Comets’ heads may be green, but never their tails
Comets, composed of frozen gases, rock and dust, are the leftover remains from the formation of the Solar System about 4.6 billion years ago. The icy bodies heat up very quickly as they approach the Sun in their orbits, causing the solid ice to turn directly into gas via a process called sublimation, which intensifies the bright-green glow. However, the green glow appears only around the head of some comets, but never in the tails trailing behind them. Now scientists from the University of New South Wales (UNSW) Sydney in Australia think they know why.
Using a vacuum chamber and lasers, the researchers were able to recreate the chemical reaction that occurs when ultraviolet radiation from the Sun breaks apart a compound called diatomic carbon, also known as dicarbon or C2. The reaction causes the icy bodies to appear green. This process, called photodissociation, was originally theorised by physicist Gerhard Herzberg in the 1930s, but has been hard to test until now. Dicarbon is made up of two carbon atoms and is highly reactive and unstable. It’s only found in extremely energetic or low-oxygen environments like stars, comets and the interstellar medium. However, on a comet, dicarbon can’t exist until the body gets close to the Sun. The Sun heats the comet’s organic matter, causing it to evaporate and enter the coma, which is the gassy envelope around the comet. The sunlight then breaks up the larger organic molecules, creating dicarbon.
As a comet gets even closer to the Sun, photodissociation breaks up the fresh dicarbon before it can move very far from the nucleus, or head, of the comet. So while the comet itself appears brighter as it approaches the Sun, the radiant green coma shrinks as the dicarbon is broken up by sunlight. The dicarbon never makes it to the comet’s tail, which is why that part of the comet doesn’t glow green.