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After about 03:22 in PBS Newshour's July 13, 2023 James Webb Space Telescope prompts scientists to rethink understanding of the universe JWST Interdisciplinary Scientist Heidi Hammel talks about planetary infrared spectroscopy via JWST, and here specifically about Uranus:

We are seeing evidence of chemistry that we had hypothesized. They're like laboratory textbook examples of chemistry; of ethane and methane and acetylene, and all this interesting dynamical stuff in the atmosphere of Uranus. And that is very interesting because that's new chemistry that we hadn't thought about.

Acetylene is quite an energetically unstable compound and on Earth is generally produced in specific high-temperature process catalytic and combustive.

Question: Did JWST spectroscopy unambiguously observe acetylene at Uranus? If so, have the results been published yet? If so, have mechanisms been proposed for how it could be produced there so far from (at least intense) sources of energy?

note: as pointed out in comments, this would not be the first observation, so discussion of earlier work is of course welcome!

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    $\begingroup$ Acetylene was proved to exist on Uranus long time ago. Here is one of the study from the 1980s: adsabs.harvard.edu/full/1986BAAS...18..789E $\endgroup$ Commented Jul 13, 2023 at 2:13
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    $\begingroup$ For what its worth, I can see an interpretation of her statement where ethane, methane and acetylene are textbook and the dynamical stuff is interesting. Not sure if this is useful for tracking down a paper, but from a presentation in April: ( meetingorganizer.copernicus.org/EGU23/EGU23-15491.html ). The novelty for them seems to be the resolution. $\endgroup$
    – Yorik
    Commented Jul 18, 2023 at 20:36
  • $\begingroup$ @Yorik Oh that's really interesting! You've done more to answer my question than either answer posts so far. My take on "textbook" was that the spectral signatures matched what you'd see in textbooks (or books of reference spectra) recorded under controlled laboratory conditions - clean and crisp spectra matching those of state-of-the-art laboratory analytical equipment. "A clear and characteristic instance or example of something." or "A real-life case that matches the theory perfectly." $\endgroup$
    – uhoh
    Commented Jul 19, 2023 at 0:33
  • $\begingroup$ @Yorik ya it seems both the spatial and the spectral resolution are emphasized: "Now, with its exceptional sensitivity and outstanding spatial and spectral resolution, JWST reveals Uranus’ stratospheric temperature and chemistry with exquisite new detail, placing new constraints on hydrocarbon abundances and temperature structure across the disk." BTW there's a beautiful simulated (not measured) JWST spectrum shown in the first figure here Webb Nearly Set to Explore the Solar System $\endgroup$
    – uhoh
    Commented Jul 19, 2023 at 0:49

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I think part of the issue here is that you seem surprised to learn that JWST detected acetylene in the atmosphere of Uranus, as though this is both unexpected and merits a certain amount of skepticism and desire for confirmation.

But, as Nilay Ghosh and Oscar Lanzi have pointed out, the presence of acetylene in the atmospheres of all Solar System gas/ice giants has been known for decades; I believe the earliest detection of acetylene -- and ethane -- in Jupiter's atmosphere dates back almost 50 years (Ridgway 1974). It's also known to exist in the atmosphere of Titan. You can see in discussions of "what kinds of studies of gas/ice giants can we do with JWST?", such as in this article by Norwood et al. (2016), that before JWST was launched it was assumed it could provide useful observations of the acetylene we already knew was there:

Perhaps the greatest contribution that JWST will make to giant-planet studies will be spatially resolved observations of the ice giants Uranus and Neptune in the mid-infrared. The mid-infrared spectra of these planets are replete with stratospheric emission features, primarily from photochemically produced hydrocarbons such as ethane (12.2 μm) and acetylene (13.7 μm).

So the answers to your first two questions would be:

  1. "Did JWST spectroscopy unambiguously observe acetylene at Uranus?" -- Undoubtedly; the failure to unambiguously observe acetylene would have been astonishing, an indication that either something was very wrong with Uranus or that something was very wrong with JWST (or at least the MIRI instrument). Since there have been reports of JWST detecting acetylene in protoplanetary systems, I very much doubt there has been any difficulty in detecting it in spectra of Uranus.

  2. "If so, have the results been published yet?" -- I can't seem to find any clear examples of this. But, again, this is irrelevant, since we've known about the presence of acetylene for decades.

As for the third question:

  1. "If so, have mechanisms been proposed for how it could be produced there so far from (at least intense) sources of energy?"

People have been working on this since at least the 1970s (when acetylene was discovered in Jupiter's atmosphere), and the answer is clearly "photolysis (photodissociation) of methane in the upper atmosphere of the planets by solar UV photons".

The photolysis of methane (and subsequent reactions among the products and unmodified methane) leads to the production of a fairly rich variety of simple hydrocarbons in addition to acetylene, including diacetylene, ethane, ethylene, propane, etc. If you're curious about the precise chemistry, this source suggests photolysis of CH$_{4}$ can lead to C$_{2}$ + H$_{2}$ + H; this leads in turn to the production of ethylene (C$_{2}$H$_{4}$). Photolysis of ethylene then gives you acetylene.

(So the "source of energy" is UV photons from the Sun.)

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  • $\begingroup$ "...you seem surprised to learn..." No, I do not seem to be anything. I've simply quoted a JPL scientist who seems both surprised and delighted. I have not expressed any surprise of my own. From the bock quote at the beginning of the post: "...all this interesting dynamical stuff..." and "...very interesting because that's new chemistry that we hadn't thought about..." suggest it's not the presence that's surprising but the chemistry - the mechanisms. This is why I've asked about the JWST data (which apparently contains "dynamical stuff") and current thinking on mechanisms. $\endgroup$
    – uhoh
    Commented Jul 17, 2023 at 22:19
  • $\begingroup$ Perhaps the fact that the comment and the answer post that choose to ignore the question and focus on first observations is causing the confusion. This is why your "this is irrelevant" response to "have the results been published yet?" is so off-base. 1) It's my question! This is Stack Exchange "your question is irrelevant" is not what we do here. 2) seeing the published data will help us understand why the astronomer is so darn excited! There be dynamics here. There be chemistry! $\endgroup$
    – uhoh
    Commented Jul 17, 2023 at 22:29
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    $\begingroup$ @uhoh "... it's not the presence that's surprising but the chemistry - the mechanisms. This is why I've asked about the JWST data " But that's not what you asked. Your first question was specifically "Did JWST spectroscopy unambiguously observe acetylene at Uranus?" -- as though there might be some doubt about the presence of acetylene. You said nothing about ethane or methane, even though Hammel mentioned them in the same breath as acetylene, so your question was not about "the chemistry" in general. $\endgroup$ Commented Jul 18, 2023 at 20:28
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    $\begingroup$ And you went on to ask, "... how it could be produced there so far from (at least intense) sources of energy?", preceded by your comment that "Acetylene is quite an energetically unstable compound and on Earth is generally produced in specific high-temperature process catalytic and combustive." The clear implication is that it's unexpected to find acetylene at Uranus, where there's no obvious way to produce it. Why else would you ask that question? $\endgroup$ Commented Jul 18, 2023 at 20:29
  • $\begingroup$ We do not try to "get into the head" of a question author. Remember, our question and answer posts are written for future readers as well as for the other authors on a given page (cf. @uhoh's lemmas). I'm simply reminding future readers that acetylene at Uranus is surprising, so they might want to keep engaged and continue reading. In short, I'm telling the reader: "Hey, you should be surprised! So keep reading!" $\endgroup$
    – uhoh
    Commented Jul 19, 2023 at 0:24
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A compilation of the known compositions of the gas giant planets (by number density), compiled from several sources indicated in the table, is given Lodders 1. References cited in the table [2-6] are given in full in the References section.

In the specific case of acetylene, $\text{C}_2\text{H}_2$, this gas is reported on all the giant planets but considerably less is found on Uranus than on the other planets.

enter image description here

More recently, the Spitzer telescope has also observed acetylene on Uranus through its infrared specrometer. Reference [7], citing [8], gives mid-IR results showing a well-defined peak identified as acetylene (photo from [7]):

enter image description here

Reference

  1. Katharina Lodders (2010), "Atmospheric Chemistry of the Gas Giant Planets", _Geochemical News 142, https://www.geochemsoc.org/publications/geochemicalnews/gn142jan10/atmosphericchemistryoftheg.

  2. Atreya, S.K, Mahaffy, P.R., Niemann, H.B., Wong, M.H., Owen, T.C. (2003), "Composition and origin of the atmosphere of Jupiter - an update, and implications for the extrasolar giant planets", Planet. Sp. Sci. *51&, 105-112.

  3. Lodders, K. 2004, Jupiter formed with more tar than ice. Astrophys. J. 611(1), 587-597

  4. Lodders, K. and B. Fegley, B. 1998,"The Planetary Scientist's Companion," Oxford University Press, New York, NY.

  5. Mahaffy, P.R., Niemann, H.B., Alpert, A., Atreya, S.K., Demick, J., Donahue, T.M., Harpold, D.N., Owen, T.C. (2000), "Noble gas abundance and isotope ratios in the atmosphere of Jupiter from the Galileo probe mass spectrometer". JGR Planets 105, 15061-15072.

  6. Wong, M.H., Mahaffy, P.R., Atreya, S.K., Niemann, H.B., Owen, T.C. (2004), "Updated Galileo probe mass spectrometer measurements of carbon, oxygen, nitrogen, and sulfur on Jupiter", Icarus 171, 153-170.

  7. Fletcher, L. N. (2022). "The Atmosphere of Uranus". Oxford Research Encyclopedias: Planetary Science. https://doi.org/10.1093/acrefore/9780190647926.013.240.

  8. Orton, G. S., Moses, J. I., Fletcher, L. N., Mainzer, A. K., Hines, D., Hammel, H. B., Martin-Torres, J., Burgdorf, M., Merlet, C., Line, M. R. (2014). "Mid-infrared spectroscopy of Uranus from the Spitzer infrared spectrometer: 2. Determination of the mean composition of the upper troposphere and stratosphere." Icarus 243 471–493.

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    $\begingroup$ (-1) Is it possible to address "Question: Did JWST spectroscopy unambiguously observe acetylene at Uranus? If so, have the results been published yet? If so, have mechanisms been proposed for how it could be produced there so far from (at least intense) sources of energy?" Currently this is just an expanded comment like this one but it doesn't currently try to answer my question as asked. $\endgroup$
    – uhoh
    Commented Jul 16, 2023 at 23:58
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    $\begingroup$ See my edit. It seems we have multiple observations of acetylene befre JWST. JWST is late to that party $\endgroup$ Commented Jul 17, 2023 at 0:33
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    $\begingroup$ "late to that party" is non sequitur My question asks specifically about JWST data. "JWST" appears in the title and in three more places in my question post. If you like, I can ask a different question about pre-JWST data and you can move this answer there instead. Or you can ask it yourself and answer it at the same time. But if there is something here that answers my question as-asked, could you flag or highlight those parts? I'm still not seeing any attempt to answer my question as asked. $\endgroup$
    – uhoh
    Commented Jul 17, 2023 at 1:08
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    $\begingroup$ "discussion of earlier work is of course welcome!" I'm going with that. $\endgroup$ Commented Jul 17, 2023 at 1:20
  • $\begingroup$ okay, I'd meant within an answer to the question not as a stand-alone post. Answer posts should strive to answer. Anyway, I'll undo my down vote and bounty for a real answer. $\endgroup$
    – uhoh
    Commented Jul 17, 2023 at 1:56

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