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# 7.11 The Origin of Ring Particles

Where do planets’ rings originate? Why don’t all the planets in our Solar System have rings? Most scientists now believe that the particles composing ring systems come from erosion or breakup of small satellites on the outskirts of the rings. Throughout the history of the solar system, interplanetary debris of various sizes has been raining onto all planetary bodies. This debris is concentrated close to giant planets, because their gravity attracts more chunks of rock and ice. Smaller particles constantly "sandblast" the inner satellites of each giant planet, dislodging dust grains. The particles knocked off a moonlet tend to spiral inward toward the planet and spread into a ring. In many cases, we find small satellites orbiting at the edge of a ring — these may be the source of the material in the ring itself.

Description of the creation of ring systems from a celestial body entering the Roche limit of a planet. Click here for original source URL.

All ring systems are within a certain distance of their planet called the Roche limit. This is about 2.5 times the radius of the planet, depending on the planet’s density and other factors. If a large object is orbiting inside the Roche limit, tidal forces will be strong enough to pull it apart. This could be another way rings form: an orbiting satellite moves too close to the planet, where tidal forces break it up. The resulting fragments would then spread out in their own Keplerian orbits. Tidal forces scale with the strength of the local gravity so are strongest around giant planets, which is propably why the terrestrial planets don’t have ring systems.

Close-up of part of the rings of Saturn. Click here for original source URL.

Impacts of debris from comet Shoemaker-Levy on Jupiter. Click here for original source URL.

Occasionally, large impacts may also break up a satellite. This would throw a substantial swarm of debris into the space near the planet, forming a thick ring like Saturn's. Another source of material could be a passing comet, like Shoemaker-Levy 9, which fragmented into more than twenty pieces before it struck Jupiter in 1994. The ring systems we see today may not date from the beginning of the Solar System, but from more recent collisions like these. When we look at Saturn's rings today, we may be seeing the debris of a satellite-shattering catastrophe in the "recent" geological past. An alien visitor to our Solar System three billion years ago might have seen a completely different configuration of ring systems!