The Destinies of the Stars

by Svante Arrhenius

Which gases should we expect to find in the atmosphere of these planets? According to the Kant-Laplace hypothesis, a theory generally credited with a sound kernel, the planets were segregated from the Sun’s substance at a time when the latter was expanded so as to include the orbits of these planets and beyond. Naturally, therefore, their atmospheres would originally be composed of the very gases that formed the outmost part of the Sun’s atmosphere, notably hydrogen. Slipher, who has photographed the spectra here reproduced of the outer planets, believes that certain strong absorption bands in the spectra of Neptune and Uranus correspond to the distinctive F and C lines of hydrogen, using Fraunhofer’s denotation (see [Fig. 13]). But because the bands in question, as shown on the figure, are very broad it is difficult to identify them with certainty. Also other gases of unknown origin enter into the vapour envelopes outside of the clouds and cause, as apparent from their spectra, a strong absorption of the sunlight reflected from the clouds below. The absorption increases with the planet’s distance from the Sun; thus, it is most pronounced on Neptune and least on Jupiter.

Fig. 12. The appearance of Saturn September 30, 1909, according to F. le Coultre of Geneva.

At any rate, the gas appendages of the heavenly bodies just considered differ in one essential respect from the atmospheres of the inner planets: Mars, Earth, Venus, and Mercury. On the Sun and on the outer planets the atmosphere gradually merges into the interior gas-masses so that no distinct boundary can be found between the rarer and the denser layers. Widely different conditions obtain on the Earth. Here the range of air is sharply defined below by the Earth’s solid crust or by the oceans. In such case alone may we speak of an atmosphere proper, of the kind that enters into our commonplace conceptions. Similar are the conditions on all stellar bodies with a solid or a liquid surface.

Fig. 13. Spectra of the major planets, compared with that of the moon. The latter corresponds to the spectrum of sunlight reflected from a planet which lacks a light-absorbing atmosphere. Photos by V. M. Slipher of the Lowell Observatory.

But it is not certain in every case that all such planets possess an atmosphere. Observations of the Moon when passing some star show that the air envelope, if present, is unable to deflect the light beam from the star, or in other words it has no perceptible power of refraction. From this we also infer that its density is very small, corresponding at most to one or two mm. (.04 to .08 inches) barometric pressure. But we have good reasons to believe that the Moon has been detached from the Earth, carrying away parts of its lightest substance, which theory is supported by the fact that the Moon’s mean density (3.3) is only six-tenths of the Earth’s (which again is 5.53 times that of water), and we might therefore have expected that the Moon in parting should have shared in the very lightest constituents of the Earth, namely its air-covering. Such was unquestionably the actual procedure, but in the course of time the Moon has lost its originally no doubt considerable atmosphere. The reason is that the molecules in a gas are in a continuous rapid motion, which is the swifter the lighter the gas and the higher its temperature. In hydrogen, the lightest gas known, the velocity amounts to 1.84 km. (1.15 miles) per second at 0° C. (32° F.). The parts of the Moon exposed to the strongest sunlight are heated to about 150° C. (300° F.). At that temperature, the average velocity of hydrogen molecules is 2.29 km. (1.43 miles) per second. But a body departing from the surface of the Moon at a rate of 2 km. (1.24 miles) per second, or more, cannot be retained by the attraction of that globe and therefore never retraces its path but speeds away for ever. In the same manner a bullet ejected from a cannon with an initial velocity of 11.2 km. (7 miles)—a velocity not even approached by present artillery—would fly away from Earth barring the resistance of the air. Thus we see that we as yet are far from the realization of the dreams of Jules Verne in his A Voyage to the Moon. At any rate, gravity on the Moon is too weak to retain hydrogen over the hottest point of the surface. This part of the gas flies away, new supplies rush in from the sides, and in a short time all traces of hydrogen have disappeared from the Moon. Probably it was mainly gathered in by the Sun, where a velocity of 613 km. (380 miles) per second is necessary if the molecules are to overcome the Sun’s attraction, while their actual velocity there amounts to only about 8 km. (5 miles) per second.

In a similar manner we find that the second lightest gas, helium, at a temperature of 150° C. (300° F.), possesses a molecular velocity of 1.62 km. (1.1 miles) per second. This is less than the 2 km. (1.24 miles) per second necessary to leave the Moon’s sphere of attraction. But all helium molecules do not move at the same speed; some are faster and some slower than the average. Those moving at a higher rate than 2 km. (1.24 miles) per second constitute a considerable fraction of the total. This fraction disappears. Equilibrium is soon restored so that in less than a second the same fraction of helium molecules is ready to depart. In this manner the Moon lost its helium atmosphere speedily, although not quite as rapidly as its hydrogen.

More slowly yet vanished the gases which are most abundant in our atmosphere, nitrogen and oxygen, but these too were not fettered for ever by the limited gravity on the Moon. The same fate befell aqueous vapour, which is nearly twice as light as oxygen. The loss of water, however, was long delayed, as we later shall learn, because new vapour masses were discharged from the lunar volcanoes. In these considerations, we should also bear in mind that the Moon no doubt was a fluid molten mass when separating from the Earth and its substance resembled the lava from our volcanoes. In this condition it remained until its exterior temperature had fallen to about 1200° C. (2200° F.). At that point, the average velocity of oxygen molecules is about 1 km. (.62 mile) per second, with variations in both directions, so that a few per cent. of them reach a sufficient velocity to leave the Moon for ever. Such gas molecules of medium weight return probably to the Earth which, as experience tells us, is ponderous enough to hold them in bonds.

All gases, that constitute any considerable fraction of the Earth’s atmosphere, and which, therefore, most likely were divided with the Moon in its parting from us, have again left that globe. The same unquestionably holds true for other stellar bodies of equal or smaller size, such as all the minor planets and for the great majority of the satellites to the major ones. Only the very largest of Jupiter’s moons, and possibly Neptune’s lonely companion, whose size is not known with certainty, might possibly surpass our Moon in ability to retain gases. Our reasoning with respect to the Moon applies also to Mercury. It is true that the molecules there must possess a velocity one and a half times as high as on the Moon if they are to leave the planet. But at the same time the temperature on Mercury’s hottest point, always turned toward the Sun, is far higher, about 400° C. (750° F.), so that the molecules there move 1.26 times as fast as similar molecules over the Moon’s hottest point. Mercury is consequently better able than the Moon to retain gases, but the difference is slight. Direct observations (see below) also lead us to believe that Mercury is very similar to the Moon in these respects. We might possibly imagine that certain gases, which on the Moon would condense into fluids or solids, on Mercury might remain volatilized on account of the high temperature and thus form an atmosphere. Such assumption, however, would be erroneous. The investigations by Schiaparelli and by all his successors show that Mercury in turning around its axis always presents the selfsame side to the Sun. The opposite side, never reached by a ray of sunlight, must assume an extremely low temperature, very close to the absolute zero (-273.7° C. or -460.6° F.) and far below any cold existing on the Moon. To this side, all bodies with an appreciable vapour pressure must distil and freeze to solid lumps or frost-coverings without perceptible vapour pressure. For these reasons, Mercury cannot possess any atmosphere to speak of. There remain in the whole series of planets and satellites in our solar system only two bodies besides the Earth which are endowed with an atmosphere in the original sense of the word—namely, Mars and Venus.