<p>There are approved proposals for Cycle 1 to point the JWST at</p>
<ul>
<li><a href="https://www.stsci.edu/jwst/phase2-public/1373.pdf" rel="noreferrer">The Jovian system</a></li>
<li><a href="https://www.stsci.edu/jwst/phase2-public/1246.pdf" rel="noreferrer">Jupiter's great red spot</a></li>
<li><a href="https://www.stsci.edu/jwst/phase2-public/1415.pdf" rel="noreferrer">Mars</a></li>
<li><a href="https://www.stsci.edu/jwst/phase2-public/1247.pdf" rel="noreferrer">Saturn and its moons and rings</a></li>
</ul>
<p>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.</p>

CC BY-SA 4.0
2022-01-11T20:18:23.110Z
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: http://dx.doi.org/10.1088/1538-3873/128/959/018004
giardia
CC BY-SA 4.0
2022-01-11T04:41:57.893Z
2022-01-11T04:51:26.853Z
2022-01-11T04:51:26.853Z
Gavin S. Yancey
<p><strong>All modes can be used. But for bright targets, observations are limited to specific filters, subarrays, regions of the target planet, or spectral intervals.</strong></p>
<p>James Norwood and colleagues wrote <a href="https://iopscience.iop.org/article/10.1088/1538-3873/128/960/025004" rel="noreferrer">a paper</a> on this topic, although the full answer is complicated because of the &quot;subset of the instrument modes&quot; part. The simplest answer to understand is for the imager, NIRCam:</p>
<p><a href="https://i.sstatic.net/OLUEe.png" rel="noreferrer"><img src="https://i.sstatic.net/OLUEe.png" alt="enter image description here" /></a></p>
<p>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 <strong>rough</strong> 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 <a href="https://iopscience.iop.org/article/10.1088/1538-3873/128/960/025004" rel="noreferrer">Norwood++2016</a> paper, and further revisions will come from in-flight testing.</p>
<p>The <a href="https://iopscience.iop.org/article/10.1088/1538-3873/128/960/025004" rel="noreferrer">Norwood++2016</a> 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?</p>
<p>Space Telescope Science Institute also put together a <a href="https://www.stsci.edu/jwst/about-jwst/history/flyers" rel="noreferrer">collection of nice 2-page JWST science flyers</a>---a lot catchier than the <a href="https://iopscience.iop.org/article/10.1088/1538-3873/128/960/025004" rel="noreferrer">Norwood++2016</a> paper---that cover what can/will be done by JWST for the targets mentioned in the original question:</p>
<ul>
<li><a href="https://www.stsci.edu/files/live/sites/www/files/home/jwst/about/history/flyers/_documents/JWST-Solar-System.pdf" rel="noreferrer">Solar System</a></li>
<li><a href="https://www.stsci.edu/files/live/sites/www/files/home/jwst/about/history/flyers/_documents/JWST-Mars.pdf" rel="noreferrer">Mars</a></li>
<li><a href="https://www.stsci.edu/files/live/sites/www/files/home/jwst/about/history/flyers/_documents/JWST-Giant-Planets.pdf" rel="noreferrer">Giant Planets</a></li>
<li><a href="https://www.stsci.edu/files/live/sites/www/files/home/jwst/about/history/flyers/_documents/JWST-GTO-ERS-solar-system.pdf" rel="noreferrer">Solar System early programs</a> (does not include first full open cycle)</li>
</ul>
<p>The Mars flyer shows some pretty neat tricks. Target is too bright? Observe its night side!</p>

CC BY-SA 4.0
2022-01-11T22:34:45.103Z
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?
uhoh
CC BY-SA 4.0
2022-01-11T23:18:39.440Z
@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.)
Paŭlo Ebermann
CC BY-SA 4.0
2022-01-11T23:22:37.640Z
that *definitely is* good enough as an answer to my question, thanks!
uhoh
CC BY-SA 4.0
2022-01-12T21:13:32.353Z
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.
giardia
CC BY-SA 4.0
2022-07-13T20:29:17.317Z
@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.
uhoh
CC BY-SA 4.0
2022-01-11T20:14:49.527Z
2022-01-11T20:14:49.527Z
giardia
<p>My <a href="https://space.stackexchange.com/questions/57675/is-there-any-point-in-space-that-the-james-webb-space-telescope-would-be-unable?noredirect=1#comment190866_57675">comment</a> says:</p>
<blockquote>
<p>...answers to the Space SE question <a href="https://space.stackexchange.com/q/57492/12102">Can James Webb take pictures of our solar system? In what ways will they differ from/compliment those from Hubble?</a> 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 <em>raison d'etre</em> to do first.</p>
</blockquote>
<p>and the following comment informs with the quote</p>
<blockquote>
<p>&quot;The very large infrared brightnesses of Mars, Jupiter, and Saturn may limit Webb observations of these planets to a subset of the instrument modes&quot;</p>
</blockquote>
<p>which is possibly quoted from NASA's <a href="https://jwst.nasa.gov/content/forScientists/faqScientists.html" rel="noreferrer">JWST Technical FAQ On A Variety Of Mission Issues, Aspects And Capabilities.</a>.</p>
<p><strong>Question:</strong> Which JWST instrument modes are compatible with observations of the bright trans-Earth planets Mars, Jupiter, and Saturn? Which aren't?</p>

CC BY-SA 4.0
2022-01-11T01:29:10.030Z
(Very) Light on details but [one](https://www.stsci.edu/jwst/science-execution/approved-programs/dd-ers/program-1373) of the approved Early Release Science programs is using all of the JWST instruments to look at Jupiter...
astrosnapper
CC BY-SA 4.0
2022-01-11T09:23:16.477Z
Small correction: Mars is one of the inner planets, not outer planets.
gerrit
CC BY-SA 4.0
2022-01-11T10:04:39.593Z
@gerrit adjusted, thanks!
uhoh
<observational-astronomy><space-telescope><planetary-science><instruments><james-webb-space-telescope>
Which JWST instrument modes are compatible with observations of the bright trans-Earth planets Mars, Jupiter, and Saturn? Which aren't?
uhoh
# Which JWST instrument modes are compatible with observations of the bright trans-Earth planets Mars, Jupiter, and Saturn? Which aren't?

(Asked by: username_0 on 2022-01-11)

My [comment](https://space.stackexchange.com/questions/57675/is-there-any-point-in-space-that-the-james-webb-space-telescope-would-be-unable?noredirect=1#comment190866_57675) 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?](https://space.stackexchange.com/q/57492/12102) 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.](https://jwst.nasa.gov/content/forScientists/faqScientists.html).

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

**Tags:** observational-astronomy, space-telescope, planetary-science, instruments, james-webb-space-telescope
> **Comments:**
> **username_1**: (Very) Light on details but [one](https://www.stsci.edu/jwst/science-execution/approved-programs/dd-ers/program-1373) of the approved Early Release Science programs is using all of the JWST instruments to look at Jupiter...  
> **username_2**: Small correction: Mars is one of the inner planets, not outer planets.  
> **username_0**: @gerrit adjusted, thanks!  

---

## 2 answers

### Answer by username_3 on 2022-01-11. Score: 11

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

* [The Jovian system](https://www.stsci.edu/jwst/phase2-public/1373.pdf)
* [Jupiter's great red spot](https://www.stsci.edu/jwst/phase2-public/1246.pdf)
* [Mars](https://www.stsci.edu/jwst/phase2-public/1415.pdf)
* [Saturn and its moons and rings](https://www.stsci.edu/jwst/phase2-public/1247.pdf)

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.

> **Comments:**
> **username_4**: 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: http://dx.doi.org/10.1088/1538-3873/128/959/018004  

---

### Answer by username_4 on 2022-01-11. Score: 7

**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](https://iopscience.iop.org/article/10.1088/1538-3873/128/960/025004) 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](https://i.sstatic.net/OLUEe.png)](https://i.sstatic.net/OLUEe.png)

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](https://iopscience.iop.org/article/10.1088/1538-3873/128/960/025004) paper, and further revisions will come from in-flight testing.

The [Norwood++2016](https://iopscience.iop.org/article/10.1088/1538-3873/128/960/025004) 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](https://www.stsci.edu/jwst/about-jwst/history/flyers)---a lot catchier than the [Norwood++2016](https://iopscience.iop.org/article/10.1088/1538-3873/128/960/025004) paper---that cover what can/will be done by JWST for the targets mentioned in the original question:

* [Solar System](https://www.stsci.edu/files/live/sites/www/files/home/jwst/about/history/flyers/_documents/JWST-Solar-System.pdf)
* [Mars](https://www.stsci.edu/files/live/sites/www/files/home/jwst/about/history/flyers/_documents/JWST-Mars.pdf)
* [Giant Planets](https://www.stsci.edu/files/live/sites/www/files/home/jwst/about/history/flyers/_documents/JWST-Giant-Planets.pdf)
* [Solar System early programs](https://www.stsci.edu/files/live/sites/www/files/home/jwst/about/history/flyers/_documents/JWST-GTO-ERS-solar-system.pdf) (does not include first full open cycle)

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

> **Comments:**
> **username_0**: 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?  
> **username_5**: @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.)  
> **username_0**: that *definitely is* good enough as an answer to my question, thanks!  
> **username_4**: 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.  
> **username_0**: @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.  

---

