[X] My Assistant

[mcaplinger]

(Started a new thread to avoid cluttering up the PJ57 thread with PJ58 discussion.)

[Bjorn Jonsson]

This is an updated plot of the values I use for correcting the color in the JunoCam images. It shows the now well known reddening of the images as the mission progresses. Interestingly, the reddening trend became more irregular once the first JunoCam anomaly occurred at PJ48. In particular, the images temporarily became less red after PJ48 but are now again getting redder - there was a very large change (reddening) from PJ57 to PJ58.



There was a very rapid reddening from PJ47 to PJ48 (i.e. immediately before the PJ48 anomaly) and again from PJ53 to PJ54. The third large change is PJ57 to PJ58. I suspect these all correspond to a lot of radiation. Smaller changes might simply be a coincidence or they might be associated with lower levels of radiation or (in some cases) mitigation measures like heating the camera between perijoves as was successfully done before PJ57 in response to the PJ56 anomaly.

[StargazeInWonder]

I keep mulling this over and had the following speculation…

Ionization caused solely by electromagnetic radiation normally has a distinct threshold effect. In the way we encounter this most painfully, we can get a sunburn from the UV in sunlight, but interestingly, we cannot get a sunburn from longer or more intense exposure to visible light. (We can get a thermal burn, of course, but that's another phenomenon.) There is an energy of ionization for a given molecule / bond and a photon either reaches that threshold or does not. For the bonds in most organic compounds, it requires UV to reach that threshold. Intense blue light will not give you a sunburn.

What we see with this reddening is directionally aligned with sunburn: It is as though operations during the mission are harming the shortest wavelength filter a significant degree; the next-shortest, less so. This doesn't display the thresholding that one expects with radiative ionization.

Is it possible that the charged particle radiation from Jupiter and the visible light radiation are somehow additive in damaging camera elements, such that the high end of the stochastic distribution of charged particles is enough to cause damage when added to the visible light of different wavelengths, making blue capable of causing somewhat of a sunburn, green less so, and red less still? If so, then is the imagery performed by JunoCam (which is overwhelmingly, of course, of Jupiter) a causative part of the damage? Put another way, might the reddening have been avoided for arbitrary spans of time if the camera had not been used to image Jupiter?

Heaps of speculation there, and too little too late in any case, but the mystery remains intriguing.

[mcaplinger]

It is as though operations during the mission are harming the shortest wavelength filter a significant degree; the next-shortest, less so.

If there has been wavelength-dependent degradation (I don't think the case is 100% made for this, but it would not be unexpected) it is almost certainly due to radiation darkening, which is caused by exposure to the jovian charged-particle environment. https://sci.esa.int/documents/34530/36042/1...rials_Doyle.pdf is a quick introduction to this and other effects in the context of Jupiter missions.

Junocam was designed with rad-hard glass* types in the front half of the lens and normal glasses farther back, shielded by the front half and also by a large cylinder of titanium. Of course, we were only required to last for 8 orbits, so for radiation effects to still be so subtle as to be debatable this late into the mission speaks to the conservatism of our approach. One could have expected the blue channel to be completely gone by this time otherwise.

It's possible that the annealing recovered some glass transmission that might have been lost, since the heater is attached to the optics and the optics thus warmer than the electronics we were trying to fix.

*I often refer to rad-hard glasses, only somewhat humorously, as "pre-darkened" -- they typically have poorer blue transmission on day 1 than their normal equivalents.