WHEN THE MAJOR PLANETS COOL
The question has been asked: "It is evident that life cannot exist at the
present time on the outer planets, since they are in a highly heated and
quasi-solar condition; but when they cool down, as cool they must, and a
solid crust is formed, may not a time come when they will be habitable? It
seems impossible to think that worlds so beautiful to our eyes and so vast
in scale are destined never to be peopled by intelligent beings."
It is clearly difficult to answer satisfactorily a question that requires
so deep a plunge into the recesses of the unknown future; yet, so far as
our knowledge goes, there is no reason to think that Jupiter will be more
habitable then than it is now. The difficulty of the small supply of light
and heat received from the Sun would apparently still remain, if indeed,
the cooling of the Sun itself would not increase it. We do not know of any
means by which our Sun could so increase its radiation as to supply to
Jupiter from 25 to 30 times as much heat as it now receives, and this
would be necessary to place it in the same favoured condition as the
Earth. If so great a change were to take place in the Sun, life would be
scorched out of existence on all planets nearer than Jupiter, and,
similarly, if the solar emission were increased to meet the necessities of
Uranus or Neptune, even Jupiter would fall a victim.
But we may consider it as a conceivable case that a planet of the exact
dimensions of Jupiter may be revolving in an annual period of the same
length as his, round some star that is capable of affording it adequate
nourishment; and so with the three other giant planets. The actual Jupiter
and Saturn of the solar system have, so far as we can tell, neither
present nor future as habitable worlds, but we can consider what would be
the case of imaginary bodies of similar dimensions in systems where the
supply of heat would be sufficient. Or we can neglect the question of
temperature altogether, as we did at first in the case of Mars.
All the four planets must shrink much in volume before their
solidification will take place. Their average density at present but
little exceeds that of water; indeed, Saturn is not so dense as water; yet
we must suppose that the same elements are in general common to the Earth
and to them all. If we assume, then, that the four planets all cool to the
point of solidification, their densities must be much increased, and their
volumes correspondingly diminished. Since all four greatly exceed the
Earth in mass, it is but natural to expect that, when they have assumed
the terrestrial condition, they will be more closely compacted than the
Earth, and their densities in consequence will be greater. It will,
however, be simpler if we assume exactly the same density for them as for
the Earth. Jupiter will then have shrunk to about one-fourth of its
present volume, and the statistics for the four planets will run as in the
following Table:
STATISTICS OF THE FOUR OUTER PLANETS IF WITH THE SAME DENSITY AS THE EARTH
PROPORTIONS OF THE PLANETS:--
Uranus Neptune Saturn Jupiter
Diameter in miles 19300 20400 36000 54000
do [Symbol] = 1 2.44 2.57 4.56 6.82
Surface, [Symbol] = 1 6.0 6.6 20.8 46.6
Mass and Volume, [Symbol] = 1 14.6 17.0 94.8 317.7
Gravity at surface, [Symbol] = 1 2.44 2.57 4.56 6.82
Rate of Fall, Feet in
the First Second 39.2 41.3 73.3 109.7
ATMOSPHERE, assuming the
total mass of the atmosphere
to be proportional to
the mass of the planet:--
Pressure at the surface in lb.
per square inch 88.2 97.0 305.8 685.0
Pressure at the surface in
"atmospheres" 6.0 6.6 20.8 46.6
Level of half-pressure in miles 1.37 1.30 0.73 0.49
Boiling point of water at
surface 127 deg.C 129 deg.C 148 deg.C 164 deg.C
Jupiter offers two peculiarities. In its shrunken condition, its diameter,
instead of being eleven times that of the Earth, will be not quite seven,
and the force of gravity at the surface will be greater than that of the
Earth in the same proportion. A man who here weighs 150 lb. will there
weigh over 1000 lb.; and the muscular effort of movement will be increased
in the same ratio. The athlete who here can clear a height 5 ft. 8 in.
will there, with like pains, surmount 10 inches; and other efforts will be
in the same proportion. The atmosphere, supposing it to be in proportion
to the mass of Jupiter, will exercise a pressure of 46-1/2 "atmospheres,"
or more than 680 lb., to the square inch. Following on this enormous
pressure at the surface would be the rapidity with which the atmosphere
would thin out in the upward direction. The level of half-pressure would
be attained by ascending less than half a mile in height; that is to say,
there would be a difference of pressure of 340 lb. on the square inch from
that experienced at the sea-level. We know from the fact that fishes live
at enormous depths in the ocean, that living organisms can be constructed
to endure great pressures, but they are not constructed to endure great
alterations of pressure. The deep-sea fishes are as instantly killed by
being brought up to the surface, as the surface fishes or the land animals
would be if they were plunged into the depths. And it is clear that on
Jupiter a low range of hills that on the Earth would be considered only
an easy climb, would be an impassable barrier, not only from the immense
exertion of mounting it, but chiefly from the unendurable change of
pressure which the ascent would involve.
The sevenfold gravity of Jupiter, taken in connection with this enormous
atmospheric pressure, would tend to make the meteorological disturbances
of the planet violent far beyond anything of which the Earth can furnish
an example. The atmosphere would possess a high viscosity, and differences
in condition, pressure and saturation would tend to accumulate, until at
length the balance would be restored with explosive suddenness and force.
Here our most violent tornadoes may reach a speed of 100 miles an hour; on
Jupiter, gales of five or six times that velocity would be common. We
cannot conceive that living organisms would be able to grow, flourish and
multiply where the conditions were so cataclysmic.
This difficulty must always exist where the planet is great in mass, and
the force of gravity high at the surface. The case of Saturn is not so
extreme as that of Jupiter, though it is probably sufficiently severe to
exclude it from the ranks of worlds that could ever be dwelt in. The
atmospheric pressure would be about 21 "atmospheres," or more than 300 lb.
on the square inch. The level of half-pressure would be reached at about
three-quarters of a mile, and the force of gravity be nearly 4-1/2 times
that of the Earth.
But the serious condition for Saturn would come from that feature which
renders it by far the most attractive of all the planets seen in the
telescope, the presence of the wonderful Ring system.
To us, viewing Saturn from afar, and from practically the same direction
as the Sun, the Rings are seen lit up; but to a dweller on Saturn, the
Rings during the day are between his world and the Sun, and hence turn
their dark side toward him. More than that, the telescope shows us that
the Rings cast a shadow on the planet; in other words, they eclipse part
of it; and this shadow changes its position with the progress of the
Saturnian year. Proctor computed that if the Rings were a hundred miles in
thickness, the equator would suffer, in consequence, total eclipse for
nearly ten days at each equinox, and partial eclipse for about forty days
more. Moving away from the equator, each higher latitude would have a
longer and longer period of eclipse in the winter half of its year; the
higher the latitude, the later after the autumnal equinox the eclipse
would begin, and the longer it would last, until about latitude 40 deg. was
reached. Here the eclipses would begin nearly three terrestrial years
after the time of the autumnal equinox. At first the Sun would be eclipsed
only in the morning and evening of each day, but the length of the daily
eclipse would increase, until the Sun was hidden the whole day long. This
period of total eclipse would last for about 6 years 8 months, terrestrial
reckoning, or with the periods of partial eclipse, 8 years and nearly 10
months. Whatever the efficiency of the Sun that afforded light and heat to
such a planet, it is clear that such eclipses must be fatal to life in two
ways: light and heat would be cut off from wide regions of the planet for
long periods of time, and terrible meteorological convulsions must follow
in the train. Here on the Earth, though a total eclipse generally lasts
only two or three minutes, the atmospheric disturbance is perceptible, and
the fall of temperature very marked, and it does not require much
reflection to see that the analogous disturbance in an atmosphere twenty
times as dense must be terrific indeed during an eclipse that lasts not a
few minutes only, but for more than six of our years.
The case of Uranus introduces us to another class of conditions fatal to
habitability. The equator of Jupiter is inclined only 3 deg. to the plane of
its orbit; the difference in its seasons is, therefore, almost
imperceptible; there is hardly any alteration in the incidence of the
solar rays; it is, as if on the Earth, the height of the Sun at noon in
mid-winter were what it actually is on the 14th of March, and its height
at midsummer the same as we observe on March 28. The inclination of the
equator of Saturn is considerably greater than that of Mars or the Earth,
so that its seasons are more pronounced, but not to an extent that would
introduce any radical difference. But for Uranus, the inclination of the
equator to the plane of the orbit is 82 deg. If this were the case for the
Earth, the noonday sun for London would be, at the spring equinox, 38-1/2 deg.
high as at present, but its altitude day by day would increase with great
rapidity, and before the end of April, the Sun at noon would be right in
the zenith, and 13 deg. above the horizon at midnight. At midsummer, indeed,
it would be only 59 deg. high at noonday, but it would be north of the zenith
instead of south, and at technical midnight, it would still be 44 deg. in
altitude, thus moving round in a very small circle, only 15 deg. in diameter.
From about April 18 to August 25--that is to say, for 129 days--the Sun
would never set, and unlike the summer day of our own polar regions now,
wherein the Sun, though always present, is always low down in the sky, for
much of that period it would pass the meridian quite close to the zenith.
As the year of Uranus is 84 times the length of our year, the London of
Uranus would have to endure not far short of 30 years continuous
scorching.
And the winter would be as long; the perpetual day of summer would be
replaced by a night as enduring. More than 29 years of unbroken darkness,
of unmitigated cold, cannot possibly ever consist with the conditions
necessary for life upon a planet. Whatever the brightness of the imagined
sun of Uranus, if for 29 years at a time that sun were below the horizon,
the water on the planet must be congealed, and during the 29 years of
unbroken day all the water would be as certainly evaporated.
Thus, though Uranus is not burdened by the enormous mass of Jupiter, nor
overshadowed, like Saturn, by a system of rings, the extraordinary
inclination of its axis introduces a condition which is as fatal to it, as
a world to dwell in, as any of the disabilities of the other planets.
It is curious that these four outer planets, that resemble each other so
strikingly in many of their conditions--in their vast size, high albedo,
low density, and vaporous envelopes, that show, in their spectra, not
merely the lines of reflected sunlight, but also special lines due to
their own atmospheres (the chief of these being common to all the four
planets)--should yet, in the inclination of their axes to the plane of
their orbits, display every possible variety. The axis of Jupiter is
almost normal to its orbit, that of Uranus lies almost in the plane of its
orbit. The axes of Saturn and Neptune have a mean inclination, but it
would appear that the rotation of Neptune is in the reverse direction to
that of planets in general, so that the true inclination is usually taken
as being the complement of the observed angle, as if the axis were turned
right over. It is uncertain whether this would have any important effect
upon the habitability of the planet, but it supplies the fourth possible
case for the position of the axis.