Comets have long been feared by humans, as acts of God to signal impending doom. Multiple disasters were said to occur after the sighting of a comet, causing the innate event-cause backward relation by humans. Just when this myth was dispelled, spectroscopic analysis of the tail of the Halley's comet in 1910 revealed the presence of cyanogen, a highly toxic gas. Quacks used this to sell gas masks, "anti-comet pills" and "anti-comet umbrellas".

When all these myths were put down, in 1965, a comet was found to emit radio waves of around 1600 MHz. An uproar was created, with some theories claiming aliens from other planets, were using the comet as an efficient way to travel, by harnessing the natural motion of the comet as a "space taxi". Other claims said that it was the emission line of a new form of matter unknown to humans, and called it mysterium.

Further studies show that all these claims are false. In 1967, it was firmly established that actually this happens:

As the comet approaches the Sun, it has to get warmer than the icy depths of the aether :) in the far reaches of our Solar System. This increased heat causes the ice in the comet to melt and causes liquid water to get formed. Further heating creates an atmosphere of water vapor around the icy dirty snowball region of the comet. The radiant Sun now photolyses the energetic vapors as

$$H_2O \longrightarrow H^+ + OH^-$$

When the comet is still quite far from the sun, both $E_1$ and $E_2$ excitation in $OH^-$ occurs. But as it gets closer to the sun, the $E_1$ excitation reduces. This is because the comet isn't a stationary body, there is relative motion between the comet and the sun. So the Doppler Effect caused by the comet's motion causes the Fraunhofer lines to overlap with the wavelength of the $E_1$ excitation. So, $E_2 \rightarrow E_1$ occurs.

Chill a moment. What are Fraunhofer lines? Where does the Doppler effect come into this? Fraunhofer lines are spectral lines of absorption which feature in the optical spectrum of the sun as dark bands. These lines happen to coincide with the emission lines of some elements when heated. So these dark lines are caused by absorption by these elements in the solar atmosphere. These lines are just the absence of a certain wavelength in the spectrum. When the comet moves, this causes a Doppler shift. The excited emission lines now overlap with the Fraunhofer absorption ones.

The energy of one of these emission lines of $OH^-$ was found to be $6.9 \times 10^{-6} eV$. This corresponds to a frequency of 1665 MHz.

$$\Delta E = h \nu \Rightarrow \nu = \frac{6.9 \times 10^{-6} \times 1.6 \times 10^{-19}}{6.626 \times 10^{-34}} = 1665 MHz$$

These lines were observed in the comet Kouhoutek in December 1973 to January 1974. Other emissions have also been found, including $CH_3OH$ & $SiO$.

When NASA scientists directed a telescope towards Chryse Planitia (in Mars, near the landing site of the PathFinder), they discovered laser radiation in its atmosphere. This natural laser was because of excitation of similar levels in $CO_2$ which is abundant in the Martian atmosphere. And Mother Nature gives it back to us humans by playing with lasers before we even 'invented' it.