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What is this bright blue object taken by the Webb telescope? includes the new JWST infrared image of Neptune and Triton. The disk of Neptune shows interesting features but I'm drawn to Neptunes's rings. I see two and in each ring there is a "blob".

I'm no ring expert, but from what I remember from the up-close Cassini images of Saturn, moons and rings are like oil and water; they don't mix. In this case that means that the orbit of a moon is usually swept clean of ring material by gravitational perturbations; ring particles need to have a slightly faster or slower period to avoid 1:1 (and perhaps other complex relationships) resonance.

So I'd like to ask:

Question: What are those blobs in each of the two bright rings of Neptune shown in the new JWST image?

If they do turn out to be Moons please help me understand why the rings appear to coincide with the moons' positions rather than fall between them to avoid resonances.


Shamelessly cropped and sharpened (and annotated) JWST image from the one in the linked question showing the blob in each of the two bright rings of Neptune:

Shamelessly cropped and sharpened (and annotated) JWST image from the one in the linked question showing the blob in each of the two bright rings of Neptune

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    $\begingroup$ It is amazing how this image seems to make Neptune look transparent. $\endgroup$ Oct 5, 2022 at 4:47

1 Answer 1

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This photo captures 7 of the 14 moons of Neptune. The ones you pointed out are Despina on the right and Galatea on the left as illustrated by this NASA website.

Now the reason why they seem to coincide with the path and position of the rings is because:

The rings of Uranus and Neptune are kept narrow by the action of shepherd moons, which "herd" the ring particles and keep them in a narrow range of orbits

They are close enough so the far photo appears as they are together.

And Wikipedia:

The arcs occupy a narrow range of orbital longitudes and are remarkably stable, having changed only slightly since their initial detection in 1980. How the arcs are stabilized is still under debate. However, their stability is probably related to the resonant interaction between the Adams ring and its inner shepherd moon, Galatea.

Also Neptune's moons and ring system scheme:

enter image description here

Note that the moons and the rings are not actually coincident but separated.

Hat tip to @OrOrg:

both moons are just a tiny bit closer to Neptune than their rings. For Galatea, it appears to be a so-called 42:43 outer Lindblad resonance, for which more information can be found in the paper An Explanation for Neptune's Ring Arcs

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    $\begingroup$ 1) "Despina on the right and Galatea on the left" how do you know? Please support that with calculations or an authoritative source. Thanks! 2) A shepherd moon (also herder moon or watcher moon) is a small natural satellite that clears a gap in planetary-ring material or keeps particles within a ring contained. I've asked why the rings seem to coincide with the orbits of the moon, and to my understanding that's not what Shepherd moons usually do (as discussed in the question. Perhaps they're not actually coincident orbits? $\endgroup$
    – uhoh
    Sep 26, 2022 at 1:32
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    $\begingroup$ I don't think speculation in prose is helpful; if you can cite published simulations or other authoritative sources and show relevance, that will be great! $\endgroup$
    – uhoh
    Sep 26, 2022 at 3:38
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    $\begingroup$ @uhoh Labeled version of the image, showing which moon is which: stsci-opo.org/STScI-01GCCVPBK1GKCQ9QM08NZEKV8Y.png $\endgroup$ Sep 26, 2022 at 6:17
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    $\begingroup$ @uhoh As DialFrost remarked, both moons are just a tiny bit closer to Neptune than their rings. For Galatea, it appears to be a so-called 42:43 outer Lindblad resonance, foro which more information can be found in the paper An Explanation for Neptune's Ring Arcs. $\endgroup$
    – OrOrg
    Sep 26, 2022 at 9:17
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    $\begingroup$ Great answer, clearly sourced and explains what the JWST shows in simple and straightforward language. $\endgroup$
    – James K
    Sep 26, 2022 at 19:21

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