CHAPTER XI

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.