The nickname of The Green Comet has taken hold to refer to C/2022 E3 (ZTF), a comet that with a bit of luck we will be able to see these days in our skies. But is what we are going to see really a green comet? And what is this color due to?
A comet in the winter sky. C/2022 E3 (ZTF) is paying us a visit these days. It will reach its closest approach to Earth between February 1 and 2, but last Saturday’s new moon has left us with a darker sky that is more suitable for sightings of this type.
If we want to see the comet we will have to take advantage of the early hours of the morning and look towards the northwest. To guide us we can take the constellation of Camelopardalis (also called the Giraffe) as a reference.
One of the characteristics that make C/2022 E3 (ZTF) so interesting is that it could be observed with the naked eye, without the need for telescopes or binoculars. However, doing without these instruments can prevent us from appreciating the characteristic that has earned it the name.
A green comet. Comets are a specific type of celestial object with a characteristic aspect, its tail. The tail appears when comets approach the Sun as they orbit it. Unlike asteroids, which are made mostly of rock, comets are an amalgamation of rock and ice.
When the comet approaches the Sun, the ice melts and a cloud of dust and gas (actually, comets usually have two tails, one made of dust and the other made of gases) is projected out of the orbit.
But not all comets are made of the same materials. In the case of C/2022 E3 (ZTF), the presence of two compounds is what gives it its characteristic color: diatomic carbon and cyanogen. As Gianluca Masi, an astronomer with the Virtual Telescope Project, explained to Newsweek magazine, “under the effects of sunlight, these two substances (diatomic carbon and cyanogen) glow green.”
Not the whole mountain is oregano. Both cyanogen and diatomic carbon are present as gas in the comet’s tail, which makes the gaseous part of the tail turn green, while the dust tail does not.
In the case of diatomic carbon, the color is produced when sunlight hits these molecules made up of two carbon atoms. The collision of ultraviolet light and the molecule causes the two atoms to separate while emitting a greenish light.
After impact, individual carbon atoms are pushed away by the comet’s gaseous tail. That is why the green color is more intense in the area of the coma, that is, the cloud that surrounds the nucleus of the comet, and less intense on the outside of the tail.
A not so unique phenomenon. Although we think of comets as white objects in the dark night sky, the truth is that they are generally colored objects. A recent example of this was comet NEOWISE, whose coloration fluctuated between red and green. Other recent “green comets” also include comet Leonard (C/2021 A1), Lulin (C/2007 N3), or Lovejoy (C/2014 Q2).
Every 50,000 years. C/2022 E3 (ZTF) is a green comet that we can see with the naked eye. But there is another characteristic that has made its passage such a renowned phenomenon: the fact that 50,000 years have passed since its last visit. That and that its orbit may not bring it back for another 50,000 years or even at all. While its color doesn’t make it unique, our chance to see it might be.
image | Dmitry Kolesnikov
