comet

90-Year-Old Hypothesis Proved Right: The Green Colored Nucleus and the not so Green Tail of a Comet

Comets hold a special place in our hearts, as spotting them means we can wish for luck. But have you ever observed their color? If you watch a comet from a less light-polluted area, you might notice that the head (nucleus) of the comet appears green, whereas the tail looks strikingly white. How does the color near the nucleus not spread through to its tail?


Astronomers had been pondering this question for some time until a Nobel prize-winning scientist, Gerhard Herzberg, proposed a hypothesis describing the mechanism behind it.


When the organic molecules present in a comet’s nucleus are subjected to large amounts of UV radiation, a highly reactive molecule known as dicarbon (C2) is formed. He theorized that the green color in the nucleus is due to the formation of dicarbon. The hypothesis could not be proved as it is difficult to replicate this phenomenon in a controlled environment on Earth, until now.

In December 2021, a UNSW led team of scientists successfully reproduced this chemical phenomenon. They experimentally verified the mechanism behind the formation of dicarbon in outer space and why the tails aren’t green. They confirmed that the latter occurs due to a chemical phenomenon called photodissociation (splitting up of atoms into ions in the presence of light).


When the comet passes near the sun, the dicarbon breaks down, due to high exposure to UV light, before it can reach the tip of the comet so that the tail never appears green. They recreated this process in a vacuum chamber using three powerful UV laser lights, perchloroethylene molecule (C2Cl4), and a speed detector.


This discovery opens up doors to other potential projects in the field of Astrobiology. Since reactions involving dicarbon produced from organic matter, found in comets and other celestial objects, might provide us with the answers that we’re searching for, like the origin of the stuff of life.


How do you think this discovery could benefit future research?

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