We Now Know Why Comets’ Heads Can Be Green but Not Their Tails

We Now Know Why Comets’ Heads Can Be Green but Not Their Tails
Image: NASA Goddard

For the longest time, scientists have been perplexed by something about comets. Why can their heads be green but never their tails?

Well, after all that time, we finally know why. In a study lead by UNSW and published in the Proceedings of the National Academy of Sciences (PNAS), the mystery surrounding comets and their green heads has been solved.

“We’ve proven the mechanism by which dicarbon is broken up by sunlight,” says Professor Timothy Schmidt, a chemistry professor at UNSW Science and the a senior author of the study.

“This explains why the green coma – the fuzzy layer of gas and dust surrounding the nucleus – shrinks as a comet gets closer to the Sun, and also why the tail of the comet isn’t green.”

Alright, that’s a lot of sciency talk. What does all that mean?

Every so often the Kuiper Belt and Oort Cloud throw out some icey comets (comprised of mostly dust, rocks and ice) which often skirt past Earth at a distance where we can observe them.

As the comets approach the Sun, they can pick up a radiant green colour at the front, but the colour never reaches the back. Surely, you’d think, the same chemical and physical reactions happening at the front of the comet would be the same as those happening at the back, right?

Well, not really. As it would turn out, the lack of green in the comets tail is due to a process of sunlight destroying diatomic carbon present in the asteroid. What is diatomic carbon? Well, it’s a green, gaseous chemical (dicarbon or C2). In this circumstance, it was created by sunlight coming into contact with organic matter present on the comet, which the sun then destroys, causing a chemical change.

green comet tail
Image: Shutterstock

That’s not to say that there’s organic life on the comets – organic matter in this circumstance simply applies to the chemical and natural makeup of the comets. This study was originally theorised by physicist Gerhard Herzberg in the 1930s, who turns out to have been right.

What made proving this theory hard for so long was the fact that dicarbon isn’t stable. It’s made of two carbon atoms and can only be found in extremely energetic or low oxygen environments like stars and comets.

On comets, in particular, it doesn’t even exist until it gets near the Sun. As the comet gets even closer to the Sun, the radiation from the star breaks down the dicarbon through a process called photodissociation.

It turns out that this is the process that causes the green colouring to not reach the back of the comet. The researchers studied this by creating a controlled environment mimicking that of the theory.

Using a UV laser to blast the chlorine atoms off of a perchloroethylene molecule, the team was able to create dicarbon in this environment. They were sent down a gas beam in a vacuum chamber, where two more lasers kicked in – one providing radiation (mimicking the Sun) and one recording the process. In line with the theory, the radiation blasted the dicarbon apart. It took nine months for the experiment to yield observable results.

“It’s extremely satisfying to have solved a conundrum that dates back to the 1930s,” Professor Schmidt added.

“Dicarbon comes from the breakup of larger organic molecules frozen into the nucleus of the comet – the sort of molecules that are the ingredients of life.

“By understanding its lifetime and destruction, we can better understand how much organic material is evaporating off comets. Discoveries like these might one day help us solve other space mysteries.”

Science rules.