What Makes the E3 Comet Green?

[exchemist]

This comet is currently in the  news. Many articles comment on its green appearance. However I have yet to come across a full explanation for the colour. The closest I've got is that it is apparently fluorescence from gases in the tail, excited by UV from the sun.  Does anyone have more information? What gases?    

Edited February 4, 2023 by exchemist

[joigus]

I've just found this, but wouldn't know why that's the case:

Quote

Color[edit]

The green color is likely due to the presence of diatomic carbon, chiefly around the comet's head.^[29] The C₂ molecule, when excited by the solar ultraviolet radiation, emits mostly in infrared, but its triplet state radiates at 518 nm (nanometers). It is produced by photolysis of organic materials evaporated from the nucleus. It then undergoes further photolysis, with a lifetime of about two days, at which time the green glow appears in the comet's head but not the tail.^[30][31] The comet researcher Matthew Knight opined that the green color of this comet is not unusual for comets with a higher gas content, but they only rarely approach the Earth as close so it provides for very good observation of the greenish hue.^[32]

(My emphasis). From:

https://en.wikipedia.org/wiki/C/2022_E3_(ZTF)

Maybe you can make something out from this.

[studiot]

Quote

https://www.bbc.co.uk/news/science-environment-64388483

A green appearance for comets is not uncommon and is usually the result of breakdown of a reactive molecule called dicarbon - two carbon atoms joined together by a double bond.

Such colour is better picked up by digital cameras, which are more sensitive to colour.

 

[exchemist]

Thanks to you both, @joigus and @studiot.

So it's dicarbon. Well well. I found this, which makes interesting reading: https://en.wikipedia.org/wiki/Diatomic_carbon

It looks as if the green comes from emission from the ³Π_g  state. This intrigues me as it looks as if it might be  a configuration equivalent to acetylene minus the H atoms, i.e. a triple bond between the atoms, leaving a σ unpaired electron on each atom. But I'm very rusty on how to interpret these molecular term symbols.

The ground state is singlet, so there are lone pairs on each atom and one of the potential π bonding orbitals is empty. It seems as if this electron-deficient molecule has quite a bit of chemistry.         

The dicarbon seems itself to be the photolysis product of relatively volatile carbonaceous compounds. Interesting that nitrogen is present, too.  

 

[sethoflagos]

42 minutes ago, exchemist said:

It looks as if the green comes from emission from the ³Π_g  state. This intrigues me as it looks as if it might be  a configuration equivalent to acetylene minus the H atoms, i.e. a triple bond between the atoms, leaving a σ unpaired electron on each atom.

... The dicarbon seems itself to be the photolysis product of relatively volatile carbonaceous compounds. Interesting that nitrogen is present, too.  

I can't give an authoritative reference, but I heard that cyanogen was involved. Cyanogen is quite a common comet constituent and since it can be formed by passing a spark discharge through a mix of acetylene and nitrogen, the mechanism may conceivably involve this overall reaction or the reverse one.

[exchemist]

1 hour ago, sethoflagos said:

I can't give an authoritative reference, but I heard that cyanogen was involved. Cyanogen is quite a common comet constituent and since it can be formed by passing a spark discharge through a mix of acetylene and nitrogen, the mechanism may conceivably involve this overall reaction or the reverse one.

Yes, cyanogen is mentioned in some of the articles about this. 

[studiot]

I found this RSC article about comet chemistry (in pdf 15 pages)

https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.1988.0064

There is a useful table of compounds expected.

and also this newer article about comet spectral analysis.

Curiously the attribute acknowledge a green oxygen line.

https://www.aanda.org/articles/aa/full_html/2019/04/aa34357-18/aa34357-18.html

Edited February 4, 2023 by studiot

[TheVat]

Another green one is going past us this week, and will be visible in the N hemisphere for the next few days before it's too close to the sun.  

From AP News:

https://apnews.com/article/comet-northern-hemisphere-nishimura-200f8cc81140387177b3436c4c3a7663

(....)Italian astronomer Gianluca Masi, founder of the Virtual Telescope Project, said in an email that the next week represents “the last, feasible chances” to see the comet from the Northern Hemisphere before it’s lost in the sun’s glare.

“The comet looks amazing right now, with a long, highly structured tail, a joy to image with a telescope,” he said.

If it survives its brush with the sun, the comet should be visible in the Southern Hemisphere by the end of September, Masi said, sitting low on the horizon in the evening twilight.

Stargazers have been tracking the rare green comet ever since its discovery by an amateur Japanese astronomer in mid-August. The Nishimura comet now bears his name.

It’s unusual for an amateur to discover a comet these days, given all the professional sky surveys by powerful ground telescopes, Chodas said, adding, “this is his third find, so good for him.”

The comet last visited about 430 years ago, Chodas said. That’s about a decade or two before Galileo invented the telescope.