of the moon's stage of volcanic activity, because already, in the pages
of my Treatise on the Moon (Chapter VI.), I have given a full account
of that portion of my present subject. I may make a few remarks,
however, on the theory respecting lunar craters touched on in my work
on 'The Moon.' I have mentioned the possibility that some among the
enormous number of ring-shaped depressions which are seen on the moon's
surface may have been the result of meteoric downfalls in long past
ages of the moon's history. One or two critics have spoken of this view
as though it were too fantastic for serious consideration. Now, though
I threw out the opinion merely as a suggestion, distinctly stating
that I should not care to maintain it as a theory, and although my
own opinion is unfavourable to the supposition that any of the more
considerable lunar markings can be explained in the suggested way, yet
it is necessary to notice that on the general question whether the
moon's surface has been marked or not by meteoric downfalls scarcely
any reasonable doubts can be entertained. For, first, we can scarcely
question that the moon's surface was for long ages plastic, and though
we may not assign to this period nearly so great a length (350 millions
of years) as Tyndall--following Bischoff--assigns to the period when
our earth's surface was cooling from a temperature of 2000° C. to
200°, yet still it must have lasted millions of years; and, secondly,
we cannot doubt that the process of meteoric downfall now going on is
not a new thing, but, on the contrary, is rather the final stage of a
process which once took place far more actively. Now Prof. Newton has
estimated, by a fair estimate of observed facts, that each day on the
average 400 millions of meteors fall, of all sizes down to the minutest
discernible in a telescope, upon the earth's atmosphere, so that on
the moon's unprotected globe--with its surface one thirteenth of the
earth's--about 30 millions fall each day, even at the present time. Of
large meteoric masses only a few hundreds fall each year on the earth,
and perhaps about a hundred on the moon; but still, even at the present
rate of downfall, millions of large masses _must_ have fallen on the
moon during the time when her surface was plastic, while _probably_ a
much larger number--including many much larger masses--must have fallen
during that period. Thus, not only without straining probabilities, but
by taking only the most probable assumptions as to the past, we have
arrived at a result which compels us to believe that the moon's surface
has been very much marked by meteoric downfall, while it renders it by
no means unlikely that a large proportion of the markings so left would
be discernible under telescopic scrutiny.
I would, in conclusion, invite those who have the requisite leisure to
a careful study of the distribution of various orders of lunar marking.
It would be well if the moon's surface were isographically charted,
and the distribution of the seas, mountain ranges, and craters of
different dimensions and character, of rills, radiating streaks, bright
and dark regions, and so on, carefully compared _inter se_, with the
object of determining whether the different parts of the moon's surface
were probably brought to their present condition during earlier or
later periods, and of interpreting also the significance of the moon's
characteristic peculiarities. In this department of astronomy, as in
some others, the effectiveness of well-devised processes of charting
has been hitherto overlooked.
FOOTNOTES:
[Footnote 8: It would still be somewhat denser, because under the
circumstances it would be somewhat cooler.]
[Footnote 9: It is thus, and not by the effects due to increasing
pressure (effects which probably do not increase beyond a certain
point), that we are to explain the fact that the earth's density as a
whole is about twice the mean density of the matters which form its
solid surface. It may be that this consideration, supported by the
results of recent experimental researches, may give a significance
hitherto not noted to the relatively small mean density of the moon.]
[Footnote 10: I have occasion to make some remarks at this stage
to avoid possible and (my experience has shown me) not altogether
improbable misconception, or even misrepresentation. The theory
enunciated above will be regarded by some, who may have read a certain
review of my Treatise on the Moon, as totally different from what I
have advocated in that work, and, furthermore, as a theory which I
have borrowed from the aforesaid review. I should not be particularly
concerned if I had occasion to modify views I had formerly expressed,
since I apprehend that every active student of science should hope,
rather than dread, that as his work proceeds he would form new
opinions. But I must point out that earlier in my book I had advocated
the theory urged above. After describing the radiations from Tycho
and other craters, I proceed as follows in chapter iv.--'It appears
to me impossible to refer these phenomena to any general cause but
the reaction of the moon's interior overcoming the tension of the
crust, and to this degree Nasmyth's theory seems correct; but it
appears manifest, also, that the crust cannot have been fractured
in the ordinary sense of the word. Since, however, it results from
Mallet's investigations that the tension of the crust is called into
play in the earlier stages of contraction, and its power to resist
contraction in the later stages,--in other words, since the crust at
first contracts faster than the nucleus, and afterwards not so fast as
the nucleus,--we may assume that the radiating systems were formed in
so early an era that the crust was plastic. And it seems reasonable to
conclude that the outflowing matter would retain its liquid condition
long enough (the crust itself being intensely hot) to spread widely,--a
circumstance which would account at once for the breadth of many of
the rays, and for the restoration of level to such a degree that no
shadows are thrown. It appears probable, also, that not only (which is
manifest) were the craters formed later which are seen around and upon
the radiations, but that the central crater itself acquired its actual
form long after the epoch when the rays were formed.']
[Footnote 11: Where several ray centres are near together, a region
directly between two ray centres would be at a level intermediate
between that of the ray centres and that of a region centrally placed
within a triangle or quadrangle of ray centres; but the latter region
might be at a higher level than another very far removed from the part
where the ray centres were near together. For instance, the space in
the middle of the triangle having Copernicus, Aristarchus, and Kepler
at its angles (or more exactly between Milichius and Bessarion) is
lower than the surface around Hortensius (between Copernicus and
Kepler), but not so low as the Mare Imbrium, far away from the region
of ray centres of which Copernicus, Aristarchus, and Kepler are the
principal.]
[Footnote 12: It is important to notice that we may derive from these
considerations an argument as to the condition of the fluid matter now
existing beneath the solid crust of the earth.]
_A NEW CRATER IN THE MOON._
Dr. Klein, a German astronomer, has recently called the attention of
astronomers to a lunar crater some three miles wide, which had not
before been observed, and which, he feels sure, was not in existence
two years ago. Astronomers have long since given up all hope of tracing
either the signs of actual life upon the moon or traces of the past
existence of living creatures there. But there are still among them
those who believe that by sedulous and careful scrutiny processes of
material change may be recognised in that seemingly inert mass. In
reality, perhaps, the wonder rather is that signs of change should
not be often recognised, than that from time to time a new crater
should appear or the walls of old craters fall in. The moon's surface
is exposed to variations of temperature compared with which those
affecting the surface of our earth are altogether trifling. It is
true there is no summer or winter in the moon. Sir W. Herschel has
spoken of the lunar seasons as though they resembled our own, but
in reality they are very different. The sun's midday height at any
lunar station is only about three degrees greater in summer than in
winter; whereas our summer sun is 47° higher in the sky at noon than
our winter sun. In fact, a midsummer's day on the moon does not differ
more from a midwinter's day, as far as the sun's actual path on the
sky is concerned, than with us the 17th of March differs from the
25th, or the 19th of September from the 27th. It is the change from
day to night which chiefly affects the moon's surface. In the lunar
year of seasons, lasting 346-2/3 of our days, there are only 11-3/4
lunar days, each lasting 29-3/4 of ours. Thus day lasts more than a
fortnight, and is followed by a night of equal length. Nor is this
all. There is neither air nor moisture to produce such effects as are
produced by our air and the moisture it contains in mitigating the heat
of day and the cold of night. Under the sun's rays the moon's surface
becomes hotter and hotter as the long lunar day proceeds, until at
last its heat exceeds that of boiling water. But so soon as the sun
has set the heat thus received is rapidly radiated away into space (no
screen of moisture-laden air checking its escape), and long before
lunar midnight a cold exists compared with which the bitterest weather
ever experienced by Arctic voyagers would be oppressively hot. These
are not merely theoretical conclusions, though even as such they could
be thoroughly relied upon. The moon's heat has been measured by the
present Lord Rosse (using his father's splendid six-feet mirror). He
separated the heat which the moon simply reflects to us from that which
her heated surface itself gives out (or, technically, he separated
the reflected from the radiated heat), by using a glass screen which
allowed the former heat to pass while it intercepted the latter. He
thus found that about six-sevenths of the heat we receive from the moon
is due to the heating of her own substance. From the entire series of
observations it appeared that the change of temperature during the
entire lunar day--that is, from near midnight to near midday on the
moon--amounts to fully 500° Fahrenheit. If we assume that the cold at
lunar midnight corresponds with about 250° below zero (the greatest
cold experienced in Arctic travelling has never exceeded 140° below
zero), it would follow that the midday heat is considerably greater
than that of boiling water on the earth at the sea-level. But the range
of change is not a matter of speculation. It certainly amounts to about
500°, and in whatever way we distribute it, we must admit, first, that
no such life as we are familiar with could possibly exist on the moon;
and, secondly, that the moon's crust must possess a life of its own,
so to speak, expanding and contracting unceasingly and energetically.
Professor Newcomb, by the way, in his fine work on Popular Astronomy,
rejects the idea that the expansions and contractions due to these
great changes of temperature can cause any disintegration at the
present time. There might, he says, be bodies so friable that they
would crumble, 'but whatever crumbling might thus be caused would soon
be done with, and then no further change would occur.' For my own part,
I cannot consider that such a surface as the moon at present possesses
can undergo these continual expansions and contractions without slow
disintegration. It seems to me also extremely probable that from time
to time the overthrow of great masses, the breaking up of arched
crater-floors, and other sudden changes discernible from the earth,
might be expected to occur. Professor Newcomb has, I conceive, omitted
to consider the enormous volumetric expansion as distinguished from
mere lateral extension, resulting from the heating of the moon's crust
to considerable depths. On a very moderate computation, the surface
of the central region of the full moon must at that time rise above
its mean position to such a degree that hundreds, if not thousands,
of cubic miles of the moon's volume lie above the mean position of
the surface there. At new moon--that is, at lunar midnight for the
same region--the same enormous quantity of matter is correspondingly
depressed. And though the actual range in vertical height at any given
point may be small, we cannot doubt that the total effect produced
by these constant oscillations is considerable. Years or centuries
may pass without any great or sudden change, but from time to time
such catastrophes must surely occur. I believe that all the cases of
supposed change in the moon, if all were regarded as proved, could be
thus fully accounted for without any occasion to assume the action of
volcanic forces properly so called.
Before considering the evidence for the new lunar volcano to which Dr.
Klein has recently called the attention of astronomers, it may be well
briefly to describe the condition of the moon's surface.
This surface, which is equal in extent to about that of the American
Continent, or to Europe and Africa together (without their islands),
is divided into two chief portions--the higher levels, which are in
the main of lighter tint, and the lower levels, which are, almost
without exception, dark. It may be remarked in passing that very
erroneous ideas are commonly entertained respecting the moon's general
colour. The moon is very far from being white, as many suppose. On the
contrary, she is far more nearly black than white. It has been well
remarked by Tyndall that if the moon were covered with black velvet
(14,600,000 square miles of that material would be required for the
purpose), she would still appear white to us, for we should see her
disc projected on the blackness of star-strewn space. The actual tint
of the moon as a whole is nearly the same as that of gray weathered
sandstone. The brightest parts, however, are much whiter; Zöllner
infers from his photometric experiments that they can be compared with
the whitest of terrestrial substances. On the other hand, the darkest
parts of the moon are probably far darker than porphyry, even if they
are not so dark that on earth we should call them black. The fact
that the low-lying parts of the moon are much darker than the higher
regions is full of meaning, though hitherto its significance does not
seem to have been much noticed. Either we must assume that these lower
regions, the so-called seas (certainly now dry), are old sea bottoms,
and owe their darkness to the quality of the matter deposited there
in remote ages, or else we must suppose that the matter which last
remained fluid when the moon's surface was consolidating was of darker
material than the rest. For such matter would occupy the lowest lunar
regions. There is here room for a very profitable study of the moon's
aspect by geologists. I doubt not that, however different the general
past history of our earth may have been from the moon's, terrestrial
regions exist where the characteristic features of the moon's surface
are more or less closely illustrated. On the American continent, for
example, there are peculiarities of geological formation which seem
to correspond closely with some of the features of the lunar globe,
presently to be noticed; and it seems to me not improbable that
geologists might find in the study of certain regions in North America
the means of interpreting the difference of tint between higher and
lower levels on the moon. If so, light would probably be thrown on very
difficult questions relating to the remote past, not only of the moon,
but of our own earth.
The lunar feature which comes next in importance to the difference
of tint between the so-called 'seas' and the higher lands is the
existence of remarkable series of radiating streaks extending from
certain important craters--centres probably of past disturbance. It
is impossible to contemplate the disc of the full moon, as seen with
a powerful telescope, without feeling that these systems of rays must
have resulted from the operation of forces of the most stupendous
nature, though as yet their true meaning is hid from us. They would be
marvellous phenomena, even if they were not so mysterious--marvellous
in their enormous extension, their singular brightness, and
their manner of traversing 'seas,' craters, and mountain-ranges
indifferently. But their chief marvel resides in the mysterious manner
of their appearance as the moon approaches her full illumination.
Other lunar features are most clearly recognised when the moon is not
full, for then the shadows which afford our only means of estimating
the height of lunar irregularities are clearly seen along the border
between the bright and dark parts of her face, and we have only to
wait until this border passes over any object we wish to study to
obtain satisfactory evidence of its nature. It is quite otherwise with
the rays. The regions occupied by these radiating streaks are neither
raised nor depressed in such sort as either to throw shadows or to
lie in shadow when surrounding regions are in sunlight. But when the
moon approaches her full illumination, the radiating regions come into
view, as bright streaks--bright even on the light-tinted lunar uplands.
A mighty system of rays can be seen extending from the great crater
Tycho in all directions. Other systems, scarcely less wonderful, extend
from the battlemented crater, Copernicus, the brilliant Aristarchus,
and the solitary Kepler. One ray from Tycho can be traced round nearly
an entire hemisphere of the moon's surface. It is specially noteworthy
of this great ray that, where it crosses the lunar Sea of Serenity,
that great plain seems to be divided as by a sort of ridge line, the
slope of the plain from either side of the ray's track being clearly
discernible when the moon is near her first quarter.
What are these mysterious ray systems? How are we to explain the
circumstance that though only the most tremendous forces seem competent
to account for bands such as these, many miles broad and many hundreds
of miles in length, there are yet none of the usual signs of the action
of volcanic forces? If mighty rifts had been formed in the moon's crust
by the outbursting action of a hot nucleus, or through the contraction
of the crust on an unyielding nucleus (for the effect would be the same
in either case), we should scarcely expect to find that after such
rifts had formed no signs of any difference of level would appear. If
lava flowed out all along the rift, one would imagine that it would
form a long dyke which would throw an obvious shadow, except under full
solar illumination. If the rift were not filled with lava, the bottom
of the rift would certainly remain in shadow long after the surrounding
region was illuminated. That lava should exactly fill the rift along
its entire length seems incredible. This might happen by a strange
chance in the case of a single rift, but not with all the rifts of a
radiating system, still less with all the rifts of all the radiating
systems. Yet I believe that neither astronomers nor geologists can form
any other opinion respecting these mysterious ray-systems than that
they were caused by what Humboldt (speaking of the earth) calls the
reaction of the interior on the crust. Nasmyth has admirably indicated
their appearance, or rather their radiating form, by filling a globular
glass shell with water, hermetically closed, and then freezing the
water. The expansion of the water bursts the glass shell, and the lines
of fracture are found to extend in a series of rays from the part of
the shell which first gave way. But this experiment of itself does not
explain the mystery of the lunar rays. Accepting the theory that the
moon's crust yielded in some such way, we have still to explain how
the rifts which were thus formed came to be covered over with matter
lying nearly at the same level as the surrounding surface. It appears
to me that the only available way of explaining this is somewhat as
follows. First, from the way in which the streaks are covered like the
surrounding region with craters, we may conclude that the streaks are
older than any except the largest craters; from the great extension
of many of them, we may safely infer that the lunar crust possessed a
large measure of plasticity when they were formed (for otherwise it
would have yielded over a smaller area). It was, therefore, probably
still hot during the era (which may have lasted millions of years) to
which the formation of the rifts belonged: accordingly the lava which
flowed out through the rifts remained liquid for a considerable time,
and was thus able to spread widely on either side of the rift, forming
a broad band of lava-covered surface, instead of a steep and narrow
dyke. This seems not only to account for the most striking peculiarity
of the bands, but to accord well with all that is known about them,
and even to suggest explanations of some other lunar features which
had appeared perplexing. I understand that in certain regions of North
America there are lava-covered rifts large enough to form geographical
features, and, therefore, fairly comparable with the lunar radiating
streaks. But as yet American geologists have not presented in an
accessible form a description of the peculiar features of the American
continent; in fact, it may be doubted whether as yet the materials for
such a description exist.
The mountain-ranges of the moon do not differ to any marked degree
from those of our own earth. They are few in number; in fact,
mountain-ranges form a less important feature of lunar than of
terrestrial geography. On the other hand, the lunar ring-mountains and
craters far exceed those of our earth in size and importance. The large
craters may, in fact, be regarded as characteristic features of lunar
scenery. There are several craters exceeding 100 miles in diameter. It
is strange to consider that though the ringed wall surrounding some of
these larger craters exceeds 10,000 feet in height, no trace of the
highest peaks of such a wall would be visible from the middle of the
enclosed plain. Conversely, an observer standing on one of the highest
peaks beside one of these great craters, would not see half the floor
of the crater, while more than half the horizon line around him would
belong to the enclosed plain, and would appear as level as the horizon
seen from a height overlooking a great prairie. These ringed plains
and larger craters seem to belong to the third great era of the moon's
history. The bright high regions and dark low levels called seas must
have been formed while the greater part of the crust was intensely
hot. The contraction of the cooling crust on the nucleus, which cooled
far less slowly, led to the formation of the great ray systems. But
though such systems extend from great craters, these craters themselves
probably attained their present form far later. The crust having in
great part cooled, the nucleus began in turn to shrink more quickly
than the crust, having more heat to part with. Thus the crust, closing
in upon the shrinking nucleus, formed the corrugations and wrinkles
which can be seen under telescopic scrutiny in nearly every part of
the visible lunar surface. The process was accompanied necessarily
by the development of great heat--the thermal equivalent of the
mechanical process of contraction. Mallet has shown that the process
of contraction at present occurring in the earth's crust gives rise
to the greater part of the heat to which volcanic phenomena are due.
If this is so in the earth's case at present, how tremendous must have
been the heat evolved by the far more rapid contraction of the moon's
mass in the remote era we are considering, when probably her heat
passed into space unchecked by the action of a dense moisture-laden
atmosphere! We can well understand that enormous volumes of heated gas
would be formed--including steam, for there is good reason to believe
that water is present in large quantities in the moon's interior. The
imprisoned gas would find an outlet at points of least resistance,
the centres, namely, of the great radiating systems of streaks. These
centres would certainly be regions of outlet. But they would not be
sufficient. We can understand then why every ray system extends from
a great crater, though that crater was really formed after the system
of radiating streaks; and we can equally understand why these central
craters are not the only or even the chief of the great craters in the
moon. Here again I would suggest that possibly the careful study of
American geology might disclose features illustrating the great lunar
craters.