Scientists solved the mystery of comet’s green shade

The team solved this mystery with the help of a vacuum chamber, many lasers, and one powerful cosmic reaction.


Comets go through a colorful metamorphosis as they cross the sky. The green color is a good omen, and many comets display this color. The green color signifies that the comet is getting more active as it gets closer to the Sun.

But strangely, this green shade disappears before it reaches the one or two tails trailing behind the comet.

Astronomers, scientists, and chemists have been puzzled by this mystery for almost 90 years. In 1930, it was suggested that this phenomenon was due to sunlight destroying diatomic carbon. The carbon is created from the interaction between sunlight and organic matter on the comet’s head. However, due to the instability of dicarbon, this theory has been hard to test.

Scientists at UNSW Sydney have finally found a way to test this chemical reaction in a laboratory – and in doing so, has proven this 90-year-old theory correct. They solved this mystery with the help of a vacuum chamber, a lot of lasers, and one powerful cosmic reaction.

Timothy Schmidt, a chemistry professor at UNSW Science and senior author of the study, said, “We’ve proven the mechanism by which dicarbon is broken up by sunlight. 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 the key player in this mystery. The molecule is highly reactive and responsible for giving many comets green. However, the molecule doesn’t exist until they get close to the Sun. As the sun starts to warm the comet up, the organic matter on the icy nucleus evaporates and moves to the coma.

Scientists, in this study, have shown that as the comet gets even closer to the Sun, the extreme UV radiation breaks apart the dicarbon molecules it recently created in a process called ‘photodissociation.’

This process destroys the dicarbon before moving 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.

Ms Jasmin Borsovszky, lead author of the study and former UNSW Science Honours student, said, “I find incredible that 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.”

Timothy Schmidt, a chemistry professor at UNSW Science and senior author of the study, said, “The findings help us better understand both dicarbon and comets. 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.”

“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.”

Scientists solved this mystery by recreating the same galactic chemical process in a controlled environment on Earth. Using a vacuum chamber, several lasers, and one powerful cosmic reaction, they pulled this off.

Prof. Schmidt said, “First, we had to make this molecule which is too reactive to store in a bottle. It’s not something we could buy from the shops. We did this by taking a larger molecule, known as perchloroethylene or C2Cl4, and blasting off its chlorine atoms (Cl) with a high-powered UV laser.”

Scientists sent the dicarbon molecules through a gas beam in a vacuum chamber. The gas chamber was around two meters long. They then pointed another two UV lasers towards the dicarbon: one to flood it with radiation, the other to make its atoms detectable. The radiation hit ripped the dicarbon apart, sending its carbon atoms flying onto a speed detector.

The team then analyzed the speed of these quickly-moving atoms to measure the strength of the carbon bond to about one in 20,000 – which is like measuring 200 meters to the nearest centimeter.

Ms. Borsovszky says, “due to the complexity of the experiment, it took nine months before they were able to make their first observation.”

“We were about to give up. It took so long to make sure everything was precisely lined up in space and time.”

“The three lasers were all invisible, so there was a lot of stabbing in the dark – quite literally.”

Prof. Schmidt says this is the first time anyone has ever observed this chemical reaction.

“It’s extremely satisfying to have solved a conundrum that dates back to the 1930s.”

Professor Martin van Kranendonk, a UNSW astrobiologist and geologist who was not involved in the study, said“This exciting research shows us just how complex processes in interstellar space are.”

“Early Earth would have experienced a jumble of different carbon-bearing molecules being delivered to its surface, allowing for even more complex reactions to occur in the leadup to life.”

Journal Reference:

  1. Jasmin Borsovszky et al. Photodissociation of dicarbon: How nature breaks an unusual multiple bonds. DOI: 10.1073/pnas.2113315118
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