Comets will often turn green as they approach the sun. But the green disappears as they get closer to it.
Researchers have recently solved this puzzle.
As comets approach the sun, the heat evaporates chemicals like carbon dioxide and methane that are frozen at the comet's core. Sunlight radiation splits these chemicals, and in the process, creates a new chemical called dicarbon that gives comets their green colour. But the radiation also breaks dicarbon as the comet gets even closer to the sun, and the comet loses its colour.
So why does this research matter?
Dicarbon comes from the breakup of all those frozen chemicals - the sort that are considered to be some of life's building blocks. One of the theories about life's origins is that billions of years ago, comets were either smashing into Earth or passing close to us. Could they have delivered these chemicals that helped start life?
Knowing why and how comets change colour helps us understand how these chemicals evaporate off comets.
And that gets us closer to figuring out where we came from.
Original article:
Comets' heads can be green, but never their tails. After 90 years, we finally know why
Original study:
Photodissociation of dicarbon: How nature breaks an unusual multiple bond