Daphnis, the ring-sculptor

Strictly speaking, Daphnis is one of many ring-sculptors — the shepherd moons that maintain some of the notable visual features of the rings of Saturn.  In the case of Daphnis, that’s the Keeler Gap, a narrow gap carved out the A ring by the gentle gravitational tug of war between Saturn and the tiny moon.

Daphnis was actually discovered by the Cassini probe in 2005, but its existence was suspected long before, when the Keeler Gap was itself discovered in images taken by the twin Voyager probes.  Voyager had already discovered the moons Prometheus and Pandora, which the Voyager team dubbed “shepherd moons” for the way their push and tug confined a group of particles to the very narrow F-ring.  So it was surmised that the other gaps would turn out to have moons creating them as well. Mimas seems to be responsible, at least in part, for both the Cassini Division and the Huygens Gap, and of course Prometheus and Pandora constrain the F ring.  Pan, discovered in 1990 from old Voyager data, is likely responsible for the Encke Gap.  And Daphnis is the sculptor of the Keeler Gap.

Or, at least, the main sculptor.  Saturn’s rings are very complex, and serve as a fascinating natural laboratory for studying gravitational interactions, and particularly the sort hypothesized to have created the solar system as we know it.  And Daphnis, like other shepherd moons, does not orbit perfectly neatly.  Its orbit is slightly inclined relative to the ringplane, and slightly elliptical as well.  Thus, it doesn’t produce a nice tidy circle, but carves out waves as it passes — waves both ahead and behind, and, as this recent Cassini image shows, sometimes it pulls off delicate tendrils of ring particles (look verrrrry closely, or just click to enlarge – you’ll see a thin wisp of material echoing the shape of the nearby wave in the ring):

That’s the closest image ever taken of Daphnis, a tiny moon roughly the same size as Mount Everest.  It appears to have striations running down its length, probably the result of accumulated ring particles — sometimes, even a tiny moon like this will manage to capture something and pull it down.  But if you want a more dramatic image of this effect, you will have to look at Daphnis near Saturn’s equinox, when the shadows are at their longest.  Then you can see what is hidden in this image: the waves aren’t just flat features.  They stand surprisingly tall.

By studying this process, scientists hope to better understand planetary formation.  Indeed, they’ve even found a few spots in Saturn’s rings where it appears that moonlets may be in the process of forming, clumping together at random until eventually one clump reaches a critical mass and begins to dominate the particles around it, gradually growing until it exhausts its immediate surroundings, carving out another gap.  Daphis itself shows signs that it may be accumulating material.  Saturn’s rings are an astonishingly and fascinatingly dynamic place.

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