Hi, I am trying to calculate if moon can possibly hold an atmosphere if we decide to terraform it. The assumption is that it is somehow miraculously completely shielded from Solar Wind, let's say by strong earth like magnetic field. I found somewhere on the web, that atmosphere of any planet will hold if the velocity of the gas molecules is less than 1/5 of the escape velocity of that planet (any link to why is it so is appreciated). So: 1. Vair <= 1/5 Ve from this we get (substitute Vair with ideal gas velocity and Ve with Escape Velocity): 2. √(3KT/Mair) <= 1/5√(2GMp/r) Now my questions are: a. What temperature T should i use in the equation, The maximum temperature in the upper atmosphere of the moon (380K)? or the average temperature of the upper atm (220K)? Maybe the average surface tmp that will be in the end? b. What do I use for r: is it top of the Thermosphere or Exosphere or maybe something else? (of course i could assume r to be Rmoon, but i just want to understand the physics here.) Thanks, Qshadow.