Why do comets emit green light on their heads?

A group of chemists have just solved the mystery that has puzzled researchers for decades that the head of a comet emits green light instead of the tail. A fleeting and elusive molecule on earth is the key to the answer.

Comets carry large chunks of ice and dust left over from the formation of the solar system, and occasionally venture across the earth from the cold boundary of the solar system. As early as A.D. 1930, Gerhard Herzberg, who later won the Nobel Prize for his research on free radicals and other molecules, speculated that the process behind the luminescence of green comets might involve a molecule composed of two carbon atoms called carbon dioxide. A new study published in the Proceedings of the National Academy of Sciences verifies Herzberg's theory.

Tim Schmidt, a chemist at the University of New South Wales in Sydney, Australia, said that carbon dioxide was so lively that the research team could not get it from the bottle. In space, it exists in stars, nebulae and comets. But when exposed to oxygen in the earth's atmosphere, carbon dioxide will react quickly and "burn", Schmidt said.

Schmidt said that this is the first time that scientists can accurately study how molecules decompose under strong ultraviolet light. In the laboratory, the team must use a vacuum chamber and three different ultraviolet lasers to simulate the environment in near-earth space. Because carbon dioxide reacts so fast, they have to use a laser to cut off a larger molecule to synthesize it in situ.

Schmidt said that they confirmed that the green light of comets comes from carbon molecules, which can absorb and emit visible light when exposed to sunlight in space. Herzberg's theory of carbon dioxide is correct, he said, even if it is not correct in mechanism-but it was in the 1930 s, Schmidt said, so "it should be understood."

In comets, when sunlight heats ice, two kinds of carbon are formed, some of which may be composed of acetylene, which is a mixture of hydrogen and carbon and is used as gas and welding fuel on earth. Schmidt said that when more complex organic molecules in comets decompose, they can be produced in space.

Hydrogen atoms are separated from acetylene molecules. Without them, the bonds between carbon atoms will be "re-tightened" to form double carbon molecules.

When the sun heats the molecules in comets, they gain energy and glow, but before they reach the tail too far, they will break down into individual carbon atoms. This explains why green light only exists around the comet's body, not around its long tail.

When a comet is exposed to enough sunlight to release gas, sunlight will continuously produce new luminescent carbon. Schmidt believes that the life span of carbon dioxide molecules is about two days at the distance between the earth and the sun.

Anita Cauquelin, an astronomer and assistant director of the MacDonald Observatory at the University of Texas, said that the team was able to understand how carbon dioxide was split in the laboratory and showed that scientists had strictly confirmed the mechanism of green comets.

Cochrane spent most of his career studying comets. She said that they can be regarded as great laboratories because telescopes can observe them well through their huge tails (usually millions of miles long) and understand what they are made of.

The research team learned about the life span of carbon dioxide molecules exposed to sunlight and measured the time required to break down their bonds. These facts will help to simulate the behavior of comets. It seems a rare event for a comet to cross the sky, but astronomers have now discovered thousands of comets. There may be an amazing number of comets farther away in the solar system. Thanks to these green snowballs, scientists have a window to understand the ancient history of the solar system.