molecules with speeds surpassing the escape velocity.
The result is that the Moon is without an atmosphere. To be sure, even the Moon’s low gravity can hold some gases if their atoms or molecules are massive enough. The atoms of the gas krypton, for instance, have a mass of 83.8 and the atoms of the gas xenon, a mass of 131.3. The Moon’s gravitational field could hold them with ease. However, these gases are so uncommon in the Universe generally, that even if they occurred on the Moon and made up its atmosphere,that atmosphere would be only a trillionth as dense as the Earth’s atmosphere, if that, and could at best be described as a “trace atmosphere.”
To all intents and purposes, as far as the problem of extraterrestrial life is concerned, such a trace atmosphere is of no consequence and the Moon can still fairly be described as airless.
All this has meaning with respect to a liquid such as water. Water is “volatile,” that is, it has a tendency to vaporize and turn into a gas. At a given temperature, there is a countertendency for the gaseous water vapor to recondense into liquid. At any particular temperature, liquid water is therefore liable to be in equilibrium with a certain pressure of water vapor, provided that water vapor is not removed from the vicinity as, for instance, by a wind.
If the water vapor is removed, equilibrium pressure is not built up and more of the liquid water vaporizes, and still more, till it is all gone. We are all familiar with the way in which the water left behind by a rainstorm evaporates until it is finally all gone. The higher the temperature, the faster the water evaporates.
Naturally, the water vapor is not removed from the Earth altogether. If it does not condense in one place, it condenses in another as dew, fog, rain, or snow, and thus the Earth holds on to its water.
If there were liquid water on the Moon, the vapor that would form would leak out into space, for the mass of the water molecule is but 18 and the Moon’s gravitational field would not hold it. The liquid water would continue to vaporize and eventually the Moon would dry up altogether. The fact that there is no air on the Moon means there is no air pressure to slow the rate of water evaporation, and the water, if it had been present, would have been lost all the more quickly.
The Moon, therefore,
must
be without water as well as without air. What’s more, any airless world would be a lifeless world—not because air is necessarily essential to life, but because an airless world is a waterless world, and water is essential.
Even the kinetic theory of gases leaves loopholes, however. The possibility remains that scraps of water, even air, can exist underground on the Moon, or in chemical combination with molecules in the soil. In that case, the small molecules would be prevented fromleaving by forces other than gravity—by physical barriers or chemical bonding.
Then, too, there may have been a time early in the history of the Moon when it had an atmosphere and an ocean,
before
it lost them both to space. Perhaps in those early days, life developed, even intelligent life, and it may have adapted itself, either biologically or technologically, to the gradual loss of air and water. It might, therefore, be living on the Moon in caverns, with a supply of air and water sealed in.
As late as 1901, the English writer H. G. Wells (1866–1946) could publish
The First Men on the Moon
and have his heros find a race of intelligent Moon beings, rather insectlike in character and highly specialized, living underground.
Even that much seems doubtful, however, since calculations show that the Moon would have lost its air and water (if any) quite rapidly. It would have retained them for many times the lifetime of a human being, of course, and if we were living on the Moon when it still had an atmosphere and ocean we could live out our life normally. The atmosphere and ocean would not last long enough, however, to
The result is that the Moon is without an atmosphere. To be sure, even the Moon’s low gravity can hold some gases if their atoms or molecules are massive enough. The atoms of the gas krypton, for instance, have a mass of 83.8 and the atoms of the gas xenon, a mass of 131.3. The Moon’s gravitational field could hold them with ease. However, these gases are so uncommon in the Universe generally, that even if they occurred on the Moon and made up its atmosphere,that atmosphere would be only a trillionth as dense as the Earth’s atmosphere, if that, and could at best be described as a “trace atmosphere.”
To all intents and purposes, as far as the problem of extraterrestrial life is concerned, such a trace atmosphere is of no consequence and the Moon can still fairly be described as airless.
All this has meaning with respect to a liquid such as water. Water is “volatile,” that is, it has a tendency to vaporize and turn into a gas. At a given temperature, there is a countertendency for the gaseous water vapor to recondense into liquid. At any particular temperature, liquid water is therefore liable to be in equilibrium with a certain pressure of water vapor, provided that water vapor is not removed from the vicinity as, for instance, by a wind.
If the water vapor is removed, equilibrium pressure is not built up and more of the liquid water vaporizes, and still more, till it is all gone. We are all familiar with the way in which the water left behind by a rainstorm evaporates until it is finally all gone. The higher the temperature, the faster the water evaporates.
Naturally, the water vapor is not removed from the Earth altogether. If it does not condense in one place, it condenses in another as dew, fog, rain, or snow, and thus the Earth holds on to its water.
If there were liquid water on the Moon, the vapor that would form would leak out into space, for the mass of the water molecule is but 18 and the Moon’s gravitational field would not hold it. The liquid water would continue to vaporize and eventually the Moon would dry up altogether. The fact that there is no air on the Moon means there is no air pressure to slow the rate of water evaporation, and the water, if it had been present, would have been lost all the more quickly.
The Moon, therefore,
must
be without water as well as without air. What’s more, any airless world would be a lifeless world—not because air is necessarily essential to life, but because an airless world is a waterless world, and water is essential.
Even the kinetic theory of gases leaves loopholes, however. The possibility remains that scraps of water, even air, can exist underground on the Moon, or in chemical combination with molecules in the soil. In that case, the small molecules would be prevented fromleaving by forces other than gravity—by physical barriers or chemical bonding.
Then, too, there may have been a time early in the history of the Moon when it had an atmosphere and an ocean,
before
it lost them both to space. Perhaps in those early days, life developed, even intelligent life, and it may have adapted itself, either biologically or technologically, to the gradual loss of air and water. It might, therefore, be living on the Moon in caverns, with a supply of air and water sealed in.
As late as 1901, the English writer H. G. Wells (1866–1946) could publish
The First Men on the Moon
and have his heros find a race of intelligent Moon beings, rather insectlike in character and highly specialized, living underground.
Even that much seems doubtful, however, since calculations show that the Moon would have lost its air and water (if any) quite rapidly. It would have retained them for many times the lifetime of a human being, of course, and if we were living on the Moon when it still had an atmosphere and ocean we could live out our life normally. The atmosphere and ocean would not last long enough, however, to
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