mass of the molecules.
A certain fraction of the molecules of any gas would be moving at speeds greater than the average for that temperature, and might exceed the escape velocity for the planet whose gravitational attraction held them. Anything moving at more than escape velocity, whether it is a rocket ship or a molecule, can, if it does not collide with something, move away forever from the planet.
Under ordinary circumstances, so tiny a fraction of the molecules of an atmosphere might attain escape velocity—and retain it through inevitable collisions until it reached such heights that it could move away without further collision—that the atmosphere would leak away into outer space with imperceptible slowness. Thus, Earth, for which the escape velocity is 11.3 kilometers (7.0 miles) per second, holds on to its atmosphere successfully and will not lose any significant quantity of it for billions of years.
If, however, Earth’s average temperature were to be substantially increased, the average speed of the molecules in its atmosphere would also be increased and so would the fraction of those molecules traveling at more than escape velocity. The atmosphere would leak away more rapidly. If the temperature were high enough, the Earth would lose its atmosphere rather quickly and become an airless globe.
Next, consider hydrogen and helium, which are gases that are composed of particles much less massive than those making up the oxygen and nitrogen of our atmosphere. The oxygen molecule (made up of 2 oxygen atoms) has a mass of 32 in atomic mass units, and the nitrogen molecule (made up of 2 nitrogen atoms) has a mass of 28. In contrast, the hydrogen molecule (made up of 2 hydrogen atoms) has a mass of 2 and helium atoms (which occur singly) a mass of 4.
At a given temperature, light particles move more rapidly than massive ones. A helium atom will move about three times as quickly as the massive and therefore more sluggish molecules of our atmosphere, and a hydrogen molecule will move four times as quickly. The percentage of helium atoms and hydrogen molecules that would be moving more rapidly than escape velocity would be much greater than in the case of oxygen and nitrogen.
The result is that Earth’s gravity, which suffices to hold the oxygen and nitrogen molecules of its atmosphere indefinitely, would quickly lose any hydrogen or helium in its atmosphere. That would leak away into outer space. If the Earth were forming under its present condition of temperature and were surrounded by cosmic clouds of hydrogen and helium, it would not have a sufficiently strong gravitational field to collect those small and nimble molecules and atoms.
It is for this reason that Earth’s atmosphere does not contain anything more than traces of hydrogen and helium, although these two gases make up by far the bulk of the original cloud of material out of which the Solar system was formed.
The Moon has a mass only 1/81 that of the Earth and a gravitational field only 1/81 as intense. Because it is a smaller body than the Earth, its surface is nearer its center, so that its small gravitational field is somewhat more intense at its surface than you would expect from its overall mass. At the surface, the Moon’s gravitational pull is 1/6 of the Earth’s gravitational pull at its surface.
This is reflected in escape velocity as well. The Moon’s escape velocity is only 2.37 kilometers (1.47 miles) per second. On Earth, a vanishingly small percentage of molecules of a particular gas might surpass its escape velocity. On the Moon, a substantial percentage of molecules of that same gas would surpass the Moon’s much lower escape velocity.
Then, too, because the Moon rotates on its axis so slowly as to allow the Sun to remain in the sky over some particular point on its surface for two weeks at a time, its temperature during its day rises much higher than does the Earth’s temperature. That further increases the percentage of
A certain fraction of the molecules of any gas would be moving at speeds greater than the average for that temperature, and might exceed the escape velocity for the planet whose gravitational attraction held them. Anything moving at more than escape velocity, whether it is a rocket ship or a molecule, can, if it does not collide with something, move away forever from the planet.
Under ordinary circumstances, so tiny a fraction of the molecules of an atmosphere might attain escape velocity—and retain it through inevitable collisions until it reached such heights that it could move away without further collision—that the atmosphere would leak away into outer space with imperceptible slowness. Thus, Earth, for which the escape velocity is 11.3 kilometers (7.0 miles) per second, holds on to its atmosphere successfully and will not lose any significant quantity of it for billions of years.
If, however, Earth’s average temperature were to be substantially increased, the average speed of the molecules in its atmosphere would also be increased and so would the fraction of those molecules traveling at more than escape velocity. The atmosphere would leak away more rapidly. If the temperature were high enough, the Earth would lose its atmosphere rather quickly and become an airless globe.
Next, consider hydrogen and helium, which are gases that are composed of particles much less massive than those making up the oxygen and nitrogen of our atmosphere. The oxygen molecule (made up of 2 oxygen atoms) has a mass of 32 in atomic mass units, and the nitrogen molecule (made up of 2 nitrogen atoms) has a mass of 28. In contrast, the hydrogen molecule (made up of 2 hydrogen atoms) has a mass of 2 and helium atoms (which occur singly) a mass of 4.
At a given temperature, light particles move more rapidly than massive ones. A helium atom will move about three times as quickly as the massive and therefore more sluggish molecules of our atmosphere, and a hydrogen molecule will move four times as quickly. The percentage of helium atoms and hydrogen molecules that would be moving more rapidly than escape velocity would be much greater than in the case of oxygen and nitrogen.
The result is that Earth’s gravity, which suffices to hold the oxygen and nitrogen molecules of its atmosphere indefinitely, would quickly lose any hydrogen or helium in its atmosphere. That would leak away into outer space. If the Earth were forming under its present condition of temperature and were surrounded by cosmic clouds of hydrogen and helium, it would not have a sufficiently strong gravitational field to collect those small and nimble molecules and atoms.
It is for this reason that Earth’s atmosphere does not contain anything more than traces of hydrogen and helium, although these two gases make up by far the bulk of the original cloud of material out of which the Solar system was formed.
The Moon has a mass only 1/81 that of the Earth and a gravitational field only 1/81 as intense. Because it is a smaller body than the Earth, its surface is nearer its center, so that its small gravitational field is somewhat more intense at its surface than you would expect from its overall mass. At the surface, the Moon’s gravitational pull is 1/6 of the Earth’s gravitational pull at its surface.
This is reflected in escape velocity as well. The Moon’s escape velocity is only 2.37 kilometers (1.47 miles) per second. On Earth, a vanishingly small percentage of molecules of a particular gas might surpass its escape velocity. On the Moon, a substantial percentage of molecules of that same gas would surpass the Moon’s much lower escape velocity.
Then, too, because the Moon rotates on its axis so slowly as to allow the Sun to remain in the sky over some particular point on its surface for two weeks at a time, its temperature during its day rises much higher than does the Earth’s temperature. That further increases the percentage of
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