Why is the comet’s head green, but its tail is not?

The head of a comet usually glows green; The tail is almost nonexistent. That includes Comet Leonard, passed closest to the sun on Monday and will be far away again.

A team of scientists has now come up with a detailed explanation for this polychromatic behavior. The molecule that makes up the emerald color is blown away by sunlight within a few days of being created near the comet’s core, leaving barely a green glow in the tail.

Timothy W. Schmidt, professor of chemistry at the University of New South Wales, Australia, said: “We have shown exactly how that happens in the lab using a UV laser, which precisely measures it. the way the molecule blows out.

When a comet – a cloud of dust and dust – gets close to the sun, it heats up and its particles turn to gas, creating a hazy atmosphere known as a coma. The atmosphere is composed of carbon-based molecules that are in turn bombarded by ultraviolet rays from the sun, breaking it apart and separating from the outer fragments. That creates a simple but fragile molecule called a dicarbon, or C₂ in chemical notation. It is two carbon atoms bonded together.

Scientists have known for more than a century that photons can knock dicarbon molecules to an excited state. Due to the quantum nature of the universe, a molecule is excited back to its ground state by emitting a photon. For dicarbons, the photon is usually one of the green light. This explains the blue color of the comet’s coma. But the apparent scarcity of dicarbon in the comet’s tail is something of a mystery.

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

From there, they showed how dicarbon molecules must absorb two photons to be blown apart, and that the lifetime of a dicarbon molecule in sunlight is about 44 hours. During that time, the molecules can travel 80,000 miles or so – pretty far. But Comet tails can stretch for millions of miles. Therefore, there will be little or no dicarbon, and no green light there.

That is largely consistent with what has been observed in comets.

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

“What they’re doing is fundamental work to interpret the observations,” said Anita Cochran, assistant director of the McDonald Observatory at the University of Texas, who was not involved in the study. “Understanding carbon in the universe is quite important because it is such a common species.”

William Jackson, professor of chemistry emeritus at the University of California, Davis, praised the work but said the story was more likely. He noted that a photo of the comet included in the paper not only showed a green coma, but also a slight greenish tinge to the tail.

“I think this is a great example of how important it is to take measurements in the lab and combine that with astronomical observations and try to understand what you see,” said Dr. Jackson.

But the bombarding sunlight has the potential to create more dicarbons in the comet’s tails and propel the molecules into various excited states. Dr. Jackson said: “It’s a bit too simplistic to say that you don’t see the C in the tail. Why is the comet’s head green, but its tail is not?

Fry Electronics Team

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