CHAPTER IX

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 deg.abs., or 95 deg.C, and its corresponding mean temperature is 69 deg. C. But water under a pressure of 11.2 lb. will boil at 93 deg. 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.

But the heat to which Mercury is exposed far transcends our terrestrial experience. In the mean it receives nearly seven times as much heat from the Sun as the Earth does, but this supply is not maintained uniformly, for Mercury moves round the Sun in a very eccentric orbit, so that when in aphelion it receives, surface for surface, only about four times as much heat as the Earth, but some six weeks later when in perihelion it receives more than eleven times. The great range of temperature due to the thinness of the atmosphere must therefore be further increased by the varying distance of the planet from the Sun.

A reference to Prof. Poynting's figures shows that the mean temperature of Mercury must approximate to 194 deg. C., while water will boil at 40 deg. C. or even lower. Here, then, is a condition the exact reverse of Mars. Water as a liquid will be rare on Mercury, not because it is congealed, but because it is evaporated; on the dark side of the planet it may, indeed, pass into ice, but on the side exposed to the Sun it must exist normally as a constituent of the atmosphere. Water in a liquid state, water that flows, must be almost unknown.

But we have good reason to believe that that which is the dark side of Mercury at one time is always dark; that which is exposed to the Sun is always exposed to it.

Since Mercury wears no concealing veil of atmosphere, and displays markings that can be identified and followed, a surprising circumstance has come to light. In 1889, Schiaparelli discovered that Mercury, instead of rotating on its axis in about 24 hours like the Earth and Mars, rotates in 88 days; that is to say, it always turns the same face towards the Sun, just as the Moon turns the same face towards the Earth. This fact, confirmed theoretically by Prof. G. H. Darwin in his development of the theory of tidal friction, puts the condition of Mercury in quite a new light. No alternation of day or night refreshes and restores the little world; one hemisphere is for ever exposed to the blasting heat of the Sun, seven times hotter for it than for the Earth; the other hemisphere is for ever exposed to the darkness and cold of outer space, a range from something like 390 deg. C. above freezing-point, to 270 deg. C. below. It is true that between the two hemispheres there is a "debatable land," for, owing to the ellipticity of the orbit, the face turned to the Sun is not exactly the same at all times, and a region about 47 deg. in width on each side of the planet, that is to say, rather more than a quarter of its entire surface, has one day and one night in each period of 88 days, but these more favoured sections can scarcely be considered more habitable than the rest.

The conditions of Mercury are so unfavourable for life that, even if this remarkable relation of rotation period to revolution did not hold good, it would still be impossible to regard it as a world for habitation. But its case shows that a further condition of habitability has to be satisfied by a planet. Size and distance from the Sun afford the first two conditions; a suitable rotation period is now seen to be a third.

And it is possible that in this very particular Venus fails to qualify. Schiaparelli, the first observer of his time, assisted by the clear Italian sky, believed that he had demonstrated that Venus, like Mercury, rotates once in her year; her day being thus equal in length to 225 of ours, and the face that she turns to the Sun being always the same.

And in her case this statement requires practically no qualification, for, her orbit being nearly circular, there is hardly any libration; a place that has the Sun in its zenith has it so for ever; one on the night side of Venus can never have a sunrise, or gladden in the daylight. The side exposed to the Sun will wither in a temperature of about 227 deg. C., in which all moisture will be evaporated; the side remote from it will be bound in eternal ice. In neither hemisphere will water exist in the liquid state; in neither hemisphere will life be possible.

But as yet the evidence is not conclusive that Venus has this long rotation period. Several observers of high rank believe that our neighbour rotates in nearly the same time as the Earth, but its markings are so faint and elusive that the problem is a difficult one. The spectroscopic method of determining the speed of rotation has been equally indecisive. Until, therefore, the rotation period has been decided, the habitability of Venus must remain in question. If it always turns the same face to the Sun, there can be no more life upon it than upon Mercury; if on the contrary it rotates in much the same time as the Earth, then, so far as we know, it may well be a habitable world. Whether it is actually inhabited is a matter at present entirely beyond our knowledge.

A page or two back we touched lightly on the eccentricity of the orbit of Mercury--lightly, because it was not the chief factor in disabling the planet for habitation. But the condition introduced by this eccentricity is one which of itself would be sufficient to put it out of court. In the six weeks in which Mercury moves from aphelion to perihelion, it approaches the Sun by fourteen millions of miles, and the heat received by it is increased 2-1/2 times. Then, in the next six weeks, it recedes as far, and there is a like diminution. In other words, six weeks makes a greater proportional change in this one planet's condition than we should experience if our Earth were transported from its own orbit to that of Mars.

But there are other members of the solar system whose orbits are so elongated that that of Mercury seems in comparison almost circular. These are the comets, some of which all but graze the surface of the Sun at perihelion, and then recede from him for periods that it takes even thousands of years to complete. But without dwelling on such extreme cases, two of the best known of the periodic comets may be taken as examples of the rest. Encke's is the comet of shortest period, returning in about 3.3 years. At perihelion it is 31 millions of miles from the Sun; one-third the distance of the Earth. It receives, therefore, at this part of its orbit, 9 times as much light and heat as the Earth. But at aphelion it retreats deep into the region of the asteroids, and is much more than four times the mean distance of the Earth. At this part of its orbit it receives but 1/17th as much heat as the Earth. By far the most famous of all the comets is that known by the name of Halley, and its mean period is 76 years. At perihelion it comes within the orbit of Venus; indeed, nearly halfway between that and the orbit of Mercury. At aphelion it recedes to thirty-five times the distance of the Earth, far beyond the orbit of Neptune. The range in its light and heat from the Sun is from 3 times that of the Earth to less than 1/1200th; or, in other words, the supply of heat at one time is nearly 4000 times that at another, and of the 76 years of its period, only 80 days are spent within the orbit of the Earth.

Comets cannot be homes of life; they are not sufficiently condensed; indeed, they are probably but loose congeries of small stones. But even if comets were of planetary size it is clear that life could not be supported on them; water could not remain in the liquid state on a world that rushed from one such extreme of temperature to another.

Between the orbits of Mars and Jupiter there are scattered an untold number of little planets commonly known as asteroids or minor planets. Minor planets indeed they are, for the one first discovered--Ceres-- probably outweighs all the rest, known and unknown, put together, though something like 700 have already been detected, and the list grows at the rate of about one a week.

As the Table shows, Ceres is so small that the Earth exceeds it in volume 5000 times; even the Moon is 90 times as large. The mass of Ceres is not known; being so small, its density is probably less than that of the Moon, so that the Earth may easily outweigh it 10,000 times. The unfavourable conditions resulting from smallness of size that the Moon presents are therefore exaggerated exceedingly in the case of Ceres; its atmosphere must approach in tenuity what we should regard as a vacuum in a terrestrial laboratory, and water as a liquid be entirely unknown. Its distance from the Sun is another hostile factor; for in consequence it receives per unit of surface only 13 per cent of the light and heat that falls on the Earth; its maximum temperature under a zenith Sun will fall far below freezing-point, the minimum on the dark side will approach the absolute zero.

With Ceres the whole of the asteroidal family can be dismissed as possible abodes of life. No astronomer can regard them as such. Yet they have their lesson to teach. Life can exist on the Earth only on the upper face of its crust, and in a very thin film of air and water; but the enormous solid bulk within, inert though it be, that supports the stage on which the great drama of life is played, is as really essential as air and water themselves. If that bulk were much smaller and less massive life could find no place upon its surface.