Why Are Some Comets Green?

Comet C/2022 E3 (ZTF) makes its way through the solar system. To the naked eye, the comet won't look as green as this picture. This image is normally achieved by a camera taking a long exposure. Alessandro Bianconi/Edu INAF/Flckr/CC BY-SA 2.0

One thing astronomers have become increasingly aware of in the past few decades is how many objects are part of our one special solar system. Whereas we once understood our system as composed of a sun, some planets and an asteroid belt, we now know there are millions of objects beyond the orbit of Neptune in the Kuiper Belt — and there's more left to discover (Planet X, anyone?).

Some of these distant objects occasionally make a visit to the inner parts of the solar system, including comets on long orbits traveling billions of miles over tens of thousands of years. One such comet made headlines recently for its distinctive green hue — but what causes a comet to appear a certain color in the first place?

Meet Comet C/2022 E3 (ZTF)

Comet C/2022 E3 around other celestial objects — Comet C/2022 E3 (ZTF) is shown in the night sky. Conrad Bohem, Osservatorio Astronomico di Trisete, INAF/Flckr/CC BY-SA 2.0

Originating in the far outer reaches of the solar system in an area called the Kuiper Belt, Comet C/2022 E3 (ZTF) was discovered in March 2022 at the Zwicky Transient Facility in California — hence the parenthetical part of its name. It was nearest to Earth on Feb. 1, 2023, the closest approach in about 50,000 years.

Astronomers estimate that the last time C/2022 E3 (ZTF) was visible was during the time that Neanderthals were alive and a major ice age covered most of the land on Earth with huge glaciers. Today, the world is obviously much different, but the awe caused by the appearance of this cosmic visitor remains much the same.

In addition to its long orbital period, another trait that made C/2022 E3 (ZTF) special was its color. The coma, the glowing atmosphere around the comet's head or nucleus, has high concentrations of diatomic carbon and cyanogen (made of bonded carbon and nitrogen). As this compound is illuminated by the sun, it glows green in the visible light spectrum. This is why many articles about C/2022 E3 (ZTF) called it the "green comet."

What Makes Comets Green?

The parts of a comet include the coma, the head (nucleus) and two tails: the dust tail and the ion or gas tail. gritsalak karalak/Shutterstock

Different elements in a comet can contribute to the green hue it appears to have for Earthly observers.

The coma of a comet doesn't actually form until somewhere around the orbit of Mars, where a comet's proximity to the sun causes ice on the rocky body to melt and create an atmosphere-like halo of dust and gas surrounding the comet. As the coma (and comet) move closer to the sun, the gases in the coma go through atomic transitions due to the effect of sunlight.

In the case of C/2022 E3 (ZTF), sunlight shining on the comet splits the diatomic carbon molecules in its tiny coma atmosphere into single carbon atoms; this is what produces the green hue around the comet's nucleus or head. The comet's dust tail still appears to be white, and its fainter gas tail appears bluish.

There have been other particularly green comets in recent years, including C/2007 N3 Lulin, C/2014 Q2 Lovejoy and C/2021 A1 Leonard, due to the same effect. Comets can also take on slightly different colors, including teal and blue, based on the chemical composition of the gases in the coma that forms around them. Some comets can even be red.

How Common Are Green Comets?

While you might have seen headlines referring to Comet C/2022 E3 (ZTF) as a "rare green" comet, these are actually two different descriptors and should be separated by a comma (not a coma)!

C/2022 E3 (ZTF) is green, certainly, but it is also rare — its rarity comes from the fact that it has that 50,000-year orbit through our solar system. Scientists believe that there have been well over a thousand generations since this comet might have been spotted by our distant homo sapiens ancestors. And computations suggest it may never be seen again.

For comets in general, appearing green as they approach the sun and pass by Earth is not uncommon; there were several recent green comets mentioned already, and we may well get to see more during our lifetimes. It all comes down to the chemical composition and how much diatomic carbon each one contains — something that was determined long ago during the Big Bang and formation of our solar system.

Now That's Interesting

Comets change color, both seasonally and on each subsequent orbit. As you might guess, comets can change color as they get closer or further from the sun, something that was studied as part of NASA's Rosetta mission to Comet 67P/Churyumov-Gerasimenko. However, it's likely that comets might also appear different colors on each subsequent orbit of the solar system, since atomic and gas composition may change as a result of the previous orbit and coma formation. Scientists can study short-period comets (those with an orbit of 200 years or less) to learn more about this, but few of the comets we've discovered and photographed so far have returned to verify this effect!