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My comment says:

...answers to the Space SE question Can James Webb take pictures of our solar system? In what ways will they differ from/compliment those from Hubble? suggest that at some point JWST might indeed bother to look directly at Jupiter and Saturn, though it's not scheduled to do so in the first cycle as there are observations more fundamental to its raison d'etre to do first.

and the following comment informs with the quote

"The very large infrared brightnesses of Mars, Jupiter, and Saturn may limit Webb observations of these planets to a subset of the instrument modes"

which is possibly quoted from NASA's JWST Technical FAQ On A Variety Of Mission Issues, Aspects And Capabilities..

Question: Which JWST instrument modes are compatible with observations of the bright trans-Earth planets Mars, Jupiter, and Saturn? Which aren't?

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    $\begingroup$ (Very) Light on details but one of the approved Early Release Science programs is using all of the JWST instruments to look at Jupiter... $\endgroup$
    – astrosnapper
    Commented Jan 11, 2022 at 1:29
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    $\begingroup$ Small correction: Mars is one of the inner planets, not outer planets. $\endgroup$
    – gerrit
    Commented Jan 11, 2022 at 9:23
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    $\begingroup$ @gerrit adjusted, thanks! $\endgroup$
    – uhoh
    Commented Jan 11, 2022 at 10:04

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There are approved proposals for Cycle 1 to point the JWST at

In those PDFs, they describe exactly what instruments they want to point where, but to summarize as far as I can tell they're planning to try to use pretty much all of the available instruments to image Jupiter and Saturn. The above mission only plans to use NIRSpec and NIRCam on Mars, but it's unclear if that's for technical reasons or if that's just all that's required for the data they want.

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    $\begingroup$ Nice links, I upvoted. About Mars, this paper by Villaneuva and colleagues in 2016 says that the long-wavelength MIRI instrument will simply saturate because of the high temperature of Mars: dx.doi.org/10.1088/1538-3873/128/959/018004 $\endgroup$
    – giardia
    Commented Jan 11, 2022 at 20:18
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All modes can be used. But for bright targets, observations are limited to specific filters, subarrays, regions of the target planet, or spectral intervals.

James Norwood and colleagues wrote a paper on this topic, although the full answer is complicated because of the "subset of the instrument modes" part. The simplest answer to understand is for the imager, NIRCam:

enter image description here

In this plot, brightness of the planets as a function of wavelength is shown as colored curves, while little black lines show the filter bandpasses and saturation limits. So, any colored segments above the black lines mean that particular target would be saturated. This gives a rough idea of which filters can be used in the imager at these planets, but these values can vary depending on parameters like what part of the planet is imaged, whether a smaller or larger subarray is used (affecting minimum readout time), and even things like scattered light from Saturn's rings (which can be much brighter than the atmosphere at some of these wavelengths). Also, the sensitivity estimates for NIRCam were revised after the Norwood++2016 paper, and further revisions will come from in-flight testing.

The Norwood++2016 paper contains similar figures for some of the spectroscopic modes, where additional complications arise. A spectrum could be saturated at some wavelengths, but not at others, so would that observation be considered successful, partially successful, or unsuccessful?

Space Telescope Science Institute also put together a collection of nice 2-page JWST science flyers---a lot catchier than the Norwood++2016 paper---that cover what can/will be done by JWST for the targets mentioned in the original question:

The Mars flyer shows some pretty neat tricks. Target is too bright? Observe its night side!

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  • $\begingroup$ Thank you for your thorough, thoughtful and abundantly-sourced answer! I'm curious about Mars, it's $1/r^2$ from Earth can vary by half an order of magnitude. Do you know if those limits are for close or far away? $\endgroup$
    – uhoh
    Commented Jan 11, 2022 at 22:34
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    $\begingroup$ @uhoh When Mars is farther away from earth, it's also on the sunny side of the telescope and thus not observable. (This is not an answer for you question, but I think this reduces the relative change in brightness as observed by the telescope.) $\endgroup$
    – Paŭlo Ebermann
    Commented Jan 11, 2022 at 23:18
  • $\begingroup$ that definitely is good enough as an answer to my question, thanks! $\endgroup$
    – uhoh
    Commented Jan 11, 2022 at 23:22
  • $\begingroup$ Actually, when determining the brightness of an extended (resolved) source, it doesn't matter how close it is to the observer. Each pixel in the observation will have the same brightness, but the distance just says whether the object takes up 100 pixels or 200 pixels. Heliocentric distance matters for reflected sunlight though. $\endgroup$
    – giardia
    Commented Jan 12, 2022 at 21:13
  • $\begingroup$ @giardia I didn't see your comment until just now. Yes of course you are right! If a walk towards a wall it doesn't get brighter! As long as the limit is not related to something like total heat deposited in the sensor and only related to per-pixel effects, then the distance between JWST and the planet doesn't really matter. $\endgroup$
    – uhoh
    Commented Jul 13, 2022 at 20:29

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