1

Because the Moon has no atmosphere, one could be created by the sublimation or evaporation of a solid or a liquid respectively, within a deep crater if the gas would not flow over the rim.
Perfluorodecalin could be one of the best fluorocarbons suited for the Moon because it is inert and stable and has a very high molar mass and thus would not escape that easy into space. By its weight the vapor of perfluorodecalin could keep it liquid on the floor of a crater with the right temperature if that crater would be deep enough.
For instance, from this table we can see perfluorodecalin has a vapor pressure of 5.47 kPa at 60⁰ C, and that temperature could be easily reached in the satellite crater Hercules G inside Hercules crater, with some parts of its floor at an elevation of minus 6500 m. being the deepest crater within the 60⁰ latitudes.
(Determined with the elevation tool on Moon Trek.)

Can it be calculated if Hercules G would be deep enough to have liquid on its floor ?

Edit: This similar question has an answer based on a scale height calculation that could also be applied here. One big difference of course is that the Moon has no atmosphere, so where would be the starting point for such a calculation, where would the atmosphere for the Hercules crater begin ?

Cornelis
  • 7,535
  • 2
  • 26
  • 77
  • 2
    Yes, it can be calculated, but it is an extremely complex calculation! It involves the gas's scale height, a measure of how quickly the gas's pressure changes with altitude. In an isothermal gas (same temperature at all altitudes) the scale height tells you how far up you must go to have the pressure drop to 1/e of that at the starting point. Its value is calcuated by H = R T / M g, where R is the universal gas constant (8314 J/kg°K), T is temperature (°K), M is the gas's average molar mass, and g is the local gravitational acceleration. In a non-isothermal gas, H is... – Tom Spilker Aug 31 '20 at 20:58
  • 2
    ...a local measure of d P / d h, with h being altitude. But on the moon the unfiltered solar UV will dissociate some of the PFD into lighter fragments, especially higher in the column, so M won't be constant. Differential absorption of solar energy will make the temperature vary a lot with altitude (the "temperature profile"), and you have to keep track of the atmospheric chemistry resulting from the UV (and other) effects to estimate the energy absorption (& radiation) at different levels. All this is done via numerical solutions to coupled differential equations - not freshman math! – Tom Spilker Aug 31 '20 at 21:11
  • 2
    The hugely variable thermal environment on the moon throws more trouble at you. Such a scenario (PFD in a deep crater) might be stable during lunar night: having a large M in the denominator helps keep the scale height short, so pressure at the crater rim is very low. But during lunar day it gets really hot, and that T in the numerator drives the scale height way up, greatly increasing the pressure at the crater rim and possibly losing a lot of the PFD, and more on the next day-night cycle, and the next... – Tom Spilker Aug 31 '20 at 21:19
  • 1
    The previous question was answered based on a scale height calculation. It's best if you include a mention of that answer in your question here so that people don't re-explain scale hight again, which could lead to duplication of answers and then closing as duplicate. Thanks! – uhoh Sep 01 '20 at 05:14
  • 1
    @uhoh You're right, I will make an edit, also because of the very useful information from @ Tom Spilker , thanks too ! – Cornelis Sep 01 '20 at 07:33
  • @TomSpilker Thank you for the valuable information in your comments. I will make an edit and try to simplify the conditions such that the question can be answered with "freshman math" ! – Cornelis Sep 01 '20 at 08:44
  • 1
    "within a deep crater if the gas would not flow over the rim" single gas molecules with enough speed may fly out of the crater. See https://en.wikipedia.org/wiki/Brownian_motion. So if there is enough pressure for liquid perfluorodecalin at the bottom of the crater, the gas above the liquid may leave the crater slowly and the pressure drops below the vapor pressure and the liquid will boil out. – Uwe Oct 19 '21 at 10:06
  • Yes, of course it would very much depend on temperature. And (not only) for the fluorocarbons there are also the rotational and vibrational motions.. There is a formula with which the (average) speed of a molecule could be calculated, so I guess one could calculate how high the crater's rim would have to be to keep the molecule inside. – Cornelis Oct 19 '21 at 13:15

0 Answers0