The Heads of Comets can be Green, but their Tails Never are; We have Finally Figured out why it’s Taken 90 Years

The Kuiper Belt and Oort Cloud occasionally fling galactic snowballs comprised of ice, dust, and boulders our way, relics from the solar system’s birth 4.6 billion years ago. As they cross the sky, these snowballs, or as we know them, comets, undergo a spectacular metamorphosis, with many comets’ heads turning a dazzling green color that brightens as they approach the Sun.

However, before it reaches the one or two tails trailing behind the comet, this green hue vanishes. This issue has perplexed astronomers, scientists, and chemists for nearly a century.

Physicist Gerhard Herzberg proposed in the 1930s that the phenomenon was caused by sunlight destroying diatomic carbon (also known as dicarbon or C2), a chemical formed by the interaction of sunlight with organic matter on the comet’s head, but because dicarbon is unstable, this theory has been difficult to test.

A new study lead by UNSW Sydney and published today in the Proceedings of the National Academy of Sciences (PNAS) has finally found a technique to verify this chemical reaction in the lab, proving this 90-year-old idea right.

“We’ve proven the mechanism by which dicarbon is broken up by sunlight,” says 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 central figure in the puzzle, is both extremely reactive and responsible for the green color of numerous comets. It’s made up of two carbon atoms fused together, and it’s only found in high-energy, low-oxygen environments like stars, comets, and the interstellar medium.

Comets don’t have dicarbon until they reach close to the Sun. The organic matter residing on the frozen nucleus evaporates and flows to the coma as the Sun warms the comet. The larger organic molecules are then broken apart by sunlight, resulting in dicarbon.

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.

Martin van Kranendonk

The research lead by UNSW has now demonstrated that as the comet approaches the Sun, strong UV light tears apart the dicarbon molecules it recently formed in a process known as ‘photodissociation.’

This process kills the dicarbon before it can travel far from the nucleus, brightening and shrinking the green coma and ensuring that the green tinge never reaches the tail. This is the first time on Earth that this chemical interaction has been explored.

“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.”

Prof. Schmidt, who has spent 15 years studying dicarbon, believes the results will 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,” 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.”

A laser show like no other

The scientists wanted to duplicate the identical galactic chemical process in a controlled environment on Earth to answer this puzzle. With the help of a vacuum chamber, a lot of lasers, and one tremendous cosmic reaction, they were able to accomplish this.

“First we had to make this molecule which is too reactive to store in a bottle,” says Prof. Schmidt. “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.”

The newly created dicarbon molecules were passed through a gas beam in a vacuum container with a length of around two meters.

The researchers then focused two more UV lasers to the dicarbon, one to flood it with radiation and the other to detect its atoms. The dicarbon was blasted apart by the radiation and its carbon atoms flew onto a speed detector.

The researchers was able to estimate the strength of the carbon bond to one in 20,000 by analyzing the speed of these fast-moving atoms, which is equivalent to measuring 200 meters to the nearest millimetre.

Ms Borsovszky claims that due to the experiment’s intricacy, it took them nine months to make their first observation.

“We were about to give up,” she says. “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.”

Solving space mysteries

There are approximately 3700 known comets in the solar system, with billions more believed to exist. The nucleus of a comet is on average 10 kilometers across, but the coma is frequently 1000 times larger.

Comets can put on amazing displays for those who are fortunate enough to view them. But comets may have done more for Earth in the past; in fact, one theory about the origin of life suggests that comets once carried the building components of life right to our doorstep.

“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.”

Prof. Schmidt, who specializes in space chemistry, wants to continue solving additional space mysteries now that the case of the missing green tail in comets has been solved. He then plans to look into diffuse interstellar bands, which are patterns of dark lines between stars that don’t match any known atom or molecule.

“Diffuse interstellar bands are a pretty big unsolved mystery,” he says. “We don’t know why the light that’s arriving on Earth often has nibbles taken out. This is just one more mystery in a huge inventory of bizarre things in space that we’re yet to discover.”

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