the earth, they might travel in all directions, and the whole universe
might, so to say, be sown with them. But now comes the question: how
can they escape from the earth against the effect of gravitation?
Corpuscles of such small weight would naturally be carried away by any
aerial current. A small rain-drop, 1/50 mm. in diameter, falls, at
ordinary air pressure, about 4 cm. per second. We can calculate from
this observation that a bacteria spore 0.00016 mm. in diameter would
only fall 83 m. in the course of a year. It is obvious that particles
of this minuteness would be swept away by every air current they met
until they reached the most diluted air of the highest strata. An air
current of a velocity of 2 m. per second would take them to a height
where the air pressure is only 0.001 mm.—_i.e._, to a height of about
100 km. (60 miles). But the air currents can never push the particle
outside of our atmosphere.
In order to raise the spores to still higher levels we must have
recourse to other forces, and we know that electrical forces can help
us out of almost any difficulty. At heights of 100 km. the phenomena
of the radiating aurora take place. We believe that the auroræ are
produced by the discharge of large quantities of negatively charged
dust coming from the sun. If, therefore, the spore in question should
take up negative electricity from the solar dust during an electric
discharge, it may be driven out into the sea of ether by the repulsive
charges of the other particles.
We suppose, now, that the electrical charges—like matter—cannot be
subdivided without limit. We must finally come to a minimum charge,
and this charge has been calculated at about 3.5.10^{-10} electrostatic
units.
We can, without difficulty, calculate the intensity of an electric
field capable of urging the charged spore of 0.00016 mm. upward against
the force of gravity. The required field-strength is only 200 volts per
metre. Such fields are often observed on the surface of the earth with
a clear sky, and they are, indeed, almost normal. The electric field of
a region in which an auroral display takes place is probably much more
intense, and would, without doubt, be of sufficient intensity to urge
the small electrically charged spores which convection currents had
carried up to these strata, farther out into space against the force of
gravity.
It is thus probable that germs of the lowest organisms known to us are
continually being carried away from the earth and the other planets
upon which they exist. As seeds in general, so most of these spores,
thus carried away, will no doubt meet death in the cold infinite space
of the universe. Yet a small number of spores will fall on some other
world, and may there be able to spread life if the conditions be
suitable. In many cases conditions will not be suitable. Occasionally,
however, the spores will fall on favorable soil. It may take one
million or several millions of years from the age at which a planet
could possibly begin to sustain life to the time when the first seed
falls upon it and germinates, and when organic life is thus originated.
This period is of little significance in comparison with the time
during which life will afterwards flourish on the planet.
The germs which in this way escape from the planets on which their
ancestors had found abode, may either wander unobstructed through
space, or they may, as we have indicated, reach outer planets, or
planets moving about other suns, or they may meet with larger
particles of dust rushing towards the sun.
We have spoken of the Zodiacal Light and that part of it which has been
designated the counter-glow. This latter glow is regularly seen in the
tropics and occasionally in that portion of our heavens which is just
opposite the sun. Astronomers ascribe the counter-glow to streams of
fine dust which are drawn towards the sun (compare page 147). Let us
assume that a seed of the diameter of 0.00016 mm. strikes against a
grain of dust which is a thousand times as large (0.0016 mm. diameter),
and attaches itself to its surface. This spore will be carried by the
grain of dust towards the sun; it will cross the orbits of the inner
planets, and it may descend in their atmospheres. Those grains of dust
do not, by any means, require very long spaces of time to pass from one
planetary orbit to another. If we assume that the spore starts with
zero velocity near Neptune (in which case the seed might originate
from the moon of Neptune; for Neptune itself, like Uranus, Saturn, and
Jupiter, is not yet sufficiently cooled to sustain life), the spore
would reach the orbit of Uranus in twenty-one years, and of Mercury in
twenty-nine years. With the same initial velocity such particles would
be twelve years in passing between the orbits of Uranus and Saturn,
four years between Saturn and Jupiter, two years between Jupiter and
Mars, eighty-four days between Mars and the earth, forty days between
the earth and Venus, and twenty-eight days between Venus and Mercury.
We see from these time estimates that the germs, together with the
grains of dust to which they have attached themselves, might move
towards the sun with much smaller velocity (from ten to twenty times
smaller) without our having to fear any loss of their germinating
powers during the transit. In other words, if these seeds adhere to the
particles, ninety or ninety-five per cent. of whose weight is balanced
by the radiation pressure, they may soon fall into the atmosphere of
some inner planet with the moderate velocity of a few kilometres per
second. It is easy to calculate that if such a particle should, in
falling, be arrested in its motion after the first second, it would
yet, thanks to the strong heat radiation from it, not be heated by more
than 100° Cent. (212° F.) above the temperature of its surroundings.
Such a temperature can be borne by the spores of bacteria without fatal
effects for much more than one second. After the particles, together
with the seed adhering to them, have once been stopped, they will
slowly descend, or will be carried down to the surface of the nearest
planet by descending convection currents.
In this way life would be transferred from one point of a planetary
system, on which it had taken root, to other locations in the same
planetary system which favor the development of life.
The seeds not caught by such particles of dust may be taken over to
other solar systems, and finally be stopped by the radiation pressure
of their suns. They cannot penetrate any farther than to spots at
which the radiation pressure is as strong as at their starting-points.
Consequently, germs from the earth, which is five times as near the sun
as Jupiter, could approach another sun within a fifth of the distance
at which germs from Jupiter would be stopped.
Somewhere near the suns, where the seeds are arrested by the radiation
pressure to be turned back into space, there will evidently be
accumulations of these seeds. The planets which circulate around their
suns have therefore more chance of meeting them than if they were not
in the vicinity of a sun. The germs will have lost the great velocity
with which they wandered from one solar system to another, and they
will not be heated so greatly in falling through the atmospheres of the
planets which they meet.
The seeds which are turned back into space when coming near a sun will
there perhaps meet with particles whose weight is somewhat greater than
the repelling power of the radiation pressure. They would, therefore,
turn back to the suns. Like the germs, and for similar reasons, these
particles would consequently be concentrated about the sun. The small
seeds have, therefore, a comparatively better chance of being arrested
before their return to space by contact with such particles, and of
being carried to the planets near that sun.
In this manner life may have been transplanted for eternal ages from
solar system to solar system and from planet to planet of the same
system. But as among the billions of grains of pollen which the wind
carries away from a large tree—a fir-tree, for instance—only one
may on an average give birth to a new tree, thus of the billions, or
perhaps trillions, of germs which the radiation pressure drives out
into space, only one may really bring life to a foreign planet on which
life had not yet arisen, and become the originator of living beings on
that planet.
Finally, we perceive that, according to this version of the theory
of panspermia, all organic beings in the whole universe should be
related to one another, and should consist of cells which are built up
of carbon, hydrogen, oxygen, and nitrogen. The imagined existence of
living beings in other worlds in whose constitution carbon is supposed
to be replaced by silicon or titanium must be relegated to the realm of
improbability. Life on other inhabited planets has probably developed
along lines which are closely related to those of our earth, and this
implies the conclusion that life must always recommence from its very
lowest type, just as every individual, however highly developed it may
be, has by itself passed through all the stages of evolution from the
single cell upward.
All these conclusions are in beautiful harmony with the general
properties characteristic of life on our earth. It cannot be denied
that this interpretation of the theory of panspermia is distinguished
by perfect consistency, which is the most important criterion of the
probability of a cosmogonical theory.
There is little probability, though, of our ever being able to
demonstrate the correctness of this view by an examination of seeds
falling down upon our earth. For the number of germs which reach us
from other worlds will be extremely limited—not more, perhaps, than a
few within a year all over the earth’s surface; and those, moreover,
will presumably strongly resemble the single-cell spores with which the
winds play in our atmosphere. It would be difficult, if not impossible,
to prove the celestial origin of any such germs if they should be found
contrary to our assumption.
THE END