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We consider the Roche critical density (rho_Roche), the minimum density of anorbiting object that, at a given distance from its planet, is able to holditself together by self-gravity. It is directly related to the more familiar"Roche limit," the distance from a planet at which a strengthless orbitingobject of given density is pulled apart by tides. The presence of a substantialring requires that transient clumps have an internal density less thanrho_Roche. Conversely, in the presence of abundant material for accretion, anorbiting object with density greater than rho_Roche will grow.  Comparing the rho_Roche values at which the Saturn and Uranus systemstransition rapidly from disruption-dominated (rings) to accretion-dominated(moons), we infer that the material composing Uranus' rings is likely morerocky, as well as less porous, than that composing Saturn's rings.  From the high values of rho_Roche at the innermost ring-moons of Jupiter andNeptune, we infer that those moons may be composed of denser material thanexpected, or more likely that they are interlopers that formed farther fromtheir planets and have since migrated inward, now being held together byinternal material strength.  Finally, the "Portia group" of eight closely-packed Uranian moons has anoverall surface density similar to that of Saturn's A ring. Thus, it can beseen as an accretion-dominated ring system, of similar character to thestandard ring systems except that its material has a characteristic densitygreater than the local rho_Roche.

Compositions and origins of outer planet systems: Insights from the Roche critical density