The Parents' Review
A Monthly Magazine of Home-Training and Culture
"Education is an atmosphere, a discipline, a life."
Terrestrial and Sun-like Planets.
By J. E. Gore, F.R.A.S.
The planets composing the solar system may be divided into three groups: -- 1. The "terrestrial planets," including Mercury, Venus, the Earth, and Mars; 2. The minor planets, or asteroids as they are sometimes called, which form a ring or zone of small planets revolving round the sun between the orbits of Mars and Jupiter; and 3. The large planets, sometimes called the major planets, which include Jupiter, Saturn, Uranus, and Neptune. The terrestrial planets, Mercury, Venus, and Mars are so-called because the present many points of resemblance to the Earth. The length of the day in each is probably nearly the same; the inclination of the planet's equator to the plane of its orbit in all very similar. Each probably possesses an atmosphere of analogous composition to our own; and each has probably its surface diversified with land and water. In the case of the Earth we have the equatorial regions, although very warm, fairly habitable by living creatures, while the region immediately surrounding the poles are probably devoid of all animal life. This state of things may possibly be reversed in Mercury and Venus. In these planets, where the total light and heat are so much greater than we receive, the equatorial regions are most probably uninhabitable, owing to the intense heat, while the regions around the poles may possibly be cool enough to form the abode of life. This is very probably the case with Venus in particular, where the total heat received from the Sun is not more than double the amount we have on Earth. The excessive brightness of its surface -- found by one astronomer to be about ten times as bright as the brightest portions of the full moon! and reflecting, according to Zöllner, as much light as freshly-fallen snow -- would imply that most of its light is reflected from a persistent stratum of dense clouds, which would of course reflect the considerably more light than the surface of the planet itself. This heavy cloud stratum would therefore mitigate to a considerable extent the great light and heat of the Sun, and if sufficiently dense and persistent, then to the inhabitants of Venus, if any there be, the Sun and stars will for ever remain unseen by them, for, like a pall, the cloudy canopy covers all. Mercury must be very much hotter than Venus, but still its polar regions may perhaps be tolerably cool, and, possibly physical conditions, of which we know little, may serve to moderate the intense heat to which its surface is apparently subjected. As, however, Mercury is much less luminous than Venus, its atmosphere is probably not so cloud-laden. This is confirmed by Mr. Denning, who finds that the markings on Mercury are much more easily seen than those on Venus, and Zöllner finds that it reflects only 12 1/2 per cent of the solar rays, very similar therefore, in this particular, to the Moon, which reflects about 12 per cent of the incident solar light. Mars, we know, possesses land and water, an atmosphere, and length of days similar to ours, and seasons which, though of longer duration, are not widely different from those which we experience. Its supply of heat is, of course, very much less but still its equatorial regions, assisted by special physical conditions, may possibly be much warmer than we might at first sight be disposed to imagine. The more equable distribution of land and water on its surface, which we know exists on Mars, would also tend to equalize the temperature, and possibly render the planet -- at least in regions near the equator -- a fitting abode for some forms of animal life. Probably, however, a slight excess of heat would be more favourable to animal existence than a deficiency of solar warmth, and on the whole I am inclined to believe that of all the planets of the solar system, Venus is the one most likely to be inhabited by sentient beings like ourselves. Next to Mars, we have a zone or ring of small planets, of the physical condition and habitability of which we know little or nothing. All we know about them is, that they are very small, and therefore, unlike the terrestrial planets on the one hand, and the "giant" planets on the other, between which two groups they are situated.
When we come to examine the large planets we find a condition of things totally different from the "terrestrial" planets. They are all very large bodies, very much larger than the Earth in volume; very much lighter in density than the Earth; rotating on their axes much more rapidly; and surrounded by atmospheres evidently much more extensive than our own. Considering the small specific gravity of these gigantic planets, we are compelled to consider that their physical condition must be wholly different from that of the Earth. The density of Jupiter does not differ much from that of the Sun. That of Saturn is about 0.68 (water being unity). That of Uranus about the same as water; and that of Neptune somewhat less. Considering their enormous absolute mass, in the case of Jupiter 317 times, and in the case of Uranus about fourteen and a-half times the mass of the Earth, and that this mass must act with great force to compress the materials from the surface towards the centre of the planet, we see at once that their condition must be utterly unlike that of the Earth. In the case of the Sun its small density is easily and satisfactorily explained by its intense heat, which is sufficient to maintain its surface in the gaseous state, but unless we assume a somewhat similar state of things in the large planets, we are unable to explain why their density should be so small. Let us see what evidence observation affords in support of this view. The "belts" of Jupiter are familiar to almost every one, at least in name. These are darkish markings on the planet's apparent surface, usually parallel to the planet's equator, and may be seen with telescopes of moderate power, though of course large instruments are not necessary to examine their details satisfactorily. These belts are not permanent features, but are subject to great and rapid changes both in form and position. Now if we consider the great distance of Jupiter from the Sun -- more than five times greater than that of the Earth -- and that the heat derived from the great central luminary is enfeebled in the ratio of the square of the distance, we can see how little effect the Sun's heat can have in producing changes in Jupiter's atmosphere. Some other cause must therefore be at work, capable of causing changes of such magnitude as to be visible a the vast distance which separates the Earth from Jupiter. Mr. Raynard, the well-known astronomer, has attempted to show that spots on Jupiter are more prevalent when spots on the Sun are most numerous, and he considers that possibly both phenomena may have their origin in the same or similar cosmical cause. This view is confirmed by Mr. Browning, who finds that the red colour of the belts coincides with the epoch of sun spot maxima. An enormous reddish spot of elliptical shape, and measuring some 25,000 miles in length by 7000 in breadth, has been visible for some years past on the southern hemisphere of the planet. Curious changes have been observed in this spot, but perhaps the most remarkable fact connected with it is that the period of Jupiter's rotation on its axis, computed from observations of this spot, has been slowly increasing since the spot first became visible. Now from analogy we may conclude that the rotation period of Jupiter is uniform, at least so uniform during a period of nine years that no observations could detect any change. The observations therefore of this red spot would seem to indicate that it has a drifting motion of its own over the disc of the planet and this, combined with the changes in appearance it has undergone, and its enormous magnitude, would imply the action of forces which have no parallel on Earth. It must be added, however, that some astronomers -- including Professor Hough and Mr. Lynn -- are of the opinion that the red spot forms "a portion of the actual body of Jupiter," and that consequently the increase in the rotation period os real and not merely apparent! As the solar heat is evidently incapable of producing the observed changes in the belts and markings on Jupiter, there seems to be no escape from the conclusion that the forces at work have their origin within the globe of the planet itself. In the case of Saturn we have a somewhat similar condition of things. "Belts" are also visible here, but of course, owing to its greater distance from the Earth, any changes which may occur in them are not so easily observed as those on Jupiter. The markings on Saturn are however undoubtedly variable, both in size and position, and, like those on Jupiter, of enormous dimensions. As the mean distance of Saturn from the Sun is greater than that of Jupiter in the proportion of 886 to 483 or about eleven to six, we have the solar heat further diminished in the ratio of the squares of these numbers, are as 121 to 26, or in other words, the heat derived by Saturn from the Sun is less than one-third of that received by Jupiter. We see therefore how small an effect the Sun's heat can have in producing changes in Saturn's atmosphere. Considering also the very small density of Saturn's globe -- not much more than half that of Jupiter -- the argument in favour of internal heat in Jupiter is further strengthened in the case of the "ringed planet."
The immense distance of Uranus from the Sun prevents us from obtaining much evidence respecting its physical condition, and this remark applies of course with greater force in the case of Neptune. Traces of belts, however, on the disc of Uranus have recently been detected with some of the monster telescopes of modern days, and observations of these markings seem to indicate that the period of rotation does not differ much from ten hours, as in the case of Jupiter and Saturn. Examined with the spectroscope, the light of Uranus exhibits a very remarkable spectrum, showing several absorption bands, one of which apparently coincides with a well known line in the spectrum of hydrogen. This, of course, implies that free hydrogen gas exists in the planet's atmosphere, and seems to suggest that the inherent heat of the planet is sufficiently great to separate water into its component gases! The spectrums of Jupiter and Saturn also present peculiarities different from that of ordinary reflected sunlight, but not in so remarkable a degree as that of Uranus.
If we consider the members of the Solar System to have been originally formed from the condensation of an enormous heated nebulous mass, as in the Nebular Hypothesis of Laplace, or by some other process of evolution, as many astronomers suppose, then it would follow that the large planets would originally be much more highly heated than the earth. They would also, owing to their greater size, cool down more slowly, so that for both these reasons we might expect to find that the large planets are still hotter than the Earth. We know, from the evidence of mines, deep Artesian wells, volcanoes, and hot springs, that even the Earth, which has long since cooled down on its surface, still retains in its interior some of its primeval heat. In the large planets the original heat probably still exists even on their surfaces. We must, however, conclude that, although hotter than the Earth, they are much cooler than the Sun, which, owing to its vastly greater size, of course parts with its heat still more slowly. In fact, there would seem to be evidence in favour of the theory that these immense planets are in an intermediate state between the Sun and the "terrestrial" planets. If we look upon them as possessing inherent heat, we may suppose them also to have some intrinsic light of their own, unless, indeed, they are merely at a dull red heat, which would emit but little light. If, however, they do emit any light, we might naturally expect to find that these larger planets would shine with greater brightness in our night sky than would be due to their relative distances from the Sun and Earth. Let us see what evidence observation affords on this point. From photometric observations, Zöllner found that the reflective power of Mars, or its "albedo," as it is called by astronomers, is 0.2672, or in other words, that Mars reflects about 26 3/4 percent of light it receives from the Sun. For Jupiter he found an "albedo" of 0.62; that is a reflective power of 62 percent, (that of white paper being 70), or more than double that of Mars and nearly four times that of the Moon, of which he found the "albedo" only 0.1736. We must recollect, however, that the reflected light from Mars is absorbed to some extent by the planet's atmosphere; in fact, a double absorption takes place, first in passing from the Sun through the atmosphere to the planet's surface, and then back again to the Earth. Making due allowance for this, however, it is evident that the brightness of Jupiter is considerably greater than that of Mars, and if we consider that a large portion of Jupiter's disc is darkened by belts and spots, we must conclude that the brighter parts of its surface must be very bright indeed -- probably considerably brighter than snow. Taking the mean distance of Jupiter from the Sun as 483 millions of miles, and that of Mars as 141 millions, we have, as light varies inversely as the square of the distance, solar light on Mars equal to 11.734 times solar light on Jupiter. Now, in opposition, the diameter of Jupiter's disc is four times that of Mars. We should therefore have the light of Mars equal to 11.734 divided by 4, or 2.933 times that of Jupiter. Now Zöllner found that Jupiter in opposition is 2.52 magnitudes brighter than a star of the first magnitude, and Mars 2.25 magnitudes brighter than the same standard. From this we have the brightness of Jupiter equal 1.2823 times brightness of Mars. But, as shown above, the light of Mars should be 2.933 times the light of Jupiter, and hence Jupiter is 1.2823 multiplied by 2.933, or 3.76 times brighter than it should be if the two planets possessed the same reflective power. Zöllner found an "albedo" (or reflective power) for Mars of 0.2672. Hence we have "albedo" of Jupiter equal to 0.2672 multiplied by 3.76, equal to 1.0046.
This result, which implies that Jupiter emits slightly more light than it receives, seems improbably, but a similar result was arrived at some years since by the American astronomer, Bond. In the case of Uranus, compared with Jupiter, we have the mean distances from the Sun, 1782 and 483 millions of miles respectively, and their respective apparent diameters, 4 seconds and 48 seconds. From these data I find that Jupiter should be 1959.84 times the brightness of Uranus. Now the stellar magnitude of Uranus being 5.46, and that of Jupiter 2.52 according to Zöllner, we have the observed light of Jupiter equal to 1556 times the light of Uranus. Hence we have the "Albedo" of Uranus (assuming Zöllner's value for Jupiter) equal to 1959.82/1556 x 0.62, or 0.78, a brightness equal to that of freshly fallen snow, a result which certainly points to a considerable amount of inherent luminosity in this distant planet, and obviously tends to confirm the spectroscopic evidence of heat sufficient to dissociate water.
If we agree to consider the large planets as being in a highly heater condition, we must, of course, abandon the idea that they can possibly form the abodes of life, and the description of some writers of the splendor of Saturn's sky with its rings, and satellites, as viewed by an inhabitant of Saturn, must be given up as a pleasant dream. The case may, however, be very different with the satellites which circle round them, and these giant planets may very possibly play the part of miniature Suns to their attendant Moons. Indeed, Saturn's system seems to form a sort of miniature of the Solar System. It has eight satellites, corresponding to the eight larger planets, revolving round the Sun, and a system of rings -- now generally admitted to consist of a swarm of minute satellites -- similar, even in its divisions to the zone of minor planets revolving between the orbits of Mars and Jupiter.
Seen from Jupiter's nearest satellite, I find that Jupiter's enormous globe would show a disc of about nineteen degrees in diameter. From the other satellites, his disc would vary from twelve to about four degrees. From Mimas, the nearest satellite of Saturn, the planet would present a disc of no less than thirty-three degrees in diameter, and, with the encircling ring system, would afford a considerable amount of light, and form a magnificent spectacle in the midnight sky of Mimas. Seen from the satellites of Uranus, the planet would show a disc varying from fifteen and a half to five degrees.
We see, therefore, that the amount of light and heat received by the satellites of these systems from their primary may possibly form a considerable addition to the scanty supply received from the sun, and that the existence of some forms of life on their surface may be more probable than their great distance from the Sun would at first sight lead us to imagine.
Typed by Kristina, Oct 2015
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