Why only comet heads are green – Wissenschaft.de

At the moment another comet is racing through the inner solar system – Comet C / 2021 Leonard was at times even visible to the naked eye. What is striking about him and many other comets is the greenish color of the head, which, however, never extends into the tail. Scientists have now clarified why this is the case and what role solar radiation plays in it. For the first time they succeeded in reproducing in a laboratory experiment how the molecule C, which consists of two carbon atoms2, which is responsible for the green color, is only created by the sun’s rays, but then disintegrates again.

Comets are icy chunks from the distant, dark outer edge of the solar system. Their origin lies in the Kuiper Belt and in the Oort Cloud, far beyond Neptune. These objects, made up of dust, organic molecules and ice, follow eccentric orbits that cause them to oscillate between extreme solar distances and the inner solar system. If a comet approaches the warmer areas of the inner solar system that are more exposed to sunlight, this is not without consequences: the sun’s heat sublimates the ice and outgassing tears dust and organic molecules out into space – this creates the comet’s tail and the solid ones The shell lying at the core of the comet, the coma.

Enigma of the green coma

Seen from earth, comets begin to shine brighter and brighter as they approach the sun, their coma enlarges and their tail becomes visible. The coma often has a noticeably greenish glow, while the tail remains whitish. However, if the comet gets closer to the sun, it loses its green color again and the coma becomes whitish. Scientists have long assumed that the green color is due to a specific molecule: the dicarbon (C2). “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.

As early as the 1930s, the physicist Gerhard Herzberg proposed an explanation for the fact that the green coloration caused by the dicarbon never spreads to the tail of comets and also disappears from the coma when closer to the sun: He postulated that the dicarbon would cause even stronger solar radiation in turn, disintegrates. “But this photodissociation was never observed directly and the mechanism of this decay is unclear,” explain the scientists. The reason for this is the great difficulty in producing the highly reactive dicarbon molecule in the laboratory and keeping it stable for a sufficiently 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 Borsovszky’s colleague Timothy Schmidt explains. 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 measurements showed that the dicarbon actually decays at a certain level of irradiation. According to the measurements, the dissociation energy of the two carbon atoms is 602.804 kilojoules per mole. “In order to break the fourfold bond of the dicarbon with the help of sunlight, the molecule must therefore absorb two photons and go through two ‘forbidden’ transitions,” report Borsovszky and her colleagues. When a comet is roughly at the level of the earth’s orbit, the solar radiation provides the necessary energy and ensures that the comet’s dicarbon breaks down, as the team found. “We have thus found the mechanism and proven that dicarbon is broken up by sunlight,” says Schmidt. After around 90 years, he and his team have confirmed Herzberg’s theory. “It’s extremely satisfying to have solved a puzzle that dates back to the 1930s,” says Schmidt.

At the same time, these results also answer the question of why only the coma of the comet has a greenish glow, but never its tail. According to the tests, it takes around 160,000 seconds for the dicarbon molecule to break under the influence of radiation. 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 green coma shrinks when the comet gets closer to the sun and also why the comet’s tail is not green,” says Schmidt.

Quelle: Jasmin Borsovszky (University of New South Wales, Sydney) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2113315118


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