What exactly is the cause of blue light of ion (plasma) tails of comets? Somewhere I have read that the source of blue light are CO+ ions which has just acquired the missing electron and became neutral molecule CO. Is this correct?


Close, but not quite right - the blue light is indeed emission from CO$^+$, but it's from the CO$^+$ ions themselves, with no need for recombination to CO; that (ionized) molecule has a strong set of energy transitions around 425 nm (4250 Angstroms), which is in the blue part of the visible spectrum:

Spectrum of Comet C/2016 R2 (Pan-STARRS)

Spectrum of Comet C/2016 R2 (Pan-STARRS), Figure 2 from Cochran and McKay (2018).

There's more on the physics of comet tails from Chris Mihos here.

  • $\begingroup$ Very nice answer! +1 The spectrum shown is only about 1.5 nm wide, so it's hard to tell if this represents most of the "blue light" from the tail or just a small fraction of it, but this sounds authoritative enough to be believable. This also finally explains to me why the tails of green comets (C₂ vibronic lines, 1, 2, 3) look blue or violet. Thanks! $\endgroup$ – uhoh Jul 13 '20 at 2:22
  • 1
    $\begingroup$ Right, this is only a zoom in on one particular band. In the paper they say that they detected “a well-developed series of bands scattered from approximately 3700 Å to 5100 Å” and that “The majority of the detected strong bands can be attributed to CO+.” This is just one of many bands they list. I don’t know how much variation there is across different comets, but in this case CO+ seems to be the dominant source of emission. $\endgroup$ – Eric Jensen Jul 13 '20 at 3:24
  • $\begingroup$ Interesting, thanks! $\endgroup$ – uhoh Jul 13 '20 at 9:09
  • 2
    $\begingroup$ Do you know if the blue color of this emission is in any way related to blue color of CO flame? I understand that during combustion CO change to quite different molecule, CO2, but after all, both molecules contain C-O bonds... $\endgroup$ – Leos Ondra Jul 13 '20 at 12:15
  • $\begingroup$ NEOWISE has a spectrum too :) groups.io/g/cometes/message/410 $\endgroup$ – pela Jul 13 '20 at 12:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.