual
colour of the material (presumably lava or some once fluid mineral
substance) that has covered these areas; and it may well be conceived
that the variety of tint is due to different characters of material, or
even various conditions of the same material coming from different
depths below the lunar surface; and we may reasonably suppose that the
same variously-coloured substances occur in the rougher regions of the
lunar surface, but that they exist there in patches too small to be
recognized by us, or are “put out” by the brightness to which polyhedral
reflexion gives rise.
Seeing that volcanic action has had so large a share in giving to the
moon’s surface its structural character, analogy of the most legitimate
order justifies us in concluding not only that the materials of that
surface are of kindred nature to those of the unquestionably volcanic
portions of the earth, but also that the tints and colours that
characterize terrestrial volcanic and Plutonian products have their
counterparts on the moon. Those who have seen the interior and
surroundings of a terrestrial volcano after a recent eruption, and
before atmospheric agents have exercised their dimming influences, must
have been struck with the colours of the erupted materials themselves
and the varied brilliant tints conferred on these materials by the
sublimated vapours of metals and mineral substances which have been
deposited upon them. If, then, analogy is any guide in enabling us to
infer the appearance of the invisible from that which we know to be of
kindred nature and which we have seen, we may justly conclude that were
the moon brought sufficiently near to us to exhibit the minute
characteristics of its surface, we should behold the same bright and
varied colours in and around its craters that we behold in and about
those of the earth; and in all probability the coloured materials of
lunar volcanoes would be more fresh and vivid than those of the earth by
reason of the absence of those atmospheric elements which tend so
rapidly to impair the brightness of coloured surfaces exposed to their
influence.
Situated as we are, however, as regards distance from the moon, we have
no chance of perceiving these local colours in their smaller masses; but
it is by no means improbable, as we have suggested, that the faint tints
exhibited by the great plains are due to broad expanses of coloured
volcanic material.
But if we fail to perceive diversity of colour upon the lunar surface,
we are in a very different position in regard to diversity of brightness
or variable light-reflective power of different districts and details.
This will be tolerably obvious to those casual observers who have
remarked nothing more of the moon’s physiography than the resemblance to
a somewhat lugubrious human countenance which the full moon exhibits,
and which is due to the accidental disposition of certain large and
small areas of surface material which have less of the light-reflecting
property than other portions; for since all parts seen by a terrestrial
observer may be said to be equally shone upon by the sun, it is clear
that apparently bright and shaded parts must be produced by differences
in the nature of the surface as regards power of reflecting the light
received.
When we turn to the telescope and survey the full disc of the moon with
even a very moderate amount of optical aid, the meagre impression as to
variety of degree of brightness which the unassisted eye conveys is
vastly extended and enhanced, for the surface is seen to be diversified
by shades of brilliancy and dullness from almost glittering white to
sombre grey: and this variety of shading is rendered much more striking
by shielding the eye with a dusky glass from the excessive glare, which
drowns the details in a flood of light. Under these circumstances the
varieties of light and shade become almost bewildering, and defy the
power of brush or pencil to reproduce them.
We may, however, realize an imperfect idea of this characteristic of the
lunar surface by reference to the self-drawn portrait of the full moon
upon Plate III. This is, in fact, a photograph taken from the full moon
itself, and enlarged sufficiently to render conspicuous the spots and
large and small regions that are strikingly bright in comparison with
what may in this place be described as the “ground” of the disc. As an
example of a wide and irregularly extensive district of highly
reflective material, the region of which Tycho is the central object, is
very remarkable. We may refer also to the bright “splashes” of which
Copernicus and Kepler are the centres. So brilliant are these spots that
they can easily be detected by the unassisted eye about the time of full
moon. Still brighter but less conspicuous by its size is the crater
Aristarchus, which shines with specular brightness, and almost induces
the belief that its interior is composed of some vitreous-surfaced
matter: the highly reflective nature of this object has often caused it
to become conspicuous when in the dark hemisphere of the moon,
unilluminated by the sun, and lighted only by the light reflected from
the earth. At these times it appears so bright that it has been taken
for a volcano in actual eruption, and no small amount of popular
misconception at one time arose therefrom concerning the conditions of
the moon as respects existing volcanic activity—a misconception that
still clings to the minds of many.
The parts of the surface distinguished by deficiency of reflecting power
are conspicuous enough. We may cite, however, as an example of a detail
portion especially remarkable for its dingy aspect, the interior of the
crater Plato, which is one of the darkest spots (the darkest well
defined one) upon the hemisphere of the moon visible to us. For
facilitating reference to shades of luminosity, Schroeter and Lohrman
assorted the variously reflective parts into 10 grades, commencing with
the darkest. Grades 1 to 3 comprised the various deep greys; 4 and 5 the
light greys; 6 and 7 white; and 8 to 10 brilliant white. The spots
Grimaldi and Riccioli came under class 1 of this notation; Plato between
1 and 2. The “seas” generally ranged from 2 to 3; the brightest
mountainous portions mostly between degrees 4 and 6; the crater walls
and the bright streaks came between these and the bright peaks, which
fell under the 9th grade. The maximum brightness, the 10th grade, is
instanced only in the ease of Aristarchus and a point in Werner, though
Proclus nearly approaches it, as do many bright spots, chiefly the sites
of minute craters, which make their appearance at the time of full moon.
In photographic pictures produced by the moon of itself, there is always
an apparent exaggeration in the relation of light to dark portions of
the disc. The dusky parts look, upon the photograph, much darker than to
the eye directed to the moon itself, whether assisted or not by optical
appliances. It may be that the real cause of this discrepancy is that
the eye fails to discover the actual difference upon the moon itself,
being insensible to the higher degrees of brightness or not estimating
them at their proper brilliance with respect to parts less bright. On
the other hand, it is probable that the enhanced contrast in the
photograph is due to some peculiar condition of the darker surface
matter affecting its power of reflecting the actinic constituent of the
rays that fall upon it.
The study of the varying brightness or reflective power of different
regions and spots of the lunar disc leads us to the consideration of the
relative antiquity of the surface features; for it is hardly possible to
regard these variations attentively without being impressed with the
conviction that they have relation to some chronological order of
formation. We cannot, in the first place, resist the conviction that the
brightest features were the latest formed; this strikes us as evident on
_primâ facie_ grounds; but it becomes more clearly so when we remark
that the bright formations, as a rule, overlie the duller features. The
elevated parts of the crust are brighter than the “seas” and other
areas; and it is pretty clear that the former are newer than the latter,
upon which they appear to be super-imposed, or through which they seem
to have extruded.[13] The vast dusky plains are in every instance more
or less sprinkled with spots and minute craters, and these last were
obviously formed after the area that contains them. One is almost
disposed to place the order of formations in the order of relative
brightness, and so consider the dingiest parts the oldest and the
brightest spots and craters the newest features, though, in the absence
of an atmosphere competent to impair the reflective power of the surface
materials, we are unable to justify this classification by suggesting a
cause for such a deterioration by time as the hypothesis pre-supposes.
As we have entered upon the question of relative age of the lunar
features, we may remark that there are evidences of various epochs of
formation of particular classes of details, irrespective of their
condition in respect of brightness, or, as we may say, freshness of
material. As a rule, the large craters are older than the small ones.
This is proved by the fact that a large object of this class is never
seen to interfere with or overlap a small one. Those of nearly equal
size are, however, seen to overlap one another as though several
eruptions of equal intensity had occurred from the same source at
different points. This is strikingly instanced in the group of craters
situated in the position 35-141 on our map, the order of formation of
each of which is clearly apparent. The region about Tycho offers an
inexhaustible field for study of these phenomena of overlapping or
interpolating craters, and it will be found, with very few exceptions,
that the smaller crater is the impinging or parasitical one, and must
therefore have been formed after the larger, upon which it intrudes or
impinges. There are frequent cases in which a large crater has had its
rampart interrupted by a lesser one, and this again has been broken into
by one still smaller; and instances may be found where a fourth crater
smaller than all has intruded itself upon the previous intruder. The
general tendency of these examples is to show that the craters
diminished in size as the moon’s volcanic energy subsided: that the
largest were produced in the throes of its early violence, and that the
smallest are the results of expiring efforts possibly impeded through
the deep-seatedness of the ejective source.
Another general fact of this chronological order is that the mountain
chains are never seen to intrude upon formations of the crater order. We
do not anywhere find that a mountain chain runs absolutely into or
through a crater; but, on the other hand, we do find that craters have
formed on mountain chains. This leads unmistakably to the inference that
the craters were not formed _before_ their allied mountain chains; and
we might assume therefore that the mountains generally are the older
formations, but that there is nothing to prove that the two classes of
features, where they intermingle, as in the Apennines and Caucasus, were
not erupted cotemporaneously.
[Illustration: PLATE XX.
OVERLAPPING CRATERS.]
Upon the assumption that the latest ejected or extruded matter is that
which is brightest, we should place the bright streaks among the more
recent features. Be this as it may, it is tolerably certain that the
cracks, whose apparently close relation to the radiating streaks we have
endeavoured to point out, are relatively of a very late formative
period. We are indeed disposed to consider them as the most recent
features of all: the evidence in support of this consideration being the
fact that they are sometimes found intersecting small craters that, from
the way in which they are cut through by the cracks, must have been _in
situ_ before the cracking agency came into operation. It is in
accordance with our hypothesis of the moon’s transition from a fluid to
a solid body to consider that a cracking of the surface would be the
latest of all the phenomena produced by contraction in final cooling.
The foregoing remarks naturally lead us to the question whether changes
are still going on upon the surface of our satellite: whether there is
still left in it a spark of its volcanic activity, or whether that
activity has become totally extinct. We shall consider this question
from the observational and theoretical point of view. First as regards
observations. This much may be affirmed indisputably—that no object or
detail visible to the earliest selenographers (whose period may be dated
200 years back) has altered from the date of their maps to the present.
When we pass from the bolder features to the more minute details we find
ourselves at a loss for materials for forming an inference; the only map
pretending to accuracy even of the larger among small objects being that
of Beer and Maedler, which, truly admirable as it is, is not very safely
to be relied upon for settling any question of alleged change, on
account of the conventional system adopted for exhibiting the forms of
objects, every object being mapped rather than drawn, and shown as it
never is or can be presented to view on the moon itself. This difficulty
would present itself if a question of change were ever raised upon the
evidence of Beer and Maedler’s map: it may indeed have prevented such a
question being raised, for certainly no one has hitherto been bold
enough to assert that any portion or detail of the map fails to
represent the actual state of the moon at the present time.
In default of published maps, we are thrown for evidence on this
question upon observations and recollections of individual observers
whose familiarity with the lunar details extends over lengthy periods.
Speaking for ourselves, and upon the strength of close scrutinies
continued with assiduity through the past thirty years, we may say that
we have never had the suspicion suggested to our eye of any actual
change whatever having taken place in any feature or minute detail of
the lunar surface; and our scrutinies have throughout been made with
ample optical means, mostly with a 20-inch reflector. This experience
has made us not unnaturally in some slight degree sceptical concerning
the changes alleged to have been detected by others. Those asserted by
Schroeter and Gruithuisen were long ago rejected by Beer and Maedler,
who explained them, where the accuracy of the observer was not
questioned, by variations of illumination, a cause of illusory change
which is not always sufficiently taken into account. A notable instance
of this deception occurred a few years ago in the case of the minute
bright crater _Linné_, which was for a considerable period declared,
upon the strength of observations of very promiscuous character, to be
varying in form and dimensions almost daily, but the alleged constant
changes of which have since been tacitly regarded as due to varying
circumstances of illumination induced by combinations of libratory
effects with the ordinary changes depending upon the direction of the
sun’s rays as due to the age of the moon. This explanation does not,
however, dispose of the question whether the crater under notice
suffered any actual change before the hue and cry was raised concerning
it. Attention was first directed to it by Schmidt, of Athens, whose
powers of observation are known to be remarkable, and whose labours upon
the moon are of such extent and minuteness as to claim for his
assertions the most respectful consideration.[14] He affirmed in 1866
that the crater at that date presented an appearance decidedly different
from that which it had had since 1841: that whereas it had been from the
earlier epoch always easily seen as a very deep crater, in October 1866
and thenceforward it presented only a white spot, with at most but a
very shallow aperture, very difficult to be detected. Schmidt is one of
the very few observers whose long familiarity with the moon entitles him
to speak with confidence upon such a question as that before us upon the
sole strength of his own experience; and this case is but an isolated
one, at least it is the only one he has brought forward. He is, however,
still firmly convinced that it is an instance of actual change, and not
an illusion resulting from some peculiar condition of illumination of
the object. It should be added also on this side of the discussion that
an English observer, the Rev. T. W. Webb, while apparently indisposed to
concede the supposition of any notable changes in the lunar features,
has yet found from his own observations that, after all due allowance
for differences of light and shade upon objects at different times,
there is still a “residuum of minute variations not thus disposed of”
which seem to indicate that eruptive action in the moon has not yet
entirely died out, though its manifestation at present is very limited
in extent. It appears to us that, if evidence of continuing volcanic
action is to be sought on the moon, the place to look for it is around
the circumference of the disc, where eruption from any marginal orifice
would manifest itself in the form of a protruding haziness, somewhat as
illustrated to an exaggerated extent in the annexed cut.
[Illustration: Fig. 46.]
The theoretical view of the question, which we have now to consider, has
led us, however, to the strong belief that no vestige of its former
volcanic activity lingers in the moon—that it assumed its final
condition an inconceivable number of ages ago, and that the high
interest which would attach to the close scrutiny of our satellite if it
_were_ still the theatre of volcanic reactions cannot be hoped for. If
it be just and allowable to assume that the earth and the moon were
condensed into planetary form at nearly the same epoch (and the only
rational scheme of cosmogony justifies the assumption) then we may
institute a comparison between the condition of the two bodies as
respects their volcanic age, using the one as a basis for inference
concerning the state of the other. We have reason to believe that the
earth’s crust has nearly assumed its final state so far as volcanic
reactions of its interior upon its exterior are concerned: we may affirm
that within the historical period no igneous convulsions of any
considerable magnitude have occurred; and we may consider that the
volcanoes now active over the surface of the globe represent the last
expiring efforts of its eruptive force. Now in the earth we perceive
several conditions wherefrom we may infer that it parted with its
cosmical heat (and therefore with its prime source of volcanic agency)
at a rate which will appear relatively very slow when we come to compare
the like conditions in the moon. We may, we think, take for granted that
the surface of a planetary body generally determines its _heat
dispersing_ power, while its volume determines its _heat retaining_
power. Given two spherical bodies of similar material but of unequal
magnitude and originally possessing the same degree of heat, the smaller
body will cool more rapidly than the larger, by reason of the greater
proportion which the surface of the smaller sphere bears to its volume
than that of the larger sphere to its volume—this proportion depending
upon the geometrical ratio which the surfaces of spheres bear to their
volumes, the contents of spheres being as the _cubes_ and the surfaces
as the _squares_ of their diameters. The volume of the earth is 49 times
as great as that of the moon, but its surface is only 13 times as great;
there is consequently in the earth a power of retaining its cosmical
heat nearly four times as great as in the case of the moon; in other
words, the moon and earth being supposed at one time to have had an
equally high temperature, the moon would cool down to a given low
temperature in about one fourth the time that the earth would require to
cool to the same temperature. But the earth’s cosmical heat has without
doubt been considerably conserved by its vaporous atmosphere, and still
more by the ocean in its antecedent vaporous form. Yet notwithstanding
all this, the earth’s surface has nearly assumed its final condition so
far as volcanic agencies are concerned: it has so far cooled as to be
subject to no considerable distortions or disruptions of its surface.
What then must be the state of the moon, which, from its small volume
and large proportionate area, parted with its heat at the above
comparatively rapid rate? The matter of the moon is, too, less dense
than the earth, and hence doubtless from this cause disposed to more
rapid cooling; and it has no atmosphere or vaporous envelope to retard
its radiating heat. We are driven thus to the conclusion that the moon’s
loss of cosmical heat must have been so rapid as to have allowed its
surface to assume its final conformation ages on ages ago, and hence
that it is unreasonable and hopeless to look for evidence of change of
any volcanic character still going on.
We conceive it possible, however, that minute changes of a non-volcanic
character may be proceeding in the moon, arising from the violent
alternations of temperature to which the surface is exposed during a
lunar day and night. The sun, as we know, pours down its heat
unintermittingly for a period of fully 300 hours upon the lunar surface,
and the experimental investigations of Lord Rosse, essentially confirmed
by those of the French observer, Marie Davy, show that under this
powerful insolation the surface becomes heated to a degree which is
estimated at about 500° of Fahrenheit’s scale, the fusing point of tin
or bismuth. This heat, however, is entirely radiated away during the
equally long lunar night, and, as Sir John Herschel surmised, the
surface probably cools down again to a temperature as low as that of
interstellar space: this has been assumed as representing the absolute
zero of temperature, which has been calculated from experiments to be
250° below the zero of Fahrenheit’s scale. Now such a severe range of
heat and cold can hardly be without effect upon some of the component
materials of the lunar surface.[15] If there be any such materials as
the vitreous lavas that are found about our volcanoes, such as obsidian
for instance, they are doubtless cracked and shivered by these extreme
transitions of temperature; and this comparatively rapid succession of
changes continued through long ages would, we may suppose, result in a
disintegration of some parts of the surface and at length somewhat
modify the selenographic contour. It is, however, possible that the
surface matter is mainly composed of more crystalline and porous lavas,
and these might withstand the fierce extremes like the “fire-brick” of
mundane manufacture, to which in molecular structure they may be
considered comparable. Lavas as a rule are (upon the earth) of this
unvitreous nature, and if they are of like constitution on the moon,
there will be little reason to suspect changes from the cause we are
considering. Where, however, the material, whatever its nature, is piled
in more or less detached masses, there will doubtless be a grating and
fracturing at the points of contact of one mass with another, produced
by alternate expansions and contractions of the entire masses, which in
the long run of ages must bring about dislocations or dislodgments of
matter that might considerably affect the surface features from a close
point of view, but which can hardly be of sufficient magnitude to be
detected by a terrestrial observer whose best aids to vision give him no
perception of minute configurations. And it must always be borne in mind
that changes can only be _proved_ by reference to previous observations
and delineations of unquestionable accuracy.
Speaking by our own lights, from our own experience and reasoning, we
are disposed to conclude that in all visible aspects the lunar surface
is unchangeable, that in fact it arrived at its terminal condition
_eons_ of ages ago, and that in the survey of its wonderful features,
even in the smallest details, we are presented with the sight of objects
of such transcendent antiquity as to render the oldest geological
features of the earth modern by comparison.