Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Dec 12, 2009 #1
    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.

  2. jcsd
  3. Dec 13, 2009 #2


    User Avatar
    Science Advisor
    Gold Member

    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.
  4. Dec 13, 2009 #3
    I now understand why the atmosphere is thin!
    I still don't understand why a lower pressure reduces boiling point?
  5. Dec 13, 2009 #4


    User Avatar
    Science Advisor
    Gold Member

    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.
  6. Dec 14, 2009 #5

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    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.
  7. Dec 14, 2009 #6
    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?
  8. Dec 14, 2009 #7


    User Avatar
    Science Advisor

    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!
  9. Dec 14, 2009 #8
    Thank you for the additional information here. Specifically:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook