Scientists Finally Solve the Mystery of Why Comets Glow Green

A team of chemists just solved the mystery of why comets' heads -- but not their tails -- glow green, which had puzzled researchers for decades. From a report: Studying an elusive molecule, which only fleetingly exists on Earth, was the key. Comets are speeding chunks of ice and dust left over from the formation of the solar system, which occasionally venture from the system's cold outer reaches to pass by Earth. Back in the 1930s, Gerhard Herzberg, who later won the Nobel prize for his research on free radicals and other molecules, guessed that the process behind the green comet glow might involve a molecule made from two carbon atoms bonded together, called dicarbon. A new study, published in the journal the Proceedings of the National Academy of Sciences, put Herzberg's theory to the test. Dicarbon is so reactive that the team behind the study couldn't get their supply of it from a bottle, says Tim Schmidt, a chemist who oversaw the study at the University of New South Wales in Sydney, Australia. In space, it exists inside stars, nebulae, and comets. But when exposed to the oxygen in Earth's atmosphere, dicarbon will quickly react and "burn up," Schmidt says. Schmidt says this is the first time scientists have been able to examine precisely how the molecule breaks apart when exposed to powerful ultraviolet rays. In the lab, the team had to simulate the environment of near-Earth space with vacuum chambers and three different ultraviolet lasers. Because dicarbon reacts so quickly, they had to synthesize it on the spot by whittling away a larger molecule with a laser.

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