The Evolution of Worlds

by Percival Lowell

The result was a revelation. The plates exposed a host of lines never previously seen; lines that do not appear in the spectrum of the Sun, nor yet in the added spectrum of the atmosphere of the Earth, but are due to the planets’ own envelopes. But this was only the starting-point of their disclosures. When in this manner he had taken the color signatures of Jupiter, Saturn, Uranus, and Neptune, an orderly sequence in their respective absorption bands stood strikingly confessed. In other words, their atmospheres proved not only peculiar to themselves and unlike what we have on Earth, but progressively so according to a definite law. That law was distance from the Sun. When the spectra were arranged vertically in ordered orbital relation outward from the Sun, with that of the lunar for comparison on top, a surprising progression showed down the column in the strange bands, an increase in number and a progressive deepening in tint. The lunar, of course, gives us the Sun and our own air. All else must therefore be of the individual planet’s own. Beginning, then, with Jupiter, we note, besides the reënforcement of what we know to be the great water-vapor bands ‘a,’ several new ones, which show still darker in the spectrum of Saturn. The strongest of these is apparently not identifiable with a band in the spectra of Mira Ceti in spite of falling near it. Passing on to Uranus, we perceive these bands still more accentuated, and with them others, some strangers, some solar lines enhanced. Thus the hydrogen lines stand out as in the Sirian stars. All deepen in Neptune, while further newcomers appear.

Thus we are sure that free hydrogen exists in large quantities in the atmospheres of the two outermost planets and most so in the one farthest off. Helium, too, apparently is there, and other gases which in part may be those of long-period stars, decadent suns, in part substances we do not know.

From the fact that these bands are not present in the Sun and apparently in no type of stars, we may perhaps infer that the substances occasioning them are not elements but compounds to us unknown. And from the fact that free hydrogen exists there alongside of them, and apparently helium, too, we may further conclude that they are of a lighter order than can be retained by the Earth.

But now, we may ask, why should these lighter gases be found where they are? It cannot be in consequence simply of the kinetic theory of gases from which a corollary shows that the heaviest bodies would retain their gases longest, because the strange gases are not apportioned according to the sizes of their hosts. Jupiter, by all odds the biggest in mass, has the least, and Saturn, the next weightiest, the next in amount. Nor can title to such gaseous ownership be lodged in the planet’s present state. For though Jupiter is the hottest and Saturn the next so, the increased mass more than makes up in restraint what increased temperature adds in molecular volatility—as we perceive in the cases of the Sun and Earth.

No; their envelopes are increasingly strange because their internal constituents are different, and as hydrogen is most abundant in Neptune, the lightest of all the gases, it is inferable that this planet’s material is lighter. As distance from the Sun determines their atmospheric clothing, so distance decides upon their bodies, too. It was all a case of primogeniture. The light strange matter that constitutes them was so because it came from the outer part of the dismembered parent orb. Neptune the outermost, Uranus the next, then Saturn and Jupiter came in that order from the several successive layers of the pristine body, while the inner planets came from parts of it deeper down. The major planets were of the skin of the dismembered body, we of its lower flesh.

Very interesting the study of these curious spectral lines from the outer planets for themselves alone; even more so for what one would hardly have imagined: that they should actually tell us something of the genesis of our whole solar system. They corroborate in so far what the meteorites have to say.

That the meteorites are solid and, except for their experiences in coming through our air, bear no marks of external heat, is a fact which is itself significant. It seems to hint not at a crash as their occasioning but at disruptive tidal strains. The parent body appears to have been torn apart without much development of heat. Perhaps, then, we had no gloriously pyrotechnic birth, but a more modest coming into existence. But about this we must ourselves modestly be content to remain for the present in the dark.

Not the least important feature of the theory I have thus outlined is that it finishes out the round of evolution. It becomes a conception sapiens in se ipso totus, teres atque rotundus. To frame a theory that carries one back into the past, to leave one there hung up in heaven, is for inconclusiveness as bad as the ancient fabulous support of the world, which Atlas carried standing on an elephant upheld by a tortoise. What supported the tortoise we were not told. So here, if meteorites were our occasioning, we must account for the meteorites, starting from our present state. This the present presentation does.

Thus do the stones that fall from the sky inform us of two historic events in our solar system’s career. They tell us first and directly of a nebula made up of them, out of which the several planets were by agglomeration formed and of which material they are the last ungathered remains. And then they speak to us more remotely but with no less certainty of a time antedating that nebula itself, a time when the nebula’s constituents still lay enfolded in the womb of a former Sun.