Whether it’s Comet Leonard, Lovejoy or Newwas: the “head” of many comets glows remarkably green, but strangely enough, their tails never shine. Now, for the first time, researchers understand why this happened in the experiment. Accordingly, the dicarbon molecule responsible for the green glow (C .) breaks down2) under ultraviolet sunlight – this photodissociation occurs before the carbon dioxide flows from the comet’s core to the tail.
when comets From the dark and cold worlds in Kuipergürtels Or the Oort cloud enters the inner solar system, and becomes active: the heat of the sun sublimates the ice of the comet’s nucleus and pulls dust and gases into space – the comet’s tail and crust are formed, the coma. The head of a comet often glows noticeably green. This is also the case with what is currently visible in the sky Comet C / 2021 Leonard the case.
Green is only on top of the comet – but why?
But the strange thing is that although the green color is typical of the nucleus and coma of many comets, it never appears on the tail of a comet. Even with a bright green comet head, it usually remains white, yellowish, or slightly purple. Also note: if a comet approaches the sun, it loses its green color again and the coma also becomes white again. but why?
The green color is clearly caused by a dicarbonate molecule (C.2), which is a compound of two carbons. “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.
First practical test of 90-year-old theory
But that doesn’t explain why a comet’s tail never turns green. Physical chemist Gerhard Herzberg postulated a hypothesis for this in the 1930s. It was suspected that the carbon dioxide would decay if the solar radiation was too strong. “But this photodissociation of dicarbonate has never been directly observed and the mechanism of this decay is unclear,” Borsovsky and colleagues explain.
The reason for this is that dicarbonate is only produced under harsh 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 so. “We first produced bicarbonate by adding a larger molecule perchloroethylene, or C2Cl4Borsovsky colleague Timothy Schmidt explains:
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
The result: although the two carbon atoms in the carbon dicarbon are very tightly bound and their dissociation energy is about 602.804 kilojoules per mole, they can decay if the radiation is high enough. “In order to break the quaternary bond of dicarbonate with the help of sunlight, the molecule must absorb two photons and pass through two ‘blocking’ transitions,” Borsovsky and colleagues say. However, their measurements confirmed that UV light in the inner solar system is sufficient for this.
About 90 years later, the research team confirmed Herzberg’s theory. “It’s very satisfying to have solved a puzzle dating back to the 1930s,” Schmidt says. “We found the mechanism and demonstrated that dicarbonate is disintegrated by sunlight.”
The tail disintegrates very quickly
Now this explained why the comas of many comets glow green, but not their tails: according to the measurement data, the average life of the dicarbon molecule in comets is about 160,000 seconds – about 44 hours. So the molecule is destroyed again no later than two days after its formation – very quickly to leave the coma and penetrate into the tail.
“This explains why the comet’s tail does not appear green, as well as why the green coma diminishes as the comet approaches the sun,” Schmidt says. Comet Leonard and its components use their colors to highlight the effect of high-energy light from the Sun on chemical processes in space. (Proceedings of the National Academy of Sciences, 2021; doi: 10.1073/pnas.2113315118)
Cowell: University of New South Wales
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