How big are the chunks of rock ice that make up Saturn's rings? Are there many objects larger than pebble size?
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$\begingroup$ it would seem that rather than pebble size, they are more like 10cm size. (Fist-sized.) But, I do not totally intuitively grasp HDE's comprehensive answer. $\endgroup$– FattieCommented Sep 23, 2016 at 17:03
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$\begingroup$ well according to the size of the rocks here on earth the sizes could be varied $\endgroup$– user26532Commented Apr 10, 2019 at 16:23
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2 Answers
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The vast majority of the particles in Saturn's rings are small, on the order of $\sim10^{-1}$ m or lower. The columnar number density, according to data from Voyager 1 and Earth-based observations, can be approximated as a function of particle radius by a power law for all particle radii $a$ in meters such that $0<a<1$, as can be seen on this log-log plot (Fig. 15.5, Cuzzi et al. (2009); taken from Fig. 8 of Marouf et al. (1982)):
Though it is only acknowledged in the original paper, the vertical axes for the three different ring regions have been shifted upward different amounts to fit all three on the same graph.
After $a=1$, there's a deviation from the law, and then a steep dropoff at about $a=3$. Obviously, particles larger than this exist, and they certainly play an important role in ring structure, but they're relatively rare.
Obviously, the trends show that smaller particles are much more common, and thus while there are indeed particles larger than pebbles - some as big as boulders, perhaps, or bigger - they are certainly few in number. Most particles are extremely small, smaller than pebbles.
This data covers observations from ring semi-major axes of $\sim$ 75,000 km to $\sim$ 135,000 km - a fairly big spread, covering most of the rings and ending near the Roche Division. The paper doesn't have one single graph of particle number density of a given size at a given semi-major axis, but it does have several subdivided plots (Fig. 15.1 and 15.2) of optical depth as a function of distance from the center of Saturn, which should give you some helpful data on total number density, if you want to make some basic assumptions about mean particle radius. This data is a bit newer, from Cassini, but the Voyager 1 data is just as helpful.
answered Sep 19, 2016 at 23:40
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$\begingroup$ Well, 10-cm diameter objects are rather larger than pebbles :-) $\endgroup$ Commented Sep 21, 2016 at 11:06
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$\begingroup$ @HDE - I'm wondering, considering only "pebble sized" (1cm sized) versus "rock sized" (10cm sized) items. Is one, or the other, of those far more common?? I think (but you should explain!) the chart suggests there are about 10,000 times more of the 1cm type than the 10cm type. Is that correct? $\endgroup$– FattieCommented Sep 23, 2016 at 17:05
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1$\begingroup$ why does the chart "stick" at 10^4 on the left side near the top?? $\endgroup$– FattieCommented Sep 23, 2016 at 17:05
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$\begingroup$ @CarlWitthoft Fair point; that might require a mention. $\endgroup$– HDE 226868 ♦Commented Sep 23, 2016 at 17:14
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Saturn's rings are composed of chunks as large as 1km in size, although the typical particle is tiny. They are spread through an area on average 10 meters thick. Also, Saturn's rings are nearly pure ice, not rocks. I don't know if we have a count of "how many" objects are larger than pebble size, given the gigantic number of particles that make up the rings I think we only have counts of the smaller moons within the rings.
