What Keeps a Ring Sharp? Small Moons Herding Particles Into Line

Particles in a ring collide, and every collision exchanges momentum, which on average moves some particles inwards and others outwards and spreads the ring. The process is analogous to the spreading of any confined collection of colliding objects and operates on timescales far shorter than the age of the solar system, so a ring with a sharp edge requires an explanation. The same reasoning says the rings themselves should not be as old as their planets, which is one of the arguments in the long-running dispute about whether Saturn's rings formed recently or have been continuously replenished. Where a sharp edge exists, something must be confining the material, and the confinement observed in several cases comes from small moons in nearby orbits.

The mechanism depends on a basic feature of orbital motion:

• •An object closer to the planet orbits faster than one further out
• •A moon just inside the ring therefore overtakes the ring particles repeatedly
• •Each passage pulls particles forward, which raises them into a higher and slower orbit, pushing the inner edge outward
• •A moon just outside is overtaken by the particles and drags them backward, lowering them and pushing the outer edge inward
• •The two effects together confine the material between them
• •The repeated nudges also produce visible waves and scalloping along the edges

The clearest examples come from Saturn and were found by spacecraft. Two small moons orbiting on either side of a narrow outer ring were identified from images returned in 1980 and provided the original demonstration of the idea, which had been proposed shortly before on theoretical grounds. Another small moon orbits within a gap in the main rings and maintains it, producing wave patterns along both edges that were photographed in fine detail by a later mission. A further moon sits within a narrow gap and keeps it open. Uranus has narrow rings with confining moons identified in the same way. The discoveries were significant beyond ring science because the same reasoning applies to gaps in the discs of dust and gas around young stars, where an unseen planet can be inferred from the gap it maintains.

A related arrangement at Saturn is odd enough to describe separately. Two small moons occupy almost the same orbit, with one very slightly closer to the planet and therefore moving slightly faster, so it gradually catches up with the other over a period of years. Rather than colliding or one overtaking the other, they exchange orbits as they approach, with the inner becoming the outer and the outer becoming the inner, after which they separate again and the process repeats every four years. The exchange happens because in the rotating frame the pair are interacting gravitationally and swap their relative energies rather than passing. The configuration was unexpected, was confirmed by spacecraft observation, and is a demonstration that stable arrangements exist which nobody would have predicted from the two-body picture.

The idea is not a complete account of ring structure and the gaps in the explanation are interesting. Many features have no identified confining moon, and although undetected small bodies are possible, searches have not found them in several cases. Some structure arises instead from orbital resonances with larger moons much further away, where particles at a distance whose orbital period is a simple fraction of a moon's period receive regularly timed tugs that clear them out, which explains several prominent divisions. Other structures appear to be self-sustaining waves in the ring material itself. The narrow rings of Uranus are still not fully explained. And the detailed shapes revealed by close spacecraft imaging include propellers, braids and clumps whose causes are argued about, so the field is considerably less settled than the textbook version suggests.

Collisions spread a ring over timescales far shorter than the age of the solar system, so a sharp edge needs explaining. An inner moon orbits faster and pulls particles forward into higher orbits, while an outer one drags them backward into lower ones, confining the material between them. Many features have no identified moon and arise from resonances with distant ones instead.