Figs. 19, 20, and 21 are three views of Mars, drawn by Mr. Nathaniel
Green, an excellent observer, who has paid special attention to this
planet. Fig. 19 shows a faintly-marked sea running north and south (the
upper part of the picture being the south, because that is the way in
which the telescope used by astronomers inverts objects.) This is one of
the markings which deceived Hooke. This picture was drawn on May 30,
1873, at half-past seven in the evening. The second picture was drawn
two days earlier, at eight in the evening; but it shows the planet as it
would have looked on May 30 at about a quarter past nine in the evening,
by which time the sea running north and south had been carried over to
the right and lost to view. But another north and south sea had come
into view on the right. The third picture shows a view taken three hours
later, or at eleven on May 28, when the planet appeared precisely as he
would have appeared at a quarter past eleven in the early morning of May
31, had weather then permitted Mr. Green to continue his observations.
You see in it the great north and south sea which Maraldi had noticed,
the other of those two which had deceived Hooke.
It will be seen from these drawings, which, be it remembered, were taken
at the telescope, that it is possible from a great number of such
drawings to make a chart of Mars, showing its lands and seas not as they
are seen in the telescope, but as they might be laid down by inhabitants
of Mars in a map or planisphere. This has been done, with gradually
increasing accuracy,--first by Sir W. Herschel, next by Beer and Mädler,
then by Phillips, and lastly by myself. (In claiming for my own chart
greater accuracy, I am simply asserting the superior completeness of the
list of telescopic drawings which I was able to consult.) The result is
shown in the accompanying chart (fig. 22), which presents the whole
surface of Mars divided into lands and seas and polar snows, with the
names attached of various observers who have at sundry times contributed
to our knowledge of the planet's features.
But now it will be asked by the thoughtful reader, how can any one
possibly be sure that the regions called continents and seas do really
consist of land and water? At any rate, the doubt might well be
entertained respecting the water. For land is a wide term, including all
kinds of rock surface, sand, earthy soil, and so forth; but it may seem
to require proof that the substance we call water really exists out
yonder in space, either in the form of snow and ice at the Martian
poles, or as flowing water in the Martian seas, or in the vaporous form
in the planet's air.
[Illustration: Fig. 22.--Chart of Mars, from 27 drawings by Mr.
Dawes.]
Very strange, then, at first must the statement seem, that we are as
sure of the existence of water in all these forms on Mars as if we had
sent some messenger to the planet who had brought back for study by our
chemists a block of Martian ice, a vessel full of Martian water, and a
flask of Martian air saturated with aqueous vapour. Indeed, I do not
know of any discovery effected by man which more strikingly displays the
power of human ingenuity in mastering difficulties which, at a first
view, seem altogether insuperable. When we know that a mass of ice as
large as Great Britain would appear at the distance of Mars a mere
bright point; that a sea as large as the Mediterranean would appear like
a faint, greenish-blue, streak; and that cloud masses such as would
cover the whole of Europe would only present the appearance of a whitish
glare, how hopeless seems the task of attempting to determine what is
the real chemical constitution of objects thus seen! It might well be
thought that no possible explanation of the method used by astronomers
could serve to establish its validity. Yet nothing can be simpler than
the principle of the method, or more satisfactory than its application
in this special case.
First, let the reader rid his mind of the difficulty arising from the
enormous distance of the celestial bodies. To do this let him note that
there are some things which a body close by can tell us no more
certainly than a remote body. For instance, we are just as certain that
Mars is a body capable of reflecting sunlight as we are that a
cricket-ball is. We know as certainly, too, that the quality of Mars is
such that more of the red of the sun's light is sent to us than of the
other colours. For we perceive that Mars is a ruddy planet. Since
distance in no way interferes with our perception of these general
facts, and others like them, we need not necessarily find in mere
distance any difficulty in the way of recognising some other facts. All
that we require to be shown before admitting the validity of the
evidence is, that it is of such a kind that distance does not affect its
_quality_, however much distance may and must affect the quantity of
evidence.
Now there is a means of taking the light which comes from a body shining
either with its own or with reflected light, and analyzing it into its
component colours. The spectroscope is the instrument by which this is
accomplished. I do not propose to describe here the nature of this
instrument, or the details of the various methods in which it is
employed. I note only that it separates the rays of different colour
coming from an object, and lays them side by side for us,--the red rays
by themselves, the orange rays by themselves, and so with the yellow,
green, blue, indigo, and violet. And not only are the rays of these
colours set by themselves, but the red rays are sorted in order, from
the deepest brown-red[11] to a tint of red (the lightest) which must
almost be called orange; the orange in order, from orange which must
almost be called red to a tint (the lightest orange) which must almost
be called yellow; the yellow, from an almost orange yellow to a yellow
just beginning to be tinged with green; the green, from an almost yellow
green (the lightest) to a green which may almost be called blue (the
darkest); the blue, from this tint to the beginning of the indigo; the
indigo, from this tint to the first rays of the violet; and lastly the
violet, through all the tints of this beautiful colour to a
blackish-brown violet, where the visible spectrum ends. All these tints
are sorted in order by the spectroscope, just as a skilful colourist
might range in due sequence a myriad tints of colour. But this is only
true of really white light, such light as comes from a glowing mass of
metal burning at a white heat. In other cases (even when the light may
seem white to the eye) some of the tints are found, when the
spectroscope spreads out the colours for us, to be missing. And we know
that this may be caused in two ways. Either the source of light never
gave out those missing tints; or, the source of light gave them out,
but some absorbing medium stopped them on their way before they reached
the spectroscope with which we examine them. There may be cases where we
cannot tell very easily which of these is the true cause. But sometimes
we can, as the instances I have now to deal with will show you.
The sun's own light shows under this method of spectroscopic analysis
millions of tints, in fact I might say millions of red tints, and so
forth, right through the spectral list of colours. But also many
thousands of tints are wanting. Imagine a rainbow-coloured ribbon, the
colours ranged along its length, so that the ribbon is black at both
ends, and that from the black of one end the colour merges into very
deep red, and thence by insensible gradations through orange, yellow,
green, blue, indigo, and violet, into the black of the other end. Then
suppose that tens of thousands of the fine threads which run athwart the
ribbon--_i.e._, the short cross threads--are drawn out. Then the ribbon,
laid on a dark background showing through the spaces where the threads
were drawn out, would represent the solar spectrum. We know then that
the light of the sun's glowing mass either wants particular tints
originally, or shines through vapours which prevent the free passage of
rays of those colours. Both causes might be at work, not one only. At
present we are not concerned with this particular point; but I only
mention that, in reality, no tints are actually wanting, though some are
very much enfeebled.
The sun's light falling on any opaque object is reflected. If the object
is white, the light gives exactly the same spectrum, only fainter. Thus,
I take a piece of white paper on which the sun's rays are falling, and
examine its light with one of Browning's spectroscopes. I get the
ordinary solar spectrum. The cold white paper gives me in fact a
spectrum which speaks of a heat so intense that the most stubborn metals
are not merely melted but vaporized in it. But this heat resides in the
sun, not in the paper.
Now, speaking generally, Mars also sends us sunlight, so that when we
spread out with the spectroscope the rays coming from this planet, we
get the solar spectrum, only of course very much enfeebled. _But_ close
examination shows that other tints besides those missing from the solar
spectrum are missing from the spectrum of Mars. He reflects to us the
sunlight, almost as it reaches him, but he abstracts from it a few tints
on his own account.
When we inquire what these tints are, we find that they are tints which
are _sometimes_ wanting even from direct sunlight. When the sun sinks
very low and looks like a great red ball through the moisture-laden
air, his spectrum is not the same exactly as that of the sun shining
high in the mid heaven. It shows other gaps than those corresponding to
the ordinary myriads of missing tints. Its red colour shows indeed that
some thing has happened to the sunlight; but, oddly enough (at first
sight at least), the gaps are chiefly in the red part of the spectrum,
just what one would expect if the sun's light showed a want instead of
an excess of ruddy light. The fact is, however, that the violet, indigo,
and blue are weakened altogether, not by the mere abstraction of tints
here and there. The red suffers under a few abstractions of tint, but
remains on the whole little weakened. Now the same gaps which at such
times appear in the spectrum of the sun are found (generally, if not
always) in the spectrum of the planet Mars, even when he is shining high
in the heavens, so that his light is _not_ at the time absorbed by the
denser portions of our air. In fact the gaps have been seen in the
spectrum of Mars when the planet has been shining higher in the heavens
than the moon, whose spectrum was found on trial (at the time) not to
show the same gaps,--as of course it must have done, and even more
markedly, if the missing tints had been abstracted by our own air.
No doubt can remain, then, that the sun's light, which reaches us after
falling on Mars, has suffered _at Mars_ the same absorption which our
own air produces on the rays of the sun when he is low down. But we know
what it is in our air which causes this absorption. It is the aqueous
vapour. We know this from several independent series of researches. It
was proved first by an American physicist, Professor Cooke of Harvard,
who found that these lines in the red are always darker when the air is
moister. Then by Janssen, who observed the spectrum of great bonfires
lit at a distance of many miles, on the Swiss mountains, finding these
same lines in the spectrum of the fire-light when the air was heavily
laden with moisture. Wherefore we know that the air of Mars must also
contain the same substance--the vapour of water--which, in our own air,
produces these dark lines. We can, indeed, understand that the ruddy
colour of Mars is in part due to this moisture, which, precisely as in
our own air it makes the sun and moon look red, would, in the air of a
planet, make the planet itself look red.
But how much follows from the discovery that there is moisture in the
air of Mars! This moisture can only come from water in sufficient
quantities. There must, therefore, be seas on Mars. We should be sure of
this from the spectroscopic evidence, even without the evidence given by
the telescope. We cannot doubt for a moment, however, knowing as we do
how the telescope shows greenish markings on Mars, that these really
are the seas and oceans of the planet. And again, the white spots at the
poles of Mars can no longer be regarded doubtfully. If we could not see
them, but knew only, from the spectroscopic evidence, that Mars must
have large seas, we should be sure that his polar regions must be
covered with everlasting ice and snow, varying with the seasons, but
always surrounding, in enormous masses, the poles themselves. Seeing
that the telescope presents spots to our view which, long before the
spectroscopic evidence had been obtained or hoped for, had been regarded
as analogues of our polar snows, we can now entertain no manner of doubt
that they really are so.
But again, recognising the presence of enormous masses of snow and ice
around the poles of Mars, and knowing that not only are there wide
oceans, seas, and lakes, but that there is an atmosphere capable of
carrying mist and cloud, how many circumstances, corresponding to those
which we associate with the wants of living creatures, present
themselves to our consideration! It remains that I should now consider
some of these points.
* * * * *
We have seen that Mars has water in all its forms, solid, liquid, and
vaporous. We perceive also that his polar regions do not extend very
much farther towards his equator than do the polar ice and snows of our
own earth. (Of course the former do not extend so far in actual
distance; I refer to their extent compared with the globe they belong
to.) It would appear then, at a first view, that the climate of Mars
cannot be very unlike that of our earth. Yet this is scarcely possible.
For Mars is so much farther than we are from the sun that he receives
less than half as much light and heat from that luminary. And it is not
easy to conceive that the deficiency can be compensated by any effects
due to the nature of the Martian air. It is more likely by far that this
air is much rarer than that it is much denser than ours. For not only
can it be shown that with the same relative quantity of air a smaller
planet would have a smaller quantity above each square mile of its
surface than would a larger one,[12] but the gravity at the surface of
the smaller planet being less, the air there is much less compressed by
its own weight (having in fact much less weight), and is therefore
rarer. Thus the probability is that the air of Mars is like that at (or
even above) the summits of our highest mountains, where we know that an
intense cold prevails. It is not that the sun's rays do not fall there
with as much heating power as at the sea-level, for experiment shows
that they fall with even greater power. But there is less air to be
warmed and to retain the heat. The difference may be compared in fact to
that between a well-watered country near the sea and an arid desert. The
sun's rays fall as fiercely on one as on the other, but because there is
no moisture in the desert to receive (after the fashion characteristic
of water) the solar heat and retain it, the heat passes away so soon as
the sun has set, and intense cold prevails, while over the well-watered
region the temperature is much more uniform, and warm nights prevail. So
is it at the summits of lofty mountains. The sun's rays are poured on
them as hotly as elsewhere, but there is little air to retain the
moisture, so that the heat passes away almost as quickly as it is
received, and during the night as much fresh snow is formed as had been
melted during the day. And so it would certainly be with Mars, if, other
things being the same, the air were as rare as it is at the summits of
our loftiest mountains. If, as seems probable, the air is still rarer
than this, the cold would be still more intense.
It would seem, then, that either some important difference exists, by
which the Martian air is enabled to retain the sun's heat even more
effectively than our air does (for the climate as indicated by the
limits of the polar snows seems the same, though the distance from the
sun is greater); or else there is some mistake in the supposition that
the same general state of things prevails on Mars as on our own earth.
I confess that though Professor Tyndall has shown clearly how the
atmosphere of a more distant planet _might_ make up for the deficient
supply of solar heat, by more effectively retaining the heat, I know of
nothing in either the telescopic or the spectroscopic evidence
respecting any of the planets which tends to show, or even renders it
likely, that any such arrangement exists,--excepting always the
peculiarity in Mars's case which we are now endeavouring to explain.
Insomuch that should any other explanation of the difficulty be
suggested, and appear to have weight in its favour, I apprehend that the
mere possibility of an atmospheric arrangement, such as has been
suggested, should not prevent our admitting this other explanation.
I am inclined to think that there is such an explanation. It seems to me
that there are good reasons for regarding Mars as a planet which has
passed to a much later stage of planetary life than that through which
our earth is now passing, and that in this circumstance some of the
peculiarities of his appearance find their explanation. As a planet
outside the earth, Mars must probably be regarded as one formed somewhat
before the earth. As a much smaller planet, he would be not only less
heated when first found (whatever theory of planetary formation we
adopt), but would also have parted much more rapidly (relatively) with
his heat, according to the same law which makes a small mass of metal
cool more quickly than a large one. If he has a rarer atmosphere he
would be a colder planet on that account also. Being also remoter from
the sun, he receives less heat from that orb, and we thus have a fourth
reason for regarding Mars as a much colder planet than our earth, both
as to inherent heat and as to heat received from without. It seems to me
that we may in this consideration find the real meaning of the
comparatively limited extension of the Martian snows. It has been well
pointed out by Professor Tyndall that for the formation of great glacial
masses, not great cold only, but great heat also is required. The snows
which fall on mountain slopes, to be compacted into ice and afterwards
to form great glaciers, were raised into the air by the sun's heat.
Every ice particle represents the action of that heat upon the particles
of water at the surface of ocean, sea, or lake, or of wet soil. If the
sun's heat suddenly died out, there would prevail an intense cold, and
the snows and ice now existing would assuredly remain. The waters also
of the earth would congeal. But no new snows would fall. The congealed
seas viewed from some remote planet would appear unchanged. For they
would not be covered with snow and broken ice, nor therefore white; but
would consist of pure ice throughout, retaining the partial transparency
and greenish colour of deep-sea water. No winds would disturb the
surface of the frozen seas, for winds have their origin in heat, and
with the death of the solar heat the winds would utterly die out also.
If we are to choose between these two explanations,--one that the snows
and ice have not the great range we should expect, because the
temperature is somehow raised despite Mars's greater distance to the
same temperature which we experience, and the other that it is not heat
but cold which diminishes the quantity of Martian snow, I conceive that
there is every reason the case admits of for accepting the latter
instead of the former explanation. As extreme cold would certainly
prevent glacial masses from being very large and deep, simply because
the stores whence the ice was gathered would be less, the snow caps of a
very cold planet would vary as readily with varying seasons as those of
a planet like our earth. For though less heat would be poured upon them
with the returning summer, less heat would be required to melt away
their outskirts.
I think we may fairly regard Mars as in all probability a somewhat old
and decrepit planet. He is not absolutely dead, like our own moon, where
we see neither seas nor clouds, neither snow nor ice, no effects, in
fine, of either heat or cold. But I think he has passed far on the road
towards planetary death,--that is, towards that stage of a planet's
existence when at least the higher forms of life can no longer exist
upon the planet's surface.
There is one peculiarity of the planet's appearance which seems
strikingly to accord with this view that Mars holds a position
intermediate between that of our earth and the moon,--as indeed we might
fairly expect from his intermediate proportions. The seas of our earth
cover nearly three-quarters of her entire globe. The moon has no visible
water on her surface. If we examine the chart of Mars at page 167, we
see that the seas and oceans of the planet are much smaller (relatively
as well as actually) than are the seas of our own earth. I have
carefully estimated their relative extent in the following simple but
effective way. I drew a chart such as the above-mentioned, but on a
projection of my own invention, in which equal surfaces on a globe are
represented by equal surfaces on the planisphere. Then I cut out with a
pair of scissors the parts representing land and the parts representing
water (leaving the polar parts as doubtful), and carefully weighed these
in a delicate balance. I found that they were almost exactly equal:
whatever preponderance there was seemed to be in favour of the land.
Thus, if we assume that, when in the same stage of planetary existence,
Mars had as great a relative extent of water surface as our earth, or
that about 72/100 of the surface of Mars were originally water, we
should have to admit that the water had so far been withdrawn into the
planet's interior as to diminish the water-surface by 22/100 (for there
are now barely 50/100). At a very fair assumption as to the slopes of
the Martian sea-bottoms, it would follow that more than half the Martian
water originally existing above the surface had been withdrawn into the
interior, as the planet's mass gradually cooled.
I am aware the assumption above mentioned is in itself somewhat daring,
and is not supported by direct evidence. But, since we have very strong
reasons for considering that the moon once had seas, which have been
withdrawn in the way suggested, and since Mars unquestionably holds a
position midway between the earth and moon as to size and presumably as
to age,[13] it seems not unreasonable to find in the character of her
seas,--less extended relatively than the earth's, but, unlike the
moon's, still existing,--the evidence that she has gone partially
through the process through which the moon has long since passed
completely.
I think it very likely that the recent discovery of two Martian
satellites will lead many to look with more disfavour than ever on the
idea that Mars may not at present be the abode of life. For moons seem
so manifestly convenient additions to a planet's surroundings, as
light-givers, time-measurers, and tide-rulers, that many will regard the
mere fact that these conveniences exist as proof positive that they are
at this present time subserving the purposes which they are capable of
subserving. I would point out, however, that our own moon must have
existed for ages before any living creatures, far less any reasoning
beings, could profit by her light, or by the regularity of her motions,
or by her action in swaying the waters of ocean. And doubtless she will
continue to exist for ages after all life shall have passed away from
the earth. Again, there can be no question that our earth would present
a most attractive scene if she were viewed from the moon, and would be a
most useful ornament of the lunar skies. Yet we have every reason to
believe that there is not a living creature on the moon at present to
profit by her light. The case may well be the same (apart from the
actual evidence that it _is_ the same) with Mars. His satellites may
long since have served most useful purposes to his inhabitants; but it
by no means follows that because if there were inhabitants on Mars now
the same purposes would still be subserved, therefore there are
inhabitants there.
Let us, however, without considering the question whether the satellites
of Mars serve such special purposes for creatures living on the planet,
consider briefly the history of their discovery, their nature, and the
laws of their motion around the planet.
Astronomers had long examined the neighbourhood of Mars with very
powerful telescopes, in the hope of discovering Martian moons. But the
hope had so thoroughly been abandoned for many years that the planet had
come to be known as "moonless Mars." The construction, however, of the
fine telescope which has been mounted at Washington, with an
object-glass twenty-six inches in diameter, caused at least American
astronomers to hope that after all a Martian moon or two might be
discovered. Taking advantage of the exceptionally favourable
opportunity presented during the planet's close approach to our earth in
the autumn of 1877, Prof. Asaph Hall, of the Washington Observatory,
paid special attention to the search for Martian moons. At last, on
August 16, 1877, he detected close by the planet a faint point of light,
which he was unable to examine further at the time (to see if it behaved
as a satellite, or as one of the fixed stars). But on the 18th he saw it
again, and determined its nature. He also saw another still fainter
point of light closer to the planet; and subsequent observations shewed
that this object also was a satellite. During the next few weeks both
the moons were observed as closely as possible, in fact, whenever
weather permitted, and the result is that we now know the true nature of
their paths.
In fig. 23 these paths are shown as they appeared in 1877. Of course the
paths themselves are not seen; but if the satellites left behind them a
fine train or wake of light, the shape of this train would be as shown
in fig. 23. The satellites themselves could not be shown at all in a
picture on so small a scale--the diameter of either would certainly be
less than the cross-breadth of the fine elliptical line representing its
track. The size of the planet is correctly indicated, and the true pose
of the planet in 1877 is shown in the figure, his southern pole being
somewhat bowed towards the earth. This is the uppermost pole; for the
figure represents the planet and his satellites' orbits as they would
appear in an astronomical telescope, which inverts objects.
[Illustration: Fig. 23.--Mars and the paths of the Martian satellites as
at present situated.]
The outer satellite is probably not more than ten miles or so in
diameter, the inner one, perhaps, the same; but neither can be measured.
In the most powerful telescopes they appear as mere points of light. Nor
is it easy to determine, from their lustre, or rather from their
faintness, their true dimensions; for we cannot compare them directly in
this respect with objects of known size, because their visibility is
partly affected by the proximity of the planet, whose overpowering light
dims their feeble rays. This remark applies with special force to the
inner satellite.
The distance of the outer satellite from Mars's centre is about 14,300
miles, from Mars's surface about 12,000 miles. The inner travels at a
distance of about 5,750 miles from the centre, and about 3,450 miles
from the surface of Mars.
The motions of the satellites as seen from Mars must be very different
from those of our own moon. Thus, our moon moves so slowly among the
stars that she requires nearly an hour to traverse a distance equal to
her own apparent diameter. The outer moon of Mars traverses a similar
distance--that is, not her own apparent diameter, but an arc on the
stellar heavens equal to our moon's apparent diameter--in about two and
a half minutes, while the inner moon moves so rapidly as to traverse the
same distance in about forty seconds. To both moons, therefore, but to
the inner in particular, Job's description of our moon as "walking in
brightness" would seem singularly applicable, so far at least as the
rapidity of their motions is concerned. Their brightness, however,
cannot be comparable to our moon's. For notwithstanding their much
greater proximity, it is easily shown that they must present much
smaller discs, and being illuminated by a more distant sun, their discs
cannot shine so brightly as our moon's. That is, not only are the discs
smaller, but their intrinsic brightness is less. Assuming the outer moon
to be ten miles, the inner fifteen miles in diameter, it is easily shown
that the two together, if full at the same time, can hardly give
one-twelfth as much light to Martians as our moon gives to us.
Yet there can be no doubt that the Martian moons must be (or _have
been_) most useful additions to the Martian skies. They do not give a
useful measure of time intermediate in length between the day and the
year, as our moon does; for the outer travels round the planet in about
thirty and a quarter hours, the inner in about seven and a half hours.
Nor can they exert an influence upon the Martian seas corresponding to
that exerted by our own moon in generating the lunar tidal wave. But
their motions must serve usefully to indicate the progress of time, both
by night and by day, for they must be visible by day unless very close
to the sun. They must be even more useful than our moon in indicating
the longitude of ships at sea, seeing that the accuracy with which a
moon indicates longitude is directly proportional to her velocity of
motion among the stars.
I have said that there does not seem to be any valid reason for
considering that now is the accepted time with these moons; their
services may have been of immense value in long past ages, and now be
valueless for want of any creatures to be benefited by them. But those
who not only believe that no object in nature was made without some
special purpose, but that we are able to assign to each object its
original purpose, should be well satisfied if they find reason for
believing that, during millions of years long, long ago, the moons
lately discovered by our astronomers were measuring time for past races
of Martians, swaying tides in wider seas than those which now lave the
shores of Martian continents, and enabling Martian travellers to guide
their course over the trackless ocean and arid desert with far greater
safety than can our voyagers by sea and land despite all the advances of
modern science.