LIFE IN OTHER WORLDS
Popular interest in astronomy is exceedingly wide, but it is very
largely confined to the idea of resemblances and differences between our
earth and the bodies of the sky. The question most frequently asked the
astronomer is, "Have any of the stars got people on them?" Or more
specifically, "Is Mars inhabited?" The average questioner will not
readily be turned off with yes or no for an answer. He may or may not
know that it is quite impossible for astronomers to ascertain anything
definite in this matter, most interesting as it is. What he wants to
find out is the view of the individual astronomer on this absorbing and
ever recurring inquiry.
We ought first to understand what is meant by the manifestation here on
the earth called life, and agree concerning the conditions that render
it possible. Apparently they are very simple. We may or may not agree
that a counterpart of life, or life of a wholly different type from
ours, may exist on other planets under conditions wholly diverse from
those recognized as essential to its existence here. The problem of the
origin of life is, in the present state of knowledge, highly speculative
and hardly within the domain of science. Here on earth, life is
intimately associated with certain chemical compounds, in which carbon
is the common element without which life would not exist. Also
hydrogen, oxygen, and nitrogen are present, with iron, sulphur,
phosphorus, magnesium and a few less important elements besides. But
carbon is the only substance absolutely essential. Protoplasm cannot be
built without it, and protoplasm makes up the most of the living cell.
Closely related to carbon is silica also, as a substitution in certain
organic compounds. Protoplasm is able to stand very low temperatures,
but its properties as a living cell cease when the temperature reaches
150 Fahrenheit.
Animal life as it exists on the earth to-day appears to have been here
many million years. The palaeontologists agree that all life originated
in the waters of the earth. It has passed through evolutionary stages
from the lowest to the highest. Throughout this vast period the
astronomer is able to say that the conditions of the earth which appear
to be essential to the maintenance of life have been pretty constantly
what they are to-day. The higher the type of life, the narrower the
range of conditions under which it thrives. Man can exist at the frigid
poles even if the temperature is 75 degrees below Fahrenheit zero; and
in the deserts and the tropics, he swelters under temperatures of 115
degrees, but he still lives. At these extremes, however, he can scarcely
be said to thrive.
We have, then, a relatively narrow range of temperatures which seems to
be essential to his comfortable existence and development: we may call
it 150 degrees in extent. Had not the surface temperature of the earth
been maintained within this range for indefinite ages, in the regions
where the human race has developed, quite certainly man would not be
here. How this equability of temperature has been maintained does not
now matter. Clearly the earth must have existed through indefinite ages
in the process of cooling down from temperatures of at least 6,000
degrees.
During this stage the temperature of the surface was earth-controlled.
Then this period merged very gradually into the stage where life became
possible, and the temperature of the surface became, as it now is,
sun-controlled. How many years are embraced in this span of periods, or
ages, we have no means of knowing. But of the sequence of periods and
the secular diminution of temperature, we may be certain.
Then there is the equally important consideration of water necessary for
the origination, support, and development of life. We cannot conceive of
life existing without it. On the earth water is superabundant, and has
been for indefinite ages in the past. There is little evidence that the
oceans are drying up; although the commonly accepted view is that the
waters of the earth will very gradually disappear. Water can exist in
the fluid state, which is essential to life, at all temperatures between
32 degrees and 680 degrees F.
Air to breathe is essential to life also. The atmosphere which envelops
the earth is at least 100 miles in depth, and its own weight compresses
it to a tension of nearly 15 pounds to the square inch at sea level.
This atmosphere and its physical properties have had everything to do
with the development of animal life on the planet. Without it and its
remarkable property of selective absorption, which imprisons and
diffuses the solar heat, it is inconceivable that the necessary
equability of surface temperature could be maintained. This appears to
be quite independent of the chemical constituents of the atmosphere, and
is perhaps the most important single consideration affecting the
existence of life on a planet. If the surface of a planet is partly
covered with water, it will possess also an atmosphere containing
aqueous vapor.
Heat, water, and air: these three essentials determine whether there is
life on a planet or not. Of course there must be nutrition suitable to
the organism; mineral for the vegetal, and vegetal for the animal. But
the narrow range of variation appears to be the striking thing:
relatively but a few degrees of temperature, and a narrow margin of
atmospheric pressure. If this pressure is doubled or trebled, as in
submarine caissons, life becomes insupportable. If, on the other hand,
it is reduced even one-third, as on mountains even 13,000 feet high, the
human mechanism fails to function, partly from lack of oxygen necessary
in vitalizing the blood, but mainly because of simple reduction of
mechanical pressure.
If, then, we conceive of life in other worlds and it is agreed that life
there must manifest itself much as it does here, our answer to the
question of habitability of the planets must follow upon an
investigation of what we know, or can reasonably surmise, about the
surface temperatures of these bodies, whether they have water, and what
are the probable physical characteristics of their atmospheres.
We may inquire about each planet, then, concerning each of these
details.
The case of Mercury is not difficult. At an average distance of only 36
million miles from the sun, and with a large eccentricity of orbit which
brings it a fifth part nearer, conditions of temperature alone must be
such as to forbid the existence of life. The solar heat received is
seven times greater than at the earth, and this is perhaps sufficient
reason for a minimum of atmosphere, as indicated by observation. If no
air, then quite certainly no water, as evaporation would supply a slight
atmosphere. But according to the kinetic theory of gases, the mass of
Mercury, only a very small fraction of that of the sun, is inadequate to
retain an atmospheric envelope. If, however, the planet's day and year
are equal, so that it turns a constant face to the sun, surface
conditions would be greatly complicated, so that we cannot regard the
planet as absolutely uninhabitable on the hemisphere that is always
turned away from the sun.
Venus at 67 millions of miles from the sun presents conditions that are
quite different. She receives double the solar heat that we do, but
possessing an atmosphere perhaps threefold denser than ours, as reliably
indicated by observations of transits of Venus, the intensity of the
heat and its diffusion may be greatly modified. What the selective
absorption of the atmosphere of Venus may be, we do not know. Nor is the
rotation time of the planet definitely ascertained: if equal to her
year, as many observations show and as indicated by the theory of tidal
evolution, there may well be certain regions on the hemisphere
perpetually turned away from the sun where temperature conditions are
identical with those on the tropical earth, and where every condition
for the origin and development of life is more fully met than anywhere
else in the solar system. Whether Venus has water distributed as on the
earth we do not know, as her surface is never seen, owing to dense
clouds under which she is always enshrouded. Her cloudy condition
possibly indicates an overplus of water.
Is the moon inhabited? Quite certainly not: no appreciable air, no
water, and a surface temperature unmodified by atmosphere--rising
perhaps to 100 degrees F. during the day, which is a fortnight in
length, and falling at night to 300 degrees below zero, if not lower.
Is Mars inhabited? The probable surface temperature is much lower than
the earth's, because Mars receives only half as much solar heat as we
do; and more important still, the atmosphere of Mars is neither so dense
nor so extensive as our own. Seasons on Mars are established, much the
same as here, except that they are nearly twice as long as ours; and
alternate shrinking and enlarging of the polar caps keeps even pace with
the seasons, thereby indicating a certainty of atmosphere whose
equatorial and polar circulation transports the moisture poleward to
form the snow and ice of which the polar caps no doubt consist.
There is a variety of evidence pointing to an atmosphere on Mars of
one-third to one-half the density of our own: an atmosphere in which
free hydrogen could not exist, although other gases might. The
spectroscopic evidence of water vapor in the Martian atmosphere is not
very strong. It is very doubtful whether water exists on Mars in large
bodies: quite certainly not as oceans, though the evidence of many small
"lakes" is pretty well made out. With very little water, a thin
atmosphere and a zero temperature, is Mars likely to be inhabited at the
present time? The chances are rather against it. If, however, the past
development of the planet has progressed in the way usually considered
as probable, we may be practically certain that Mars has been inhabited
in the past, when water was more abundant, and the atmosphere more dense
so as to retain and diffuse the solar heat.
Biologists tell me that they hardly know enough regarding the extreme
adaptability of organisms to environment to enable them to say whether
life on such a planet as Mars would or would not keep on functioning
with secular changes of moisture and temperature. The survival of a race
might be insured against extremely low temperatures by dwelling in
sub-Martian caves, and sufficient water might be preserved by
conceivable engineering and mechanical schemes; but the secular
reduction of the quantity and pressure of atmosphere--it is not easy to
see how a race even more advanced than ourselves could maintain itself
alive under serious lack of an element so vital to existence. Both
Wallace, the great biologist, and Arrhenius, the eminent chemist (but
biologist, astronomer, and physicist as well), both reject the
habitation theory of Mars, regarding the so-called canals as quite like
the luminous streaks on the moon; that is, cracks in the volcanic crust
caused by internal strains due to the heated interior. Wallace, indeed,
argues that the planet is absolutely uninhabitable.
The asteroids, or minor planets? We may dismiss them with the simple
consideration that their individual masses are so insignificant and
their gravity so slight that no atmosphere can possibly surround them.
Their temperatures must be exceedingly low, and water, if present at
all, can only exist in the form of ice.
Jupiter, the giant planet, presents the opposite extreme. His mass is
nearly a thousandth part of the sun's, and is sufficient to retain a
very high temperature, probably approximating to the condition we call
red-hot. This precludes the possibility of life at the outset, although
the indications of a very dense atmosphere many thousand miles in depth
are unmistakable.
Of Saturn, one thirty-five hundredth the mass of the sun, practically
the same may be said. Proctor thought it quite likely that Saturn might
be habitable for living creatures of some sort, but he regarded the
planet as on many accounts unsuitable as a habitation for beings
constituted like ourselves. Mere consideration of surface temperature
precludes the possibility of life in the present stage of Saturn's
development; but the consensus of opinion is to the effect that life may
make its appearance on these great planets at some inconceivably remote
epoch in the future when the surface temperature is sufficiently reduced
for life processes to begin. Discoveries of algae flourishing in hot
springs approaching 200 degrees Fahrenheit make it possible that these
beginnings may take place earlier and at much higher temperatures than
have hitherto been thought possible.
A century ago, when the ring of Saturn was believed to be a continuous
plane, this was a favorite corner of the solar system for speculation as
to habitability; but now that we know the true constitution of the
rings, no one would for a moment consider any such possibility.
Conditions may, however, be quite different with Saturn's huge satellite
Titan, the giant moon of the solar system. Its diameter makes it
approximately the size of the planet Mars; and although it is much
farther removed from the sun, its relative nearness to the highly
heated globe of Saturn may provide that equability of temperature which
is essential to life processes.
Also the three inner Galilean moons of Jupiter, especially III which is
about the size of Titan, are excellently placed for life possibilities,
as far as probable temperature is concerned, but we have of course no
basis for surmising what their conditions may be as to air and water,
except that their small mass would indicate a probable deficiency of
those elements.
Uranus and Neptune are planets so remote, and their apparent disks are
so small, that very little is known about their physical condition. They
are each about one-third the diameter of Jupiter, and the spectrum of
Uranus shows broad diffused bands, indicating strong absorption by a
dense atmosphere very different from that of the earth. Indications are
that Neptune has a similar atmosphere.
It is possible that the denser atmospheres of these remote planets may
be so conditioned as to selective absorption that the relatively slender
supply of solar heat may be conserved, and thus insure a relatively high
surface temperature when the sun comes into control. If our theories of
origin of the planets are to be trusted, we may rather suppose that
Uranus and Neptune are still in a highly heated condition; that life has
not yet made its appearance on them, but that it will begin its
development ages before Saturn and Jupiter have cooled to the requisite
temperature.
Comets? In his _Lettres Cosmologiques_ (1765) Lambert considers the
question of habitability of the comets, naturally enough in his day,
because he thought them solid bodies surrounded by atmosphere, and
related to the planets. The extremes of temperature at perihelia and
aphelia to which comets are subjected did not bother him particularly.
After calculating that the comet of 1680, "being 160 times nearer to the
sun than we are ourselves, must have been subjected to a degree of heat
25,600 times as great as we are," Lambert goes on to say: "Whether this
comet was of a more compact substance than our globe, or was protected
in some other way, it made its perihelion passage in safety, and we may
suppose all its inhabitants also passed safely. No doubt they would have
to be of a more vigorous temperament and of a constitution very
different from our own. But why should all living beings necessarily be
constituted like ourselves? Is it not infinitely more probable that
amongst the different globes of the universe a variety of organizations
exist, adapted to the wants of the people who inhabit them, and fitting
them for the places in which they dwell, and the temperatures to which
they will be subjected? Is man the only inhabitant of the earth itself?
And if we had never seen either bird or fish, should we not believe that
the air and water were uninhabitable? Are we sure that fire has not its
invisible inhabitants, whose bodies, made of asbestos, are impenetrable
to flame? Let us admit that the nature of the beings who inhabit comets
is unknown to us; but let us not deny their existence, and still less
the possibility of it."
Little enough is really known about the physical nature of comets even
now, but what we do know indicates incessant transformation and
instability of conditions that would render life of any type exceedingly
difficult of maintenance.
A word about Sir William Herschel's theory of the sun and its
habitability. He thought the core of the sun a dark, solid body, quite
cold, and surrounded by a double layer, the inner one of which he
conceived to act as a sort of fire screen to shield the sun proper
against the intense heat of the outer layer, or photosphere by which we
see it. Viewed in this light, the sun, he says, "appears to be nothing
else than a very eminent, large and lucid planet, evidently the first,
or, in strictness of speaking, the only primary one of our system.... It
is most probably also inhabited, like the rest of the planets, by beings
whose organs are adapted to the peculiar circumstances of that vast
globe." But physics and biology were undeveloped sciences in Herschel's
days.
Herschel knew, however, that the stars are all suns, so that he must
have conceived that they are inhabited also, quite independently of the
question whether they possess retinues of planets, after the manner of
our solar system.
This again is a question to which the astronomer of the present day can
give no certain answer. So immensely distant are even the nearest of
these multitudinous bodies that no telescope can ever be built large
enough or powerful enough to reveal a dark planet as large as Jupiter,
alongside even the nearest fixed star. Whatever may be the process of
stellar evolution, there doubtless is an era of many hundreds of
millions of years in the life of a star when it is passing through a
planet-maintaining stage. This would likely depend upon spectral type,
or to be indicated by it; and as about half of the stars are of the
solar type, it would be a reasonable inference that at least half of the
stars may have planets tributary to them.
In such a case, the chances must be overwhelmingly in favor of vast
numbers of the planets of other stellar systems being favorably
circumstanced as to heat and moisture for the maintenance of life at the
present time. That is, they are habitable, and if habitable, then
thousands of them are no doubt inhabited now. But astronomers know
absolutely nothing about this question, nor are they able to conceive at
present any way that may lead them to any definite knowledge of it.
There is, indeed, one piece of quasi-evidence which might reasonably be
interpreted as implying that it is more likely that the stars are not
attended by families of planets than that they are.