Short existence of liquid water on Mars (explanation related to thin atmosphere)

[ajassat]

I understand that liquid water cannot exists on Mars for extended periods of time because:

a) The atmosphere is too thin
b) Resulting in a lower boiling point water
c) Which leads to sublimification of solid ice into gaseous vapour

Quite specifically, I would like to understand the following:

1. Why a lower atmospheric pressure reduces boiling point
2. The energy changes resulting in sublimification of solid ice into gaseous vapour

I'm fine with mathematical explanantions too.

Regards,
Adam

 

[Chronos]

Mars thin atmosphere is susceptible to solar stripping. Mars does not have a magnetic field like Earth so solar radiation rips through it and carries off its atmoshpere. Lower atmospheric pressure also results in lower boiling point. Water boils at a lower temperature on Earth at high altitudes for the same reason. Imagine a bottle of soda. Under sufficient pressure, carbon dioxide remains dissolved in the soda. Reduce the pressure and it is released. Ice sublimates more easily into water vapor under low atmospheric pressure for the same reason.

 

[ajassat]

Mars thin atmosphere is susceptible to solar stripping. Mars does not have a magnetic field like Earth so solar radiation rips through it and carries off its atmoshpere. Lower atmospheric pressure also results in lower boiling point. Water boils at a lower temperature on Earth at high altitudes for the same reason. Imagine a bottle of soda. Under sufficient pressure, carbon dioxide remains dissolved in the soda. Reduce the pressure and it is released. Ice sublimates more easily into water vapor under low atmospheric pressure for the same reason.

I now understand why the atmosphere is thin!
I still don't understand why a lower pressure reduces boiling point?

 

[Chronos]

Liquids include dissolved gasses. Part of the reason they stay dissolved is the atmospheric pressure pushing against them. Release the pressure and they more easily escape the liquid.

 

[Vanadium 50]

I still don't understand why a lower pressure reduces boiling point?

Think about the surface of the liquid. It begins to evaporate when a molecule is moving fast enough to "escape". If pressure is pushing this molecules back into the liquid, it needs more energy to start the evaporation process - so the boiling point goes up.

This is not very exact. One gets a better picture thinking about equilibria and partial pressures, but this should give you a flavor of what's going on.

 

[ajassat]

Think about the surface of the liquid. It begins to evaporate when a molecule is moving fast enough to "escape". If pressure is pushing this molecules back into the liquid, it needs more energy to start the evaporation process - so the boiling point goes up.

This is not very exact. One gets a better picture thinking about equilibria and partial pressures, but this should give you a flavor of what's going on.

Thank you. It does allow me to picture what is going on.
I also gather that this can be descibed by Thermodynamics, mathematically?
If this is so could you show me how?

 

[Wallace]

Yes, this is basic thermodynamics. You could start by looking at the wiki article on 'boiling point', or other physics sites like wolfram scienceworld. I can't remember the maths of the top of my head, but I think boiling point goes as the natural log of the gas pressure? Would have to check the details though.

Note that this happens on earth. When standing on the top of Mt. Everest, where the atmospheric pressure is lower than at sea level, water boils at something like 80 degrees Celsius instead of the 100 we are used to. This means you can't really make a good hot chocolate to celebrate you ascent to the summit, because you simply can't get water hot enough to properly dissolve the chocolate before it simply boils away!

 

[ajassat]

Yes, this is basic thermodynamics. You could start by looking at the wiki article on 'boiling point', or other physics sites like wolfram scienceworld. I can't remember the maths of the top of my head, but I think boiling point goes as the natural log of the gas pressure? Would have to check the details though.

Note that this happens on earth. When standing on the top of Mt. Everest, where the atmospheric pressure is lower than at sea level, water boils at something like 80 degrees Celsius instead of the 100 we are used to. This means you can't really make a good hot chocolate to celebrate you ascent to the summit, because you simply can't get water hot enough to properly dissolve the chocolate before it simply boils away!

Thank you for the additional information here. Specifically: