It ain't easy saying why comets are green

·2-min read

Every so often, space throws galactic balls of ice, dust and rocks earthwards - 4.6-billion-year-old leftovers from the formation of the solar system best known as comets.

As they cross they sky, many turn a radiant green at the head which gets brighter as they approach the sun - but strangely the glow disappears before reaching the comet's tail.

It's something that has long puzzled science.

Physicist Gerhard Herzberg proposed in the 1930s that the phenomenon was due to the destruction of dicarbon, a chemical created from the interaction between sunlight and organic matter on the comet's head.

As dicarbon isn't stable, it's a theory which has been hard to test.

However a UNSW Sydney-led study has finally found a way to assess it in a laboratory using a vacuum chamber and multiple lasers and has shown the 90-year-old explanation to be correct.

"We've proven the mechanism by which dicarbon is broken up by sunlight," said senior study author Professor Timothy Schmidt.

"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 up of two carbon atoms stuck together and can only be found in extremely energetic or low oxygen environments.

It doesn't exist on comets until they approach the sun but as they begin to warm, the organic matter living on the icy nucleus evaporates and moves to the coma.

Sunlight then breaks up these larger organic molecules, creating dicarbon.

The UNSW-led team were able to show that as the comet hurtles sunward, extreme ultraviolet radiation breaks apart the dicarbon molecules it recently created in a process called photodissociation.

This destroys the dicarbon before it can move far from the nucleus, causing the green coma to get brighter and shrink, and making sure the green tinge never makes it into the tail.

"I find it incredible someone in the 1930s thought this is probably what's happening, down to the level of detail of the mechanism of how it was happening, and then 90 years later, we find out it is what's happening," said lead author Jasmin Borsovszky.

"Herzberg was an incredible physicist and went on to win a Nobel Prize for Chemistry in the 1970s. It's pretty exciting to be able to prove one of the things that he theorised."

Ms Borsovszky said due to the complexity of the experiment it took nine months to make the first observation.

"We were about to give up," she said.

There are about 3700 known comets in the solar system, although it's suspected there could be billions.

The findings have been published in Proceedings of the National Academy of Sciences

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