Juno Perijove 35 - Jupiter, July 21, 2021 Options

[Brian Swift]

...It would nevertheless be very interesting if someone (Brian maybe?) would process the PJ20_40 and PJ35_68 images and use identical color correction for PJ20_40 and PJ35_68. If the result is similar to mine (i.e. redder PJ35) it would completely rule out a bug in what I'm doing. A confirmed color change would be an interesting result regardless of what's causing it.

Here you go. Same processing run, same/fixed/constant brightness setting, linear to sRGB encoded 16-bit, 5292 × 7162 16-bit png available in Flickr download:
Jupiter PJ20_40 and PJ35_68

[Bjorn Jonsson]

Here you go. Same processing run, same/fixed/constant brightness setting, linear to sRGB encoded 16-bit, 5292 × 7162 16-bit png available in Flickr download:

Thanks! This is very similar to what I am getting and seems to confirm that there is a color change from PJ20 to PJ35. Here is a plot from a few images I processed. It shows the blue/red ratio in the South Tropical Zone (STrZ) for several perijoves. This is from decompanded images. There are some inaccuracies due to various factors (for example the illumination geometry varies a bit) but the trend is clear. The units are arbitrary in the sense that I didn't attempt to make B/R=1 for any specific perijove. As noted above, this is the STrZ but everything else in the images has also been getting redder.



The images are clearly getting redder.

Interestingly, the 1600x1600 pixel map projected color images released at the missionjuno website together with each raw image have not been getting redder. Now I am wondering how the color is corrected in these map projected images.

[mcaplinger]

Now I am wondering how the color is corrected in these map projected images.

They are simply white-balanced using a gray-world algorithm.

[Brian Swift]

...Interestingly, the 1600x1600 pixel map projected color images released at the missionjuno website together with each raw image have not been getting redder. Now I am wondering how the color is corrected in these map projected images.

I believe the the missionjuno images include a correction for illumination intensity change due to change in SOLAR_DISTANCE.
JNCE_2019149_20C00040_V01.json: "SOLAR_DISTANCE": "7.9340e+08 <km>",
JNCE_2021202_35C00068_V01.json: "SOLAR_DISTANCE": "7.5293e+08 <km>",
So, Jupiter has gotten about 5% closer to the Sun, and illumination has increased by about 11%.
But I don't see how that would lead to reddening. (Unless a SOLAR_DISTANCE correction was only being applied to one color channel)

[mcaplinger]

I believe the the missionjuno images include a correction for illumination intensity change due to change in SOLAR_DISTANCE.

Nope. The only files that use this value are the PDS-archived RDR files, which are all normalized for solar distance as described in the documentation.

The outreach map-projected RGB images on missionjuno have a lambertian shading correction applied and are then arbitrarily white-balanced, there is no explicit response function applied. The component map-projected images don't have the shading correction or the white balance, but are otherwise photometrically out of the camera (they have some blemish removal.)

[Floyd]

If grey balanced, wouldn't that counter an increase in the averaged red channel? You may see your reddening in the un-grey balanced map-projected images.

[mcaplinger]

If grey balanced, wouldn't that counter an increase in the averaged red channel? You may see your reddening in the un-grey balanced map-projected images.

Yes, certainly. And you can definitely see a color difference if you simply look at these two images in raw form.

The question is whether this is 1) a change in Junocam, 2) a change in Jupiter, or 3) a change in the illumination geometry. (I think we can rule out the illumination itself changing color.)

I vote for 2, and there are independent observations that suggest this is happening, at least for some areas.

One could perhaps look at the colors of the moons in various Junocam images over time to see if there has been a gross change in instrument response unrelated to the jovian atmosphere. Europa is probably the most color-neutral target.

[Bjorn Jonsson]

I decided to process two Hubble images from calibrated data. I used an image from 2019-04-09 (a few days after PJ19) and an image from 2020-09-20 (a few days after PJ29). More recent data than 2020-09-20 was not available.



I deliberately didn't correct for Jupiter's rotation between the three exposures since doing so messes up the overall color balance a bit unless additional corrections are applied to the images. The lack of correction for Jupiter's rotation isn't a problem in large, low contrast areas like the STrZ - useful color measurements are possible there.

There is no obvious *global* color change here. In particular the STrZ has the same (or very nearly so) whitish color in both of these images.

These images are processed from images obtained with filters centered at 395 nm, 502 nm and 631 nm. This is not optimal for true color so the overall color in the above images is only approximate.

I also took a quick look at Ganymede's color in a PJ24 image vs. the color in a PJ34 image - luckily they have some common terrain coverage. I'm *probably* seeing some reddening in PJ34 compared to PJ24 but I'm not particularly confident in the accuracy of these measurements because of the low resolution of the PJ24 image.

I strongly suspect that the color change discussed in earlier posts is due to changes in illumination geometry and/or a change in JunoCam but I'm aware that none of the above conclusively proves that this is the case. I think it would be interesting to study this more deeply. I'm not so sure about the practical value of doing so though since I don't think the JunoCam images can ever be used for high-accuracy photometry of Jupiter.

[Bjorn Jonsson]

Here is a quick comparison of southern hemisphere JunoCam images from PJ20, PJ29 and PJ35. The reddening in the PJ35 image compared to PJ20 is everywhere and not confined to specific belts or zones. IMO this is unlikely to be a real, global color change on Jupiter.



The processing of the three images is identical. They are not processed to make the color accurate/correct in any specific image. The main intention is to show the color change.

I'm seeing some tentative evidence that the reddening of the images may have started (or greatly accelerated) shortly after PJ20, possibly at PJ22. This is not easy to measure accurately though since the difference between two consecutive perijoves is small.

[JRehling]

Here are images taken by Christopher Go in May 2019 and Aug 2021. I downscaled the images 2x as resolution isn't the point here.

As he's imaging from Earth, there's always the possibility of some random variation due to conditions on this planet, but it doesn't seem to show up discernibly in his work.

Aside from many individual features changing shape and size, it seems like things are close to the same. If anything, it looks like Jupiter is slightly bluer now than in 2019. FWIW.

Attached thumbnail(s)

 

[Bill Harris]

What does Hubble show of Jupiter? I realize that HST is primarily used as a deep-sky instrument, but it has produced iconic planetary images, and it doesn't have atmospheric effects.

--Bill

[JRehling]

As an instrument, HST is wonderful and could deliver a perfect answer to this, at least over the set of observations, which are more sparing than amateurs'. (HST may image Jupiter only about once a year, while amateurs image it every day apart from solar conjunction.) However, the public releases are not carefully controlled by any single set of creative choices and many public releases of HST images of the planets are obviously candy-colored reflecting varying aesthetic decisions by the image processing individuals and not real changes in the planets.

If someone would like to take raw HST images (which are publicly available) from two separate dates and process them in comparable fashion, that would give us a clean result.

[mcaplinger]

If someone would like to take raw HST images (which are publicly available) from two separate dates and process them in comparable fashion, that would give us a clean result.

Which Bjorn did back on post 23.

[JRehling]

Indeed. My negligence in reading the thread only at the end, skipping what I missed earlier.

[fredk]

About the colour shift, I don't know the details of the raw data or the processing steps: Is the raw data being used directly proportional to the intensity (up to nonlinearity of the sensor), ie no black-level offsets, companding, or gamma adjustments? And are all the processing steps being done linearly on the linear data, before converting to display gamma/sRGB at the final step? In particular any white balancing (RGB multipliers) or overall brightness scaling should be done on the linear data.

I'm wondering whether a combination of a brightening Jupiter as it approaches perihelion, coupled with some nonlinear processing steps, could account for the colour shift due to a different effect on different colour channels. I guess any actual sensor nonlinearity would be far too small to account for this.