Enceladus Plume Search, Nov. 27 Options

[David]

I'm wondering if it's appropriate to talk about "plumes"? I may have the terminology not quite right, but in hearing "plumes" I think about geysers and things like the Ionian volcanoes, which erupt intermittently. The impression I'm getting is more of continuous venting of material. Do we have any data to support either possibility -- that the steam is being emitted from Enceladus continuously, or that it gets burped out from time to time? The images of the plumes suggested localization, but I couldn't tell if they implied intermittency.

[tallbear]

[quote=jmknapp,Nov 29 2005, 07:18 AM]
We're set for a Christmas rerun:



THE DEC 25 OBSERVATION IS NOT GOING TO HAPPEN

[Guest_BruceMoomaw_*]

One thing to keep in mind is all those dark speckles flocking around the fresher-looking crevasses. They positively jump out at you in many of the better photos, and http://www.aas.org/publications/baas/v37n3/dps2005/450.htm confirms that the team has been thinking the same thing I thought about when I first saw them:

"Among the most mysterious newly-discovered features are small, sub-kilometer-sized dark spots and circular pits that sometimes cluster in a honeycomb like patterns near faults and scarps. Their origin is unknown, but perhaps the pits and dark spots identify sites of explosive venting of subsurface volatiles through fractures or volcanic conduits."

Surely these are vents, in which case the plumes are actual expelled liquid material rather than just water vapor boiling off the surface of an area of warmed solid ice. As for their darkness: I understand that there's probably a small amount of methane mixed with the liquid, and thus some of it is mixed in with the part of that expelled liquid that refreezes into ice immediately around the top of the vent -- after which solar and Saturnian radiation could turn the trapped methane into dark carbon compounds (or just plain elemental carbon), as is supposed to be the case with Pluto's dark patches.

[jmknapp]

[quote=tallbear,Dec 2 2005, 10:14 PM]
[quote=jmknapp,Nov 29 2005, 07:18 AM]
We're set for a Christmas rerun:
THE DEC 25 OBSERVATION IS NOT GOING TO HAPPEN

[/quote]

Oh... pity. Are they too busy with Titan approach stuff at that time?

[The Messenger]

Regarding Enceladus' density, Wikipedia quotes the current Cassini-derived estimate of 1.61 g/cm^3.

Conversely, in 1994 an article was published in Icarus where the abstract states:

"Using the observed shape alone, without any other assumptions other than that the satellite is in hydrostatic equilibrium at its present orbital radius, we place an upper bound on the mean density of 1.12 +/- 0.05 g/cu cm. Thus, the mean density of Enceladus is probably little more than that of water-ice and we conclude that this satellite is markedly deficient in rock."

My how things change.

The abstract continues:

...We now know the mean densities of all the primary Saturnian satellites [/b] in the sequence from the coorbital satellites, Janus and Epimetheus, through to the outer satellite Iapetus (the densities of the small, secondary satellites in Trojan-type orbits are still unknown)."

Gotta be careful of the "we now knows" I guess.

I keep arguing with Bruce, with everybody really, that these constant revisions of solar masses and/or gravity anomalies are symptomatic of a weak second-order gravimetric effect that is STILL causing gross underestimates of outer planet and moon masses.

This is why it is so imparative that Cassini makes the gravity runs as scheduled. If the concept is correct, every moon of Saturn will yield unrealistic gravity anomalies upon closest-approach - much like Ganymede, only worse.

[ugordan]

Since I haven't seen anyone else do it, here's an animated GIF I put together from the 4 images that used the same exposure (which best shows details in the plumes). The images were resized to compensate for the changing distance. The GIF rapidly runs back and forth and vividly shows the parallax effect of Cassini's viewing geometry. I don't believe the changes in the appearance of the plumes are due to their temporal variability, they appear to be constant.

If you concentrate on the Saturnshine lit right side of Enceladus, you get a better feel of the rotating view. Since we're viewing the tiger stripes broadside (so they're nicely sorted out distance-wise), I think it's fairly obvious that we're seeing plumes from all the tiger stripes because the ones farther from the limb would exhibit more parallax than the near ones. It's evident there's a wide range of parallax motion. Most of the plumes seem to be located on the far side of the point around which Cassini's view is rotating, which seems to fit with the fact more tiger stipes actually are on the far side of the limb.

[Decepticon]

^ Now thats cool!

Nice job.

[David]

I don't believe the changes in the appearance of the plumes are due to their temporal variability, they appear to be constant.

If you concentrate on the Saturnshine lit right side of Enceladus, you get a better feel of the rotating view. Since we're viewing the tiger stripes broadside (so they're nicely sorted out distance-wise), I think it's fairly obvious that we're seeing plumes from all the tiger stripes because the ones farther from the limb would exhibit more parallax than the near ones.

Thanks! That is a very helpful summation of the images. But looking at it, I get the impression that the three bursts that I can associate with the tiger stripes are actually double -- which you might expect if the bursts are not coming from the middle of the stripes, but in parallel lines down each side of the stripes. Is this possible, or is it just an illusion?

[jmknapp]

I'll try to answer some of the questions posed in the last day:

ugordan: the escape velocity used by Views of the Solar System is based on the Voyager-era derived mass of Enceladus.  As jmknapp pointed out, the closer Cassini flybys have really helped to pin down the mass, which proved to be much higher than expected.  Thus the calculated density rose from 1-1.1 g/cc from the Voyager-era mass to 1.61 g/cc using the Cassini-derived mass.  So the escape velocity is 240 m/sec. 

Thanks for addressing that. When ugordan mentioned that the escape velocity might be lower once the escaping object came under the influence of Saturn, I thought it was a good point. But I guess that since Enceladus is in free fall already around Saturn, the latter is out of the picture gravitationally for such calculations & the escape velocity remains at 240 m/sec.

I did calculate what the gravity towards Saturn of a particle on the surface of Enceladus would be, and was surprised to see that Saturn has almost six times as much gravitational pull on said particle as Enceladus itself does.

ugordan: nice animation. That must have taken quite a bit of care to align the images.

[volcanopele]

THE DEC 25 OBSERVATION IS NOT GOING TO HAPPEN

Unless you have heard something I haven't, I wouldn't exactly put the nail on that coffin JUST yet. That may change...

[mars loon]

Since I haven't seen anyone else do it, here's an animated GIF I put together from the 4 images that used the same exposure (which best shows details in the plumes). The images were resized to compensate for the changing distance. The GIF rapidly runs back and forth and vividly shows the parallax effect of Cassini's viewing geometry. I don't believe the changes in the appearance of the plumes are due to their temporal variability, they appear to be constant.

Thats beautiful. thank you

[JRehling]

I keep arguing with Bruce, with everybody really, that these constant revisions of solar masses and/or gravity anomalies are symptomatic of a weak second-order gravimetric effect that is STILL causing gross underestimates of outer planet and moon masses.

I'm glad that you specified "with everybody really". Bruce is not a messiah of the physics establishment, just part of the congregation.

What you suggest is a major revision in physics. What is the track record of major revisions in physics proposed on Internet chat rooms? Indulge me a moment to address the medium and not the message: You have a better chance of winning the lottery, being hit by lightning, and bitten by a rattlesnake all at the same time than revising the field of physics rather than being dead wrong.

To address the content in one small way, if the mass of Jupiter were grossly underestimated, then the Galileo probe would have arrived at grossly higher velocities than expected, and it would have undergone grossly higher accelerations and temperatures -- destroying it.

To address the content in a larger way: If you have something that wouldn't be shredded in a moment by a competent physicist, you should submit it to a peer-reviewed publication. Yes, prevailing mindsets can be resistant to new ideas, but if the prevailing physics is dead wrong and yours is dead right, then everyone would be obliged to accept your idea. To the contrary (to address your content in one more way), if the prevailing physics was highly erroneous in these matters, it would not be remotely possible to have a tour like Galileo's and Cassini's keep putting the probe in the desired location on orbit after orbit. If Ganymede's mass were very much different than expected, Galileo would have left the desired path very quickly and by very much. It didn't. So I think you have an idea that only avoids refutation if you keep it vague, nonquantitative, and sheltered from serious critique. And as such, it subtracts from any forums in which you discuss it.

[tfisher]

Hear, hear, JRehling. Standard physics is well tested far past the point where any anomalies could be identified with Cassini trajectory. Frankly, any speculation about modifications to gravitational theory are pointless unless general relativity is the starting point. Messenger, I'm guessing that you don't have that background? If you do, great, start a thread and throw out some equations. I'm about to finish a Ph.D in mathematics, and I've always wanted a good excuse to learn GR at a deeper level. I'd be happy to follow along your line of thinking. Otherwise, maybe leave the unfounded (and misguided) speculation elsewhere?

[ugordan]

Thanks for addressing that. When ugordan mentioned that the escape velocity might be lower once the escaping object came under the influence of Saturn, I thought it was a good point. But I guess that since Enceladus is in free fall already around Saturn, the latter is out of the picture gravitationally for such calculations & the escape velocity remains at 240 m/sec.

Don't be so quick to dismiss my reasoning. If your argument held, it would mean Enceladus would not experience and tidal effect due to it being in free fall towards Saturn. That neglects the effect of there being quite a reasonable difference between the pull on the near and the far side of Enceladus. You can't apply an inertial system and expect to have all measured forces accounted for. You said it yourself, Saturn's gravity is much stronger than Enceladus' and only because *locally* the centripetal force of Encleadus' orbit cancels the much stronger Saturnian gravity doesn't mean the effect holds at any other distance. If the particles can get just far enough that their orbital speed (which they basically inherited from Enceladus) is too small or too large (if the particles are directed away from Saturn), they will quickly escape from Enceladus. As I said before, a particle needs not 240 m/s to escape to a height of 500 km (yes, I pulled the figure out of thin air) above the moon and be essentially free. The escape velocity is just a theoretical value, applies only to a one-body system with no other forces in play.

I hope you can now see where I'm getting at.

Oh and about the GIF, thanks, but really it wasn't all that difficult to do...

[ugordan]

I probably misunderstood what The Messenger was trying to say, but I got the impression he wasn't suggesting new physics. I thought he meant that we're hasty to deduce a moon's mass from a single gravitational run or so, assuming the mass distribution is uniform, having no lumps as in Ganymede's case. In this case, if we were to fly above such a lump, we'd assume a greater overall density for the moon. Hence the need for more gravity passes to establish a good understanding on the moon's interior.

But, I might be wrong, Messenger might have indeed proposed all new physics and I got him completely wrong...