Green Comet Puzzle Solved – Test shows why many comet heads glow green, but not their tails

Comet Leonard, Lovejoy or Neovice: The “head” of many comets glows remarkably green, but strangely enough, their tail never glows. Researchers now understand for the first time why this is the case in an experiment. Thus, the dicarbon molecule that causes the green glow breaks down (c₂) This photosynthesis occurs under the sun’s ultraviolet light – and before the dicarbon flows from the center of the comet to the tail.

When Comets From dark, cold areas Kuiperkertels Or the Oort cloud enters the inner solar system and is active: the sun’s heat lifts the comet’s nucleus, pulling dust and gases into space – forming the tail and comet’s shell, coma. The head of the comet often glows green. This is what is currently seen in the sky Comet C / 2021 Leonard Case.

Only the head of the comet is green – but why?

However, the strange thing is that although the color green is common in the nucleus and coma of many comets, it never appears on the tail of a comet. Although the head of the comet is bright green, it is usually white, yellow or slightly purple. Also noteworthy: if the comet approaches the Sun, it loses its green color again and the coma turns white again. But why?

It is clear that the green color is due to the dicarbon molecule (c.₂), A mixture of two carbon atoms. “This molecule is found in flames, comets, stars and a wide range of galaxies,” explained Jasmine Borsowski and her colleagues at the University of New South Wales in Sydney. Organic molecules in comet coma are formed when they decompose under the influence of solar radiation.

The first practical test for a 90-year-old theory

But this does not explain why the tail of a comet does not turn green. The hypothesis was put forward by physicist Gerhard Herzberg in the 1930s. He suspected that if the solar radiation was too strong, the dicarbon would decompose itself. “But this photosynthesis of dicarbon has never been directly observed and the mechanism of this decay is not clear,” explains Porzowski and his colleagues.

The reason for this is: dicarbon is produced only under extreme conditions and is highly reactive. Therefore, the molecule is difficult to form and keep in the laboratory for a long time. But now the research team has succeeded. “We first made dicarbon by adding a large molecule called perchloroethylene or C.₂Cl₄, Bombed with a powerful UV laser and stole its chlorine atoms, ”explains Porzowski’s colleague Timothy Schmidt.

The generated dicarbon molecules were sent through a two-meter-long vacuum chamber where they had to be sent back with two UV lasers. It should simulate radiation close to the sun. Using particle detectors and spectrometers, the team succeeded in recording the excitation and binding states of molecules.

Photosynthesis confirmed

Effect: Although the two carbon atoms in dicarbon are very tightly bonded and their deflection energy is 602.804 kilojoules per mole, they will decompose if the radiation is high enough. “In order to break the fourfold bond of dicarbon with the help of sunlight, the molecule must absorb two photons and undergo two ‘inhibited’ transformations,” said Borsowski and his colleagues. However, their measurements confirmed that the UV light in the inner solar system was sufficient.

About 90 years later, the research team confirmed Hersberg’s theory. “Solving a puzzle from the 1930s is very satisfying,” says Schmidt. “We invented the mechanism and proved that dicarbon is broken down by sunlight.”

The tail disintegrates very quickly

This has now clarified why the coma of many comets glows green, but not their tail: According to measurement data, the average lifespan of a dicarbon molecule in a comet is about 160,000 seconds – about 44 hours. So the molecule is not destroyed two days after its formation – very quickly to leave the coma and penetrate into the tail.

“This explains why the comet’s tail is not green, and why the green coma contracts as the comet approaches the sun,” says Schmidt. Comet Leonard and its concepts use their color to directly observe the effect of high-energy light from the sun on chemical reactions in space. (Processes of the National Academy of Sciences, 2021; doi: 10.1073 / pnas.2113315118)

Quelle: University of New South Wales

December 21, 2021

– Nadja Podbregar