Why does the head of a comet turn green, while its tail turns green?

Scientists have been searching for an explanation for the color difference between a comet’s head and tail for nearly a century. For example, physicist and Nobel Prize winner Gerhard Herzberg believed that this phenomenon was the result of sunlight breaking down diatomic carbon (C2). Diatomic carbon is a chemical produced by the effect of sunlight on the organic matter at the head of a comet. But because this substance is unstable, this theory has been difficult to test.

A team led by Jasmine Borsovsky from the University of New South Wales in Sydney, Australia, has finally found a way to test the chemical reaction in question in the lab. Herzberg’s theory has been proven correct.

The molecule responsible for the green color of many comets consists of two carbon atoms ‘sticking together’ and occurs only in highly energetic or oxygen-depleted environments such as stars, comets and the interstellar medium.

In comets, this chemical is formed only when they get close to the sun. Due to the heating of the comet, the organic matter on the icy surface evaporates and ends up in its coma – the temporary “atmosphere”. Once there, the organic molecules are broken down by sunlight, which can create diatomic carbon. Diatomic carbon is highly reactive and is responsible for the green color of many comets.

Borsovsky and her team have now shown that as the comet approaches the sun, newly formed diatomic carbon molecules disintegrate under the influence of the sun’s intense ultraviolet rays – a process called photodissociation. This process destroys the diatomic coal before it can move away from the core. For this reason, the green can never reach the tail.