Does the Moon have an atmosphere?
Yes it does.
The lunar escape velocity is 2.4 kilometers per second. For an atmosphere at a constant temperature, the heavier atoms move more sluggishly than the lighter ones. The rule of thumb is that if you multiply the average velocity of a particular atomic specie and get a number equal to the escape velocity or higher, that specie of atom in the atmosphere will evaporate away. On the Moon, the average temperature of the atmosphere there would be about 240 Kelvins or so, and the typical speed of heavy oxygen atoms would be about 0.5 kilometers per second. Since 6 x 0.5 = 3.0 kilometers per second, and this is higher than the lunar escape velocity of 2.4 kilometers/sec, any atoms lighter than oxygen would have escaped the lunar atmosphere. But, in principle, this leaves heavier atoms such as sulfur with a thermal velocity of about 0.3 kilometers per second and the heavy gas xenon with 0.1 kilometer/sec still under the 'rule of six' rule. This means that a very thin gas of sulfur, argon, zenon and similar heavy gases could still survive.
In 1933, astronomers Flynn and Mendillo reported in the issue of the journal Science ( vol. 261 page 184) that they had detected sodium emission from around the Moon by occulting the Moon with a special mask and the searching for the optical emission line of sodium. They were unable to search closer than 1000 kilometers from the lunar surface due to the quality of their optical filters. This search was repeated in 1994 by A. Potter, T. Morsan and L. Gillian with modern technology, and they detected the sodium cloud 'exosphere' all the way to the lunar surface. It was reported in the 1994 issue of the Bulletin of the American Astronomical Society, vol. 26, No. 13 page 1104 and 1106. Overall, the atmosphere is a million times less dense than the Earth's.
Return toAsk the Astronomer