Comet ISON exhibiting a green glow on solar approach
© NASA
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.