Dreams of an astronomer

by Camille Flammarion

Venus

On Venus the Sun’s heat is still very considerable, and in the torrid zone it would be insupportable to us. But, on the other hand, this planet is surrounded by a dense atmosphere, which on account of the presence of water-vapour proves the existence of water on the planet. The higher regions of the atmosphere are occupied by a thick layer of clouds which hardly ever allows our gaze to reach the ground, but which is equally opposed to the passage of the Sun’s rays. Much more than half the solar radiation is reflected by that layer of cloud, and we can suppose, on the whole, that the upper limit of a tolerable temperature is not passed at the surface of this planet. And as we have already said, since there is water and an atmosphere, we have no reason to doubt the habitability of Venus.

Mars

As regards Mars, which commences the outer series of planets, we obtain a still more satisfactory result. We can clearly recognise the subdivision of the surface into water and dry land. Its atmosphere has properties which agree with those of our atmosphere. Not only is there certain evidence of the existence of water-vapour, but spectroscopic studies have proved also that the principal components of the atmosphere are the same as those of the Earth’s atmosphere—that is to say that there is oxygen and nitrogen. Sometimes the soil is hidden by groups of clouds, sometimes they disappear to appear again in other places. Its poles are encased in snow and ice, the white area of which varies in extent according to the seasons. Besides a number of enigmatical facts—we need only refer to the canals and their doubling—there are numerous meteorological phenomena on Mars which frequently occur in our own atmosphere. Although the temperature of Mars is, on account of its farther distance from the Sun, sensibly below that of the Earth, the difference is not sufficiently considerable to oppose a serious obstacle to the stable existence of organised matter in the torrid or temperate zone. The torrid zone of Mars must correspond approximately in climate to our temperate zone. We can therefore finally declare with entire conviction that the conditions offered by Mars are suitable for life as we know it on Earth.

Jupiter, Saturn, Uranus, and Neptune

With the planets Jupiter, Saturn, Uranus, and Neptune we enter a region quite different from the preceding one. These bodies all possess atmospheres of great density in which, on Jupiter for instance, we can observe immense revolutions and violent cataclysms. They also contain water-vapour, but besides that they contain a substance characterised by a strong absorption of certain red rays. This gas is found in a small quantity on Jupiter, but in much greater quantities on Saturn and Uranus. On the latter planet the atmosphere, apart from the water-vapour contained in it, does not seem to have any analogy with ours; strong absorption-bands show in the less refrangible portions of the spectrum. As we have seen that oxygen is not absolutely necessary to organic life, the single fact that the outer planets have an atmosphere may suffice to show that our first condition is fulfilled. The presence of water-vapour in their atmosphere proves the existence of water and fulfils the second condition. As regards the third condition, Jupiter also seems to satisfy that, at least in equatorial regions, especially if we take into account that radiation of heat into space is much limited by the thick atmosphere filled with clouds. The farther we go away from the Sun the more does the third condition become precarious, and while we may have some doubt concerning Saturn, it cannot be denied that on Uranus and Neptune the solar heat is insufficient to support organised life in a durable way.

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(The atmospheres might be formed of gases which would make the radiation almost zero and would produce a relatively high temperature. This certainly happens in the case of Mars, whose temperature is not lower than that of our globe, where the polar snows are less dense than ours and melt more completely in the course of the summer.)

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But another peculiarity presents itself which may reverse all our ideas relative to the planets from Jupiter outwards. Certain observations tend to show that Jupiter is not yet cooled down, that its real nucleus is still fused or perhaps even in the gaseous state, and that it has not yet formed a solid crust upon which life might develop. Besides the phenomena directly observed or revealed in the spectroscope, the very small specific gravity of these planets also supports that hypothesis—the density of Saturn is about the same as that of cork. It is very difficult to form an exact idea of the constitution of these planets. On the other hand, it is possible to believe that one of the extreme planets, Uranus for instance, is sufficiently cooled to possess at least a liquid surface which, on account of the internal heat of the planet, may have preserved for a certain period a temperature sufficient to entertain life even after the solar heat is no longer sufficiently powerful But these are only hypotheses.