CRACKS AND RADIATING STREAKS.
We have hitherto confined our attention to those reactions of the moon’s
molten interior upon its exterior which have been accompanied by
considerable extrusions of sub-surface material in its molten or
semi-solid condition. We now pass to the consideration of some phenomena
resulting in part from that reaction and in part from other effects of
cooling, which have been accompanied by comparatively little ejection or
upflow of molten matter, and in some cases by none at all. Of such the
most conspicuous examples are those bright streaks that are seen, under
certain conditions of illumination, to radiate in various directions
from single craters, and some of the individual radial branches of which
extend from four to seven hundred miles in a great arc on the moon’s
surface.
There are several prominent examples of these bright streak systems upon
the visible hemisphere of the moon; the focal craters of the most
conspicuous are Tycho, Copernicus, Kepler, Aristarchus, Menelaus, and
Proclus. Generally these focal craters have ramparts and interiors
distinguished by the same peculiar bright or highly reflective material
which shows itself with such remarkable brilliance, especially at full
moon: under other conditions of illumination they are not so strikingly
visible. At or nearly full moon the streaks are seen to traverse over
plains, mountains, craters, and all asperities; holding their way
totally disregardful of every object that happens to lay in their
course.
The most remarkable bright streak system is that diverging from the
great crater Tycho. The streaks that can be easily individualized in
this group number more than one hundred, while the courses of some of
them may be traced through upwards of six hundred miles from their
centre of divergence. Those around Copernicus, although less remarkable
in regard to their extent than those diverging from Tycho, are
nevertheless in many respects well deserving of careful examination:
they are so numerous as utterly to defy attempts to count them, while
their intricate reticulation renders any endeavour to delineate their
arrangement equally hopeless.
The fact that these bright streaks are invariably found diverging from a
crater, impressively indicates a close relationship or community of
origin between the two phenomena: they are obviously the result of one
and the same causative action. It is no less clear that the actuating
cause or prime agency must have been very deep-seated and of enormous
disruptive power to have operated over such vast areas as those through
which many of the streaks extend. With a view to illustrate
experimentally what we conceive to have been the nature of this
actuating cause, we have taken a glass globe and, having filled it with
water and hermetically sealed it, have plunged it into a warm bath: the
enclosed water, expanding at a greater rate than the glass, exerts a
disruptive force on the interior surface of the latter, the consequence
being that at the point of least resistance, the globe is rent by a vast
number of cracks diverging in every direction from the focus of
disruption. The result is such a strikingly similar counterpart of the
diverging bright streak systems which we see proceeding from Tycho and
the other lunar craters before referred to, that it is impossible to
resist the conclusion that the disruptive action which originated them
operated in the same manner as in the case of our experimental
illustration; the disruptive force in the case of the moon being that to
which we have frequently referred as due to the expansion which precedes
the solidification of molten substances of volcanic character.
Our illustration, Plate XIX., is a photograph from one of many glass
globes which we have cracked in the manner described: a careful
comparison between the arrangement of the divergent cracks represented
in the photograph and those seen spreading from Tycho and other lunar
craters will, we trust, justify us in what we have stated as to the
similarity of the causes which have produced such identical results.
The accompanying figures will further illustrate our views upon the
causative origin of the bright streaks. The primary action rent the
solid crust of the moon and produced a system of radiating fissures
(Fig. 42): these immediately afforded egress for the molten matter
beneath to make its appearance on the surface simultaneously along the
entire course of every crack, and irrespective of all surface
inequalities or irregularities whatever (Fig. 43). We conceive that the
upflowing matter spread in both directions sideways and in this manner
produced streaks of very much greater width than the cracks or fissures
up through which it made its way to the surface.
[Illustration: Fig. 42.
ILLUSTRATIVE OF THE RADIATING CRACKS WHICH PRECEDE THE FORMATION OF
THE BRIGHT STREAKS.]
In further elucidation of this part of our subject we may refer to a
familiar but as we conceive cogent illustration of an analogous action
in the behaviour of water beneath the ice of a frozen pond, which, on
being fractured by some concentrated pressure, or by a blow, is well
known to “star” into radiating or diverging cracks, up through which the
water immediately issues, making its appearance on the surface of the
ice simultaneously along the entire course of every crack, and on
reaching the surface, spreading on both sides to a width much exceeding
that of the crack itself.
[Illustration: Fig. 43.
ILLUSTRATIVE OF THE RADIATING BRIGHT STREAKS.]
If this familiar illustration be duly considered, we doubt not it will
be found to throw considerable light on the nature of those actions
which have resulted in the bright streaks on the moon’s surface. Some
have attempted to explain the cause of these bright streaks by assigning
them to streams of lava, issuing from the crater at the centre of their
divergence and flowing over the surface, but we consider such an
explanation totally untenable, as any idea of lava, be it ever so fluid
at its first issue from its source, flowing in streams of nearly equal
width, through courses several hundred miles long, up hills, over
mountains, and across plains, appears to us beyond all rational
probability.
[Illustration: PLATE XVIII.
ARISTARCHUS & HERODOTUS.]
It may be objected to our explanation of the formation of these bright
streaks, that so far as our means of observation avail us, we fail to
detect any shadows from them or from such marginal edges as might be
expected to result from a sideway-spreading outflow of lava from the
cracks which afforded it exit in the manner described. Were the edges of
these streaks terminated by cliff-like or craggy margins of such height
as 30 or 40 feet, we might just be able at low angles of illumination
and under the most favourable circumstances of vision, to detect some
slight appearance of shadows; but so far as we are aware, no such
shadows have been observed. We are led to suppose that the impossibility
of detecting them is due not to their absence but to the height of the
margins being so moderate as not to cast any cognizable shadow, inasmuch
as an abrupt craggy margin of 10 or 15 feet high would, under even the
most favourable circumstances, fail to render such visible to us.
Reference to our ideal section of one of these bright streaks (Fig. 45),
will show how thin their edges may be in relation to their spreading
width.
The absence of cognizable shadows from the bright streaks has led some
observers to conclude that they have no elevation above the surface over
which they traverse, and it has therefore been suggested that their
existence is due to possible vapours which may have issued through the
cracks, and condensed in some sublimated or pulverulent form along their
courses, the condensed vapours in question forming a surface of high
reflective properties. That metallic or mineral substances of some kinds
do deposit on condensation very white powders, or sublimates, we are
quite ready to admit, and such explanation of the high luminosity of the
bright streaks, and of the craters situated at the foci or centres of
their divergence is by no means improbable, so far as concerns their
mere brightness. But as we invariably find a crater occupying the centre
of divergence, and such craters are possessed of all the characteristic
features and details which establish their true volcanic nature as the
results of energetic extrusions of lava and scoria, we cannot resist the
conclusion that the material of the crater, and that of the bright
streaks diverging from it, are not only of a common origin, but are so
far identical that the only difference in the structure of the one as
compared with the other is due to the more copious egress of the
extruded or erupted matter in the case of the crater, while the
restricted outflow or ejection of the matter up through the cracks would
cause its dispersion to be so comparatively gentle as to flood the sides
of the cracks and spread in a thin sheet more or less sideways
simultaneously along their courses. There are indeed evidences in the
wider of the bright streaks of their being the result of the outflow of
lava through _systems of cracks_ running parallel to each other, the
confluence of the lava issuing from which would naturally yield the
appearance of one streak of great width. Some of those diverging from
Tycho are of this class; many other examples might be cited, among which
we may name the wide streaks proceeding from the crater Menelaus and
also those from Proclus. Some of these occupy widths upwards of 25
miles—amply sufficient to admit of many concurrent cracks with confluent
lava outflows.
We are disposed to consider as related to the fore-mentioned radiating
streaks, the numerous, we may say the multitudinous, long and narrow
chasms that have been sometimes called “canals” or “rills,” but which
are so obviously _cracks_ or chasms, that it is desirable that this name
should be applied to them rather than one which may mislead by implying
an aqueous theory of formation. These cracks, singly and in groups, are
found in great numbers in many parts of the moon’s surface. As a few of
the more conspicuous examples which our plates exhibit we may refer to
the remarkable group west of Treisnecker (Plate XI.), the principal
members of which converge to or cross at a small crater, and thus point
to a continuity of causation therewith analogous to the evident relation
between the bright streaks and their focal craters. Less remarkable, but
no less interesting, are those individual examples that appear in the
region north of (below) the Apennines (Plate IX.), and some of which by
their parallelism of direction with the mountain-chain appear to point
to a causative relation also. There is one long specimen, and several
shorter in the immediate neighbourhood of Mercator and Campanus (Plate
XV.); and another curious system of them, presenting suggestive
contortions, occurs in connection with the mountains Aristarchus and
Herodotus (Plate XVIII.). Others, again, appear to be identified with
the radial excrescences about Copernicus (Plate VIII.). Capuanus,
Agrippa, and Gassendi, among other craters, have more or less notable
cracks in their vicinities.
Some of these chasms are conspicuous enough to be seen with moderate
telescopic means, and from this maximum degree of visibility there are
all grades downwards to those that require the highest optical powers
and the best circumstances for their detection. The earlier
selenographers detected but a few of them. Schroeter noted only 11;
Lohrman recorded 75 more; Beer and Maedler added 55 to the list, while
Schmidt of Athens raised the known number to 425, of which he has
published a descriptive catalogue. We take it that this increase of
successive discoveries has been due to the progressive perfection of
telescopes, or, perhaps, to increased education, so to speak, of the
eye, since Schmidt’s telescope is a much smaller instrument than that
used by Beer and Maedler, and is regarded by its owner as an inferior
one for its size. We doubt not that there are hundreds more of these
cracks which more perfect instruments and still sharper eyes will bring
to knowledge in the future.
While these chasms have all lengths from 150 miles (which is about the
extent of those near Treisnecker) down to a few miles, they appear to
have a less variable breadth, since we do not find many that at their
maximum openings exceed two miles across; about a mile or less is their
usual width throughout the greater part of their length, and generally
they taper off to invisibility at their extremities, where they do not
encounter and terminate at a crater or other asperity, which is,
however, sometimes the case. Of their depth we can form no precise
estimate, though from the sharpness of their edges we may conclude that
their sides approach perpendicularity, and, therefore, that their depth
is very great; we have elsewhere suggested ten miles as a possible
profundity. In a few cases, and under very favourable circumstances, we
have observed their generally black interiors to be interrupted here and
there with bright spots suggestive of fragments from the sides of the
cracks having fallen into the opening.
In seeking an explanation of these cracks, two possible causes suggest
themselves. One is the expansion of subsurface matter, already suggested
as explanatory of the bright streaks; the other, a contraction of the
crust by cooling. We doubt not that both causes have been at work, one
perhaps enhancing the other. Where, as in the cases we have pointed out,
there are cracks which are so connected with craters as to imply
relationship, we may conclude that an upheaving or expansive force in
the sublunar molten matter has given rise to the cracks, and that the
central craters have been formed simultaneously, by the release, with
ejective violence, of the matter from its confining crust. The nature of
the expansive force being assumed that of solidifying matter, the wide
extent of some chasms indicates a deep location of that force. And depth
in this matter implies lateness (in the scale of selenological time) of
operation, since the central portions of the globe would be the last to
cool. Now, we have evidence of comparative lateness afforded by the fact
that in many cases the cracks have passed through craters and other
asperities which thus obviously existed before the cracking commenced;
and thus, so far, the hypothesis of the expansion-cracking is supported
by absolute fact.
It may be objected that such an upheaving force as we are invoking,
being transitory, would allow the distended surface to collapse again
when it ceased to operate, and so close the cracks or chasms it
produced. But we consider it not improbable that in some cases, as a
consequence of the expansion of subsurface matter, an upflow thereof may
have partially filled the crack, and by solidifying have held it open;
and it is rational to suppose that there have been various degrees of
filling and even of overflow—that in some cases the rising matter has
not nearly reached the edge of the crack, as in Fig. 44, while in others
it has risen almost to the surface, and in some instances has actually
overrun it and produced some sort of elevation along the line of the
crack, like that represented sectionally in Fig. 45. It is probable that
some of the slightly tumescent lines on the moon’s surface have been
thus produced.
[Illustration: PLATE XIX.
GLASS GLOBE CRACKED BY INTERNAL PRESSURE.]
[Illustration: Fig. 44.]
[Illustration: Fig. 45.]
We have suggested shrinkage as a possible explanation of some cracks. It
could hardly have been the direct cause of those compound ones which are
distinguished by focal craters, though it may have been a co-operative
cause, since the contracting tendency of any area of the crust, by so to
speak weakening it, may have virtually increased the strength of an
upheaving force and thus have aided and localized its action. We see,
however, no reason why the inevitable ultimate contraction which must
have attended the cooling of the moon’s crust, even when all internal
reactions upon it had ceased, should not have created a class of cracks
without accompanying craters, while it would doubtless have a tendency
to increase the length and width of those already existing from any
other cause. Some of the more minute clefts, which presumably exist in
greater numbers than we yet know of, may doubtless be ascribed to this
effect of cooling contraction. In this view we should have to regard
such cracks as the latest of all lunar features. Whether the agency that
produced them is still at work—whether the cracks are on the increase—is
a question impossible of solution: for reasons to be presently adduced,
we incline to believe that all cosmical heat passed from the moon, and
therefore that it arrived at its present, and apparently final,
condition ages upon ages ago.
Besides the ridges spoken of on p. 140, and regarded as cracks up
through which matter has been extruded, there are numerous ridges of
greater or less extent, which we conceive are of the nature of wrinkles,
and have been produced by tangential compression due to the collapse of
the moon’s crust upon the shrunken interior, as explained and
illustrated in Chap. III. The distinguishing feature of the two classes
of phenomena we consider to be the presence of a serrated summit in
those of the extruded class, while those produced by “wrinkling” action
have their summits comparatively free from serration or marked
irregularity.