<p>Seems to me that moons are regularly subjected to tidal forces that comets, asteroids and the like are not.
Repeated stress, such as rocks in a rock tumbler, or two circles of glass on a <a href="https://en.wikipedia.org/wiki/Amateur_telescope_making#Mirror_making" rel="nofollow">mirror maker's table</a>, tends to smooth things out. </p>

<p>I'm sure there's a pile of academic papers on sub-disruptive levels of strain tending to increase the symmetry of many types of systems, likely even some formal proofs, but I can't search at that level this morning.</p>

CC BY-SA 3.0
2015-07-12T13:37:01.343Z
2015-07-12T13:37:01.343Z
Wayfaring Stranger
<p>I can only speculate but here goes nothing.</p>

<p>67P/Churyumov–Gerasimenko apparently came from the <a href="http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko" rel="nofollow">Kuiper Belt</a>. There are a lot of KBO (Kuiper Belt Objects) in that region, so it is not unreasonable to expect some of these objects to have crashed into other objects of about the same mass, resulting in irregular shapes. </p>

<p>Small moons (or other moons in the Solar System) have been isolated from other objects of the same size. This allows gravity work work undisturbed for a longer period of time than those in the Kuiper Belt, molding the object into more or less a spherical shape. </p>

<p>That is my best stab at the question, though I can't imagine some other cause that would result in such a drastic irregular shape in something as large as a comet (relatively speaking).</p>

<hr>

<p>I just found something interesting. So the smallest moon in the solar system is <a href="http://www.ciclops.org/media/ma/2011/6956_16708_0.pdf" rel="nofollow">Aegaeon</a>. It happens to be a couple of orders of magnitude smaller than the comet mentioned above. In addition, this moon appears to have an oblate spheroidal shape to it. It may just be the case that we have only seen spherical moons, but there are many more moons out in the cosmos that have odd shapes to them.</p>

CC BY-SA 3.0
2015-06-12T02:07:20.953Z
So you're saying that several kilometers wide pieces of ice might flow under their feeble gravity on timescales of billions of years? I don't know.
user6784
CC BY-SA 3.0
2015-06-12T04:11:29.860Z
Are you referring to the comet or the moons? 

If you are referring to the comet, then I never said that gravity would mold it into a spherical shape. 

If you are referring to the moons, it is unlikely that anything other than its own gravitational pull will shape the moons the way they are (unless they are near something incredibly massive I would imagine).
firest
CC BY-SA 3.0
2015-06-12T07:24:56.743Z
To further @Charles' answer a little. Any irregularly shaped objects coming close to a stable orbit would undoubtedly be quickly shaken out of the orbit due to the irregular shape, being flung out in to the outer regions of the solar system or crashing towards the planet. The mostly spherical shape allows the moons to maintain a stable orbit. I would suggest that this is the reason we don't see dumbbell shaped moons.
theotheraussie
CC BY-SA 3.0
2015-06-12T12:08:53.573Z
Ah, yes, of course. Similar to how lunar orbits decay. If it were so easy we would not have Enceladus or Mimas I think as the smallest moon considered to be at hydrostatic equilibrium (several hundred kilometers and that only because ice is weak enough to flow even on Earth). And disregard my "answer", I only meant to comment.
user6784
CC BY-SA 3.0
2015-06-12T01:01:50.053Z
2015-06-12T04:15:15.533Z
2015-06-12T04:15:15.533Z
firest
<p>67P/Churyumov–Gerasimenko has a highly irregular dumbbell shape. But the sample of comet shapes observed is very small, so I wonder if irregular shape is the norm for comets and for small moons. Many known moons are no larger than this ~4 km diameter comet. The smallest moons imaged are modestly irregular, basically just elongated, at least they don't have a waist like 67/P. The most elongated moon is perhaps ~135/60 km Prometheus.</p>

<p>Is there reason to believe that moons which are too small for hydrostatic equilibrium get more rounded than comets of similar mass and composition? Moons differ from comets in several ways. AFAIK: Moons in general have a very different gravitational environment, a much more stable distance to the Sun, experience more frequent impacts, another formation history if not captured. Composition and density depends on the formation distance from the Sun, although even ~300 km Hyperion has a similar density as 67/P.</p>

<p>Should we expect the small moons in general, and the moons of Pluto especially, to be rounded or dumbbell shaped? If 67P is a merged binary, isn't it more likely for two objects to merge if they are moons instead of comets, since neighboring moons have low relative speed? ~100-200 km Janus and Epimetheus look as if they could merge fairly calmly. Does the lack of (observed) dumbbell shaped moons tell us something about 67P, that its shape is a rare outlier for example?</p>

<comets><formation><natural-satellites><hydrostatic-equilibrium>
Is there a mechanism that makes small moons more rounded than comets?
LocalFluff
# Is there a mechanism that makes small moons more rounded than comets?

(Asked by: username_0 on 2015-06-06)

67P/Churyumov–Gerasimenko has a highly irregular dumbbell shape. But the sample of comet shapes observed is very small, so I wonder if irregular shape is the norm for comets and for small moons. Many known moons are no larger than this ~4 km diameter comet. The smallest moons imaged are modestly irregular, basically just elongated, at least they don't have a waist like 67/P. The most elongated moon is perhaps ~135/60 km Prometheus.

Is there reason to believe that moons which are too small for hydrostatic equilibrium get more rounded than comets of similar mass and composition? Moons differ from comets in several ways. AFAIK: Moons in general have a very different gravitational environment, a much more stable distance to the Sun, experience more frequent impacts, another formation history if not captured. Composition and density depends on the formation distance from the Sun, although even ~300 km Hyperion has a similar density as 67/P.

Should we expect the small moons in general, and the moons of Pluto especially, to be rounded or dumbbell shaped? If 67P is a merged binary, isn't it more likely for two objects to merge if they are moons instead of comets, since neighboring moons have low relative speed? ~100-200 km Janus and Epimetheus look as if they could merge fairly calmly. Does the lack of (observed) dumbbell shaped moons tell us something about 67P, that its shape is a rare outlier for example?

**Tags:** comets, formation, natural-satellites, hydrostatic-equilibrium

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## 2 answers

### Answer by username_1 on 2015-07-12. Score: 1

Seems to me that moons are regularly subjected to tidal forces that comets, asteroids and the like are not.
Repeated stress, such as rocks in a rock tumbler, or two circles of glass on a [mirror maker's table](https://en.wikipedia.org/wiki/Amateur_telescope_making#Mirror_making), tends to smooth things out.

I'm sure there's a pile of academic papers on sub-disruptive levels of strain tending to increase the symmetry of many types of systems, likely even some formal proofs, but I can't search at that level this morning.


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### Answer by username_2 on 2015-06-12. Score: 1

I can only speculate but here goes nothing.

67P/Churyumov–Gerasimenko apparently came from the [Kuiper Belt](http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko). There are a lot of KBO (Kuiper Belt Objects) in that region, so it is not unreasonable to expect some of these objects to have crashed into other objects of about the same mass, resulting in irregular shapes.

Small moons (or other moons in the Solar System) have been isolated from other objects of the same size. This allows gravity work work undisturbed for a longer period of time than those in the Kuiper Belt, molding the object into more or less a spherical shape.

That is my best stab at the question, though I can't imagine some other cause that would result in such a drastic irregular shape in something as large as a comet (relatively speaking).

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I just found something interesting. So the smallest moon in the solar system is [Aegaeon](http://www.ciclops.org/media/ma/2011/6956_16708_0.pdf). It happens to be a couple of orders of magnitude smaller than the comet mentioned above. In addition, this moon appears to have an oblate spheroidal shape to it. It may just be the case that we have only seen spherical moons, but there are many more moons out in the cosmos that have odd shapes to them.

> **Comments:**
> **username_3**: So you're saying that several kilometers wide pieces of ice might flow under their feeble gravity on timescales of billions of years? I don't know.  
> **username_2**: Are you referring to the comet or the moons?   If you are referring to the comet, then I never said that gravity would mold it into a spherical shape.   If you are referring to the moons, it is unlikely that anything other than its own gravitational pull will shape the moons the way they are (unless they are near something incredibly massive I would imagine).  
> **username_4**: To further @Charles' answer a little. Any irregularly shaped objects coming close to a stable orbit would undoubtedly be quickly shaken out of the orbit due to the irregular shape, being flung out in to the outer regions of the solar system or crashing towards the planet. The mostly spherical shape allows the moons to maintain a stable orbit. I would suggest that this is the reason we don't see dumbbell shaped moons.  
> **username_3**: Ah, yes, of course. Similar to how lunar orbits decay. If it were so easy we would not have Enceladus or Mimas I think as the smallest moon considered to be at hydrostatic equilibrium (several hundred kilometers and that only because ice is weak enough to flow even on Earth). And disregard my "answer", I only meant to comment.  

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