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5 mins read 17 Jan 2022

Why can a comets head be green but never their tail?

UNSW scientists have finally shown why the heads of comets are green, confirming a 90 year old chemistry hypothesis.

Comet Leonard showcasing its green di-carbon head. Credit: Damian Peach.

Comets have been a fairly popular topic recently, with the world being witness to an existing interplanetary visitor across both the north and southern hemisphere. Comet Leonard (C/2021 A1) has over the last few months been observed, photographed and filmed as it traversed the skies - sometimes even having an apparent closeness to some of the most famous objects in our Solar system, like Jupiter and Venus.  

In many photographs, including those captured by amateur Australian and New Zealand astrophotographers - Leonard, like other comets, displayed a beautiful bright electric pastel green at the front of the comet, just ahead of the bow shock as it ploughs around the Solar system. 

And now, after 90 years, it has finally been shown that the green glow of some comets is due to sunlight destroying diatomic carbon (C2).

Comets are balls of ice, dust, and rock left over from the formation of our Solar system. These interplanetary snowballs occasionally are thrown towards Earth from the Kuiper Belt and the Oort Cloud.

If you’ve ever had the chance to observe or photograph, you’ll know that they come in a variety of colours, including green. Comets become green as they get closer to the sun, however, the green shade puzzlingly disappears before it reaches the comet's tail.

In the 1930s Gerhard Herzberg theorised that this was due to sunlight destroying diatomic carbon (C2). Dicarbon is created when sunlight interacts with organic matter on the comets head, however, dicarbon is very unstable so this is hard to test.

Comet Leonard showing the green colour around the head of the comet but not the tail. Credit: Rami Mandow.

Now, a new UNSW study published in the journal Proceeding of the National Acadamy of Sciences (PNAS), has finally found a way to test in a laboratory if the breakup of dicarbon is responsible for the green glow of comets. In doing so, they have proven Herzberg’s 90 year old theory correct.

“We’ve proven the mechanism by which dicarbon is broken up by sunlight,” said Timothy Schmidt, a chemistry professor at UNSW Science and senior author of the study. 

“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, the key player of this mystery, is highly reactive being made up of two carbon atoms stuck together. Dicarbon is only found in extremely energetic and low oxygen environments like stars, comets, and the interstellar medium. The UNSW study has shown that as comets get even closer to the sun, the extreme UV radiation, breaks apart the dicarbon in a recently created process called photodissociation.

Photodissociation destroys the dicarbon before it can move far from the nucleus of the comet which causes the green coma to brighten and shrink. This is the first time the chemical reaction has been studied on Earth.

“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,” says Ms Jasmin Borsovszky, lead author of the study and former UNSW Science Honours student.

“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.”

Professor Schmidt has been studying dicarbon for 15 years and says that the study and its findings help us better understand both comets and dicarbon.

“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,” he says.

“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.”

Comet Leonard with its long extended tail. Credit: Andy Casely.

To solve this mystery, the team needed to create chemical processes that would normally occur in space on Earth. This was done with a lot of lasers in a vacuum chamber. The dicarbon was created in the lab and then sent through a gas beam in a vacuum chamber, which was around 2m long. The team 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.

By analysing the speed of the atoms flying off, the team could measure the strength of the carbon bond to about one in 20,000 – which is like measuring 200 metres to the nearest centimetre.

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

“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.” 

Now that the case of the missing green tail in comets is solved, Prof. Schmidt, who specialises in space chemistry, wants to continue solving other space mysteries. 

Next, he hopes to investigate diffuse interstellar bands: patterns of dark lines between stars that don’t match any atom or molecule we know of.  

The article is available in the journal, PNAS.