a distinct orb, as presenting the same general characteristics that we
ascribe to our earth in its primary stage as a planet. In one respect
the moon, even at that early stage, may have differed from the earth.
I refer to its rotation, the correspondence between which and its
revolution may probably have existed from the moon's first formation.
But this would not materially have affected the relations with which we
have to deal at present. We may apply, then, to the moon the arguments
which have been applied to the discussion of the first stages of our
earth's history.
Adopting this view, we see that at the first stage of its existence
as an independent planet, the moon must have been an intensely heated
gaseous globe, glowing with inherent light, and undergoing a process of
condensation, 'going on at first at the surface only, until by cooling
it must have reached the point where the gaseous centre was exchanged
for one of combined and liquefied matter.' To apply now to the moon at
this stage the description which Dr. Sterry Hunt gives of the earth.
'Here commences the chemistry of the moon. So long as the gaseous
condition of the moon lasted, we may suppose the whole mass to have
been homogeneous; but when the temperature became so reduced that the
existence of chemical compounds at the centre became possible, those
which were most stable at the elevated temperature then prevailing
would be first formed. Thus, for example, while compounds of oxygen
with mercury, or even with hydrogen, could not exist, oxides of
silicon, aluminium, calcium, magnesium, and iron, might be formed and
condensed in a liquid form at the centre of the globe. By progressive
cooling still other elements would be removed from the gaseous mass,
which would form the atmosphere of the non-gaseous nucleus.' 'The
processes of condensation and cooling having gone on until those
elements which are not volatile in the heat of our ordinary furnaces
were condensed into a liquid form, we may here inquire what would be
the result on the mass of a further reduction of temperature. It is
generally assumed that in the cooling of a liquid globe of mineral
matter congelation would commence at the surface, as in the case of
water; but water offers an exception to most other liquids, inasmuch as
it is denser in the liquid than in the solid form. Hence, ice floats
on water, and freezing water becomes covered with a layer of ice which
protects the liquid below. Some metals and alloys resemble water in
this respect. With regard to most other earthy substances, and notably
the various minerals and earthy compounds like those which may be
supposed to have made up the mass of the molten globe, the case is
entirely different. The numerous and detailed experiments of Charles
Deville and those of Delesse, besides the earlier ones of Bischoff,
unite in showing that the density of fused rocks is much less than that
of the crystalline products resulting from their slow cooling, these
being, according to Deville, from one-seventh to one-sixteenth heavier
than the fused mass, so that if formed at the surface they would, in
obedience to the laws of gravity, tend to sink as soon as formed.'
Here it has to be noted that possibly there existed a period (for
our earth as well as for the moon) during which, notwithstanding the
relations indicated by Dr. Hunt, the exterior portions of the moon
were solid, while the interior remained liquid. A state of things
corresponding to what we recognise as possible in the sun may have
existed. For although undoubtedly any liquid matter forming in the
sun sinks in obedience to the laws of gravity towards the centre, yet
the greater heat which it encounters as it sinks must vapourise it,
notwithstanding increasing pressure, so that it can only remain liquid
near the region where rapid radiation allows of sufficient cooling
to produce liquefaction. And in the same way we may conceive that
the solidification taking place at any portion of the surface of the
moon's or the earth's liquid globe, owing to rapid radiation of heat
thence, although it might be followed immediately by the sinking of the
solidified matter, would yet result in the continuance (rather than the
existence) of a partially solid crust. For the sinking solid matter,
though subjected to an increase of pressure (which, in the case of
matter expanding on liquefaction, would favour solidification), would
nevertheless, owing to the great increase of heat, become liquefied,
and, expanding, would no longer be so much denser[8] than the liquid
through which it was sinking as to continue to sink rapidly.
Nevertheless, it is clear that after a time the heat of the interior
parts of the liquid mass would no longer suffice to liquefy the solid
matter descending from the surface, and then would commence the process
of aggregation at the centre described by Dr. Hunt. The matter forming
the solid centre of the earth consists probably of metallic and
metalloidal compounds of elements denser than those forming the known
portions of the earth's crust.[9] In the case of the moon, whose mean
density is very little greater than the mean density of the matter
forming the earth's crust, we must assume that the matter forming the
solid nucleus at that early stage was relatively less in amount, or
else that we may attribute part of the difference to the comparatively
small force with which lunar gravity operated during various stages of
contraction and solidification.
In the case of the moon, as in that of the earth, before the last
portions became solidified, there would exist a condition of imperfect
liquidity, as conceived by Hopkins, 'preventing the sinking of the
cooled and heavier particles, and giving rise to a superficial crust,
from which solidification would proceed downwards. There would thus
be enclosed between the inner and outer solid parts a portion of
uncongealed matter,' which may be supposed to have retained its
liquid condition to a late period, and to have been the principal
seat of volcanic action, whether existing in isolated reservoirs or
subterranean lakes, or whether, as suggested by Scrope, forming a
continuous sheet surrounding the solid nucleus.
Thus far we have had to deal with relations more or less involved
in doubt. We have few means of forming a satisfactory opinion as to
the order of the various changes to which, in the first stages of
her existence as a planet, our moon was subject. Nor can we clearly
define the nature of those changes. In these matters, as with the
corresponding processes in our earth's case, there is much room for
variety of opinion.
But few can doubt that, by whatever processes such condition may have
been attained, the moon, when her surface began to form itself into its
present appearance, consisted of a globe partially molten surrounded
by a crust at least partially solidified. Some portions of the actual
surface may have remained liquid or viscous later than others but at
length the time must have arrived when the radiating surface was almost
wholly solid. It is from this stage that we have to trace the changes
which have led to the present condition of the moon's surface.
It can scarcely be questioned that those seismologists are in the
right who have maintained in recent times the theory that in the
case of a cooling globe, such as the earth or moon at the stage just
described, the crust would in the first place contract more quickly
than the nucleus, while later the nucleus would contract more quickly
than the crust. This amounts, in fact, to little more than the
assertion that the process of heat radiation from the surface would be
more rapid, and so last a shorter time than the process of conduction
by which in the main the nucleus would part with its heat. The crust
would part rapidly with its heat, contracting upon the nucleus; but
the very rapidity (relative) of the process, by completing at an early
stage the radiation of the greater portion of the heat originally
belonging to the crust, would cause the subsequent radiation to be
comparatively slow, while the conduction of heat from the nucleus
to the crust would take place more rapidly, not only relatively but
actually.
Now it is clear that the results accruing during the two stages into
which we thus divide the cooling of the lunar globe would be markedly
different. During the first stage forces of tension (tangential) would
be called to play in the lunar crust; during the later stage the forces
would be those of pressure.
Taking the earlier stage, during which the forces would be tensional,
let us consider in what way these forces would operate.
At the beginning, when the crust would be comparatively thin, I
conceive that the more general result of the rapid contraction of
the crust would be the division of the crust into segments, by the
formation of numerous fissures due to the lateral contraction of
the thin crust. The molten matter in these fissures would film over
rapidly, however, and all the time the crust would be growing thicker
and thicker, until at length the formation of distinct segments would
no longer be possible. The thickening crust, plastic in its lower
strata, would now resist more effectively the tangential tensions,
and when yielding would yield in a different manner. It was at this
stage, in all probability, that processes such as those illustrated by
Nasmyth's globe experiments took place, and that from time to time the
crust yielded at particular points, which became the centres of systems
of radiating fissures. Before proceeding, however, to consider the
results of such processes, let it be noted that we have seen reason to
believe that among the very earliest lunar formations would be rifts
breaking the _ancient_ surface of the lunar crust. I distinguish in
this way the ancient surface from portions of surface whereof I shall
presently have to speak as formed at a later time.
Now let us conceive the somewhat thickened crust contracting upon
the partially fluid nucleus. If the crust were tolerably uniform in
strength and thickness we should expect to find it yielding (when
forced to yield) at many points, distributed somewhat uniformly over
its extent. But this would not be the case if--as we might for many
reasons expect--the crust were wanting in uniformity. There would
be regions where the crust would be more plastic, and so readier to
yield to the tangential tensions. Towards such portions of the crust
the liquid matter within would tend, because there alone would room
exist for it. The down-drawing, or rather in-drawing, crust elsewhere
would force away the liquid matter beneath, towards such regions of
less resistance, which would thus remain at (and be partly forced
to) a higher level. At length, however, the increasing tensions thus
resulting would have their natural effect; the crust would break
open at the middle of the raised region, and in radiating rifts,
and the molten matter would find vent through the rifts as well as
at the central opening. The matter so extruded, being liquid, would
spread, so that--though the radiating nature of the rifts would still
be indicated by the position of the extruded matter--there would
be no abrupt changes of level. It is clear, also, that so soon as
the outlet had been formed the long and slowly sloping sides of the
region of elevation would gradually sink, pressing the liquid matter
below towards the centre of outlet, whence it would continue to pour
out so long as this process of contraction continued. All round the
borders of the aperture the crust would be melted, and would continue
plastic long after the matter which had filled the fissures and flowed
out through them had solidified. Thus there would be formed a wide
circular orifice, which would from the beginning be considerably above
the mean level of the moon's surface, because of the manner in which
the liquid matter within had been gathered there by the pressure of
the surrounding slopes.[10] Moreover, around the orifice, the matter
outflowing as the crust continued to contract would form a raised wall.
Until the time came when the liquid nucleus began to contract more
rapidly than the crust, the large crateriform orifice would be full
to the brim (or nearly so), at all times, with occasional overflows:
and as a writer who has recently adopted this theory has remarked, 'We
should ultimately have a large central lake of lava surrounded by a
range of hills, terraced on the outside,--the lake filling up the space
they enclosed.'
The crust might burst in the manner here considered, at several
places at the same--or nearly the same--time, the range of the
radiating fissures, depending on the extent of the underlying lakes
of molten matter thus finding their outlet; or there might be a
series of outbursts at widely separated intervals of time and at
different regions, gradually diminishing in extent as the crust
gradually thickened and the molten matter beneath gradually became
reduced in relative amount. Probably the latter view should be
accepted, since, if we consider the three systems of radiations from
Copernicus, Aristarchus, and Kepler, which were manifestly not formed
contemporaneously, but in the order in which their central craters have
just been named, we see that their dimensions diminished as their date
of formation was later. According to this view we should regard the
radiating system from Tycho as the oldest of all these formations.
At this very early stage of the moon's history, then, we regard the
moon as a somewhat deformed spheroid, the regions whence the radiations
extended being the highest parts, and the regions farthest removed from
the ray centres being the lowest.[11] To these lower regions whatever
was liquid on the moon's surface would find its way. The down-flowing
lava would not be included in this description, as being rather viscous
than liquid; but if any water existed at that time it would occupy the
depressed regions which at the present time are called Maria or Seas.
It is a question of some interest, and one on which different opinions
have been entertained, whether the moon at any stage of its existence
had oceans and an atmosphere corresponding in relative extent to
those of the earth. It appears to me that, apart from all the other
considerations which have been suggested in support of the view that
the moon formerly had oceans and an atmosphere, it is exceedingly
difficult to imagine how, under any circumstances, a globe so large
as the moon could have been formed under conditions not altogether
unlike, as we suppose, those under which the earth was formed (having
a similar origin, and presumably constructed of the same elements),
without having oceans and an atmosphere of considerable extent. The
atmosphere would not consist of oxygen and nitrogen only or chiefly,
any more than, in all probability, the primeval atmosphere of our
own earth was so constituted. We may adopt some such view of the
moon's atmosphere--_mutatis mutandis_--as Dr. Sterry Hunt has adopted
respecting the ancient atmosphere of the earth. Hunt, it will be
remembered, bases his opinion on the former condition of the earth by
conceiving an intense heat applied to the earth as now existing, and
inferring the chemical results. 'To the chemist,' he remarks, 'it is
evident that from such a process applied to our globe would result the
oxidation of all carbonaceous matter; the conversion of all carbonates,
chlorides, and sulphates into silicates; and the separation of the
carbon, chlorine, and sulphur in the form of acid gases; which,
with nitrogen, watery vapour, and an excess of oxygen, would form an
exceedingly dense atmosphere. The resulting fused mass would contain
all the bases as silicates, and would probably nearly resemble in
composition certain furnace-slags or basic volcanic glasses. Such we
may conceive to have been the nature of the primitive igneous rock,
and such the composition of the primeval atmosphere, _which must have
been one of very great density_.' All this, with the single exception
of the italicised remark, may be applied to the case of the moon. The
lunar atmosphere would not probably be dense at that primeval time,
even though constituted like the terrestrial atmosphere just described.
It would perhaps have been as dense, or nearly so, as our present
atmosphere. Accordingly condensation would take place at a temperature
not far from the present boiling-point, and the lower levels of
the half-cooled crust would be drenched with a heated solution of
hydrochloric acid, whose decomposing action would be rapid, though
not aided--as in the case of our primeval earth--by an excessively
high temperature. 'The formation of the chlorides of the various bases
and the separation of silica would go on until the affinities of the
acid were satisfied.' 'At a later period the gradual combination of
oxygen with sulphurous acid would eliminate this from the atmosphere
in the form of sulphuric acid.' 'Carbonic acid would still be a large
constituent of the atmosphere, but thenceforward (that is, after the
separation of the compounds of sulphur and chlorine from the air) there
would follow the conversion of the complex aluminous silicates, under
the influence of carbonic acid and moisture, into a hydrated silicate
of alumina or clay, while the separated lime, magnesia, and alkalies
would be changed into bicarbonates, and conveyed to the sea in a state
of solution.'
It seems to me that it is necessary to adopt some such theory as to
the former existence of lunar oceans in order to explain some of the
appearances presented by the so-called lunar seas. As regards the
present absence of water we may adopt the theory of Frankland, that the
lunar oceans have withdrawn beneath the crust as room was provided for
them by the contraction of the nucleus. I think, indeed, that there
are good grounds for looking with favour on the theory of Stanislas
Meunier, according to which the oceans surrounding any planet--our own
earth or Mars, for example--are gradually withdrawn from the surface to
the interior. And in view of the enormous length of the time-intervals
required for such a process, we must consider that while the process
was going on the lunar atmosphere would not only part completely with
the compounds of sulphur, chlorine, and carbon, but would be even
still further reduced by chemical processes acting with exceeding
slowness, yet effectively in periods so enormous. But without insisting
on this consideration, it is manifest that--with very reasonable
assumptions as to the density of the lunar atmosphere in its original
complex condition--what would remain after the removal of the chief
portion by chemical processes, and after the withdrawal of another
considerable portion along with the seas beneath the lunar crust, would
be so inconsiderable in quantity as to accord satisfactorily with
the evidence which demonstrates the exceeding tenuity of any lunar
atmosphere at present existing.
These considerations introduce us to the second part of the moon's
history,--that corresponding to the period when the nucleus was
contracting more rapidly than the crust.
One of the first and most obvious effects of this more rapid nuclear
contraction would be the lowering of the level of the molten matter,
which up to this period had been kept up to, or nearly up to, the
lips of the great ringed craters. If the subsidence took place
intermittently there would result a terracing of the interior of the
ringed elevation, such as we see in many lunar craters. Nor would
there be any uniformity of level in the several crater floors thus
formed, since the fluid lava would not form parts of a single fluid
mass (in which case, of course, the level of the fluid surface would
be everywhere the same), but would belong to independent fluid masses.
Indeed it may be noticed that the very nature of the case requires us
to adopt this view, since no other will account for the variety of
level observed in the different lunar crater-floors. If these ceased to
be liquid at different times, the independence of the fluid masses is
by that very fact established; and if they ceased to be liquid at the
same time, they must have been independent, since, if communication had
existed between them, they would have shown the uniformity of surface
which the laws of hydrostatics require.[12]
The next effect which would follow from the gradual retreat of
the nucleus from the crust (setting aside the withdrawal of lunar
seas) would be the formation of corrugations,--in other words, of
mountain-ranges. Mallet describes the formation of mountain-chains as
belonging to the period when 'the continually increasing thickness of
the crust remained such that it was still as a whole flexible enough,
or opposed sufficient resistance of crushing to admit of the uprise of
mountain-chains by resolved tangential pressures.' Applying this to the
case of the moon, I think it is clear that--with her much smaller orb
and comparatively rapid rate of cooling--the era of the formation of
mountain-chains would be a short one, and that these would therefore
form a less important characteristic of her surface than of the
earth's. On the other hand, the period of volcanic activity which would
follow that of chain-formation would be _relatively_ long continued;
for regarding this period as beginning when the thickness of the moon's
crust had become too great to admit of adjustment by corrugation, the
comparatively small pressure to which the whole mass of the moon had
been subjected by lunar gravity, while it would on the one hand cause
the period to have an earlier commencement (relatively), would on the
other leave greater play to the effects of contraction. Thus we can
understand why the signs of volcanic action, as distinguished from the
action to which mountain-ranges are due, should be far more numerous
and important on the moon than on the earth.