A comet’s tail is made from the volatile materials inside the comet vaporising, which carries dust and gas out with them. This detritus reflects sunlight, leaving it glowing.
Strangely, however, while many comets glow green as they streak across the sky, this hue never reaches their tails.
This mystery has been puzzling scientists since the 1930s, when it was suggested that diatomic carbon created by the interaction of sunlight and organic matter on the comet’s head was being destroyed by sunlight.
This theory has historically been difficult to test because dicarbon is not stable, but now scientists have been able to experiment on it under laboratory conditions.
“We’ve proven the mechanism by which dicarbon is broken up by sunlight,” says Timothy Schmidt, a chemistry professor at the University of New South Wales.
“This explains why the green coma – the fuzzy layer of gas and dust surrounding the nucleus – shrinks as a comet gets closer to the Sun, and also why the tail of the comet isn’t green.”
Dicarbon is made of two carbon atoms and is only found in extremely high-energy or low-oxygen environments. The molecule does not exist until the comet gets close to the Sun, as organic matter living on the icy body evaporates as heat warms it up.
As the comet gets even closer to the Sun, extreme UV radiation breaks apart the dicarbon molecules it recently created in a process called ‘photodissociation’.
This process destroys the dicarbon before it can move far from the nucleus, causing the green coma – the part around the nucleus of the comet – to get brighter and shrink, ensuring the green hue never reaches the tail.
The scientists created dicarbon molecules that they then sent through a gas beam in a two metre-long vacuum chamber. Two ultraviolet lasers were then pointed towards the molecule – one to flood it with radiation, the other to make its atoms detectable. The atoms were sent flying into a speed detector, letting them measure the strength of the carbon bond.
This was not an easy task; it took nine months for the researchers to make their first observation, as the light from all the lasers used is invisible.
Now that the discovery has been made, the findings might help scientists better understand the other 3700 comets in the known solar system.
“Dicarbon comes from the breakup of larger organic molecules frozen into the nucleus of the comet – the sort of molecules that are the ingredients of life,” Professor Schmidt said.
“By understanding its lifetime and destruction, we can better understand how much organic material is evaporating off comets. Discoveries like these might one day help us solve other space mysteries.”