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Unmanned Spaceflight.com _ Cassini's ongoing mission and raw images _ Enceladus Plume-Orbital Effects?

Posted by: nprev Dec 6 2006, 03:02 AM

One interesting question that needs to be answered is just how long Enceladus' plume has lasted over geological time. Just out of curiosity, has anybody calculated how much "thrust" is generated by this continuous outgassing?

Reason I ask is that there just might be enough cumulative effect to cause axial instability, and therefore possibly explain some of the moon's odd geology. Perhaps the hot spot is migratory?

Posted by: ugordan Dec 6 2006, 08:17 AM

OTOH, the thrust is really, really, really too weak to change the moon's orbit. I'd say the plume is the result of strange stuff happening inside Enceladus, not the cause. It's the chicken and the egg problem.

Posted by: tasp Dec 6 2006, 02:19 PM

If most of the plume follows ballistic trajectory and recontacts the surface, there is that much less thrust.

If the south polar area is consistently the outgassing site, the cummulative effect is to be pushing on the Encelodosian orbit at a right angle. As Enceladus goes 360 degrees around Saturn, any effect on the orbital inclination will be canceled out, assuming constant average thrust around the orbit.

If materials are escaping the Saturn system primarily in the same 'southerly' direction, then the plumes are working against the mass of the entire Saturnian system.

I suspect the total effect over the life of the solar system to be infinitesimal.

Posted by: dvandorn Dec 6 2006, 07:25 PM

Bringing it down to spacecraft terms, though, while the outgassing won't have any appreciable affect on Enceladus' orbit (translation), how about its orientation in regards the rest of the Saturnian moon system (attitude)?

It takes a lot less energy to change something's attitude than to change its orbit...

-the other Doug

Posted by: ugordan Dec 6 2006, 09:08 PM

That, too, will be miniscule. The "thrust" is fixed to Enceladus so it amounts to so little over the course of an orbit in the sense that it effectively cancels out the momentum (half the orbit is pushing the other way). In any case, Saturn's tides would overwhelm the small nonzero resultant momentum change.

Posted by: nprev Dec 7 2006, 07:14 PM

Asked and answered--good stuff, gentlemen, thanks. I was unsure just how energetic this process really is.

Posted by: mchan Dec 8 2006, 03:55 AM

QUOTE (ugordan @ Dec 6 2006, 01:08 PM)

Agreed miniscule. But just a nit, the vector of the "thrust" in this case appears to be orthogonal to the plane of the orbit so there is no dependency on the position within the orbit.

Posted by: ugordan Dec 8 2006, 07:58 AM

Yeah, but that orthogonal portion of the vector was already covered by tasp when he showed that its effect will cancel out. I was aiming at small offsets in the vector and in case it didn't precisely go through the center of mass of Enceladus (in which case it would also exert a torque, apart from orbital effects).
On second thought, I'm not sure this other effect would cancel itself out over the course of an orbit, but it certainly is infinitesimally small not to make any difference at all. For example, let's say the polar plumes are as "powerful" as Saturn V first stage engines -- 3500 tons of thrust. Apply this to a moon 1.08 ×10^17 tons in mass and you can clearly see where that'll get you. Nowhere. The delta-V amounts to 0.00001 m/s after one year. After 100 000 years you get a whopping 1 m/s delta-V of the entire moon. This is total delta-V, the torque effect would be even weaker than this.

Posted by: ngunn Dec 8 2006, 01:01 PM

Tasp dealt with the component perpendicular to the orbit also. It doesn't 'cancel out' but it does have to act against the whole mass of Saturn so any effect will be infinitesimal. I found that statement surprising at first but it makes sense if you think it through. Also (therefore) for it to be non-zero some of the ejected material would have to escape the whole Saturn system, not just the moon itself.

Posted by: tty Dec 8 2006, 09:13 PM

QUOTE (ugordan @ Dec 8 2006, 08:58 AM)

The delta-V amounts to 0.00001 m/s after one year. After 100 000 years you get a whopping 1 m/s delta-V of the entire moon. This is total delta-V, the torque effect would be even weaker than this.

That may sound completely negligible, but it would amount to 10 ms-1 per million years and 45 kms-1 over the age of the solar system.

tty

Posted by: ugordan Dec 9 2006, 12:53 AM

Ahh, but it is totally insignificant to the orbital evolution of the moon. Much larger perturbations occur for example from Saturn's tides and orbital pumping such as orbital resonances with other moons. Both of which have been going on for billions of years. No one is suggesting this outgassing lasted 4.5 billion years so that's really pushing the extrapolation a bit too far. A large impactor during the past (and there were certainly enough of those) influenced Enceladus much, much more than puny south polar plumes can.

Posted by: dvandorn Dec 9 2006, 03:24 AM

Yes, but...

I seem to recall that various analyses of Enceladus and the location of its "hot spots" indicated that the entire moon may have heeled over during its lifetime, repositioning its polar areas to equatorial regions and vice-versa. If the plumes were originally pushing at Enceladus from, say, 20 degrees south, could they have (very slowly) pushed the entire body around such that the plumes are now coming from the south pole?

-the other Doug

Posted by: ugordan Dec 9 2006, 11:56 AM

IIRC, the diapir-induced orientation of the moon had to do with tidal effects, not plume thrust effects. Also, if the plumes really imparted rotational torques, why would the rotation stop at the south pole?