Here’s a tricky question. Why are the heads or “nucleus” of some comets green, yet their tails aren’t?
After all, a comet’s tail is made from the same material as the nucleus and subject to the same conditions – Solar wind and radiation, and yet somehow, they appear to be entirely different colours. Their heads can appear green but comets’ tails almost never are.
Why is that? Up until recently we simply didn’t know for sure!
Image of Comet 2022/C (ZTF) taken by Michael Egger on December 18, 2022
Early Theory about why comets glow green first proposed in the 1930s.
Astronomers had a hint at what the green colour could be, although proving it took time and some modern laboratory technology.
In the 1930s, German chemist Gerhard Herzberg suggested that Sunlight was destroying a molecule called diatomic carbon (C2), which is also known as dicarbon. This could possibly be created by the interaction of Sunlight with the organic material on the surface of the comets, but there was no way to test and confirm his theory.
(Organic material or “organics” does not mean materials from living organisms or produced by them. The term refers to complex, carbon-based molecules. They’re some of the same materials that make life as found here in Earth. They’re formed and destroyed by natural processes. Organics are found in vast quantities in the Solar System and throughout the Universe.)
The main issue with testing this theory behind green comets is that the dicarbon molecule is extremely unstable. It’s made up of two carbon atoms stuck together. This can only happen in extremely low pressure or in energetic environments. Interstellar space or the outer reaches of the Solar System are two such places. Studying dicarbon on Earth under conditions like those found on a comet was problematic to put it mildly. There was also no clear answer as to why the tails of comets aren’t also green.
UNSW Scientists delve into the problem
A team lead by the University of New South Wales (UNSW) researcher, Professor Timothy Schmidt, finally found the answer by creating a way to study dicarbon here on Earth. Using a high-power ultraviolet laser, they stripped chlorine atoms of an organic molecule called perchloroethylene which is also a chemical commonly found in dry-cleaning solvents. This left just dicarbon molecules.
The remaining dicarbon was kept in a vacuum chamber and sent travelling along a gas beam some two metres long. Two powerful lasers were then focussed on the dicarbon, one to hit it with further radiation to simulate the conditions in space. The final laser was to make the carbon atoms visible so their behaviour could be studied. The lasers tore the dicarbon atoms apart and sent them into a sensor to measure their speed. This could be used to work out the energy in the reaction.
Announcing their results in December 2021 team leader, Professor Schmidt, commented this was the first time anyone has ever observed this chemical reaction here on Earth.
“It’s extremely satisfying to have solved a conundrum that dates back to the 1930s.” he said.
The mystery solved – Solar radiation breaks up the material it only recently created
After months of frustrating experimentation, once reactions around dicarbon were observed, the reasons comets glow green became clear.
As the comet approaches the Sun, sunlight falling on the surface of the comet breaks up various organic molecules producing dicarbon. Then as the comet gets even closer to the Sun, solar UV radiation breaks up the unstable dicarbon molecule it only recently created. This process is called photodissociation and produces the characteristic green glow around the centre or “nucleus” of the comet.
As dicarbon is unstable and breaks up quickly once exposed to UV light from the Sun, it simply doesn’t have time to spread to the comet’s tail.
This is why comet tails are not bright green.
It also explains why the green glow surrounding a comet will sometimes get brighter and smaller the closer the comet gets to the Sun – more dicarbon is being produced by the greater intensity of the Sun’s radiation and this dicarbon is even more quickly being broken up. While the green glow is commonly observed surrounding comets, it not seen on all of them. This is because the types of organics needed to produce dicarbon simply aren’t on those comets’ surface to begin with, or periodic comets that have already made many visits to the inner Solar System might have had these materials stripped away by previous encounters with the Sun’s radiation.
If you’d like to read more about why some comets are green, you can find a news article from UNSW here.
Green in photos – not visually
One thing to note it that the greenish hues and other colours seen on images of comets are not likely to be seen visually even if you’re using a pair of binoculars. Along with most astronomical objects including nebulae, they will appear in “black and white” when you view them.