Water ice, sublimation, vacuum.

[Bushranger]

It has been discovered that there is water ice in the perpetually shaded craters on the Moon and on Mars.  I have always understood the freeze-drying process to be an acceleration of the sublimation process of water when frozen and subjected to a vacuum.  Therefore, I do not understand how there could be water ice on either the Moon or Mars.  Why has not the ice on the Moon (and Mars, et. al.)sublimated and dissipated into space especially inasmuch as it has no atmosphere?  The only reason I can think of for that not to happen is that there has to be some involvement with oxygen for sublimation to take place. 

An explanation, especially accompanied by a chemical formula that demonstrates the process of sublimation would be most welcome.

[Endy0816]

Sublimation requires some heating to occur while the pressure remains below that of the Triple point. This is purely changing what phase the water is in.

 

Water's Phase Diagram

 

 

 

 

 

 

https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_Chemistry_(OpenSTAX)/10%3A_Liquids_and_Solids/10.4%3A_Phase_Diagrams

 

 

 

 

[Bushranger]

6 hours ago, Endy0816 said:

Sublimation requires some heating to occur while the pressure remains below that of the Triple point. This is purely changing what phase the water is in.

 

Water's Phase Diagram

 

 

 

Can you explain it to me just like I was six years old?  If the temperature in my freezer is below 0 degrees Centigrade (32 degrees Feirnhight)...actually the temperature is way colder than 0 degrees C (as per your chart)...it is -17.77 C how is it that ice sublimates in my freezer...or doesn't it?   Please put you answer into the context for someone who is not a scientist.

 

 

 

 

https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_Chemistry_(OpenSTAX)/10%3A_Liquids_and_Solids/10.4%3A_Phase_Diagrams

[Sensei]

Quote

Can you explain it to me just like I was six years old?  If the temperature in my freezer is below 0 degrees Centigrade (32 degrees Feirnhight)...actually the temperature is way colder than 0 degrees C (as per your chart)...it is -17.77 C how is it that ice sublimates in my freezer...or doesn't it?

It does not. Notice horizontal line where is 101 on the left vertical arrow. That's normal air pressure line (101 kPa) on the Earth (including your freezer).

Sublimation triple point is at 0.6 kPa = ~600 Pa, and temperature 0.01 C. Big dot on the image above.

 

[Bushranger]

3 hours ago, Sensei said:

It does not. Notice horizontal line where is 101 on the left vertical arrow. That's normal air pressure line (101 kPa) on the Earth (including your freezer).

Sublimation triple point is at 0.6 kPa = ~600 Pa, and temperature 0.01 C. Big dot on the image above.

 

So to summarize what you are saying, It is too cold for any sublimation of any water ice discovered in the moon's permanently shaded craters?

[Enthalpy]

The Moon has no significant atmospheric pressure, so the equilibrium would tell that ice sublimes. But this equilibrium can take very long to achieve if the ice is cold. Comets can keep ice for billions of years and release vapour only when passing nearer to the Sun.

This leaves open the question of the origin of this ice on the Moon, since we don't find significant free water elsewhere. I could imagine - with no figures to support nor invalidate it - that the solar wind brings hydrogen that somehow reacts with local oxygen, say from silicates.

The situation is quite different on Mars, which has an atmosphere with significant pressure. Water vapour in the atmosphere, even if in tiny proportion, can freeze on a soil cold enough, and sublime when the ground warms up. Very similar to Earth, just colder and with less vapour pressure.