ATMOSPHERE AND PHYSICS OF THE STELLAR BODIES
In a certain sense, we are justified in speaking of an atmosphere of
suns and stars. These bodies consist mainly of a comparatively dense
mass surrounded by a layer of very attenuated gas. The density of our
Sun is about 1.4 times that of water. In other stars it is considerably
lower, in some cases only a few hundredths of the density of water.
This applies particularly to those stars of variable magnitude, the
Cepheid type, named after their longest and best known representative,
the mysterious star Delta in the constellation Cepheus, and in general
to young stars. In any case the stars are gaseous throughout on account
of their high temperature. An exception must be made for the clouds
of matter precipitated by easily condensed vapours, such as gaseous
carbon, which clouds float in the outer strata, and are responsible for
the bright astral light.
The stars just mentioned belong to the comparatively young stellar
bodies, while the Sun in common with other yellow stars is considerably
older. Correlated with their age is undoubtedly the greater mean
density of the yellow stars. Around many of the young stars, for
instance around the brilliant Altair, the principal member of the
constellation Aquila, a gas shell of great expansion has been observed,
usually consisting of hydrogen but frequently also of helium. These
extensive gas appendages may be considered as a kind of atmosphere
surrounding the stars in question. Their density is no doubt
exceedingly small. Our own central orb, the Sun, is also endowed with
rarefied gases outside of the luminous clouds. By their absorption of
light they cause the dark lines in the Sun’s spectrum, after their
discoverer named Fraunhofer lines. The greatest height from the
surface of the Sun is reached by hydrogen, mixed with a small quantity
of helium and with a gas, unknown on Earth, which we call coronium
because it has been observed in the Sun’s corona. These gases may be
looked upon as the atmosphere of the Sun.
Similar conditions obtain no doubt on the major planets, which possess
a density not essentially different from that of the Sun. They have,
moreover, practically the same period of revolution around their own
axis, Jupiter 9.9 hours, Saturn 10.3, and Uranus (probably) 10.8 hours.
Judging by their density, they are, in all probability, like the Sun,
gaseous throughout except for the heavy cloud formations which appear
to constitute the outer limit of these stellar bodies. Their interior,
like that of the Sun, may contain comparatively sluggish gas-masses,
as certain peculiar patches appear on their exterior, similar to the
sunspots and persisting for long intervals, sometimes over a year. The
best known example of the kind is the so-called red spot on Jupiter,
which has remained since 1878, although it is not so pronounced now as
during its early days (see Fig. 11). Characteristic for these planets
are certain bands of a marked delineation and running parallel with the
equator (see Figs. 11 and 12). They are caused by the rapid peripheral
motion of these planets, with Jupiter 28 and with Saturn 24 times that
of the Earth.
[Illustration:
Fig. 11. The planet Jupiter in 1909 in Mercator’s projection by
F. le Coultre of Geneva. The “red spot” over which the
clouds are dispersed, is found at 355° Long. and 20° S.
Lat. in the outcurving of the dark band. South direction
is upward, as on all pictures obtained with astronomical
tubes.
]
Which gases should we expect to find in the atmosphere of these
planets? According to the Kant-Laplace hypothesis, a theory generally
credited with a sound kernel, the planets were segregated from the
Sun’s substance at a time when the latter was expanded so as to include
the orbits of these planets and beyond. Naturally, therefore, their
atmospheres would originally be composed of the very gases that formed
the outmost part of the Sun’s atmosphere, notably hydrogen. Slipher,
who has photographed the spectra here reproduced of the outer planets,
believes that certain strong absorption bands in the spectra of
Neptune and Uranus correspond to the distinctive F and C lines of
hydrogen, using Fraunhofer’s denotation (see Fig. 13). But because
the bands in question, as shown on the figure, are very broad it is
difficult to identify them with certainty. Also other gases of unknown
origin enter into the vapour envelopes outside of the clouds and cause,
as apparent from their spectra, a strong absorption of the sunlight
reflected from the clouds below. The absorption increases with the
planet’s distance from the Sun; thus, it is most pronounced on Neptune
and least on Jupiter.
[Illustration: Fig. 12. The appearance of Saturn September 30, 1909,
according to F. le Coultre of Geneva.]
At any rate, the gas appendages of the heavenly bodies just considered
differ in one essential respect from the atmospheres of the inner
planets: Mars, Earth, Venus, and Mercury. On the Sun and on the outer
planets the atmosphere gradually merges into the interior gas-masses so
that no distinct boundary can be found between the rarer and the denser
layers. Widely different conditions obtain on the Earth. Here the range
of air is sharply defined below by the Earth’s solid crust or by the
oceans. In such case alone may we speak of an atmosphere proper, of
the kind that enters into our commonplace conceptions. Similar are the
conditions on all stellar bodies with a solid or a liquid surface.
[Illustration:
Fig. 13. Spectra of the major planets, compared with that of
the moon. The latter corresponds to the spectrum
of sunlight reflected from a planet which lacks a
light-absorbing atmosphere. Photos by V. M. Slipher of the
Lowell Observatory.
]
But it is not certain in every case that all such planets possess an
atmosphere. Observations of the Moon when passing some star show that
the air envelope, if present, is unable to deflect the light beam from
the star, or in other words it has no perceptible power of refraction.
From this we also infer that its density is very small, corresponding
at most to one or two mm. (.04 to .08 inches) barometric pressure. But
we have good reasons to believe that the Moon has been detached from
the Earth, carrying away parts of its lightest substance, which theory
is supported by the fact that the Moon’s mean density (3.3) is only
six-tenths of the Earth’s (which again is 5.53 times that of water),
and we might therefore have expected that the Moon in parting
should have shared in the very lightest constituents of the Earth,
namely its air-covering. Such was unquestionably the actual procedure,
but in the course of time the Moon has lost its originally no doubt
considerable atmosphere. The reason is that the molecules in a gas are
in a continuous rapid motion, which is the swifter the lighter the gas
and the higher its temperature. In hydrogen, the lightest gas known,
the velocity amounts to 1.84 km. (1.15 miles) per second at 0° C.
(32° F.). The parts of the Moon exposed to the strongest sunlight are
heated to about 150° C. (300° F.). At that temperature, the average
velocity of hydrogen molecules is 2.29 km. (1.43 miles) per second.
But a body departing from the surface of the Moon at a rate of 2 km.
(1.24 miles) per second, or more, cannot be retained by the attraction
of that globe and therefore never retraces its path but speeds away
for ever. In the same manner a bullet ejected from a cannon with an
initial velocity of 11.2 km. (7 miles)--a velocity not even approached
by present artillery--would fly away from Earth barring the resistance
of the air. Thus we see that we as yet are far from the realization of
the dreams of Jules Verne in his _A Voyage to the Moon_. At any rate,
gravity on the Moon is too weak to retain hydrogen over the hottest
point of the surface. This part of the gas flies away, new supplies
rush in from the sides, and in a short time all traces of hydrogen have
disappeared from the Moon. Probably it was mainly gathered in by the
Sun, where a velocity of 613 km. (380 miles) per second is necessary if
the molecules are to overcome the Sun’s attraction, while their actual
velocity there amounts to only about 8 km. (5 miles) per second.
In a similar manner we find that the second lightest gas, helium, at
a temperature of 150° C. (300° F.), possesses a molecular velocity of
1.62 km. (1.1 miles) per second. This is less than the 2 km. (1.24
miles) per second necessary to leave the Moon’s sphere of attraction.
But all helium molecules do not move at the same speed; some are
faster and some slower than the average. Those moving at a higher rate
than 2 km. (1.24 miles) per second constitute a considerable fraction
of the total. This fraction disappears. Equilibrium is soon restored
so that in less than a second the same fraction of helium molecules is
ready to depart. In this manner the Moon lost its helium atmosphere
speedily, although not quite as rapidly as its hydrogen.
More slowly yet vanished the gases which are most abundant in our
atmosphere, nitrogen and oxygen, but these too were not fettered
for ever by the limited gravity on the Moon. The same fate befell
aqueous vapour, which is nearly twice as light as oxygen. The loss of
water, however, was long delayed, as we later shall learn, because
new vapour masses were discharged from the lunar volcanoes. In these
considerations, we should also bear in mind that the Moon no doubt was
a fluid molten mass when separating from the Earth and its substance
resembled the lava from our volcanoes. In this condition it remained
until its exterior temperature had fallen to about 1200° C. (2200° F.).
At that point, the average velocity of oxygen molecules is about 1 km.
(.62 mile) per second, with variations in both directions, so that a
few per cent. of them reach a sufficient velocity to leave the Moon for
ever. Such gas molecules of medium weight return probably to the Earth
which, as experience tells us, is ponderous enough to hold them in
bonds.
All gases, that constitute any considerable fraction of the Earth’s
atmosphere, and which, therefore, most likely were divided with the
Moon in its parting from us, have again left that globe. The same
unquestionably holds true for other stellar bodies of equal or smaller
size, such as all the minor planets and for the great majority of the
satellites to the major ones. Only the very largest of Jupiter’s moons,
and possibly Neptune’s lonely companion, whose size is not known with
certainty, might possibly surpass our Moon in ability to retain gases.
Our reasoning with respect to the Moon applies also to Mercury. It is
true that the molecules there must possess a velocity one and a half
times as high as on the Moon if they are to leave the planet. But at
the same time the temperature on Mercury’s hottest point, always turned
toward the Sun, is far higher, about 400° C. (750° F.), so that the
molecules there move 1.26 times as fast as similar molecules over the
Moon’s hottest point. Mercury is consequently better able than the Moon
to retain gases, but the difference is slight. Direct observations (see
below) also lead us to believe that Mercury is very similar to the
Moon in these respects. We might possibly imagine that certain gases,
which on the Moon would condense into fluids or solids, on Mercury
might remain volatilized on account of the high temperature and thus
form an atmosphere. Such assumption, however, would be erroneous. The
investigations by Schiaparelli and by all his successors show that
Mercury in turning around its axis always presents the selfsame side
to the Sun. The opposite side, never reached by a ray of sunlight,
must assume an extremely low temperature, very close to the absolute
zero (-273.7° C. or -460.6° F.) and far below any cold existing on the
Moon. To this side, all bodies with an appreciable vapour pressure
must distil and freeze to solid lumps or frost-coverings without
perceptible vapour pressure. For these reasons, Mercury cannot possess
any atmosphere to speak of. There remain in the whole series of planets
and satellites in our solar system only two bodies besides the Earth
which are endowed with an atmosphere in the original sense of the
word--namely, Mars and Venus.
We reach the same conclusion when investigating the ability of the
planets to reflect the sunlight falling upon them. The bodies which
possess an atmosphere hold also suspended therein clouds of water or
ice, and also of dust, whirled up from below. These floating particles
reflect light far more efficiently than the solid or fluid surface of
a planet. The Moon can now reflect 7.3 per cent. of the sunlight and
Mercury 6.9 per cent. (H. N. Russell, _Proceedings Nat. Acad._, 1916).
These numbers lie so close that they may be considered practically the
same within errors of observation.
It is therefore probable that Mercury is as devoid of an atmosphere as
the Moon. The opposite extreme is represented by Venus, which reflects
not less than 59 per cent. of the sunlight received, according to
H. N. Russell. Terrestrial clouds were found by Abbot to return 65
per cent. We believe from astronomical observations that the entire
surface of Venus is hidden behind a thick opaque cloud-covering.
The slight difference between O.65 and O.59 may be due to errors of
observation, but also to a small absorption of light in those parts of
Venus’s atmosphere which are outside of the clouds. Saturn and Jupiter
are very similar to Venus in this regard with 63 and 56 per cent.
respectively. The gases above the clouds on these planets extinguish
to a considerable extent the sunlight reflected from the clouds, as
apparent from their spectra. (Compare Fig. 13.) Hence the value 0.63
given by Russell for Saturn is probably too high. Concerning Jupiter
it has been observed that its red light becomes deeper when the
sunspots are few, but whiter when the spots are numerous. The sunspots
have been found to favour the formation of high clouds, such as cirrus,
and this would seem to apply also to Jupiter; when spots are plenty,
the clouds are high, and consequently the absorbing layers above, which
cause the red colour, are thinner, so that Jupiter will then shine with
a whiter--less red--lustre than when the sunspots are rare.
The two outmost planets, Uranus and Neptune, return, according to
Russell, 63 and 73 per cent. respectively of the sunlight received.
These figures are probably too high. They do not agree well with
Slipher’s records of their spectra (Fig. 12).
There now remains Mars. This planet approaches the Moon inasmuch as
it reflects only 15.4 per cent. of the sunlight arriving to the orb.
Everything points to the conclusion that the atmosphere of Mars is very
thin. Lowell estimates, on somewhat meagre grounds, however, that on
each square metre of the planet rests only 22 per cent. of the mass of
air supported by each square metre of the Earth’s surface.
It would naturally be very interesting to ascertain the amount of
sunlight our Earth throws back into space. This we cannot measure, as
we cannot place our instruments outside of the Earth’s cloud-mists
nor can we read them there. Not less than 52 per cent. of the Earth
is covered with clouds, whose whiteness (Latin: _Albedo_) is 65. Thus
the clouds alone return 0.52 × 0.65 = 0.338 parts of the sunlight. Of
this portion a fraction amounting to about 4 per cent. is extinguished
in the air above. The remainder is 0.325. Atmosphere and suspended
dust reduce the sunlight over the cloud-free part, _i. e._ 48 per
cent. of the Earth, by 60 per cent, half of which returns to space,
while the other half reaches the ground in the form of light from the
sky, and of this fraction again about 4 per cent. is reflected into
space; these two items added give 0.15. Finally, the 40 per cent.
sunlight directly received by the Earth’s surface is reflected to the
extent of 6 per cent. by the oceans and by the generally moist ground;
deserts and bare rocks reflect about twice as much, but their total
area is comparatively small; of this 6 per cent. reflected light, 70
per cent. reaches outside space; thus we obtain 0.48 × 0.40 × 0.06 ×
0.70 = 0.008. In all, therefore, the amount of reflected sunlight is
0.338 + 0.15 + 0.008 = 49.6 per cent. If the air were free from clouds,
the reflexion-number or Albedo would be 33 per cent., or considerably
higher than that of Mars. When now half or a little more (52%) of the
Earth’s surface is overcast with clouds and this portion therefore has
the whiteness of Venus, the figure 49.6 (Russell calculates the figure
45) for the entire Earth naturally falls closer--almost 3.6 times--to
59, the figure of Venus, than to 15.4, the figure for Mars. We may
also compare the value 33 per cent., which applies to the cloud-free
portion of the Earth, with the value 15.4 per cent. for Mars, which is
almost without clouds, and with the value 7.3 per cent. for the Moon,
which has neither clouds nor dust, because it lacks an atmosphere. We
can then conclude that the atmosphere of our Earth holds almost three
times as much dust suspended over each square metre as does Mars, and
this in spite of the smaller gravitational force on Mars, which is
about 37.5 per cent. of that on Earth. Taking proper account of the
low temperature on Mars we may easily compute, by means of a formula
given by Stokes, that a particle of dust should sink 2.3 times slower
on Mars than on Earth. When, nevertheless and in spite of frequent
but thin mists, so few particles of dust float in the atmosphere of
Mars, the conception inevitably comes to our mind that the air on
that planet must be extremely rarefied so that the wind-puffs have
little power to raise the dust from the ground. Lowell estimated the
barometric pressure at the surface of Mars to be about 64 mm. (2.52
inches), and Proctor gives about twice this figure. There appears to
be ground for considering already the former value too high; both are
very uncertain. If we accept that of Lowell, we find that each square
metre of the surface of Mars supports a column of air, whose mass is
only about one-fifth of the mass resting on each square metre of ocean
surface on Earth.
[Illustration:
Fig. 14. The planet Venus, with sunlit atmosphere (to the
left), as observed by Langley at the Venus passage
December 6, 1882.
]
The dense clouds which float above Venus have long ago led to the
assumption that the atmosphere of that planet must be far deeper than
that of the Earth. Its strong refractory power has also contributed
to this belief. When Venus is close to the sun-disc the dark body
stands forth surrounded by a ring of light (see Fig. 14). It is,
however, recognized that this phenomenon requires no greater air
density than that on Earth for its appearance. In this connection we
should remember that the inside limit of the vapour shell which we
in this manner observe, is the cloud-wall, not the ground. And these
clouds, we have every reason to believe, float on account of the heat
prevailing at a great height in the atmosphere, so high in fact that
they form an impenetrable wall already where the cirrus clouds
appear in our sky. If these suppositions are correct, the light-ring
mentioned is caused by a quarter only of the air-masses on Venus, and
its total air-covering must be far deeper than that of the Earth.
The latter occupies probably in this respect as well as in reference
to position in space a middle ground between Mars with its extremely
thin and Venus with its comparatively dense atmosphere. If so, we
might expect the atmosphere on Mercury to be denser yet, while we
already have seen that it is almost wholly lacking on that planet.
The explanation is that Mercury has lost its spin around its own axis
and therefore always presents the same side to the Sun--just as the
Moon and probably all other satellites turn one side only toward their
respective central bodies--hence the opposite side becomes so cold
that all gases are there condensed to fluids or solids except the two
most volatile ones, hydrogen and helium, which on the other hand leave
the planet on its hot side. If Venus, therefore, as held by several
astronomers from Schiaparelli to Lowell, always turned one side only
toward the Sun, this planet also would be without any perceptible
air-covering. According to investigations by Bjelopolsky of Venus’s
spectrum, which investigations, however, are in complete disagreement
with corresponding measurements by Slipher, that planet has a period
of rotation on its own axis of about 29 hours. This figure is very
uncertain and a new determination is therefore highly desirable.
In order to understand the atmospheres of the planets, it is of great
interest to ascertain the composition of the air that surrounds the
Earth. Our knowledge in these matters has grown considerably of late.
We shall in the main follow the presentation by Dr. Wegener of Marburg.
We know at present with considerable accuracy which gases enter into
the air. Besides the previously well-known nitrogen and oxygen which
contribute the bulk, 78.1 and 20.9 per cent. respectively, of the total
volume at the earth’s surface, we find water vapour in proportions
changing with localities and times, and it is for this reason left
out when fixing the various percentages; further, carbon dioxide 0.03
volume per cent. and the rare gases discovered by Rayleigh and Ramsay,
argon, 0.932 per cent., neon 0.0012 per cent., helium 0.0004 per cent.,
krypton 0.000005 per cent., and xenon 0.0000006 per cent. Each one of
these constituents diminishes in quantity with height in accordance
with the so-called barometer-formula and the rate is the more rapid
the heavier the gas. Krypton and xenon, therefore, which are two and
one half and four times heavier than oxygen, occur mainly in the lower
strata. The percentage of helium on the other hand, a gas eight times
lighter than oxygen, should increase rapidly with height. If the air
consisted of a mixture of oxygen and helium at 0° C. (32° F.) the
former would decrease to one half at an elevation of 5 km. (3.1 miles)
but the latter not before we had ascended 40 km. (25 miles) (eight
times higher than for oxygen, as the weights are in the ratio of 1 to
8). At that altitude, oxygen would have decreased in the proportion
1:2^8 = 1:256. When, as actually is the case, there is 50,000 times
more oxygen than helium at the surface this ratio should decrease
in the proportion 128:1 at a height of 40 km. (25 miles). Ninety
kilometers (56 miles) above the surface helium should overbalance
oxygen and thereafter rapidly gain in preponderance. This holds true
provided no agitation takes place in the form of vertical currents of
air.
Similar laws apply to all light gases which do not turn into fluids or
solids at low temperatures. Aqueous vapour, on the other hand, which
when cooled condenses to clouds, diminishes much faster than the nearly
twice as heavy oxygen, because the temperature rapidly decreases as we
move upward or at a rate of about 5° C. per km. (14.5° F. per mile)
up to 2.5 km. (1.5 miles) and of 8° C. per km. (23° F. per mile) at a
height of 8.5 km. (5.3 miles). The quantity of water vapour shrinks to
one half at 1.9 km. (slightly more than a mile) above ground. Carbon
dioxide again follows the barometer-formula applicable to other gases
because it occurs in such minute quantity that it never condenses to
clouds. In fact, it is water vapour alone which must be treated as an
exception. Carbon dioxide is nearly one and one half times heavier than
the other gases of the atmosphere on an average. It should therefore
diminish in the proportion 1:2^{1.5} = 1:2.8 in a vertical distance of
5 km. (3.1 miles) while the density of the air decreases only in the
ratio 1:2. Several determinations of the presence of carbon dioxide
in the atmosphere as high up as 3.8 km. (2.33 miles) have been made,
by S. A. Andree among others, but the percentage of this gas remains
constant within the errors of observation. The same holds true to a
height of 7 km. (4.35 miles) for the proportion between oxygen and
nitrogen, although we might have expected a perceptible change as
oxygen is 14 per cent. heavier than nitrogen. How shall we explain this
fact which seemingly contradicts the theory just advanced?
The explanation is quite simple. The preceding statements hold true for
a mass of air at perfect rest. But, if the air is violently agitated,
the composition becomes homogeneous all through. We know that in the
barometric cyclones and anticyclones strong rising and descending air
currents flow. The composition of the atmosphere, therefore, becomes
the same as far up as this mixing action prevails. These currents
produce another effect, namely, a fall of temperature with rising
height. Because when a gas is transported upward the surrounding
pressure decreases, resulting in expansion and consequent cooling. It
is well known that a gas is heated when (rapidly) compressed, a quality
formerly made use of in the pneumatic fire-tool to ignite tinder. It is
evident that conversely a gas must cool off when expanding. If now the
mixing of the air were extremely rapid the thermometer would fall very
close to 10° C. (18° F.) with each km. (.62 miles) rise in elevation.
If, on the other hand, the air stood perfectly still in a vertical
direction, the temperature would remain constant at all heights
over the same point. Between these two extremes, we find the actual
condition, inasmuch as the temperature of the atmosphere decreases
upward 5° to 8° C. per km. (14.5° to 23° F. per mile) as observed
during balloon ascensions.
This applies to the so-called “troposphere”--mixing-zone. One of the
most remarkable discoveries in recent times, made by Teisserenc de Bort
and Assman, is the fact that the decrease of temperature with height
does not continue indefinitely but only up to a certain elevation,--in
middle Europe about 11 km. (7 miles), in Lapland about 7 km. (4.5
miles), and at the equator about 15 km. (10.5 miles)--and above this
point the temperature remains constant. We now meet the peculiar
condition that the temperature of this upper layer, which is called
stratosphere--“film-zone”--is lowest over the equator, because it
commences at a great height there, and lowest over the polar regions,
where it extends farther down. The stratosphere has received its
name from the fact that it consists so to speak of lamellæ almost
parallel to the earth’s surface and moving in a horizontal direction
while vertical motions are absent. The winds in these strata have a
marked westerly direction (_i. e._ they are east winds) and they become
stronger the higher the stratum--at an altitude of 83 km. (52.5 miles)
their velocity is about 100 m. (330 ft.) per second. In the troposphere
on the other hand west winds predominate. The wind direction in the
stratosphere was observed on the so-called luminous night-clouds which
were found as high as 80 km. (50 miles) above the Earth. These strata
consequently revolve slower around the earth’s axis than the solid
body of the planet itself. At an elevation of 80 km. (50 miles) the
rotational speed has decreased to 65 per cent. of the angular velocity
of the earth’s surface. We have reason to believe that the very highest
strata stand still, that is do not take part in the earth’s rotation on
its axis. This would follow if outside space were not entirely devoid
of vapour so that our atmosphere would merge imperceptibly into the
exceedingly attenuated gas masses of interplanetary space.
As high as the mixing-zone extends, so high also is the composition of
the air constant and like that at the surface of the earth. But above
this limit--in Scandinavia, we might say above an elevation of 10 km.
(6.2 miles)--commences a rapid tapering of the heavy gases, while the
percentage of the light ones correspondingly rises. Foremost among the
latter is hydrogen, with only half the weight of helium. The presence
of hydrogen in the atmosphere has been shown by Boussingault, and
the proportion in which it occurs has later been measured by Armand
Gautier. It is about one three hundredth part of one per cent. It
increases extremely rapidly with height in the stratosphere so that 80
km. (50 miles) above the earth and upward hydrogen is more abundant
than all other known gases of the atmosphere at the same altitudes.
We reproduce below a somewhat revised table by Dr. Wegener of Marburg,
who has made the most recent computation of the percentages of the
various constituents of air at different heights. Consideration has
been given to the fact that the composition of the air does not change
except as regards the percentage of moisture within the troposphere,
which is assumed to reach a height of 10 km. (6.2 miles). As usual in
similar cases the percentages refer to volume.
-------------+-----------------+----------+--------+----------
_Height_ | _Pressure_ | | |
-----+-------+--------+--------+ | |
_in | _in | _in | _in |_Hydrogen |_Helium |_Nitrogen
km._| miles_| mm._ | inches_| 2_ | 4_ | 28_
-----+-------+--------+--------+----------+--------+----------
0 | 0 |760 |29.9 | 0.0033 | 0.0005 | 78.1
-----+-------+--------+--------+----------+--------+----------
10 | 6.2 |197 | 7.75 | 0.0033 | 0.0005 | 78.1
-----+-------+--------+--------+----------+--------+----------
30 | 18.6 | 8.95 | .352 | ---- | ---- | 85
-----+-------+--------+--------+----------+--------+----------
50 | 31.0 | 0.45 | .0177 | 1 | ---- | 88
-----+-------+--------+--------+----------+--------+----------
70 | 43.5 | 0.045 | .00177| 13 | 1 | 80
-----+-------+--------+--------+----------+--------+----------
90 | 55.8 | 0.0157| .00062| 68 | 5 | 26
-----+-------+--------+--------+----------+--------+----------
110 | 68.2 | 0.0116| .00046| 94 | 5 | 1
-----+-------+--------+--------+----------+--------+----------
130 | 80.6 | 0.0097| .00038| 96 | 4 | 0
-----+-------+--------+--------+----------+--------+----------
210 | 130.2 | 0.0055| .00022| 99 | 1 | ----
-----+-------+--------+--------+----------+--------+----------
310 | 192.6 | 0.0032| .00013| 100 | ---- | ----
-----+-------+--------+--------+----------+--------+----------
410 | 254.2 | 0.0021| .00008| 100 | ---- | ----
-----+-------+--------+--------+----------+--------+----------
510 | 316.2 | 0.0016| .00006| 100 | ---- | ----
-----+-------+--------+--------+----------+--------+----------
-------------+-----------------+--------+-------+---------+-------
_Height_ | _Pressure_ | | | |
-----+-------+--------+--------+ | |_Carbon |
_in | _in | _in | _in |_Oxygen |_Argon | Dioxide |_Water
km._| miles_| mm._ | inches_| 32_ | 39.9_ | 44_ | 18_
-----+-------+--------+--------+--------+-------+---------+-------
0 | 0 |760 |29.9 | 20.9 | 0.937 | 0.03 | 1.41
-----+-------+--------+--------+--------+-------+---------+-------
10 | 6.2 |197 | 7.75 | 20.9 | 0.937 | 0.03 | 0.14
-----+-------+--------+--------+--------+-------+---------+-------
30 | 18.6 | 8.95 | .352 | 15 | 0.29 | 0.0064 | 0.5
-----+-------+--------+--------+--------+-------+---------+-------
50 | 31.0 | 0.45 | .0177 | 10 | 0.10 | 0.0014 | 1.7
-----+-------+--------+--------+--------+-------+---------+-------
70 | 43.5 | 0.045 | .00177| 6 | 0.05 | 0.0005 | ----
-----+-------+--------+--------+--------+-------+---------+-------
90 | 55.8 | 0.0157| .00062| 1 | ---- | ---- | ----
-----+-------+--------+--------+--------+-------+---------+-------
110 | 68.2 | 0.0116| .00046| 0 | ---- | ---- | ----
-----+-------+--------+--------+--------+-------+---------+-------
130 | 80.6 | 0.0097| .00038| ---- | ---- | ---- | ----
-----+-------+--------+--------+--------+-------+---------+-------
210 | 130.2 | 0.0055| .00022| ---- | ---- | ---- | ----
-----+-------+--------+--------+--------+-------+---------+-------
310 | 192.6 | 0.0032| .00013| ---- | ---- | ---- | ----
-----+-------+--------+--------+--------+-------+---------+-------
410 | 254.2 | 0.0021| .00008| ---- | ---- | ---- | ----
-----+-------+--------+--------+--------+-------+---------+-------
510 | 316.2 | 0.0016| .00006| ---- | ---- | ---- | ----
-----+-------+--------+--------+--------+-------+---------+-------
Under the name of each gas its molecular weight is given as a measure
of corresponding specific weight. The quantity of water vapour was
not included when the percentages of the other gases were calculated,
because it changes considerably with locality and time. The number
given in the table for water is the mean for the entire globe--it
corresponds to 11.4 grammes per cubic metre (.31 oz. per cu. yd.)--or
the amount present in air saturated with moisture at 16.5° C.
(61.7° F.). The bulk of the water vapour forms a layer strongly
concentrated toward the surface of the earth. Carbon dioxide also
tapers rapidly with increasing height because its density is 1.5
times greater than that of air. This is apparent from the molecular
weight 44 stated under carbon dioxide, while the average molecular
weight of air is 29. Faster yet do krypton, with a molecular weight of
83, and xenon, with a molecular weight of 131, decrease as we ascend
in the atmosphere. These gases, like neon, whose percentage first
increases slightly with height, and argon, which decreases upward as
shown in the table, do not perceptibly influence the processes of
nature. The reverse is true about water vapour and carbon dioxide,
which nourish the plants and also protect the Earth against a too
rapid heat radiation into space. We well remember how abruptly the
temperature changes in the course of the day in the dry desert
climate, while corresponding variation is comparatively slight in
humid climates (compare page 86). This is the result of the ability of
water vapour to arrest the radiation from the Earth. Carbon dioxide is
about evenly distributed over the globe--although somewhat sparser
over highlands--and its heat-conserving and equalizing influence is,
therefore, not so manifest as that of moisture. Only by the most
accurate investigations has this influence been demonstrated.
In Wegener’s table a gas is included, called Geocoronium, whose
existence in the air has not been directly proved. Conspicuous is,
however, the green light displayed at great altitude by the Northern
Light arches, a green color which does not, as far as we are aware,
belong to any known constituent of the air. It is true that the
corresponding spectral line (557 µµ) lies very close to a line
belonging to krypton, but the latter is a heavy gas which cannot occur
to any traceable extent in the high strata, more than 300 km. (186
miles) above the earth, where occasionally the Northern Light arches
appear--their favoured height according to measurements by Störmer
is about 120 km. (75 miles). Wegener assumes, therefore, that this
green line belongs to an hitherto unknown substance, Geocoronium,
which should be five times lighter than hydrogen. Recent researches
present great difficulties to the acceptance of this assumption, and
for this reason further discussion of the problem will be omitted.
Above a height of 210 km. (130 miles), this gas, according to Wegener,
would preponderate. If such postulated gas does not exist, hydrogen
completely dominates in these regions and down to 85 km. (53 miles)
above the Earth. Because hydrogen is so light, the density of the
air in the range of a barometric pressure below 0.02 mm. (.0008
inches) increases but slowly as we descend toward the Earth. This
uppermost part of the atmosphere may appropriately be designated as
the hydrogen-zone. Even within this range, the “shooting stars” meet
sufficient resistance to flame into light at a height of about 120
km. (75 miles) and dissolve into dust which turns dark about 85 km.
(53 miles) above the Earth. E. C. Pickering recognized the spectrum
of hydrogen in the light of meteors passing at great height, but
decomposed water vapour might possibly be its source. Meteors crossing
lower strata show the spectrum of nitrogen. Nitrogen becomes important
from a height of about 85 km. (53 miles) downward and from 75 km. (46.5
miles) to the surface of the Earth it predominates. As a consequence
the pressure increases rapidly as we approach the ground. In these
regions or up to 80 km. (50 miles) floated the highest luminous night
clouds, observed by Jesse, indicating that here commenced a new range,
the nitrogen-zone. Only the heavy meteorites are able to penetrate
into the nitrogen sphere, which checks their speed and causes them to
explode, and thereafter the remnants fall with a velocity compatible
with the air resistance they meet. To these parts descend also the
lowest rays of the Northern Lights, the so-called draperies--Störmer
observed them once at a height of 37 km. (23 miles). Finally, water
vapour presents itself in appreciable quantities at an elevation of
about 10 km. (6.2 miles), where the troposphere commences. We now meet
the highest clouds, cirrus (with the exception of the “luminous night
clouds” observed only in the years 1883–1892 after the eruption on
Krakatoa). To these heights, reach the vertical air currents which are
essential to cloud formations. Only light clouds, however, float at
these altitudes; the heavy clouds (alto-cumulus) do not rise above 4
to 5 km. (2.5 to 3.1 miles) and the rain clouds proper (cumuli) occur
only below a height of 2 km. (1.25 miles). This is the result of the
downward concentration of water vapour within the troposphere.
If gravity decreased in intensity, the effect would be the same as if
the gases were lighter. On Venus, the intensity of gravity is eight
tenths of that on Earth. The difference is slight. If everything else
were similar the various air-zones would reach 25 per cent. higher on
Venus than they do on our globe. But one essential condition is varied
by the far higher temperature on our neighbour. The proportion of
moisture in the air is thereby vastly increased. The dense clouds rise
to much greater heights than on Earth. If there be ten times as much
water in the air on Venus as there is in the air on Earth--which might
fairly represent the actual condition--the heavy rainclouds would there
rise to a height of more than 10 km. (6.2 miles), and their smaller
weight on Venus would also contribute to their buoyancy. The light
cirrus clouds should appear as high as about 30 km. (18.5 miles) above
the ground. Under such circumstances, we cannot expect but that the
planet must be entirely hidden from our sight as well as from the rays
of the Sun.
On Mars, the intensity of gravity is 2.68 times smaller than on Earth.
In consequence barometric pressure falls 2.68 times slower with
increasing height there than here. The same ratio holds for decrease in
temperature and for shrinkage of proportion of moisture when comparing
conditions on the two planets. The strong cold precludes anything but
insignificant quantities of water vapour. The air on Mars is similar
to the atmosphere on Earth in and above the cirrus-region. The clouds
existing there are not only extremely thin and transparent--it is well
known that cirrus clouds throw no shadows--but they are confined to
small fractions of the planet’s sky. They are replaced by light mists.
We shall later return to the consequences of these peculiarities.
With help of the spectroscope we have ascertained that the gases on
the Sun are, in the main, also arranged according to specific weights,
so that the lightest reach the greatest heights. Somewhat similar
conditions obtain in the gas-appendages of the stars (compare page
119).