Researchers from the University of New South Wales (UNSW) have published a new study examining why only comet heads look green and not their tails.
Comets are frozen masses of gas, rock and dust that are left over from the formation of a solar system. As they approach the sun, their ice sublimates from solid form directly to a gas, contributing to a clear green glow in some comets, but not all.
The researchers used a vacuum chamber and lasers to recreate the chemical reaction between ultraviolet radiation from the sun and diatomic carbon (dicarbon). The process, known as photodissociation, breaks the two carbon atoms apart and gives the comets their green appearance.
Dicarbon can only exist in extremely energetic or oxygen-poor environments such as stars or comets, as it is highly reactive and unstable. As a comet approaches the sun, it heats up and evaporates to form the comet’s coma.
At the same time, dicarbon is formed by the breakdown of other carbon-based molecules that make up the comet. But before the dicarbonate can join the tail, it is photodissociated, causing the comet’s head to glow brighter green, but not the tail.
“We have proven the mechanism by which dicarbon is degraded by sunlight. This explains why the green coma, the cloudy layer of gas and dust that surrounds the nucleus shrinks when a comet gets closer to the sun, and also why the tail of the comet is not green, “said Timothy Schmidt, senior author of the study and professor of chemistry at UNSW Science.
You can read more from the survey here.