outer portions are cooled to a suitable temperature (below 55°) by
radiation, and because the cooling does not proceed so far that the
open sea would continually be frozen over, and that the temperature on
the Continent would always remain below freezing-point. We owe this
favorable intermediate stage to the fact that the radiation from the
sun balances the loss of heat by radiation into space, and that it is
capable of maintaining the greater portion of the surface of the earth
at a temperature above the freezing-point of water. The temperature
conditioning life on a planet is therefore maintained only because,
on the one side, light and heat are received by radiation from the
sun in sufficient quantities, while on the other side an equivalent
radiation of heat takes place into space. If the heat gain and the heat
loss were not to balance each other, the term of suitable conditions
would not last long. The temperature of the earth-crust could sink
in a few hundreds or thousands of years from 1000° to 100°, because
when the earth was at this high temperature its radiation into space
predominated over the radiation received from the sun. On the other
hand, about a hundred million years have passed, according to Joly,
since the age when the ocean originated. The temperature of the earth,
therefore, required this long space of time in order to cool down from
365° (at which temperature water vapor can first be condensed to liquid
water) to its present temperature. The cooling afterwards proceeded at
a slower rate, because the difference between the radiations inward and
outward was lessened with the diminishing temperature of the earth.
Various methods have been applied in estimating these periods. Joly
based his estimate on the percentage of salt in the sea and in the
rivers. If we calculate how much salt there is in the sea, and how much
salt the rivers can supply to it in the course of a year, we arrive at
the result that the quantity of salt now stored in the ocean might have
been supplied in about a hundred million years.
We arrive at still higher numbers when we calculate the time which
must have elapsed during the deposition of all the stratified and
sedimentary layers. Sir Archibald Geikie estimates the total thickness
of those strata, supposing them to have been undisturbed, at 30,000
m. (nearly 20 miles). He concludes, further, from the examination of
more recent strata, that every stratum one metre in thickness must
have required from 3000 to 20,000 years for its formation. We should,
therefore, have to allow a space of from ninety to six hundred million
years for the deposition of all the sedimentary strata. The Finnish
geologist Sederholm even fixes the time at a thousand million years.
Another method again starts from the consideration that, while the
temperature of the surface of the earth remains fairly steady owing to
the heat exchange between solar radiation and terrestrial radiation
into space, the interior of the earth must have shrunk with the
cooling. How far this shrinkage extends we may estimate from the
formation of the mountain chains which, according to Rudzki, cover
1.6 per cent. of the earth’s surface. The earth’s radius should
consequently have contracted by about 0.8 per cent., corresponding to
a cooling through about 300°, which would require two thousand million
years.
Quite recently the renowned physical chemist Rutherford has expounded
a most original method of estimating the age of minerals. Uranium and
thorium are supposed to produce helium by their slow dissociation, and
we know how much helium is produced from a certain quantity of uranium
or thorium in a year. Now Ramsay has determined the percentage of
helium in the uranium mineral fergusonite and in thorianite. Rutherford
then calculates the time which would have passed since the formation of
these minerals. He demands at least four hundred million years, “for
very probably some helium has escaped from the minerals during that
time.” Although this estimate is very uncertain, it is interesting to
find that it leads to an age for the solid earth-crust of the same
order of magnitude as the other methods.
During this whole epoch of almost inconceivable length of between one
hundred million and two thousand million years, organisms have existed
on the surface of the earth and in the sea which do not differ so very
much from those now alive. The temperature of the surface may have
been higher than it is at present; but the difference cannot be very
great, and will amount to 20° Cent. (36° F.) at the highest. The actual
mean temperature of the surface of the earth is 16° Cent. (61° F.). It
varies from about -20° Cent. (-4° F.) at the North Pole, and -10° Cent.
(+14° F.) at the South Pole to 26° Cent. (79° F.) in the tropical zone.
The main difference between the temperatures of the earth’s surface in
the most remote period from which fossils are extant and the actual
state rather seems to be that the different zones of the earth are now
characterized by unequal temperatures, while in the remote epochs the
heat was almost uniformly distributed over the whole earth.
The condition for this prolonged, almost stationary state was that the
gain of heat of the earth’s surface by radiation from the sun and the
loss of heat by radiation into space nearly balanced each other. That
the replenishing supply by radiation from an intensely hot body—in
our case the sun—is indispensable for the existence of life will be
evident to everybody. Not everybody may, however, have considered that
the loss of heat into cold space or into colder surroundings is just as
indispensable. To some people, indeed, the assumption that the earth as
well as the sun should waste the largest portions of their vital heat
as radiation into cold space appears so unsatisfactory that they prefer
to believe radiation to be confined to radiation between celestial
bodies; there is no radiation into space, in their opinion. All the
solar heat would thus benefit the planets and the moons in the solar
system, and only a vanishing portion of it would fall upon the fixed
stars, because their visual angles are so small. If that were really
correct, the temperature of the planets would rise at a rapid rate
until it became almost equal to that of the sun, and all life would
become impossible. We are therefore constrained to admit that “things
are best as they are,” although the great waste of solar heat certainly
weakens the solar energy.
The opinion that all the solar heat radiated into infinite space is
wasted, starts moreover from a hypothesis which is not proved, and
which is highly improbable—namely, that only an extremely small
portion of the sky is covered with celestial bodies. That might
certainly be correct if we assumed, as has formerly been done, that
the majority of the celestial bodies must be luminous. We do not
possess, however, any reliable knowledge of the number and size of the
dark celestial bodies. In order to account for the observed movements
of different stars, it has been thought that there must be in the
neighborhood of some of them dark stars of enormous size whose masses
would surpass the mass of our sun, or, at least, be equal to it. But
the largest number of the dark celestial bodies which hide the rays
from the stars behind them probably consist of smaller particles,
such as we observe in meteors and in comets, and to a large extent of
so-called cosmical dust. The observations of later years, by the aid
of most powerful instruments, have shown that so-called nebulæ and
nebulous stars abound throughout the heavens. In their interior we
should probably find accumulations of dark masses.
The light intensity of most of the nebulæ is, moreover, far too weak to
permit of their being perceived. We have, therefore, to imagine that
there are bodies all through infinite space, and about as numerous as
they are in the immediate neighborhood of our solar system. Thus every
ray from the sun, of whatever direction, would finally hit upon some
celestial body, and nothing would be lost of the solar radiation, nor
of the stellar radiation.
As regards the radiation-heat exchange, the earth might be likened to
a steam-engine. In order that the steam-engine shall perform useful
work, it is necessary not only that the engine be supplied with heat
of high temperature from a furnace and a boiler, but also that the
engine be able to give its heat up again to a heat reservoir of lower
temperature—a condenser or cooler. It is only by transferring heat
from a body of higher temperature to another body of lower temperature
that the engine can do work. In a similar way no work can be done on
the earth, and no life can exist, unless heat be conferred by the
intermediation of the earth from a hot body, the sun, to the colder
surroundings of universal space—_i.e._, to the cold celestial bodies
in it.
To a certain extent the temperature of the earth’s surface, as we shall
presently see, is conditional by the properties of the atmosphere
surrounding it, and particularly by the permeability of the latter for
the rays of heat.
If the earth did not possess an atmosphere, or if this atmosphere were
perfectly diathermal—_i.e._, pervious to heat radiations—we should
be able to calculate the mean temperature of the earth’s surface,
given the intensity of the solar radiation, from Stefan’s law of the
dependence of heat radiation on its temperature. Starting from the not
improbable assumption that, at a mean distance of the earth from the
sun, the solar rays would send 2.5 gramme-calories per minute to a body
of cross section of 1 sq. centimetre at right angles to the rays of the
sun, Christiansen has calculated the mean temperatures of the surfaces
of the various planets. The following table gives his figures, and
also the mean distances of the planets from the sun, in units of the
mean distance of the earth from the sun, 149.5 million km. (nearly 93
million miles):
─────────┬─────────┬───────────┬──────────┬───────────────┬──────────
│ Radius │ Mass │ Mean │ Mean │ Density
Planet ├─────────┴───────────┤ distance │ temperature │ according
│ According to See │ │ │ to See
─────────┼─────────┬───────────┼──────────┼───────────────┼──────────
Mercury │ 0.341 │ 0.0224 │ 0.39 │ + 178°(332°) │ 0.564
Venus │ 0.955 │ 0.815 │ 0.72 │ + 65° │ 0.936
Earth │ 1 │ 1 │ 1 │ + 6.5° │ 1
Moon │ 0.273 │ 0.01228│ 1 │ + 6.5°(105°)│ 0.604
Mars │ 0.53 │ 0.1077 │ 1.52 │ - 37° │ 0.729
Jupiter │ 11.13 │ 317.7 │ 5.2 │ - 147° │ 0.230
Saturn │ 9.35 │ 95.1 │ 9.55 │ - 180° │ 0.116
Uranus │ 3.35 │ 14.6 │ 19.22 │ - 207° │ 0.388
Neptune │ 3.43 │ 17.2 │ 30.12 │ - 221° │ 0.429
Sun │ 109.1 │ 332,750 │ 0 │ +6200° │ 0.256
─────────┴─────────┴───────────┴──────────┴───────────────┴──────────
In the case of Mercury, I have added another figure, 332°. Mercury
always turns the same side to the sun, and the hottest point of this
side would reach a temperature of 397°; its mean temperature, according
to my calculation, is 332°, while the other side, turned away from the
sun, cannot be at a temperature much above absolute zero, -273°. I have
made a similar calculation for the moon, which turns so slowly about
its axis (once in twenty-seven days) that the temperature on the side
illuminated by the sun remains almost as high (106°) as if the moon
were always turning the same face to the sun. The hottest point of
this surface would attain a temperature of 150°, while the poles of
the moon and that part of the other side which remains longest without
illumination can, again, not be much above absolute zero temperature.
This estimate is in fair agreement with the measurements made of the
lunar radiation and the temperature estimate based upon it. The first
measurement of this kind was made by the Earl of Rosse. He ascertained
that the moon disk as illuminated by the sun—that is to say, the full
moon—would radiate as much heat as a black body of the temperature
110° Cent. (230° F.). A later measurement by the American Very seems
to indicate that the hottest point of the moon is at about 180°, which
would be 30° higher than my estimate. In the cases of the moon and
of Mercury, which do not possess any atmosphere to speak of, this
calculation may very fairly agree with the actual state of affairs.
The temperature of the planet Venus would be about 65° Cent. (149° F.)
if its atmosphere were perfectly transparent. We know, however, that
dense clouds, probably of water drops, are floating in the atmosphere
of this planet, preventing us from seeing its land and water surfaces.
According to the determinations made by Zöllner and others, Venus
would reflect not less than 76 per cent. of the incident light of the
sun, and the planet would thus be as white as a snow-ball. The rays of
heat are not reflected to the same extent. We may estimate that the
portion of heat absorbed by the planet is about half the incident heat.
The temperature of Venus will therefore be reduced considerably, but it
is partly augmented again by the protective action of this atmosphere.
The mean temperature of Venus may, hence, not differ much from the
calculated temperature, and may amount to about 40° (104° F.). Under
these circumstances the assumption would appear plausible that a very
considerable portion of the surface of Venus, and particularly the
districts about the poles, would be favorable to organic life.
Passing to the earth, we find that the temperature-reducing influence
of the clouds must be strong. They protect about half of the earth’s
surface (52 per cent.) from solar radiation. But even with a perfectly
clear sky, not all the light from the sun really reaches the earth’s
surface; for finely distributed dust is floating even in the purest
air. I have estimated that this dust would probably absorb 17 per cent.
of the solar heat. Clouds and dust would therefore together deprive the
earth of 34 per cent. of the heat sent to it, which would lead to a
reduction of the temperature by about 28°. Dust and the water-bubbles
in the clouds also prevent the radiation of heat from the earth, so
that the total loss of heat to be charged to clouds and dust will
amount to about 20° (36° F.).
It has now been ascertained that the mean temperature of the earth is
16° (61° F.), instead of the calculated 6.5° (43.7° F.). Deducting the
20° due to the influence of dust and clouds, we obtain -14° (7° F.), and
the observed temperature would therefore be higher than the calculated
by no less than 30° (54° F.). The discrepancy is explained by the
heat-protecting action of the gases contained in the atmosphere, to
which we shall presently refer (page 51).
There are but few clouds on Mars. This planet is endowed with an
atmosphere of extreme transparency, and should therefore have a high
temperature. Instead of the temperature of -37° (35° F.), calculated,
the mean temperature seems to be +10° (+50° F.). During the winter
large white masses, evidently snow, collect on the poles of Mars, which
rapidly melt away in spring and change into water that appears dark to
us. Sometimes the snow-caps on the poles of Mars disappear entirely
during the Mars summer; this never happens on our terrestrial poles.
The mean temperature of Mars must therefore be above zero, probably
about +10°. Organic life may very probably thrive, therefore, on Mars.
It is, however, rather sanguine to jump at the conclusion that the
so-called canals of Mars prove its being inhabited by intelligent
beings. Many people regard the “canals” as optical illusions; Lowell’s
photographs, however, do not justify this opinion.
As regards the other large planets, the temperatures which we have
calculated for them are very low. This calculation is, however, rather
illusory, because these planets probably do not possess any solid or
liquid surface, but consist altogether of gases. Their densities, at
least, point in this direction. In the case of the inner planets,
Mars and our moon included, the density is rather less than that of
the earth. Mercury stands last among them, with its specific gravity
of 0.564. There follows a great drop in the specific gravities of
the outer large planets. Saturn, with a density of 0.116, is last in
this order; the densities of the two outermost planets lie somewhat
higher—by 0.3 or 0.4 about—but these last data are very uncertain.
Yet these figures are of the same order of magnitude as that assumed
for the sun—0.25—and we believe that the sun, apart from the small
clouds, is wholly a gaseous body. It is therefore probable that
the outer planets, including Jupiter, will also be gaseous and be
surrounded by dense veils of clouds which prevent our looking down
into their interior. That view would contend against the idea that
these planets can harbor any living beings. We could rather imagine
their moons to be inhabited. If these moons received no heat from their
planets, they would assume the above-stated temperatures of their
central bodies. Looked at from our moon, the earth appears under a
visual angle, 3.7 times as large as that of the sun. As the temperature
of the sun has, from its radiation, been estimated at 6200° Cent., or
6500° absolute, the moon would receive as much heat from the earth as
from the sun, if the earth had a temperature of about 3100° Cent., or
3380° absolute. When the first clouds of water vapor were being formed
in the terrestrial atmosphere, the earth’s temperature was about 360°,
and the radiation from the earth to the moon only about 1.25-thousandth
of that of the sun. The present radiation from the earth does not
even attain one-twentieth of this value. It is thus manifest that
the radiation from the earth does not play any part in the thermal
household of the moon.
The relations would be quite different if the earth had the 11.6 times
greater diameter of Jupiter, or the diameter of Saturn, which is
9.3 times greater than its own. The radiation from the earth to the
moon would then make up about a sixth or a ninth of the actual solar
radiation, taking the temperature of the earth’s surface at 360°. We
can easily calculate, further, that Jupiter and Saturn would radiate
as much heat against a moon at a distance of 240,000 or 191,000 km.
respectively (since the distance of the moon from the earth amounts
to 384,000 km.) as the sun sends to Mars—taking the temperature of
those planets at 360° Cent. Now we find, near Jupiter as well as near
Saturn, moons at the distances of 126,000 and 186,000 km. respectively,
which are smaller than those mentioned, and it is not inconceivable
that these moons receive from their central bodies sufficient heat
to render life possible, provided that they be enveloped by a
heat-absorbing atmosphere. The conditions appear to be less favorable
for the innermost satellites of Jupiter and Saturn. When their planets
are shining at the maximum brilliancy, their light intensity is only
a sixth or a ninth of the solar light intensity, which upon these
satellites is itself only one-twenty-seventh or one-ninetieth of the
intensity on the earth. During the incandescence epoch of these planets
their moons will certainly for some time have been suitable for the
development of life.
That the atmospheric envelopes limit the heat losses from the planets
had been suggested about 1800 by the great French physicist Fourier.
His ideas were further developed afterwards by Pouillet and Tyndall.
Their theory has been styled the hot-house theory, because they thought
that the atmosphere acted after the manner of the glass panes of
hot-houses. Glass possesses the property of being transparent to heat
rays of small wave lengths belonging to the visible spectrum; but it is
not transparent to dark heat rays, such, for instance, as are sent out
by a heated furnace or by a hot lump of earth. The heat rays of the sun
now are to a large extent of the visible, bright kind. They penetrate
through the glass of the hot-house and heat the earth under the glass.
The radiation from the earth, on the other hand, is dark and cannot
pass back through the glass, which thus stops any losses of heat, just
as an overcoat protects the body against too strong a loss of heat by
radiation. Langley made an experiment with a box, which he packed with
cotton-wool to reduce loss by radiation, and which he provided, on the
side turned towards the sun, with a double glass pane. He observed
that the temperature rose to 113° (235° F.), while the thermometer
only marked 14° or 15° (57° or 59° F.) in the shade. This experiment
was conducted on Pike’s Peak, in Colorado, at an altitude of 4200 m.
(13,800 ft.), on September 9, 1881, at 1 hr. 4 min. P.M., and therefore
at a particularly intense solar radiation.
Fourier and Pouillet now thought that the atmosphere of our earth
should be endowed with properties resembling those of glass, as regards
permeability of heat. Tyndall later proved this assumption to be
correct. The chief invisible constituents of the air which participate
in this effect are water vapor, which is always found in a certain
quantity in the air, and carbonic acid, also ozone and hydrocarbons.
These latter occur in such small quantities that no allowance has been
made for them so far in the calculations. Of late, however, we have
been supplied with very careful observations on the permeability to
heat of carbonic acid and of water vapor. With the help of these data
I have calculated that if the atmosphere were deprived of all its
carbonic acid—of which it contains only 0.03 per cent. by volume—the
temperature of the earth’s surface would fall by about 21°. This
lowering of the temperature would diminish the amount of water vapor
in the atmosphere, and would cause a further almost equally strong
fall of temperature. The examples, so far as they go, demonstrate that
comparatively unimportant variations in the composition of the air
have a very great influence. If the quantity of carbonic acid in the
air should sink to one-half its present percentage, the temperature
would fall by about 4°; a diminution to one-quarter would reduce the
temperature by 8°. On the other hand, any doubling of the percentage of
carbon dioxide in the air would raise the temperature of the earth’s
surface by 4°; and if the carbon dioxide were increased fourfold, the
temperature would rise by 8°. Further, a diminution of the carbonic
acid percentage would accentuate the temperature differences between
the different portions of the earth, while an increase in this
percentage would tend to equalize the temperature.
The question, however, is whether any such temperature fluctuations
have really been observed on the surface of the earth. The geologists
would answer: yes. Our historical era was preceded by a period in which
the mean temperature was by 2° (3.6 F.) higher than at present. We
recognize this from the former distribution of the ordinary hazel-nut
and of the water-nut (_Trapa natans_). Fossil nuts of these two species
have been found in localities where the plants could not thrive in
the present climate. This age, again, was preceded by an age which,
we are pretty certain, drove the inhabitants of northern Europe from
their old abodes. The glacial age must have been divided into several
periods, alternating with intervals of milder climates, the so-called
inter-glacial periods. The space of time which is characterized by
these glacial periods, when the temperature—according to measurements
based upon the study of the spreading of glaciers in the Alps—must
have been about 5° (8° F.) lower than now, has been estimated by
geologists at not less than 100,000 years. This epoch was preceded by
a warmer age, in which the temperature, to judge from fossilized plants
of those days, must at times have been by 8° or 9° (14° or 16° F.)
higher than at present, and, moreover, much more uniformly distributed
over the whole earth (Eocene). Pronounced fluctuations of this kind in
the climate have also occurred in former geological periods.