Are the Planets Inhabited?

by E. Walter Maunder

VENUS, MERCURY AND THE ASTEROIDS

Of all the planets, Venus appears, to the unassisted eye, by far the loveliest. When seen in the early morning before sunrise—its “western elongation”—or after sundown in the evening—its “eastern elongation”—and still more as it attains its greatest brilliancy, it has attracted attention everywhere and in all ages. It then shines with brilliance ten times as great as Jupiter in opposition, and the brightest members of the heavenly host look pale and dim beside it. It is emphatically the morning or the evening star, Lucifer, or Vesper, herald or follower of the Sun; it can even assert itself in the presence of the Lord of Day, for it has often been seen at noonday by watchers who knew where to look; sometimes by the general crowd.

But in the telescope Venus appears less satisfying. It is a pretty spectacle indeed to watch the phases of the gleaming little globe of silver, for, like the Moon under varying illumination from the Sun, it undergoes change of apparent shape. But the surface of the planet yields little detail, and that little is illusive and ill-defined. The clear-cut outlines and black shadows of the Moon have no place here, nor do the ruddy plains and blue-grey “seas” of Mars find any analogues. All that can be observed beyond the changes of phase are a few faint, ill-defined patches, where the molten silver of the general surface is slightly dimmed and tarnished, and perhaps one or two spots, not less evasive and difficult to fix, that exceed the rest of the surface in brightness.

This very difficulty in making out the markings on Venus is hopeful for our search; it points to a veiling over the planet, a veiling by an atmosphere. And the statistics of the Table show that Venus closely resembles our Earth in size and mass, and therefore probably in atmospheric equipment. If we assume that the atmosphere of any planet is in direct proportion to its mass—and as Venus is so nearly the twin of the Earth there is no reason to expect any great difference between the two in this respect—the atmosphere of Venus would have a pressure of about 11·2 lb. on the square inch, and the level of half pressure would be nearly four miles above the surface. In other words the atmosphere would be both thinner and deeper than that of the Earth, but the difference would not be important in amount.

But Venus is nearer to the Sun than the Earth, and receives nearly double the light and heat. Its theoretical equatorial temperature is 368°abs., or 95°C, and its corresponding mean temperature is 69° C. But water under a pressure of 11·2 lb. will boil at 93° C, so that at the equator of Venus the upper limit for water as a liquid is just passed, but, for the planet in general, a fairly safe margin is maintained. Here then is sufficient explanation why the topography of Venus is concealed. The atmosphere will always be abundantly charged with water-vapour, and an almost unbroken screen of clouds be spread throughout its upper regions. Such a screen will greatly protect the planet from the full scorching of the Sun, and tend to equalize the temperature of day and night, of summer and winter, of equator and poles. The temperature range will be slight, and there will be no wide expanses of polar ice. Water that flows will be abundant everywhere.

So far all the facts connected with Venus are favourable for life, even though the picture called up to the mind may not seem inviting to us. For views of the heavens must be rare; the Sun must seldom pierce through the cloud veil; there is no moon and the stars must be almost always hidden. The Earth with its Moon might form a beautiful ornament at times in the midnight sky if the cloud-shell should occasionally open, but on the whole, the planet is shut up to itself in a perpetual vapour-bath, and its condition will approach that of some of the most humid countries in the terrestrial tropics during the height of their rainy seasons.

But it would seem that life both of plants and animals, under such conditions, might flourish and be abundant. The mean temperature would not, in general, be high enough to drive off the water as steam, nor low enough to congeal it into ice; it would remain water—water that flows.

But there is still a possible hindrance to life on Venus, a hindrance that actually exists in the case of Mercury.

Mercury, the “Twinkler,” is not an easy object in our Northern latitudes, but, in countries near the tropics, is often quite conspicuous, a little scintillating gem of light in the bright sky, before sunrise or after sunset. In the telescope it is not so attractive as Venus, partly because it is smaller, partly because, though it receives more than three times as much light from the Sun, it is duller in hue. Yet it is not quite so secretive as its neighbour, and a certain number of markings have been detected upon its disc, markings which, like those of the Moon, appear to be permanent.

A glance at the Table will show that this was to be expected. In size, Mercury comes between the Moon and Mars, and the atmospheric veil ought therefore to be, as it evidently is, very slight and transparent; offering little or no hindrance to an observer scanning it from another world. The other necessary consequences of small size and mass will follow; the feeble force of gravitation, the languid atmospheric circulation, the extreme range of temperatures, the low temperature at which water will boil.