The impact of the Lunar Prospector spacecraft in a lunar polar crater was used in an (unsuccessful) attempt to detect the release of vapor from volatilized water ice. The short-time evolution of such an impulsively produced atmosphere is discussed. The effects of the initial vapor temperature (and thus its initial molecular thermal velocity) and the nature of the gas-surface interaction are examined parametrically. It is shown that the exosphere quickly diffuses to the nightside of the Moon. The results have implications for observations of impulsive gas releases on airless bodies in general.