The Lunar Environment
Section M 3.
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Why the Moon Loses Gasses

A gas is a collection of particles bouncing around. The hotter the gas is, the faster the particles move. The lighter the gas particles are, the faster they move at a given temperature.

On the earth, the gravity field is relatively high, and the range of surface temperatures isn't hot enough to cause the gas particles to reach escape velocity. So lightweight gasses stay in the atmosphere.

On the moon, surface temperatures reach 200 degrees F, causing molecules of the lighter gasses hydrogen) to move faster to lunar escape velocity. So they escape into free space.

Why this leads to water in special places on the lunar surface

For lightweight substances to stick to the surface of the moon, they have to stay in a solid or liquid state. In regions of extreme cold, these substances can remain frozen in a solid state. The only such region on the surface of the moon is in double-shadowed craters near the south pole.

Double-shadowed craters are craters whose rims are shadowed by other craters, 100% of time. This has allowed these surface regions to remain very, very cold for a few billion years. Anything that lands there, stays there, especially if it ends up covered with a layer of regolith for protection against solar wind and cosmic ray sputtering, and the Moon's slight, but not insignificant, inclination of 1 20'.

Beneath the surface

A foot or two beneath the surface, the moon is a constant temperature, a few degrees below the freezing point of water. Except for transient impacts, these temperature conditions have existed for a couple of billion years.

We have no data about whether volatiles are trapped in the lunar refrigerator throughout the surface of the moon. The deepest we have dug is a single Apollo core sample which went less than a yard deep; so we probably won't know for sure until we get there and start digging.

The Lunar Environment

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