Why Is the 'Christmas Comet' Green?

A comet passing close to the Earth this month is extremely green. Here's why that is.

Comet 46P/Wirtanen
Getty Imagesdavidhajnal

This month, the Earth is being visited by a comet that is nearly close enough to see with the naked eye. If you do manage to spot it, you might notice that it gives off a green hue. A green color is pretty uncommon in space, so where does the comet get this color from?

The answer, unsurprisingly, is . Comets are made of a whole lot of different chemicals, and each comet gets its own unique mixture. Some comets are made of mostly water, while others have rock, or metal, or random other materials. The comet currently passing by Earth, comet 46P/Wirtanen, has a substantial amount of cyanogen.

Comets like Wirtanen spend most of their time in the colder and more distant parts of the solar system, far from the Sun. Wirtanen, in particular, tends to hang out mostly near the orbit of Jupiter. But occasionally, these comets fly closer to the Sun. When that happens, the comets heat up, and some of the materials on the surface of those comets—like ice, or cyanogen—start to evaporate.

When you look at Wirtanen and see that green glow, you’re not seeing the comet itself. The actual solid core of the comet is a tiny dot in the center of a giant cloud of cyanogen gas. Inside that cloud, the gas interacts with ultraviolet light from the Sun and becomes ionized. The ultraviolet light is so energetic that it rips electrons away from the cyanogen atoms and makes them electrically charged.

If you’ve ever seen a neon light before, you’re familiar with what happens next: The gas starts to glow. Neon lights work by the same principle, except here on Earth we use electricity instead of ultraviolet light to ionize gases. Once a gas is ionized, it starts to glow, and the color it produces depends on what the gas is made of. Hydrogen glows red, helium glows yellow, and cyanogen glows green.

In 1910, scientists first spotted cyanogen in Halley’s Comet prior to one of its famous flybys, and when people thought they could be poisoned by the gas. Technically, cyanogen is toxic to humans, but the cyanogen from Halley’s Comet—or from Wirtanen, for that matter—poses no harm to people on Earth.

If you want to see the comet for yourself, you’ll likely need some binoculars or a telescope, plus some pretty dark skies. Right now the comet is sitting , and you can track its position with . You can also with this livestream from the Virtual Telescope Project.

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