Saturn's Coolest #10: Prometheus/Pandora
I'm cheating here. These two aren't worlds, they're rocks. And there's two of them in this slot, but that's because it's the two of them working together that makes them interesting.
These two potato-shaped rocks (each about eighty or so miles across the long way) orbit just outside the main part of Saturn's rings; their orbits are only a few thousand miles apart. What makes them interesting is that between the two of them there's a very thin ring - the F ring - and it's the presence of these two moons that "traps" material in this ring; their gravity herds tiny particles of ring-material into an orbit between the two; they clear out particles in their own orbits and maintain a thin ring, looking almost threadlike, between them.
But the F ring isn't just a thread. Take a look - it has ripples. (When we first took pictures of it with Voyager, it looked braided.)
Those are waves in the ring material caused by gravity - they're like ripples in a pond. Prometheus, on the inside, moves slightly faster around Saturn than the ring material; Pandora moves slightly slower. So as they pass alongside the ring, their feeble gravity pushes and pulls on the tiny grains making up the ring, just a little, and it creates a ripple moving through the ring. It's easier to see Prometheus' effect in that video because in this part of its orbit, it's the one closer to the ring.
What's more, the two of them affect each other. Every time Prometheus laps Pandora, their gravitational pulls kick each other and change their orbits slightly - between Voyager's flyby and Cassini's arrival, they wound up about twenty degrees off from where we expected them to be. The kick isn't predictable and doesn't follow a set pattern - it's "chaotic", to use the correct word - but it remains inside certain parameters, and judging from the deposits and cratering on these two moons, they - like the rest of the system - have been here a really long time.
This isn't nearly the weirdest gravitational effect in the Saturn system. It whets the appetite to hear about this pair first, but it barely even scratches the surface of how weird gravity gets here. Stay tuned.
These two potato-shaped rocks (each about eighty or so miles across the long way) orbit just outside the main part of Saturn's rings; their orbits are only a few thousand miles apart. What makes them interesting is that between the two of them there's a very thin ring - the F ring - and it's the presence of these two moons that "traps" material in this ring; their gravity herds tiny particles of ring-material into an orbit between the two; they clear out particles in their own orbits and maintain a thin ring, looking almost threadlike, between them.
But the F ring isn't just a thread. Take a look - it has ripples. (When we first took pictures of it with Voyager, it looked braided.)
Click to viewThose are waves in the ring material caused by gravity - they're like ripples in a pond. Prometheus, on the inside, moves slightly faster around Saturn than the ring material; Pandora moves slightly slower. So as they pass alongside the ring, their feeble gravity pushes and pulls on the tiny grains making up the ring, just a little, and it creates a ripple moving through the ring. It's easier to see Prometheus' effect in that video because in this part of its orbit, it's the one closer to the ring.
What's more, the two of them affect each other. Every time Prometheus laps Pandora, their gravitational pulls kick each other and change their orbits slightly - between Voyager's flyby and Cassini's arrival, they wound up about twenty degrees off from where we expected them to be. The kick isn't predictable and doesn't follow a set pattern - it's "chaotic", to use the correct word - but it remains inside certain parameters, and judging from the deposits and cratering on these two moons, they - like the rest of the system - have been here a really long time.
This isn't nearly the weirdest gravitational effect in the Saturn system. It whets the appetite to hear about this pair first, but it barely even scratches the surface of how weird gravity gets here. Stay tuned.