Comets’ Heads Can be Green, but Never Their Tails. After 90 Years, We Finally Know Why

upstart writes:

Comets' heads can be green, but never their tails. After 90 years, we finally know why: The team solved this mystery with the help of a vacuum chamber, a lot of lasers, and one powerful cosmic reaction.:

[... C]omets -- go through a colourful metamorphosis as they cross the sky, with many comets' heads turning a radiant green colour that gets brighter as they approach 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 a century. In the 1930s, physicist Gerhard Herzberg theorised the phenomenon was due to sunlight destroying diatomic carbon (also known as dicarbon or C2), a chemical created from the interaction between sunlight and organic matter on the comet's head -- but as dicarbon isn't stable, this theory has been hard to test.

A new UNSW Sydney-led study, published today in Proceedings of the National Academy of Sciences (PNAS), has finally found a way to test this chemical reaction in a laboratory -- and in doing so, has proven this 90-year-old theory correct.

"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."

[...] This is the first time this chemical interaction has been studied here on Earth.

Journal Reference:
Jasmin Borsovszky, Klaas Nauta, Jun Jiang, et al. Photodissociation of dicarbon: How nature breaks an unusual multiple bond [open], Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.2113315118)

Original Submission

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