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 (CO). The full suite of cometary ices was investigated by the Rosetta mission, but these are the big five. Under typical, cold conditions, the ices remain frozen, but as the comet nears the Sun, they start to heat up.

The first thing that happens to a comet, as it approaches the Sun, is that the amount of ultraviolet light striking it becomes great enough that it can start ionizing the weakest molecule there: carbon monoxide. This creates an abundance of the CO+ ion, which streams directly away from the Sun. This turns into a blue ion tail, and is the first comet-like feature to appear as a comet begins to heat up.

*When Comet ISON was the same distance from the Sun as Jupiter, there was only an ion tail (in blue) present. As it neared the Sun further, additional features developed.* (NASA, ESA, J.-Y. LI (PLANETARY SCIENCE INSTITUTE), AND THE HUBBLE COMET ISON IMAGING SCIENCE TEAM)

The ion tail always points directly away from the Sun, and is always blue in color. As the comet gets even closer to the Sun, however, somewhere around the orbit of Mars, it heats up further. As the nucleus of the comet gets hot, more of the ices melt and diffuse away from the surface, creating a large, diffuse set of particles around the nucleus. This diffuse region is known as the coma of a comet, and is made of a mix of gas and dust.

Once this coma gets created, it has no option but to get struck by sunlight. The pressure from the sunlight striking the coma pushes the dust particles out of the coma and away from the Sun, creating a second, yellow/white tail: a dust tail. Although the blue ion tail always points directly away from the Sun, the dust tail curves, as the comet moves in its elliptical orbit around the Sun.

Comet McNaught, as imaged in 2006 from Victoria, Australia. The dust tail is white and diffuse (and curved), while the far fainter ion tail is thin, narrow, blue, and points directly away from the Sun. (SOERFM / WIKIMEDIA COMMONS)

The ion tail is narrow, as all the ions of a specific type are the same size. The dust tail is wide, since dust particles vary in size and so are given a variety of speeds. And finally, large particles can be sheared off the comet, creating what’s known as a debris stream. This stream will continue in the same elliptical orbit that a comet follows, but will spread out along the path over time. When a planet (like Earth) passes through the debris stream, it creates a meteor shower. Yes, Earth is not the only planet that experiences them; even worlds like Mercury without an atmosphere can have meteor showers!

As they orbit the Sun, comets and asteroids can break up a little bit, with debris between the chunks along the path of the orbit getting stretched out over time, and causing the meteor showers we see when the Earth passes through that debris stream. (NASA / JPL-CALTECH / W. REACH (SSC/CALTECH))

But the coma is more than dust. There is also gas, created from the sublimated compounds that were part of the comet. There aren’t merely simple ices and rocks on this body, but more complex molecules made out of these fundamental building blocks: mostly hydrogen, oxygen, carbon, and nitrogen. Two molecules that are of particular interest are cyanide/cyanogen (CN: a carbon-nitrogen bond) and diatomic carbon (C2: a carbon-carbon bond).

The green color of the coma of Comet ISON lit up the skies in 2013. The green color isn’t a rarity, but rather tells us about the gas composition and the ultraviolet light content striking the comet as it approaches close to the Sun. (ADAM BLOCK / MOUNT LEMMON SKYCENTER / UNIVERSITY OF ARIZONA)

This teal or blue-green color comes about because when these gases are stimulated by the ultraviolet light present in sunlight, their bound electrons get kicked to higher energy levels: a basic rule of atomic transitions. But electrons don’t remain forever in a higher-energy state; they drop down to lower energy levels. And when they do, some of those transitions result in an emission line that falls in a part of the electromagnetic spectrum that human eyes are sensitive to.

Electron transitions in the hydrogen atom, along with the wavelengths of the resultant photons, showcase the effect of binding energy and the relationship between the electron and the proton in quantum physics. Every atom and molecule has its own unique set of spectral lines, and an excited atom will have its electrons drop in energy levels, sometimes releasing visible light. (WIKIMEDIA COMMONS USERS SZDORI AND ORANGEDOG)

When you see that green color, it’s an indicator of a combination of things:

that the coma contains large amounts of CN and C2 molecules,
that the comet is active (outgassing) and warm (close to the Sun), and
that the potential for a schism or eruption is at its highest.

As of August 9th, there’s a relatively close comet just 70 million miles (113 million km) from Earth: Comet C/2017 S3 (PANSTARRS). It has a green glow, it’s relatively bright right now, and it’s still headed towards the Sun. It’s being called the “Incredible Hulk” comet for its green color.

*Even though it’s in the process of disintegrating, there’s still a chance for a final, spectacular outburst from Comet PanSTARRS C/2017 S3, known as the ‘Incredible Hulk’ comet.* (BENCE GUBEAR / TWITTER USER @VIVSTOITSIS)

But it isn’t angry, nor is it unusual. While its closest approach to Earth was August 7th/8th, its closest approach to the Sun won’t be until August 15th, and this will be when it’s most likely to have its icy nucleus split apart, which sometimes happens. Whenever an event like that occurs, there is the spectacular opportunity of the comet brightening tremendously. Even though it’s relatively close to the Sun, it’s still visible in the night sky from most locations on Earth.

If it experiences such an eruptive event, it could become visible, despite its proximity to the Sun, to the naked eye.

A telescopic closeup of Comet Lovejoy (C/2014 Q2) from January 17, 2015, showing structure in the ion gas tail, in the form of streamers and discontinuities. The green color in the coma is unmistakable, and is often a portent of a spectacular outburst. (Alan Dyer /VW PICS/UIG via Getty Images)

Just before dawn on the morning of August 15th will be the best chance of seeing it, if we’re lucky enough to get a brightening event, from anywhere on Earth. (Including the Southern Hemisphere!)

At its closest point to Earth in 3D-space, Comet PanSTARRS C/2017 S3 will be 70 million miles (113 million km) away, but it still has yet to reach its closest approach to the Sun. This screenshot reflects where the comet is the night of August 7th/8th. (THE SKY LIVE, VIA THESKYLIVE.COM/C2017S3-INFO)

But even though things aren’t looking particularly good for this comet, there’s always a chance it will surprise us. Furthermore, the features that you can expect for this comet — the ion tail, the dust tail, the coma, and the nucleus — are common to practically all comets that enter our inner Solar System. When a comet gets warm enough, it creates an extended, gas-rich cloud known as a coma around its nucleus. If the coma contains carbon-nitrogen and carbon-carbon bonds, the Sun’s ultraviolet light will excite the electrons inside it, causing them to emit a green glow when they drop down in energy. And whenever you see that green glow, know that there’s a chance of the comet’s nucleus splitting apart. It may not happen this time, or even most times, but there’s a chance for a visually spectacular show. When it comes to skywatching, it’s hard to ask for more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.