Why are only comet heads green?

At the moment, another comet is racing through the inner solar system – Comet C/2021 Leonard was sometimes visible with the naked eye. What is striking about him and many other comets is the green color of the head, which, however, never extends to the tail. Scientists have now explained why this happens and what role solar radiation plays in it. For the first time, they succeeded in reproducing in a laboratory experiment how the C molecule, which consists of two carbon atoms, reproduces₂, which is responsible for the green color, is created only by sunlight, but then disintegrates again.

Comets are icy bits from the far, dark outer edge of the solar system. Their origin lies in the Kuiper belt and in the Oort cloud, far from Neptune. Composed of dust, organic particles, and ice, these bodies follow eccentric orbits that oscillate between the extreme distances of the sun and the inner solar system. If a comet approaches the warmer regions of the inner solar system that are more exposed to the sun, it will not be without consequences: the sun’s heat tolerates the ice and releases dust and organic molecules into space – this creates the comet’s tail and solids. The shell that falls into the heart of the comet, coma.

green zombie puzzle

When viewed from Earth, comets begin to shine brighter and brighter as they approach the Sun, their coma swells and their tails become visible. The coma often has a noticeably greenish glow, while the tail remains white. However, if a comet approaches the sun, it loses its green color again and the coma becomes white. Scientists have long assumed that the green color is caused by a specific molecule: dicarbon (C .).₂). “This molecule is found in flames, comets, stars and the interstellar medium,” explains Jasmine Borsovsky of the University of New South Wales in Sydney and colleagues. In a coma, comets are 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 color caused by carbon dioxide never diffuses into the tail of comets and also disappears from the coma when it is close to the sun: he hypothesized that the carbon dioxide might cause the stronger solar radiation in turn to disintegrate. “But this photodissociation has never been directly observed and the mechanism of this decay is unclear,” the scientists explain. The reason for this is the great difficulty in producing the highly reactive molecule dicarbonate in the laboratory and keeping it stable for a sufficiently long time. But the research team has now succeeded in doing so. “We first produced bicarbonate by adding a larger molecule perchloroethylene, or C₂Cl₄ Borsovsky’s colleague Timothy Schmidt explains it. The generated dicarbon particles were passed through a two-meter vacuum chamber, where they had to pass two UV lasers again. This should mimic radiation near the sun. Using particle detectors and spectrometers, the team succeeded in recording the excitation and binding states of the molecules.

Photodissociation confirmed

Measurements showed that the carbon dioxide actually decays at a certain level of radiation. According to the measurements, the dissociation energy of the two carbon atoms is 602.804 kilojoules per mole. “In order to break the quadruple bond of carbon dioxide with the help of sunlight, the molecule must absorb two photons and pass through two ‘forbidden’ transitions,” Borsovsky and colleagues report. When the comet is roughly at the level of Earth’s orbit, the solar radiation provides the necessary energy and ensures the comet’s bicarbonate breaks down, the team found. “So we found the mechanism and demonstrated that dicarbonate is disintegrated by sunlight,” says Schmidt. About 90 years later, he and his team confirmed Herzberg’s theory. “It’s very satisfying to have solved a puzzle dating back to the 1930s,” Schmidt says.

At the same time, these findings also answer the question of why only comet comas have a green glow, and never their tail. According to tests, the dicarbon molecule takes about 160 thousand seconds to break down under the influence of radiation. So the molecule is destroyed no later than two days after its formation – too quickly to leave the coma and penetrate into the tail. “This explains why the green coma diminishes as a comet approaches the sun, and also why the comet’s tail does not appear green,” Schmidt says.

Coyle: Jasmine Borsovsky (University of New South Wales, Sydney) et al, Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2113315118

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