Comets’ heads may be green, but never their tails, study finds

Many comets’ heads glow a bright green as they stream across the night sky — but why? 

Comets, comprised of frozen gases, rocks 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, causing the solid ice to turn directly into gas via a process called sublimation, which intensifies the bright green glow.