Comets tend to glow green, but only from their heads, not their tails. This has baffled scientists for decades. In the 1930s, Gerhard Herzberg speculated that the green may be from a molecule made of two carbon atoms, called a "dicarbon." But how would they test the theory? Dicarbon exists in space, but quickly reacts with oxygen and burns up in earth's atmosphere.
A recent experiment from the University of New South Wales in Sydney, Australia, confirmed that the green glow of a comet is due to dicarbon, but it wasn't easy. Read how they produced dicarbon and introduced it to a simulated space environment at Popular Science. They learned that Herzberg was right about dicarbon, but wrong about the mexhnism that made the green glow. -via Fark
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