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Posted by: angel1801 Mar 19 2006, 06:18 PM

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.

Posted by: dilo Mar 25 2006, 10:05 AM

Not so close, but very intriguing long-exposure image of Enceladus embedded in it's E-ring...
http://saturn.jpl.nasa.gov/multimedia/images/raw/casJPGFullS19/N00055895.jpg
and above main rings...
http://saturn.jpl.nasa.gov/multimedia/images/raw/casJPGFullS19/N00055814.jpg

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

Posted by: nprev Mar 25 2006, 11:43 AM

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...

Posted by: dilo Mar 25 2006, 04:11 PM

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):

Posted by: BruceMoomaw Mar 25 2006, 10:14 PM

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.

Posted by: ynyralmaen Mar 26 2006, 12:13 AM

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

Posted by: mchan Mar 26 2006, 05:22 AM

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

Posted by: BruceMoomaw Mar 26 2006, 05:41 AM

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.)

Posted by: dilo Mar 26 2006, 08:10 AM

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!)

Posted by: BruceMoomaw Mar 26 2006, 10:18 AM

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.)

Posted by: BruceMoomaw Mar 26 2006, 10:51 AM

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.

Posted by: edstrick Mar 26 2006, 11:48 AM

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.

Posted by: ugordan Mar 26 2006, 04:43 PM

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.

Posted by: dilo Mar 26 2006, 10:39 PM

Thanks for the highlight, Gordan!

I think we experienced closest approach.
Look at these interesting links:
http://www.rssd.esa.int/SA-general/Projects/GAIA_files/LATEX2HTML/node124.html
http://aanda.u-strasbg.fr:2002/papers/aa/full/2001/44/aah2819/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

Posted by: dilo Mar 26 2006, 11:03 PM

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):

Posted by: Gsnorgathon Mar 26 2006, 11:39 PM

Say - is that 100kg/sec figure for the total amount of erupted material, or just for the amount that acheives escape velocity? And if the latter, what does that make the rate for the total erupted mass?

Posted by: RGClark Mar 27 2006, 12:42 AM

Someone will check this, but there are 3600 seconds in hour, so 24*3600 = 86,400 seconds in a day, so 365*86,400 = 31,536,000 seconds in a year. Assuming the larger 450 kg/s outgassing rate, this amounts to 14,191,200,000 = 1.4 x 10^10 kg in a year.
Enceladus mass is 1.08×10^20 kg. So the entire mass could be outgassed in less than 10 billion years.
But its mass has a large portion of rock, so the water portion probably would be outgassed in less time than the age of the solar system.


- Bob Clark

Posted by: BruceMoomaw Mar 27 2006, 01:53 AM

The 100 kg/second figure is just for the water that actually achieves escape velocity -- about 1% of the total outflow from the geysers, according to Carolyn Porco's "Science" article. (And a small fraction of that probably comes back eventually and impacts Enceladus again.)

My revised calculations:
Total mass of Enceladus 1.08 x 10^20 kg (courtesy of Dilo, whose figure I've now confirmed in several pieces on the Internet, based on the new Cassini data).

Percentage of that mass which is the rocky core: 57% ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2200.pdf ).

The rest is a matter of simple division -- which, simple or not, I screwed up very, very badly earlier. Fortunately Dilo was here to correct me. That new estimate of 15 billion years to totally exhaust the moon's remaining ice supply is based on a steady loss rate of 100 kg/second -- it could be several times higher overall, especially given that huge eruption indirectly detected by Cassini during its approach in Feb. 2004, which instantly doubled the total mass of the E Ring for several months.

Posted by: nprev Mar 27 2006, 02:01 AM

Fascinating, gentlemen, as always. One major implication seems to be that Enceladus could well have maintained at least this level of activity throughout its history to date....

Posted by: dvandorn Mar 27 2006, 02:18 AM

The problem is, we have no way of knowing how long Enceladus has been expelling mass, or at what mean rate. It's possible (though extremely unlikely) that it *never* had active geysers or plumes until about, say, 50 years ago. It's also possible (and more likely) that it's been venting for its entire lifetime. It's also possible (and probably most likely) that it has had epochs of geyser activity and epochs without -- we just happen to be visiting during an active epoch.

And we have no real way of knowing how long such epochs may last, or what percentage of the lifetime of Enceladus have been active epochs. Or how much mass it may have expelled during active epochs.

And... I seriously doubt Cassini has the ability to answer *any* of these questions.

Sure did a good job of raising them, though, didn't it?

-the other Doug

Posted by: JRehling Mar 27 2006, 02:18 AM

I think by far our best measure of the cumulative lifetime mass lost through eruptions on Enceladus will come from examining the ancient surfaces of the north. If a spheroid loses mass, and part of the surface area remains relatively unmolested, then the loss must be pretty limited. Granted, a lot of Enceladus's crust has been utterly wiped clean and unusual buckling may have hidden some of the contraction, but there see no way to take a big part of the surface of a world with radius X and "stick" it onto a world with radius 3/4 X without annihilating small craters, etc. When someone does "real math" to analyze this, I expect they'll find that the mass could not have shrunk by so much as 20% in the last 4 GY.

Posted by: RGClark Mar 27 2006, 01:34 PM

Are you sure that 100 kg/s figure represents only 1% of the total amount of outflow? I haven't seen anything to suggest the total outflow is 10,000 kg/s.


Bob Clark

Posted by: dilo May 6 2006, 07:34 PM

Between many spectacular pictures taken on May 4, http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=74843 of Enceladus + rings is very intriguing.
Here a zoom based on Wide+Narrow cameras (last is zoomed/enhanced):

Plumes are clearly visible.

Posted by: tasp May 6 2006, 11:22 PM

Would it be worthwhile to look for stellar occulations by Enceladus that could be seen from Keck (or any of the other big 'scopes)?

Might be some ground based equipment brought to bear on pinning down some of the trace constituents that maybe Cassini wasn't equipped for.

BTW, earth based radar has been used to study the surface of Titan, any chance the reflectivity of the plumes would be sufficient for detection?

Perhaps the plumes can be monitored after the Cassini mission.

Posted by: BruceMoomaw May 7 2006, 01:23 AM

No chance at all of detecting them by Earth radar -- they're too rarified for that by (I imagine by 8 or 9 orders of magnitude) -- but it's possible that UV or IR observations with powerful Earth-based or orbiting telescopes may be able to monitor changes in the plumes' total output, at least. After all, it was Earth-based observations that first revealed the E Ring and the fact that there was something peculiar about Enceladus.

Posted by: dilo May 28 2006, 06:04 AM

Beautiful view of plumes and G ring (unfortunately, only half res image):
http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=76958

Posted by: dilo May 28 2006, 06:57 AM

And here a couple of processed versions, in order to see faint details: