Ice on the moon

[thetexan]

TL;DR

How is ice possible on the moon?

NASA keeps talking about establishing a permanent moon base presence on the moon, made possible because they discovered water ice on the southern portions of the moon.

How is it possible that there is any ice on the moon. If there were ice on the moon it would have been there for hundreds of thousands, perhaps millions of years. Why would that ice not have sublimated by now, especially in the vacuum of space? Unless there is a supply of water that keeps replenishing the ice.

This sounds fishy.

Tex

 

[.Scott]

From NASA Moon South Pole page:

Lighting and Terrain​ At the lunar South Pole, the Sun hovers below or just above the horizon, creating temperatures upwards of 130°F (54°C) during sunlit periods. Even during these periods of illumination, soaring mountains cast dark shadows and deep craters protect perpetual darkness in their abysses. Some of these craters are home to permanently shadowed regions that haven’t seen sunlight in billions of years and experience temperatures as low as -334°F (-203°C).

Ideally, we would have the highest temperature in the very low temperature areas. But since the range of temperatures for a "permanently shadowed region" is likely small (because any heating up has to be done through conduction), we can use the -203C value.

So next you need to find a water vapor pressure calculator that will accept numbers that cold. Here's one.
Plug in -203C, and you get 4.477e-28kPa (or much lower is you use the Tetens formula).
So, it is sublimating - but even when the universe is a million times older, it will not be done.

 

[Baluncore]

How is it possible that there is any ice on the moon.

Ice could remain in a deep crater at either pole, where the sunlight never shines. Temperatures there can remain near -246°C, 27K. Water would be stable there, unlike ice on a comet that is driven off by sunlight to form a tail.

Water, from the almost non-existent atmosphere, would accumulate in a deep polar crater. Internal water could rise through the surface rocks, or fall from space onto the surface.

 

[thetexan]

I'm not familiar with the vapor pressure calculations. But I hear you saying that, even after millions of years ice could still be present. Is that because the rate of sublimation is very slow, or because of excessively low temps, or because there is so much ice?

In any case, this sounds physically possible to you guys? I don't know enough about it to talk inteligently about it. I just know that ice cubes in my freezer sublimate and disappear after a few months.

Tex

 

[.Scott]

The vapor pressure tells you how eager the ice is to sublimate - to become a vapor.
Vapor pressure drops exponentially as the temperature drops.
The water vapor pressure of the ice in those permanently shaded regions of the moon is so low, that there are more water molecules that happen across this ice and stick to it that there are ice molecules that escape to space. Even a molecule that momentarily escapes from that ice would be travelling so slowly that it wouldn't get very far.
Combustion products from the Apollo Lunar Landers would have included water vapor. A lot of that vapor would have become trapped in these ice fields.

 

[Baluncore]

I just know that ice cubes in my freezer sublimate and disappear after a few months.

Your freezer is not as cold as the space beyond Jupiter.
Comets are made of ice, and have long lifetimes, unless they collide with something. The tail is material out-gassed as a result of sunlight falling on the comet while near perihelion. Ice from space or the surface of the Moon, that does not escape into space, will be trapped, to accumulate in deep polar craters on the moon.