There can be green comets, and we are still finding out why.
Green is an unusual color in astronomy. It is the color to which our eyes are most delicate, and yet few things in the night sky in fact appear green. There are, for example, no green stars, just yellow-white, red, and blue ones. There can be green comets, and we are still discovering why.
Comets can appear green since of the chemistry of the gases vented by the comet. When ionized, these gasses release a brilliant green radiance, which can make the coma appear green.
The rate at which C2 breaks apart likewise helps explain why the green radiance of a comets coma fades as the comet gets closer to the Sun.
The green color of a comet is short-term, and appears only as it passes near the Sun.
Comets can appear green due to the fact that of the chemistry of the gases vented by the comet. When ionized, these gasses release a brilliant green radiance, which can make the coma appear green.
The structure of a comet. Credit: NMM London
While the coma can appear green, even to the naked eye, the tail doesnt. If the tail is just ionized gas streamed off the coma, should not it appear green?
The longstanding idea is that in time these substances are disintegrated by ultraviolet radiation. By the time they reach the tail the particles have actually been broken down to atoms, hence no green color. But showing this is incredibly tough. Diatomic carbon, which is the most typical source of the green glow, is exceptionally reactive with oxygen and nitrogen gas, so its unsteady in a typical lab.
To study C2, the group utilized a vacuum chamber, exposed C2Cl4 to an effective laser to break it apart, then vented the chlorine gas to leave diatomic carbon. The rate at which C2 breaks apart also assists explain why the green radiance of a comets coma fades as the comet gets closer to the Sun.
Other astrochemistry research studies reveal that diatomic carbon isnt widely present on the surface area of a comet. Instead, the molecule is formed through chain reaction set off by sunlight. As a comet nears the Sun it slowly forms a greenish radiance. The green color of a comet is short-term, and appears only as it passes near the Sun.
Reference: Borsovszky, Jasmin, et al. “Photodissociation of dicarbon: How nature breaks an uncommon several bond.” Proceedings of the National Academy of Sciences 118.52 (2021 ).
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