As comets streak across the night sky, their radiant heads often glow green—but not their long tails. In the 1930s, physicist Gerhard Herzberg suggested that the lime halo may be produced by sunlight destroying diatomic carbon, C2. But because diatomic carbon is so unstable, no one on Earth has been able to test that theory—until now. Researchers in Australia used a vacuum chamber and powerful lasers to simulate the comet environment, proving that the breakdown of C2 produces the emerald hue.
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