E. Walter Maunder

“ From the consideration of the planets in our own system, we have seen how stringent and how many are the conditions imposed for Life to be possible. Round our Sun there is but a narrow zone in which a habitable world may circle; in this zone there is room for but few worlds, and we actually know of three alone, the Earth, the Moon, and Venus. We know that the Earth can be and is inhabited; that the Moon is not and cannot be inhabited; and that Venus, though of habitable size, may yet be subject to the fatal disqualification of always turning the same face to the Sun. ”

“ It seems impossible to believe that Life, so rare a fruit of the universe, intelligent Life, conscious Life, to which the long course of evolution has been so manifestly leading up all through the long ages, should have [Pg 160] no better destiny than a final and hopeless extinction; that this Earth and all the efforts and aspirations of the long generations of men should have no worthier end than to swing, throughout the eternal ages, an empty, frozen heap of dust, circling round the extinct cinder that was once its Sun. If we look backward, we seem to discern clear signs of progress ”

“ This is what is meant when we speak of a world being “inhabited.” It would not, for example, at all content us if we could ascertain that Jupiter was covered by a shoreless ocean, rich in every variety of fish; or that the hard rocks of the Moon were delicately veiled by lichens. Just as no richness [Pg 3] of vegetation and no fulness and complexity of animal life would justify an explorer in describing some land that he had discovered as being “inhabited” if no men were there, so we cannot rightly speak of any other world as being “inhabited” if it is not the home of intelligent life. ”

“ But if Jupiter be a semi-Sun, still a source of heat, perhaps even of light, can it yield the means of life to its satellites? For Jupiter is sun-like, not merely in its own condition, but also in that it is the centre and ruler of a system of its own. We know already of eight satellites revolving round it.
Of these eight, only four—the four discovered by Galileo, in the first days of his possession of a telescope—need be considered; the other four are of the same order of size as the asteroids, and are indeed much smaller than Ceres. ”

“ The range of conditions through which we find life on this Earth is as nothing to the range given by the varied sizes and positions of the different planets; and even on our Earth, life in the unfavoured regions—the tops of mountains, the polar snows, the waterless deserts, the ocean depths—is only possible because there are more favoured regions close at hand, and there are, as it were, “crumbs that fall from the rich man’s table.” ”

“ We know that this, our Earth, is a habitable globe, for we ourselves are living upon it. We know what constitutes the physical basis of our life, and under what conditions on this Earth it flourishes, and under what conditions it is destroyed. ”

“ There are only two of the heavenly bodies that appear to be more than points of light, only two that show a surface to the naked eye, and the Sun, being so much the brighter of the two, and the obvious source of all our light and heat, and the fosterer of vegetation, readily takes the premier place in interest. In the present day we know too much about the Sun for anyone to suppose that it can be the home of organic life; but it is not many years since its habitability was seriously suggested even by so high an authority as Sir William Herschel. ”

“ The form and size of the flame are equally defined by exterior conditions; they are imposed upon it by the shape of the burner and the pressure of the gas passing through it. The form of the engine is as its designer has made it. But the form of the living organism is imposed upon it from within; and, as far as we can tell, is inherent in it. Here is the wonder and mystery of life: the power of the living organism to assimilate dead matter, to give it life and bring it into the law and unity of the organism itself. ”

“ But a radiating body radiates from its surface, while the store of heat from which that radiation is kept up is supplied by its volume. It follows, therefore, that a large and heavy world must differ from a small light world, not merely in scale, but also in kind.
The surface of a world is all that we see of it; it is, therefore, very commonly all that we consider. But unseen, and hence often unconsidered, beneath the surface lies its substance or mass, and it is this that determines the state and condition of the surface; it is the underlying power. ”

“ 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. ”

“ Only those members of our solar system that shine by reflecting the light of the Sun can be cool enough for habitation; the true stars cannot [Pg 35] be inhabited, for, whatever their quality and order, they are all suns, and must necessarily be in far too highly heated a condition to be the abode of life. Many of them may, perhaps, be a source of light and heat to attendant planets, but there is no single instance in which such a planet has been directly observed; no dark, non-luminous body has ever been actually seen in attendance on a star. ”

“ The mass of the planet is 317 times that of the Earth; it is indeed nearly three times as great as that of all the other planets put together. But the aggregation of so vast an amount of material is of itself a source of heat; the chief source at the present time of the enormous output of heat from the Sun is ascribed to its gradual contraction; the slow falling of its substance, if we may so express it, a little nearer to its centre. The great mass of Jupiter points to its inherent store of heat being much greater than that of any other planet. ”

“ 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 temperatures, on the one hand, are sufficient to convert into a permanent gas every substance with which we are acquainted; the pressures, on the other hand, apart from the high temperatures, would probably solidify every element, and the Sun, as a whole, would present itself to us as a comparatively small solid globe, with a density like that of platinum. With both factors in operation, we have the result already given: a huge globe, more than one hundred times the diameter of the Earth, yet only one-fourth its density, and gaseous probably throughout the whole of its enormous bulk. ”

“ For a wind of such violence mows down buildings and trees as a scythe the blades of grass; and were tornadoes moving at a rate of 300 miles an hour as common upon the Earth as spots are upon the Sun, it would be stripped bare of plants and animals, as well as of men and of all their works. ”

“ The size of Mars taught us that we have in it a planet with an atmosphere of but one half the density of that prevailing on the top of our highest mountain; the distance of Mars from the Sun showed us that it must have a mean temperature close to that of freezing mercury. What chance would there be for life on a world the average condition of which would correspond to that of a terrestrial mountain top, ten miles high and in the heart of the polar regions? But Mars in the telescope does not look like a cold planet. ”

“ Between the earth and man arose the leaf. Between the heaven and man came the cloud. His life being partly as the falling leaf and partly as the flying vapour.” [7]
The leaf and the cloud are the signs of a habitable world. The leaf—that is to say, plant life, vegetation—is necessary because animal life is not capable of building itself up from inorganic material. This step must have been previously taken by the plant. The cloud, that is to say water-vapour, is necessary because the plant in its turn cannot directly assimilate to itself the nitrogen from the atmosphere. ”

“ If we take the Earth at noonday at the time of the equinox, then a square yard at the equator has the Sun in its zenith, and is fully presented to its light and heat. But, as we move away from the equator, we find that each higher latitude is less fully presented to the Sun, until, when we reach latitude 641⁄2°—in other words just outside the Arctic Circle—7 square yards are presented to the Sun so as to receive only as much of the solar radiation as 3 square yards receive at the equator. We may take, then, latitude 641⁄2° as representing Mars, while the equator represents the Earth. ”

“ Every world must have its Past and its Future, as well as its Present. For some worlds the conditions are so fixed that, like Jupiter and Saturn, they are not now worlds that can be dwelt in, they never were in that condition, and they never can be; their enormous mass forbids it. Mercury and the Moon at the other end of the planetary scale are also permanently disabled; their insignificant size excludes them. There was also a time when the Earth was not a world of habitation ”

“ On the Earth we find animated existence confined to the surface of the crust of the globe, to the lower and denser strata of the atmosphere, and to the film of water that constitutes the oceans. It does not exist in the heart of the rocks forming the body of the planet nor in the void of space surrounding it outside the atmosphere. ”

“ We have simply to ask with regard to each heavenly body which we pass in review: “Are its physical conditions, so far as we can ascertain them, such as would render the maintenance of life possible upon it?” The question is not at all as to how life is generated on a world, but as to whether, if once in action on a particular world, its activities could be carried on. ”

“ If, from some other globe in the depths of space, this world of ours could have been watched during the long aeons that elapsed from its first separation from the solar nebula down to the time when it first possessed a surface of land and water, and from that time, again, throughout the hypothetical one hundred million years that preceded the advent of man, then, during all those aeons, those imagined observers would have had under their scrutiny a world as yet without inhabitant. ”

“ Its procuring depends on the intelligence of the organisms that stand in need of it.... As a planet ages, any organisms upon it will share in its development. They must evolve with it, indeed, or perish. At first they change only, as environment offers opportunity, in a lowly, unconscious way. But, as brain develops, they rise superior to such occasioning.... The last stage in the expression of life upon a planet’s surface must be that just antecedent to its dying of thirst.... With an intelligent population this inevitable end would be long foreseen.... ”

“ Of that great change—when non-living matter first became endowed with life, became so endowed not by the action and intervention of other living matter, but without it—we have no knowledge, no experience. And so long as this continues to be the case, that change, the greatest physical change that has yet taken place in the history of the universe, the first change of the non-living into the living, is outside the reach of science ”

“ So wide a range of temperature must be fatal to living organisms, particularly when the range is repeated at short, regular intervals of time. But this range of temperature comes directly from the length of the Moon’s rotation period; for the longer the day of the Moon, the higher the temperature which may be attained in it; the longer the night, the greater the cold which will in turn be experienced. We learn, therefore, that the time of rotation of a planet is an important factor in its habitability. ”

“ And when we consider how puny are men as we see them on this our planet, how minute their greatest works, how superhuman any undertaking would be which could demonstrate our existence to observers on another planet, we must admit that it is a marvel that there should be any evidence forthcoming that could bear one way or another on the solution of a problem so difficult. ”