[X] My Assistant

[angel1801]

I was using the solar system simulator for Enceladus for March 20, 2006 and I noticed something.

Cassini will be within 391,000km (at 17:00:00 UT) of almost all of the area that is poorly imaged right now. That is the area from about 20W to 150W longitude. Will this area be imaged on this orbit?

If so, one would have about 2km/pixel resolution.

[dilo]

Not so close, but very intriguing long-exposure image of Enceladus embedded in it's E-ring...
http://saturn.jpl.nasa.gov/multimedia/imag...9/N00055895.jpg
and above main rings...
http://saturn.jpl.nasa.gov/multimedia/imag...9/N00055814.jpg

In the last one, the southern plume is barely visible trough enhancement:

[nprev]

Yep...I can see it. Nice, job, Dilo, and thank you!

Does anyone have an estimate (or even a wild guess) concerning the duration and cumulative mass of these eruptions yet? I am beginning to believe that they are more or less constant during the present epoch; if so, how long would it take for this tiny moon to completely disappear--or at least lose enough mass to cease activity?

Almost makes you wonder if Mimas and/or some of the other satellites fell below this critical point at some time in the perhaps not-so-distant past...

[dilo]

Does anyone have an estimate (or even a wild guess) concerning the duration and cumulative mass of these eruptions yet? I am beginning to believe that they are more or less constant during the present epoch; if so, how long would it take for this tiny moon to completely disappear--or at least lose enough mass to cease activity?

Science articles report an estimated water loss on the order of 150-450 Kg/s.
Even if continuated, this would require several billions years to make total consumption of Enceladus mass...

This is another elaboration which better shows faint (smoothed) features and bright, sharpened ones using a color coding (start image is N00055814):

[Guest_BruceMoomaw_*]

Actually, by my recent calculations, it would take several TRILLION years to remove Enceladus' current ice supply -- which means that, unless the geysers were erupting at a stupendously higher rate earlier in the moon's history, they can't have played any significant role in the fact that Enceladus now has an unusually large rocky core as compared to the other Saturnian moons. That core was there from the beginning.

[ynyralmaen]

Does anyone have an estimate (or even a wild guess) concerning the duration and cumulative mass of these eruptions yet? I am beginning to believe that they are more or less constant during the present epoch; if so, how long would it take for this tiny moon to completely disappear--or at least lose enough mass to cease activity?

Although the eruptions seem to have been going at least since Cassini's arrival at Saturn, MIMI instrument observations of the energetic particles show that the plume activity varies on the scale of days to weeks. Cassini only needs to cross the orbital distance of Enceladus to determine this - particles in the radiation belts are lost to the moon and possibly to E-ring particles, and the signatures of these losses have been seen to vary.

The magnetometer team also report that the plume was more active during first of the 2005 flybys than during the other two. Also, UVIS detected an apparent outburst in activity during approach to Saturn in early 2004.

So, the plume activity level isn't constant, but I doubt that Cassini's seen evidence of it varying enough to severely change the estimate of billions/trillions of years to remove its ice supply.
Geraint

[mchan]

Actually, by my recent calculations, it would take several TRILLION years to remove Enceladus' current ice supply...

The Oort Cloud comets -- TRILLIONS of km from the Sun...

Is it just me, or is Bruce trying to one up Carl?

[Guest_BruceMoomaw_*]

Not consciously. Actually, "billions" is a word that comes up with monotonous regularity whenever one tries to discuss the Solar System; but "trillions" is still relatively uncommon in that connection and worthy of note. (Besides, I will never cease to marvel that -- when one remembers the Oort Cloud -- the Solar System actually extends all the way to the halfway point between the Sun and the nearest stars.)

[dilo]

Bruce, I confirm that number is on the order of 10 billion years (Enceladus mass is 1.1e20 Kg)...

About your marvel, sincerly I never believed is possible to have a Sun-bounded comet orbiting 2 light years away... In the solar system past history we had, for sure, many stars passing at lower distance from Sun and they would rip off similar object. So, unless I missed something, the external boundary of Oort cloud must lie at a fraction of light year from Sun. (sorry for the OT!)

[Guest_BruceMoomaw_*]

True; but by "nearest stars" I meant the nearest stars over the lifetime of the Solar System -- which, as you say, is a period during which some stars have brushed much closer to us than Proxima Centauri is right now. (I'll have to look up the latest information on the estimated size of the Oort Cloud, and for that matter of the Kuiper Belt, to make sure I get this part right.)

[Guest_BruceMoomaw_*]

Dilo is right -- somehow, in "my recent calculations", I screwed up big time. Since the current estimate from Cassini data is that about 43% of Enceladus' mass is ice, then, assuming that it has been losing that ice permanently to space at the rate of 100 kg/second (as estimated in Tokar's article in the "Science" issue on Enceladus), then it would take only about 15 billion years for that total mass of ice to disappear. And we know, from Cassini's Feb. 2005 observations, that there are occasional eruptions during which the amount of water vapor expelled by Enceladus dramatically increases.

This puts a whole new complexion on things -- if the moon's geysers have been operating at this rate since the early days of the Solar System, it is indeed true that they have played a major role in reducing its ice/rock ratio.

[edstrick]

My arm waving understanding is that any star approaches within closer than 2 light years is very rare or statistically less than 50% chance over the age of the solar system. I'm not sure on the number but I think it's something rather surprisingly large.

And Oort cloud comets have typical aphelions of a light month or so. Part of the problem is larger orbits interact with the radial tides and vertical gravity field of the galaxy and progressively get peturbed and are eventually lost. That was an essential arguement against the proposed Nemesis sub-stellar object that hypothetically caused periodic extinctions... Such an object in an orbit with a ?27? (is that the number) million year period would last a few hundred million years and be lost to interstellar space.

Sedna... it was being discussed a bit ago in some forum... may not be an Oort cloud object.. it's in way too short period an orbit for an inner-Oort cloud object (as predicted.. NONE are known yet) yet has a perihelion way too far out for solar system planetary peturbations to have pushed it out that far. That's a big reason they are looking at late or just post nebular phase close stellar encounters from another star in the cluster or association the sun formed in.... far more likely than any encounter over the age of the solar system with a random "field" star.

[ugordan]

Another high phase set was taken that really brings out the plumes. Here, 7 images were stacked to improve S/N ratio and magnified 2x. The sun was practically directly below, at the 6 o' clock position. The image on right uses a color map to bring out the extent of the plumes.

[dilo]

Thanks for the highlight, Gordan!

My arm waving understanding is that any star approaches within closer than 2 light years is very rare or statistically less than 50% chance over the age of the solar system.

I think we experienced closest approach.
Look at these interesting links:
http://www.rssd.esa.int/SA-general/Project...ML/node124.html
http://aanda.u-strasbg.fr:2002/papers/aa/f...19/aah2819.html
based on last article, we had 50000 approaches within 1pc during the Solar system history... if they are randomically distibuted, this means at least 1 very close approach below 1000 au

[dilo]

Last Cassini images show very strong plume activity... we have also multi-filter images and here below I made a IR+Green+UV combination (normal and enhanced version):