A GLOBE of shining silver, pure and spotless, brightest object in all the heavens, save for the Sun and Moon, the planet longest known, perhaps, certainly the second, at worst -- Venus, the planet about which we know less than of any other of the solar system, with the single exception of Pluto, last discovered, and most distant of the Sun's children.
Venus has been known ten thousand years -- has been known to be different from the stars for all recorded time. Fifteen hundred years before Europeans accepted the planetary theory that made the Earth and the other planets circle the sun, the Mayans of Central America had observed and computed the orbit of Venus in many ways.
Yet we know more about Neptune, undiscovered till telescopes reached large dimensions -- so dim and distant as to be invisible in small telescopes. We know Neptune's mass with great accuracy. We know the length of Neptune's day.
We know the mass of Venus only to a fair approximation. We can say of its day only that it is more than twenty, certainly, and less than fifty, probably around thirty days.
Venus is the nearest of the major planets. Mars has always been the center of interest with regard to speculation on possible inhabitation. Yet, in many ways, Venus should be considered before Mars.
The answer is easy. We can see enough of Mars, know enough about it to make intelligent speculations. Venus is the unknown, the most mysterious of the planets.
FIRST, of the planets that might bear life. To consider the possibilities of life on Venus, we must first lay down some basic foundation of data on life -- and on Venus. We can get the life data -- data that will be all-embracing enough to be remembered in the discussions on all the planets -- later on.
We can delineate limits. First: We use the term "life as we know it" rather too loosely. It is senseless to expect all life to consist of rabbits and elephants, human beings and goldfish. Life as we know it must mean something broader than life in the forms we know. By sufficient use of the imagination, we can conceive of life built up of pure forces. Life is a complex thing, recognizable by its ability to use obscure natural law to overcome evident laws of nature.
An unsupported body heavier than the surrounding medium will fall. That is an evident law of nature. But unsupported birds don't drop; they fly, by calling on the obscure laws of chemistry for power, the hidden mechanisms of nervous energy for control, and the laws of kinetic gas flow for support. Life must obey natural law, but it can amplify and bend to its needs less-common laws.
It must be complex. If it consists of pure force, those forces are so far beyond our imagining -- certainly our knowledge -- as to make it useless to speculate on them.
Life as we know it is chemical. It is a chemical reaction continually maintained in a state of unbalance while it is life: death is the balance finally attained by that reaction. We do not know the full processes of life in chemical form, but we know a good many fundamental rules.
First: Any complex chemistry, any delicate sensitivity of nervous tissues, must take place in solution. Drying is inevitably and invariably fatal to any known form of life. Certain spores, in view of this, protect themselves against drying by means of waxy or otherwise resistant husks; but if they are actually dried, they die.
Second: There must be available a supply of chemical energy. Conceivably, a planet might have native masses of iron and quantities of water. Low forms of life could exist on this chemical energy -- the rusting of iron -- without the benefit of an active, gaseous atmosphere. But, inasmuch as chemical energy tends to escape, there usually are no solid or liquid chemicals available. Further, that requires a concentration of life in those spots. Then, generally speaking, life depends on an atmosphere of an active gas. An atmosphere one thousand miles deep consisting of helium, argon and nitrogen would be practically useless, since none of these gases is active enough to support life. There must be atmosphere, and an active atmosphere.
Third: There must be sufficient, but not too much, warmth. That range is wider than we normally think -- because pressure of the active atmosphere makes a great difference, and because liquids other than water may form the solution.
What liquids are possible? The liquid must be active -- active as the gaseous atmosphere. Benzene is a liquid at quite low temperatures, but is not a good medium for life, probably an impossible one. At even lower temperature ethane gas becomes and remains a liquid, but is not suitable because it is inactive.
Water we normally think of as inactive. That is a serious error. It seems inactive only because it has already attacked viciously every substance it can reach, and reacted with it. Sulphuric acid is really less active than water, in some respects. Sulphuric acid forms a salt with calcium CaSO4, and can no longer react with it. Water starts in when sulphuric acid is exhausted, and attacks the compound to form CaSO42H2O.
There are millions of tons of that compound, and we say water is inactive because we came along too late to see it at work. Put a little water on CaO -- quicklime -- and watch the distinctly violent action. Water attacks viciously sodium, potassium and calcium metals, which we keep in the inactive liquids like benzene and gasoline. Iron certainly has short shrift with water present.
Because of that voracious and catholic activity, water will dissolve to some extent practically every known substance. Animals and plants found a wonderful medium for their life processes, used it, and through evolution discovered ways to prevent the solution of their needed tissues.
The second-best solvent in the world, or universe, is ammonia -- not ammonia water, but the liquid ammonia used in refrigerating plants. Ammonia -- NH3 -- is also an active substance, forming complex compounds similar to water's hydrated calcium sulphate. Our own types of life can't exist in ammonia, but there is no reason to believe other types couldn't. Unfortunately, ammonia is unstable, with the result that here on Earth we spend millions synthesizing it from nitrogen and hydrogen as fertilizer for plants that can't live in it, and can't live without it.
Sulphur dioxide -- SO2 -- is another possibility, an active liquid under slightly different conditions, a bit more pressure and a bit lower temperature. It also could be a basis for life.
NOW, so far as gases go, we can name a few, but whereas the possible liquids are by no means exhausted with those three -- though they are the most important possibilities -- the gases are more limited. Under the conditions of Earth, so far as temperature and pressure go, there are only three that can be considered: oxygen, of course; flourine; and chlorine. All three of these are active gases at our temperatures, entering readily into the compounds used by life forms. Flourine is the most active, and would be a possibility under lower temperature and lower pressure, for while the activity of any chemical declines with the temperature, the activity of a gas declines also with the pressure, since pressure represents concentration in gases.
The possible substances, under any conditions, are fluorine, oxygen, chlorine, bromine, hydrogen and iodine. That is the order of declining activity. To make them equally possible, the temperature must be increased. Thus, fluorine would be active enough to support life at our pressure and a temperature of -80° or so; oxygen at -20°; chlorine at -10°; bromine at 70° above zero, and iodine at a temperature of at least 200° above zero.
You noticed the omission of hydrogen? Temperature is not as important to hydrogen as is pressure. Immense pressure can be applied to hydrogen and the activity of hydrogen increases rapidly under that influence. For instance, nitrogen and hydrogen -- which can combine to ammonia -- break down to hydrogen and nitrogen normally. But the Haber process of making ammonia simply mixes the two gases under great pressure -- and they combine quite actively, heat, as usual, being applied to speed the reaction.
But, while fluorine and chlorine cannot stand much greater pressure than Earth's atmosphere at any low temperature, because of liquefaction, oxygen and hydrogen can be compressed terrifically at very low temperatures, and hydrogen withstands any conceivable pressure without liquefaction at any temperature above the critical temperature which is far, far below zero centigrade -230° below. But pressure applied to either bromine or iodine immediately restores it to liquid, which naturally removes it from the atmosphere. Incidentally, there are compounds, HF, HCl, and HBr -- the hydrogen compounds of each of those gases that are liquid under sufficient pressure, and are some of the other possible liquids of life.
Any life-bearing planet must have carbon. All chemistry, unlike Gaul, is divided into two parts: inorganic chemistry, dealing with ninety-one of the ninety-two elements; and organic chemistry, dealing with the other one -- carbon. And there are more organic compounds than in all inorganic chemistry. No complex chemistry could exist without carbon, because carbon can tie onto carbon atoms in a way other elements cannot. One other element weakly imitates carbon in this -- silicon, a first cousin of the carbon atom -- but too weakly to replace carbon.
We've laid down a basis now. One more thing, and we can rule out many a planet: temperature. If the temperature is too high, as in the Sun itself compounds cannot form. There can be no life there. If the temperature is too low, chemical reactions are slowed to paralysis; they cannot take place. Sodium, virulently active at our temperature, becomes so inactive that it can be kept indefinitely in a water container -- ice then -- on a planet near absolute zero, as is Pluto, and yet well above the temperature of liquid hydrogen. If even that prodigiously active substance is paralyzed, what chance have life's delicate activities.
Right now we can eliminate Pluto from consideration as a life-bearer of any chemical type; the temperature is too low. Mercury we eliminate because it is too hot for complex delicate organic substances.
VENUS has a warm -- but not too warm -- temperature. It apparently has a deep atmosphere. It is about the size of Earth -- so nearly so that they are almost twins. Gravity on Venus is 85% of that on Earth. But -- the vast clouds obscure the secrets, clouds that roll continuously, unbroken, through all astronomical history. By their very secrecy they tell us some things -- though no man has ever seen the true surface of the "Veiled Planet."
There must be water; it is difficult to imagine that any substance other than water would form those clouds, though our spectroscope tests show no water vapor. Still, they may be clouds of ice particles, with so little vapor present as to be unobservable.
Further, we can make our tests only on the thinnest, outermost layers of the atmosphere, above the clouds, since we cannot see beneath them. The measured depth of visible atmosphere above the clouds is considerably less than one mile, so our tests are not delicate. There is not material enough there to affect the spectroscope very heavily. We can, however, find no oxygen, no water vapor.
By direct observation we see only mirroring clouds, white and reflective, as characterless as a mirror. But that is not all. The deepest probe man has is not light, but gravitation, that passes unhindered through all things. We can -- as I will tell later -- learn more about the internal structure of Saturn than we can about Earth itself! Nine moons of varying size circle Saturn, each influenced and utterly bound by Saturn's mass. The character of that binding and the way they move, tells us a great deal about the structure of Saturn far below the cloud layer that covers the planet.
But Venus has no moon, no revealing satellite. All the information we can get of Venus is size, shape, distance from the Sun and, of course, a measure of its reflecting power. That is important -- that albedo, or mirror power; for, while Venus, being three quarters as far from the Sun as Earth, receives, due to the inverse-square law of radiation, twice as much heat from the Sun, the mirroring clouds send a lot of it right back where it came from -- space.
But we can calculate the temperature of Venus -- from this data -- fairly easily. It's high, very high for a planet. On the equator the seas are boiling, slowly throwing out great bubbles of steam. Only the very poles of the planet are remotely livable, hotter even there than Earth's equatorial regions.
Far below the cloud layer lies Venus' rocky surface -- hot, mist-wrapped, and whipped by eternally driving winds that blow only endless, world-wrapping mists. The spectroscope shows us no rotation of Venus, and if the planet revolved even one twentieth as swiftly as Earth, it would tell us. If Venus rotated one thirtieth as rapidly as Earth the equator would bulge slightly under centrifugal force. But neither effect is seen. Venus is a blank, secretive, cloud-wrapped world.
If we went by these indications alone, we might guess Venus had an eternal day. But if this were true, the side facing forever into space would be as cold as space, and there could be no clouds, only vast glaciers of fallen snow. So Venus does turn. But calculation shows only that it must turn more than once in fifty days. Astronomers conclude it probably has a day equal to thirty of ours.
But we don't know. Silvery, mirrored, wonderfully brilliant, our nearest neighbor shines in the morning or evening sky, never more than 46° from the Sun. Silvery, mirrored, unknown, our nearest neighbor is the greatest mystery of the system. Inhabited? We don't know; can't say. We can only guess, and estimate that it is probably not. We know more, in some ways, of Pluto than of Venus, and no telescope can ever tell us much of Venus.
COMING NEXT MONTH: The Double World
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