In this nice NYTimes 'survey article' "Our Vast Solar System and Its Many Explorers" there is a NEOWISE image of Comet C/2014 Q2 (Lovejoy). See here also.

The NASA description of the image says:

The red color is caused by the strong signal in the NEOWISE 4.6-micron wavelength detector, owing to a combination of gas and dust in the comet's coma.

So I understand that this is a false color image, and the image at 4.6-micron is overlaid using red. So I should really ask: why is Comet C/2014 Q2 (Lovejoy) (and probably many) comet actually bright at 4.6 microns "...owing to a combination of gas and dust in the comet's coma." Is the gas fluorescing and the dust scattering? Or is this thermal radiation from the dust? The explanation in quotes doesn't really explain why "the red color is caused by the strong signal (around) 4.6-micron wavelength (is) owing to a combination of gas and dust in the comet's coma."

enter image description here

below: The more familiar green glow of this (and many) comet from here.

enter image description here

  • $\begingroup$ 4.6 micron is a strong peak for CO emission. No idea if that's a big contributor in this particular image. $\endgroup$
    – BowlOfRed
    Aug 4 '16 at 18:07

Mainzer et al. 2014 characterize the performance of the reactivated NEOWISE. Having run out of cryogenic coolant for the original WISE's 12 and 22 μm bands, it only detects in the 3.4 and 4.6 μm bands. The comet looks red in the false-color infrared image because, relative to the 4.6 μm "W2" band, it emits less in the 3.4 μm "W1" band than stars do. Figure 3 of the paper shows a similar image in which an asteroid also looks red.

Section 4.1 discusses comets, attributing the 4.6 μm band detection to a combination of reflected sunlight and thermal radiation from dust, 4.26 μm emission from CO2, and a little 4.67 μm emission from CO. They estimate how much is due to gas vs. dust by comparison with ground-based observations near the same times, as in Figure 17.

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    $\begingroup$ OK I see now. The red dot located at about 0.5 μm represents the ground measurement in the visible. Together with the measurement in the “W1” band around 3.4 μm (green triangle) a baseline spectrum for the dust can be estimated. The strong excess in the “W2” band around 4.6 μm is then attributed to some combination of CO and CO2, emission. Thank you! This is a great summary of the types of measurements and the current situation, as well as the bigger statistical picture of what can be done (earlier in the paper). $\endgroup$
    – uhoh
    Aug 5 '16 at 0:58
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    $\begingroup$ Just to be perfectly clear, it's a totally artificial, false-color image right? The "red" has no connection to what a human would see looking with their Mk 1 eyeball - right?? $\endgroup$
    – Fattie
    Aug 5 '16 at 13:27
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    $\begingroup$ Yes, IR bands (W1, W2) appear to be mapped to (cyan, red). $\endgroup$
    – Mike G
    Aug 5 '16 at 13:56
  • $\begingroup$ @JoeBlow I believe in the few cases where people have been able to perceive infrared it does in fact appear red, but that's not what the false color is based on necessarily. $\endgroup$
    – called2voyage
    Aug 5 '16 at 18:23

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