The following calculator estimates the planetary noontime (subsolar) temperature on a planet and computes the root-mean-squared (RMS) thermal velocity of a gas on the planet. This speed may be compared to the "escape velocity" (ESC) required for an object to escape from the surface of the planet and never return. If the V_RMS and V_ESC are nearly the same then the gas will vanish - evaporate - rather quickly into space.

In general, at equilibrium, gas molecules follow a Maxwellian distribution, so that at any temperature there will always be some molecules with sufficient velocity to escape - evaporate off the surface of the planet and never return:

The calculator estimates the "gas half life" on the surface of the planet. This is the time required for the gas to diminish to half its original abundance.

See The Physics of Atmospheres by John T. Houghton, p64 for details of the half life calculation. Note the following:

• Subsolar (noontime, equatorial) temperatures are estimated based solely on distance to the sun and planetary albedo. Effects of fluid circulation (which will even out temperatures on the planet surface) and the greenhouse effect are not included.
• The fact that a gas could have a long lifetime on a planet does not mean that it exists in any appreciable abundance in the atmosphere. Xenon has a long lifetime on most planets, but little is present in atmospheres.
• The lifetimes are estimated using noontime surface temperatures at the planetary equator. The exact lifetime will depend on atmospheric circulation rates, planetary rotation period, atmosphere radiation absorption characteristics and the sensible heat transfer from the surface to the atmosphere.
• The age of the universe is estimated to be about 10¹⁷ seconds