No. 32: Mar-Apr 1984
"It has long been realized that the Earth's upper atmosphere cannot be in thermodynamic equilibrium, and during the last decade astronomers have made telescopic observations of nonequilibrium processes taking place in the upper atmospheres of our Earth-like neighbors, Mars and Venus. A preliminary analysis by Michael Mumma, of Goddard Space Flight Center in Maryland, and his colleagues. indicated that processes analogous to optical pumping in laboratory lasers were taking place and led to the coining of the term 'natural lasers.' Now, Deming and colleagues from the Goddard team have taken new observations of emission from Mars and Venus at wavelengths near 10 micrometers. and modelled them to show that stimulated emission -- the effect which makes lasers so powerful -- accounts for up to seven per cent of the total emission. This is not a large amplification factor by laboratory standards, particularly for a CO2 laser. But, the authors speculate that the sheer size of the natural lasers could make them useful tools in the future for communicating with distant civilizations beyond our own planetary system."
The atmospheres of Mars and Venus are almost pure CO2 . The CO2 molecules are excited by the absorption of energetic solar photons; then, thermally emitted photons at about 10 micrometers from lower reaches of the atmosphere collide with the excited molecules, stimulating them to emit another 10-micrometer photon, thus doubling the number of photons. This is typical laser action. Deming and Mumma speculate that the natural laser action existing in the Martian atmosphere could be intensified and focussed into an intense beam of infrared radiation of enormous power by placing two large mirrors in orbit, creating a space-borne analog of a laboratory laser. With this huge laser, one could conceive of communicating with neighboring stellar systems.
(Taylor, F.W.; "Natural Lasers on Venus and Mars," Nature, 306:640, 1983.)