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The head of a comet often glows green; The queue is mostly absent. This includes Comet LeonardIt made its closest transit to the sun on Monday and is moving away again.
A team of scientists has now found a detailed explanation for this multicolored behavior. The molecule responsible for the emerald color is broken down by sunlight within a few days of being formed near the comet’s nucleus, leaving almost nothing in the tail to glow green.
“We showed exactly how this happens in the lab by measuring exactly how the molecule breaks down, using UV lasers,” said Timothy W. Schmidt, professor of chemistry at the University of New South Wales in Australia.
A comet, a clump of ice and dust, heats up as it approaches the sun, and its ice turns into gas, creating a cloudy atmosphere known as a coma. The atmosphere contains carbon-based molecules, which in turn are bombarded with ultraviolet light from the sun, breaking it down and stripping it of its outer parts. This produces a simple but fragile molecule known as dicarbon or C₂ in chemical notation. It is two carbon atoms bonded together.
Scientists had known for the better part of a century that photons could bring dicarbon molecules into an excited state. Due to the quantum nature of the universe, an excited molecule returns to its ground state by emitting a photon. For the dicarbon, the photon is usually one of the green light. This would explain the green color of the comet comas. But the apparent scarcity of dicarbon in the comet’s tails was something of a mystery.
Thus, Dr. Schmidt recreated what happened in his labs. They started with molecules made up of two carbon atoms and four chlorine atoms to produce dicarbons, and they used a laser to strip the chlorines, leaving only the dicarbon. They then used another laser to break up the dicarbon and measured exactly how much energy was required.
Based on this, they showed how dicarbon molecules must absorb two photons to break apart, and that a dicarbon molecule bathed in sunlight has a lifespan of about 44 hours. At that time, the molecules could travel as much as 80,000 miles – quite a distance. But comet tails can extend millions of miles. Thus, there would be little or no green glow there.
This largely agrees with what has been observed in comets.
Dr. Schmidt’s team presented their findings last month. An article published in the Proceedings of the National Academy of Sciences.
“What they’re doing is fundamental work that’s fundamental to explaining the observations,” said Anita Cochran, deputy director of the University of Texas McDonald Observatory, who was not involved in the research. “Understanding carbon in the universe is very important because it’s such a common species.”
William Jackson, a professor of chemistry at the University of California at Davis, praised the work, but said there was probably more to the story. He noted that a photo of a comet featured in the paper showed not only a green coma, but also a slight green tint to the tail.
Dr. “I think this is a great example of the importance of doing laboratory measurements and combining them with astronomical observations and trying to understand what you’re seeing,” Jackson said.
But bombarding sunlight likely produces additional dicarbons in the comet’s tails and puts the molecules in various excited states. Dr. “It’s a bit too simplistic to say you don’t see C₂ in the queue,” Jackson said.
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