Popular Science Monthly/Volume 11/June 1877/On the Physical Condition of Jupiter and Saturn

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NOT many years ago the planets Jupiter and Saturn were regarded as solid bodies, their small relative density being supposed to be due either to the peculiar arrangement of the materials which compose them, or to the small specific gravity of the materials themselves. Within a few years, however, the nebular hypothesis has gained so strong a hold on the minds of astronomers, that the larger planets are regarded in a very different light as respects their physical condition from what they were a few years ago. It is now thought that at least Jupiter and Saturn, and probably Uranus and Neptune, have not yet cooled down from their heated and nebulous condition sufficiently to be habitable globes, and that, owing to their relatively high temperature, they have very extensive atmospheres.

It is quite possible for us to guess at the extent and density of their atmospheres, but, unless we subject our guesses to calculation, it cannot be said that they are of much use to us, since the planets are too far from us to allow direct observations to settle any points in dispute. Let us, therefore, assume Jupiter and Saturn to be globes of much higher temperature than the earth's, and that this temperature varies from the surface upward inversely as the square of the distance from the centre of the planet. Let us further assume that gases there follow the same law of expansion as terrestrial gases, and that this law holds for all temperatures above 32° Fahr. With these assumptions, it is possible for us to subject our guesses to calculation, and thus learn how far they are admissible.

As respects the telescopic appearance of those planets, it is plain that we have but little evidence of an atmosphere outside the highest cloud-layers. That their atmospheres extend above the highest clouds is perhaps certain; but if we attempt to explain the physical appearance of those planets, the condition of the atmosphere above the clouds may be left out of the consideration. Let us assume the density of the atmosphere at the upper surface of the clouds to be equal to that of our atmosphere at the height of about fifteen miles, and we shall perhaps regard it as sufficiently rare. The height of the upper cloud-layers above the surface proper of the planets will now depend on the surface-temperature and on the mass of the atmosphere. The law of density for different heights above the surface of the planets is almost independent of the mass of the atmosphere, unless we consider the mass as very great in comparison with that of the planet which it surrounds.

Let us suppose the density of the atmosphere at the surface of the planet to be equal to that of water—and such a density is certainly too great for either Jupiter or Saturn—it is then possible for us to calculate the height of the cloud-layers to which we have already referred, if the temperature be given; or, if the height be given, to calculate the temperature of the surface of the planet. We have seen it suggested that, if this height be assumed equal to 2,000 miles, it will enable us to explain the telescopic appearances of Jupiter and Saturn. This height for the former planet requires a surface-temperature of 45,000°, and for the latter 20,000° Fahr. It seems to be quite evident that these temperatures are much too high. That Jupiter and Saturn give out more light than they receive from the sun, seems nearly certain; but a temperature of 20,000° would seem to imply a much greater intrinsic splendor than either Jupiter or Saturn possesses.[1]

Prof. Proctor, in one of his articles, assumes the height of the highest cloud-layers to be 100 miles above the surface of the planet. This implies a temperature of 1,800° for the surface of Jupiter, and of 600° for that of Saturn. We must now conclude that the cloud-layers which form the outlines of the disks of Jupiter and Saturn are scarcely 100 miles high on Saturn, and considerably less on Jupiter; for we have assumed the surface-density much too high, and probably the density of the upper cloud-layers much too small. We cannot certainly consider the temperature of either planet above 2,000° Fahr.

The Herschelian figure of Saturn can probably be explained by the surface cloud-layers in its atmosphere. We need a greater number of and more careful observations on Jupiter and Saturn.

  1. These calculations are based on a mathematical paper read before the American Philosophical Society, November 3, 1876.