Popular Science Monthly/Volume 64/November 1903/Life in Other Worlds

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By F. J. ALLEN, M.A., M.D., Cantab.,


THE question 'whether life exists in any other worlds than our own' is one in which very many persons feel an interest, and about which much has been said and written; but if the man of ordinary education has any ideas on the subject, they are generally mistaken; and even scientists are prone to regard it too exclusively in the light of their particular science, and thus to conceive and propagate fallacies which might be easily avoided.

Of course no absolute answer to the question can be given until perchance some hitherto undreamt-of means shall be discovered for the observation of distant worlds. We can hardly forward the matter by mere speculation on general grounds, such as the law of probabilities, or the relative position of worlds in the universe. Nevertheless there is one method by which we can at least guard ourselves against erroneous speculation, and prepare the way for discovery when the opportunity comes; and that method is, to find out the conditions on which terrestrial life depends, and then to search other worlds and find if possible whether they provide similar or parallel conditions.

Though the ultimate nature of life is as yet unknown to us, its secrets are being gradually unraveled by research; and it becomes more and more apparent that the phenomena of life are but special and intricate developments of physical action. The most prominent and perhaps most fundamental characteristic of life is what may be called the energy traffic, or the function of trading in energy; and the phenomena of assimilation, growth, movement, etc., are the outward and visible signs of this traffic. Living substance possesses in the highest degree the property of absorbing radiant energy, as heat or light, storing it in a potential form, and subsequently expending it in active forms such as motion, mechanical work, heat and electricity.

Actions of this kind are not unknown in the inorganic world. For example, the atmosphere and ocean absorb the energy of light and heat from the sun, store it temporarily and convert it subsequently into the energy of wind and wave, lightning and thunder. But in living substance there exists a more finely coordinated energy-trading system, evolved out of the chemical capacities of a small number of elements acting under the physical conditions which prevail on our planet.

The energy traffic depends, no doubt, on causes at present unknown to us; and some biologists are wont to personify these causes as a 'vital force,' an influence (sometimes regarded as external) which modifies the behavior of matter and energy without affecting their quantity. But as we can not exclude the probability of similar directing and modifying influences in the inorganic world, it is best provisionally to regard the causes of life as present alike in living and not-living substances—conspicuous in living substance because coordinated, but hardly observed in not-living substance, owing to incoordination. If a simile be needed, it may be found in the behavior of light: for certain properties of light, though ever present, become evident only when coordinated by polarization; and polarization, be it noted, is a purely physical action.

While every element present in living substance may assist in the work, the energy-traffic is carried on chiefly amongst the four elements, nitrogen, oxygen, carbon and hydrogen. Nitrogen is remarkable for the instability of its chemical compounds—the readiness with which they change their composition—and there is little doubt that on this property depends the extreme sensitiveness of living substance. Carbon and hydrogen have the property of combining together to build up complex compounds, with great storage of potential energy; whereas the same compounds during their oxidation expend their energy in the form of mechanical work, heat, etc. The energy traffic consists of two alternating phases: (1) the accumulation of energy, or 'anabolic' phase, which is always coincident with deoxidation and the formation of complex chemical compounds: and (2) the dispersion of energy, or 'catabolic' phase, coincident with oxidation of the complex substances, which are thereby converted into simpler substances, as carbonic acid and water. In these processes nitrogen is intimately concerned: it is believed to act as the carrier, taking up each element or group of elements and passing it on in a new state of combination.

All the energy of life is derived ultimately from the sun. A little of this comes indirectly through lightning, which in passing through the air forms ammonia and oxides of nitrogen. These, being carried by rain into the ground, are the constant source of nitrogen for vegetable, and indirectly for animal life. A much larger quantity of energy is well known to be taken direct from the sunshine by plants and used in their anabolic processes. This energy is appropriated by animals in their food; and whether in the vegetable or in the animal, it assists in many alternations of anabolism and catabolism before it is completely dispersed.

The range of temperature suited to terrestrial life is comparatively narrow. All vital actions are suspended temporarily, some permanently, if subjected to a temperature near the freezing point; while the highest that most organisms can bear lies somewhere between 35° and 45° Centigrade (95° and 113° Fahrenheit). Only the spores of certain bacteria can survive boiling. It is therefore probable that if the general temperature of the earth's surface rose or fell 40° (a small amount relatively), the whole course of life would be changed, even perchance to extinction. The record of the fossiliferous rocks shows us that for countless millions of years a large portion of the earth's surface has had a temperature much the same as it now has; it is even probable that the surface temperature never greatly exceeded 40° C, though the interior was, and is, very hot.

Water plays an indispensable part in both the environment and the internal chemistry of life. It forms more than half the weight of most living things; and all the actively living parts of animals and plants (e. g., the nuclei and protoplasm of cells) consist of water holding the other ingredients in solution or suspension.

Every one of the conditions above mentioned (supply of energy, particular elements, range of temperature, abundance of water) is essential to life—i. e., such life as is known to us; and it is difficult to avoid the conclusion that this life is really the outcome of the conditions existing on our earth, and that only in worlds with identical conditions can identical life exist.

It is quite odd how, in spite of the advance of biological science and the acceptance of the principles of evolution, the notion still prevails that life in other worlds is similar to that of our earth. We find astronomers searching for the absorption bands of chlorophyll in the spectrum of Mars; marks on planets are described as probable vegetation; some worlds are supposed to be uninhabitable because they have no atmosphere, others because the temperature is too high for the existence of 'protoplasm.' All this indicates a very contracted view of the nature of life. Chlorophyll, respiration, vegetable, animal and protoplasm are earthly phenomena which may exist nowhere else: their place may be taken in other worlds by other phenomena no less wonderful. What we know of terrestrial life gives us reason to think that the same principles which produce life under earthly conditions may produce life of a different type under different conditions; e. g., where the temperature is different, and a different set of elements are available.

It must be freely admitted that we do not know what elements could take the place of nitrogen, carbon, etc., under conditions differing from those on our earth. All speculations concerning this question have been based on misconceptions of the functions of the elements in life. We may only venture so far as to say that certain elements suggest possibilities of energy traffic by reason of the varied character of their compounds: such are phosphorus, sulphur, iodine and iron. Other elements, such as aluminium and silicon, are remarkable for the monotony of their known chemical actions....


In order that any world may support life comparable with the life that we know, the following conditions, among others, are necessary, namely, (1) A supply of radiant energy which is intermittent or variable, (2) one or more elements which (like nitrogen) are very sensitive to changes of energy, and (3) one or more elements which (like carbon and hydrogen) are capable of alternately accumulating and dispersing energy by means of opposite chemical changes.

Since popular ideas as to the physical conditions of other worlds are generally hazy and often far from correct, it may not be amiss to recount such particulars as astronomical research has revealed in the members of the solar system.

In looking for a world where the conditions are most like our own, we naturally turn first to Venus. Her size and gravitation are nearly the same as those of the earth, and her atmosphere may therefore be expected to have a similar density and to hold approximately the same gases. As she is nearer the sun, the general temperature would be considerably higher, and the equatorial region might be too hot for our life; but there might nevertheless be a suitable temperature nearer the poles. But our speculations are damped by a suspicion, strong but not fully confirmed, that Venus has no day and night, but always keeps the same side toward the sun. If this is really the case, then the sunny side must be always burning hot and quite dry, while the opposite side must be always encased in ice—nay more, in a mixture of ice and solidified atmospheric gases. The life of such a world must be very different from any that we know.

After Venus, Mars is the planet whose conditions seem most to resemble those of our world. But there are far greater differences than are generally supposed. Mars is so small that he can not provide much heat from within, and so far from the sun that he receives comparatively little heat from without. His gravitation is so slight that the atmosphere is rare and nearly cloudless, and therefore heat must be readily lost by radiation. Thus on theoretical grounds Mars should be intensely cold: in fact his surface should be constantly in the condition of the highest mountain-tops of our world, only receiving less heat than they do from the sun. At such low temperature and pressure, water could never exist in the liquid form, though it might be solid or gaseous. But water is very possibly absent from Mars. Dr. Johnstone Stoney has calculated, by application of the dynamic theory of gases, that any water vapor introduced into the atmosphere of that planet would escape into space, the gravitation being there insufficient to retain it. Professor G. H. Bryan, calculating from slightly different data, questions Dr. Stoney's conclusions; but at all events Mars's gravitation is very near the dividing line between the ability and inability to retain water. If water is absent from Mars, then the polar caps and other seeming evidence of its presence must be due to some other fluid or gas, having heavier molecules and lower freezing and boiling temperatures. Dr. Stoney himself holds that carbon dioxide would give the appearances of vapor, frost and snow, which are seen with the telescope; and there are still heavier gases which might be imagined to be present. In any case, the conditions seem quite incompatible with life of our earthly type.

As already hinted, the atmosphere of a world depends on its gravitation. Gases tend to diffuse into space, unless retained by adequate attraction. Our earth can hardly retain so light and mobile a gas as hydrogen; Mars may have difficulty in retaining the less mobile vapor of water; but the gravitation of the moon is too slight to retain any known gas, hence she has no atmosphere and no water. Yet this is not sufficient reason for assuming the absence of life. The surface of the moon is usually considered to have been for a long time in an inert state. If it had been so, the accumulation of meteoric stones and dust during ages would have covered it with a uniform veil. Instead of this, the surface presents much variety of tint and texture, indicating a still continuing geological activity; and some changes in its markings are said to have been observed in recent years. Professor Lapworth, regarding it with a geologist's eye, feels convinced that the moon is an active and living world. The geological activity may be the result of the extremes of temperature which are produced by the regular alternation of a half-month's sunshine and a half-month's darkness. At the same time such extremes might awaken to vital activity elements which behave as dead on this earth.

In contrast to the moon are the very large planets, Jupiter and Saturn. Owing to the high gravitation, the atmosphere of such planets is very dense, and so loaded with opaque particles that we can not see through it to the body of the planet within. But though the body is beyond our scrutiny, we can infer that it is very hot, even at the surface; for if the solar system was formed (as is assumed) by condensation of a nebula, the heat of condensation must be proportionately greater and longer retained in a large world than in a small one. Thus for the purposes of life on these great planets the energy radiating from within may be available, and indeed may largely exceed the energy received from the sun at so great a distance. The satellites of these planets may resemble our moon, except that they receive much less energy from the sun.

Of Uranus and Neptune we know very little; but their large size leads us to suppose that their physical conditions may have some resemblance to those of Jupiter and Saturn.

What can we say of the possibility of life in the sun? The visible surface or photosphere has a temperature so high that even iron exists there as a gas, and almost all chemical compounds known to us may be dissociated. The deeper parts are doubtless far hotter still, with chemical possibilities or impossibilities beyond our comprehension. In fact the chemistry of the sun is of so different an order from any that we have experience of, that we can not reconcile it with any of our notions of life. But if such a vast fund of energy can run to waste without producing life except in distant planets, there must be in nature an extravagance such as almost to justify those persons who imagine our own world to be the only one bearing life, and the whole universe to be made for man.

These few particulars, which we have learned concerning the physical conditions prevailing in other worlds of our solar system, are distinctly against the probability of their possessing life similar to that existing in this world. If they contain life, even life depending on the same principles, it must be quite different in its manifestations, and not easily recognizable with the telescope. If any germ of life should escape from this world and land upon another member of the solar system, it must pretty certainly perish for want of the necessary conditions. The same might be said of a germ from another world landing on our earth—if indeed we have any right to speak of a 'germ' from another world, since the word is geomorphic, and actually assumes that similarity which we hold to be improbable in life under different conditions.

Whether any members of other solar systems resemble our own in physical conditions is naturally a matter of pure speculation; but there is no a priori impossibility, since the spectroscope shows that many of the fixed stars contain the same elements as our sun, and have about the same temperature.


We are considering only such life as depends on the same principles as earthly life. We may admit the possibility of other kinds of life, having nothing in common with such life as we know, but at present we have no grounds for speculation concerning them. Keeping within the bounds of legitimate induction, we are led to the following conclusions:

1. If life is essentially a function of the elements nitrogen, oxygen, carbon and hydrogen, acting together, then it can probably occur only on exceptional worlds, with conditions closely resembling those of our own earth. Such conditions are not present in any other world in our solar system, nor can they be expected to occur frequently in members of other systems.

2. On the other hand, if different conditions can awaken a capacity for exalted energy traffic among other elements than those just named, then the universe seems to provide immense possibilities of life, whose variety and magnificence may far exceed anything that we can imagine.