Green comet riddle solved – experiment shows why many comet heads glow green, but not their tails –

Whether Comet Leonard, Lovejoy or Neowise: The “head” of many comets is strikingly green, but strangely enough, their tail never shines. Researchers have now for the first time understood why this is so in an experiment. According to this, the dicarbon molecule responsible for the green glow breaks down (C2) under the UV light of the sun – and this photodissociation occurs before the dicarbon can flow from the comet’s core to the tail.

When comets come into the inner solar system from the dark, cold regions of the Kuiper Belt or the Oort Cloud, they become active: The heat of the sun sublimes the ice of the comet’s core and tears dust and gases out into space – the tail and the comet’s shell are formed Coma. The comet’s head often glows noticeably green. This is also the case with comet C / 2021 Leonard, which is currently visible in the sky.

Dicarbon is responsible for the green color. Whether the two atoms of the dicarbon are linked by a double, triple or quadruple bond is a matter of dispute and depends on the conditions. © Komet: Franco Tognarini/ Getty images

Green only on the comet’s head – but why?

The strange thing, however, is that although the green coloration is typical of the nucleus and coma of many comets, it never appears on the comet’s tail. Even with a bright green comet head, it usually remains whitish, yellowish or slightly purple. Also noticeable: If the comet comes closer to the sun, it loses its green color again and the coma also becomes whitish again. But why?

It is clear that the green coloring is due to the molecule dicarbon (C.2), a compound of two carbon atoms. “This molecule is found in flames, comets, stars and the diffuse interstellar medium,” explain Jasmin Borsovszky from the University of New South Wales in Sydney and her colleagues. In the cometary coma it is formed when organic molecules disintegrate under the influence of solar radiation.

First practical test for a 90-year-old theory

But that doesn’t explain why the comet’s tail never turns green. The physical chemist Gerhard Herzberg postulated a hypothesis for this in the 1930s. He suspected that dicarbon decomposes itself if the solar radiation is too strong. “But this photodissociation of the dicarbon was never observed directly and the mechanism of this decay is unclear,” explain Borsovszky and her colleagues.

The reason for this: Dicarbon is only produced under extreme conditions and is highly reactive. Hence, the molecule is difficult to make in the laboratory and to keep for a long time. But the research team has now succeeded in doing this. “We first produced the dicarbon by adding the larger molecule perchlorethylene, or C2Cl4, bombarded it with a strong UV laser and thus stole its chlorine atoms, ”explains Borsovszky’s colleague Timothy Schmidt.

The generated dicarbon molecules were passed through a two-meter-long vacuum chamber, where they had to pass two UV lasers again. This should simulate the radiation near the sun. Using particle detectors and spectrometers, the team succeeded in recording the excitation and binding states of the molecules.

Photodissociation confirmed

The result: Although the two carbon atoms in the dicarbon are very tightly bound and their dissociation energy is around 602.804 kilojoules per mole, they can decay if the radiation is sufficiently high. “In order to break the fourfold bond of the dicarbon with the help of sunlight, the molecule has to absorb two photons and go through two ‘forbidden’ transitions,” report Borsovszky and her colleagues. However, their measurements confirmed that the UV light in the inner solar system is sufficient for this.

After around 90 years, the research team has thus confirmed Herzberg’s theory. “It’s extremely satisfying to have solved a puzzle that dates back to the 1930s,” says Schmidt. “We found the mechanism and proved that dicarbon is broken up by sunlight.”

Disintegrates too quickly for the tail

This has now also clarified why the coma of many comets glows green, but not their tail: According to the measurement data, the average lifespan of a dicarbon molecule in the cometary is around 160,000 seconds – around 44 hours. The molecule is therefore destroyed no later than two days after its formation – too quickly to leave the coma and penetrate into the tail.

“That explains why the comet’s tail is not green and also why the green coma shrinks when the comet gets closer to the sun,” says Schmidt. Comet Leonard and its conspecifics use their color to make directly visible what influence the high-energy light from the sun has on chemical processes in space. (Proceedings of the National Academy of Sciences, 2021; doi: 10.1073/pnas.2113315118)

Quelle: University of New South Wales

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