Why is a comet's head green, but not its tail

The head of a comet often glows green; the tail usually does not. Including comet Leonardo , which made its closest passage to the sun...


The head of a comet often glows green; the tail usually does not. Including comet Leonardo, which made its closest passage to the sun on Monday and leaves.

A team of scientists have now come up with a detailed explanation of this multichromatic behavior. The molecule responsible for the emerald hue is destroyed by sunlight a few days after its creation near the comet’s nucleus, leaving almost nothing glowing green in the tail.

“We have shown exactly how it works in the lab using UV lasers, measuring exactly how the molecule breaks down,” said Timothy W. Schmidt, professor of chemistry at the University of New South Wales in Australia.

When a comet – a clump of ice and dust – approaches the sun, it heats up and its ices turn into gas, producing a hazy atmosphere known as a coma. The atmosphere contains carbon-based molecules which in turn are bombarded with ultraviolet light from the sun, shattering it and ripping off the outer pieces. This generates a simple but fragile molecule known as dicarbon, or C₂ in chemical notation. They are two carbon atoms bonded together.

Scientists have known for nearly a century that photons can cause dicarbon molecules to go 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 dicarbon, the photon is usually that of green light. This explained the green color of comas in comets. But the apparent shortage of dicarbon in the tails of comets was something of a mystery.

So Dr. Schmidt recreated what was going on in their lab. To produce dicarbon, they started with molecules made up of two carbon atoms and four chlorine atoms and used a laser to remove chlorine, leaving only dicarbon. Then they used another laser to break down the dicarbon, measuring exactly how much energy was needed.

From there, they showed how dicarbon molecules had to absorb two photons to be blown out, and the lifespan of a sun-bathed dicarbon molecule is around 44 hours. During this time, the molecules could travel about 80,000 miles – quite far. Corn comet tails can stretch for millions of kilometers. So there would be little or no dicarbon, and no green glow, there.

This largely matches what has been observed in comets.

Dr. Schmidt’s team shared their findings last month in an article published in Proceedings of the National Academy of Sciences.

“What they’re doing is the groundwork that’s fundamental to explaining the sightings,” said Anita Cochran, deputy director of the McDonald Observatory at the University of Texas, who was not involved in the research. “Understanding carbon in the universe is quite important because it is such a common species. “

William Jackson, professor emeritus 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 photograph of a comet included in the document shows not only a green coma but also a slight tint of green in the tail.

“I think this is a great example of the importance of taking measurements in the laboratory and combining them with astronomical observations, and trying to figure out what you are seeing,” Dr Jackson said.

But the sun’s bombardment likely produces additional dicarbon in the tails of comets and projects the molecules into a variety of excited states. “It’s a little too simple to say you don’t see C₂ in the tail,” Dr Jackson said.

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Newsrust - US Top News: Why is a comet's head green, but not its tail
Why is a comet's head green, but not its tail
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