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There’s no question that comets have been regarded as some of the most beautiful things in the night sky for thousands of years. But why are their heads often green but never their tails?

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Go to and use the code SciShowSpace at  checkout to get $5 off any order. [♪ INTRO] Few things in the night sky  are as beautiful as comets. They’ve been recorded by ancient people  all over the world for thousands of years.

And understandably so! No other astronomical object visible  to the naked eye shines so colorfully. And scientists have been interested  in their color for quite a while.

One question, in particular, has puzzled  astronomers for nearly a century: if comets’ heads are often green, why  is that never true for their tails? Now, thanks to cutting-edge research,  scientists may finally have the answer. A comet’s green color isn’t just beautiful; it’s also unstable and easily  destroyed by the Sun’s powerful rays.

Comets are icy objects that were left behind after the solar system formed  four and a half billion years ago. Today, these space snowballs largely  orbit the Sun in the vast expanse beyond Neptune, in areas called  the Kuiper belt and the Oort cloud. And when they’re pulled onto paths that  carry them into the inner solar system, they become visible from Earth.

As a comet gets closer to the Sun, it  begins to heat up, which transforms it from basically a ball of ice into the  more extended object we’re familiar with. In general, comets, when we see them, can  be broken down into four distinct parts. The most substantial is the nucleus, which  is the hard, icy object at its center.

Around the nucleus swirls the  coma, an envelope of gas and dust that has evaporated from the  nucleus as the comet heats up. Then, there are the tails. Most comets have two distinct tails.

The dust tail consists of bits of  the coma that have drifted away as the comet hurtles through space, kind of like the dust behind a car as  it drives down a dirt road. But when the Sun’s ultraviolet light  hits particles in those bits of coma, it can knock some of their electrons  off, forming electrically-charged ions. This creates the ion tail.

And because it interacts with the Sun’s  powerful solar wind, the ion tail always points directly away from the Sun,  regardless of which way the comet is moving. What makes comets so intriguing is that each of these components  can have a different color. A dust tail reflects sunlight, giving  it a yellow-white hue, while the carbon monoxide molecules in the ion tail emit their own light with a ghostly blue color.

The coma also emits its own light, but  rather than blue, it can shine bright green. Astronomers have puzzled for more than a century over why all these colors  can exist at the same time. After all, if both the ion and dust  tails form from material in the coma, why aren’t they green?

The answer seems to come down to chemistry. You see, as the comet gets closer to the  Sun, it warms up, and the coma forms. The rising gas expels complex  carbon-based molecules into space, where they are torn apart by  the Sun’s ultraviolet light.

As the molecules are broken down, they rearrange, creating an unusual molecule called dicarbon, which is two carbon atoms bonded  to each other and nothing else. Scientists were able to figure out as  early as 1868 that dicarbon was the likely source of a comets’ green color, but they couldn’t figure out why  it didn’t spread to its tail. Dicarbon shines green because of the  arrangement of its electron energy levels.

Molecules can emit light as their electrons fall from a higher energy level to a lower one. The exact change in energy corresponds to a particular color of light, in this case, green. But a potential answer on how  this color doesn’t reach the tail wasn’t suggested until the 1930s: Perhaps UV light didn’t just create  dicarbon, it also destroyed it.

And if that destruction happened fast enough, the molecule wouldn’t survive  long enough to enter the tail. But the technology to actually test that  hypothesis didn’t exist at the time. That’s because dicarbon is unstable and can only be produced in  energetic, low-oxygen environments.

Space has plenty of those; Earth, not so much. But, in a 2021 paper,  researchers set up an experiment to simulate the space environment around a comet. The team shined ultraviolet lasers  on the molecule perchloroethylene, which knocked off the chlorine atoms  and left behind a dicarbon molecule.

And the experiment was run in a vacuum chamber to simulate the conditions in a comet’s coma. At that point, another UV laser was turned on, mimicking the intense light  of the Sun on the coma. The researchers observed that the  light split apart, or photodissociated, the dicarbon, exactly as had  been predicted in the 1930s.

They were able to calculate the average  lifetime of a dicarbon atom in the coma, which turned out to be around two days, which was also in line with previous estimates. And, voilà, the resolution of  a 150-year-old cosmic mystery! As a comet hurtles through  space, it forms dicarbon, but it can’t drift too far into the tail  because it gets destroyed way too fast.

A result like this can seem  kind of unimportant, after all, scientists had the basic idea figured  out almost a hundred years ago! But getting confirmation of our  predictions is a really big deal because it gives future researchers  the confidence to make their own. Astronomers have made all sorts  of predictions that may not be testable in our lifetimes, so  every success is an encouragement that the scientific method is leading  humanity in the right direction.

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Thanks so much for watching, and thanks again to Magic Spoon for sponsoring  this episode of SciShow Space. [♪ OUTRO]