C/2014 Q2 (Lovejoy) is a long-period comet discovered on 17 August 2014 by Terry Lovejoy. This photograph was taken from Tucson, Arizona, using a Sky-Watcher 100mm APO telescope and SBIG STL-11000M camera. (JOHN VERMETTE / WIKIMEDIA COMMONS)

This Is Why Comets Glow An Eerie Green Color

The ices and rock aren’t green, and neither are the tails. So where does a comet’s green color come from?

Every so often, with extreme regularity, comets will plunge from beyond the orbit of Neptune into the inner Solar System. From well beyond the orbit of Saturn, they remain cold, frozen, and in a dormant state; although they’re always moving, nothing about them changes. But when they start to approach the orbit of Jupiter, being in close proximity to the Sun changes things.

The outer parts of the comet heat up, the frozen ices on the surface start to sublimate, and the radiation and wind from the Sun start to push the surface molecules away. Before long, your comet glows with not just the reflected light from the Sun, but with two tails — one grey, one blue — and an eerie, green coma around the center. Here’s why that happens.

The comet that gives rise to the Perseid meteor shower, Comet Swift-Tuttle, was photographed during its last pass into the inner Solar System in 1992. This comet, which gives rise to the Perseid meteor shower, also displayed a spectacular green coma. (NASA, OF COMET SWIFT-TUTTLE)

Comets are made out of a mix of rocky components, similar to what makes up the Earth’s mantle, dust, and ices. Ice doesn’t just mean water-ice (H2O), but also volatile components like dry ice (solid CO2), methane (CH4), ammonia (NH3), and carbon monoxide…