[X] My Assistant

[dilo]

http://www.nasa.gov/mission_pages/cassini/...i-20070322.html

The neutral gas particles ejected from the geysers on Enceladus form a donut-like torus around Saturn. As these particles become electrically charged, they are captured by Saturn's magnetic field, forming a disk of ionized gas, or plasma, which surrounds the planet near the equator. The particles weigh down the magnetic field so much that the rate of rotation of the plasma disk slows down slightly. This slippage causes the radio period, controlled by the plasma disk rotation, to be longer than the planet's actual rotation period.
[...] The day measured by Cassini is some six minutes longer than the day recorded by NASA's Voyager spacecraft in the early 1980s, a change of nearly 1 percent. [...] Based on the new observations, scientists now think there are two possible reasons for the change in radio period. The first theory is that the geysers on Enceladus could be more active now than in Voyagers' time. The second is that there may be seasonal variations as Saturn orbits the sun once every 29 years.

[Patteroast]

Interesting! One question... why is this effect only seen on Saturn? You'd think it would happen as well on Jupiter or perhaps Neptune, with active moons in the magnetosphere of each... all I can think of for Jupiter is that it's immense strength is enough to overcome any effects from gas released from Io or Europa? Also, is there any evidence for gas escape along Triton's orbit?

[tasp]

We might have more than one mechanism at work around these various objects.

Triton's (relatively) stronger gravity and lower temperature than Enceladus would reduce leakage rate. For Titan, we have an object that apparently alternates between the realm of the Saturnian magnetosphere and the solar wind. Penetration of the magnetosphere might 'turbulize' leakage effluents away from the Titanian orbit rather effectively. Enceladus is deeper into the Saturnian realm, but experiences a drastically weaker effect than Io in it's perpetual 'deep fry'.

Additionally, compare the mass of the Enceledosian water based e-ring to the tenuous Io sodium torus.

[Rob Pinnegar]

I think the problem at Neptune is that any gas escaping from Triton would be just that -- gas -- and it presumably wouldn't be able to form a ring around Neptune. The particles in the E ring, on the other hand, are solid, although they're microscopically tiny. (On that note -- is Io's sodium torus composed of individual sodium atoms, or "clumps"?)

You guys have probably more or less nailed the major points, as regards Jupiter vs. Saturn, viz. (1) vastly stronger field at Big Jupe, and (2) Io's much stronger gravity compared with Enceladus.

There's a third point to consider though: perhaps sodium isn't as efficient at disrupting magnetic fields as water. I'm only speculating here, but maybe it's got something to do with sodium being monatomic, while water's a polar molecule? This is a *total* guess; chemistry was never my Big Thing.

[ngunn]

Free neutral sodium atoms don't sound very likely - I'd expect positive ions and free electrons.

[edstrick]

"...... why is this effect only seen on Saturn? You'd think it would happen as well on Jupiter or perhaps Neptune,..."

It's probably because Saturn's magnetic field is weird-beyond-aberrant. It's TOO regular.. it's almost perfectly aligned with the planet's rotation axis and has almost no non-dipole (quadrupole, etc. structore). The result is that the emissions apparently are from structures in the torus and not "hardwired" to planetary-core-attached irregularities in the internal field that are expressed (for example) in the auroral regions.

[martin peters]

Free neutral sodium atoms don't sound very likely - I'd expect positive ions and free electrons.

Brown observed the strong resonant scattering of solar photons at the Fraunhofer D1 and D2 lines (589.0 and 589.6 nm in the rest frame) in the Io torus in 1972. For a nice summary, go to Io Plasma Torus

martin

[Littlebit]

I was kinda hoping Cassini would find a couple of big composite brake shoes on either side of the ring. This sounds more like Enceladus is throwing ice cubes in the blender