Enceladus Plume Search, Nov. 27 Options

[jmknapp]

Interesting item in the science plan kernel (S16) just released to the NAIF website:

OBSERVATION_ID: S1629

SEQUENCE: S16

OBSERVATION_TITLE: Plume Search

SCIENCE_OBJECTIVE: Hope to detect/observe plumes, whether from volcanic activity or geysers.

OBS_DESCRIPTION: Point and stare.

SUBSYSTEM: ISS

PRIMARY_POINTING: ISS_NAC to Enceladus (0.0,5.0,0.0 deg. offset)

REQUEST_ID: ISS_018EN_PLUMES001_PRIME

REQUEST_TITLE: ENCELADUS Geyser/Plume Search

REQ_DESCRIPTION: 1;ENCELADUS Geyser/Plume Search 1x1xNPp -- 3 different exposures

BEGIN_TIME: 2005 NOV 27 19:00:00 UTC

END_TIME: 2005 NOV 27 20:00:00 UTC

[volcanopele]

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. And as BruceMoomaw tried to point out, this indicates that Enceladus has a much larger rock fraction than the other satellites (only Dione at 1.4 g/cc comes close; interesting coincidence that density corresponds to geologic activity, hmm).

And good on ya for remembering the secondary spin-orbit resonance, more on that later... But yes, that is one possibility.

jmknapp: regarding the jetting mechanism, there are a couple of hypotheses that may account for our observations. First, the water ice could be heated from below enough to allow for enhanced sublimation, which would carry along with it entrained particles. This is the possibility mentioned by Dr. Porco. This hypothesis has the main advantage of not needing as much heat as the second possibility, so you don't need nearly as much energy. The layer heating the ice from below could be a mix of water and ammonia (and thus heated to 170K or so) and the sublimating ice can still be pure water vapor, as was seen by INMS. The second hypothesis is that there is a pressurized chamber of liquid water which is connected to the surface via fractures, to the tiger stripes. This gas then jets out into space. This possibility requires enough heating to produce pure liquid water, and thus quite a bit more energy would be required. Right now both possibilities, at least as evaluated by late August, are possible, though these new images may require a reassessment. Until we can say one way or the other, we have to keep both possibilities open.

Composition: believe it or not, we ALREADY flew through the plume back in July. So we have INMS measurements of the gases, a UVIS occultation, and CDA data on particle flux and sizes. Summaries of these results, as released thus far, can be found at:

http://photojournal.jpl.nasa.gov/catalog/PIA07723
http://photojournal.jpl.nasa.gov/catalog/PIA03553
http://photojournal.jpl.nasa.gov/catalog/PIA03552
http://saturn.jpl.nasa.gov/multimedia/prod...RM_Esposito.pdf (transcript of the talk can be found at http://saturn.jpl.nasa.gov/multimedia/prod..._transcript.doc )

In summary, what everyone had been calling at atmosphere is actually this plume, not a traditional atmosphere so to speak.

tasp: 1) How fast are these plumes turning Enceladus inside out? Depends on how long this has been going on. We know the mass flux (to within an order of magnitude), so I would presume such calculations could be done. at least to find out how much mass Enceladus could have lost.
2) How much material falls back on to the surface, and how much is permanently lost to the E ring? The material likely escapes Enceladus' grasp and goes directly into the E-ring. however, the bright surface of Enceladus, and its fairly uniform grain size distribution except for very young locations, suggests that Enceladus is coated with E-ring particles.
3) If the Cassini extended mission lasts till the next Saturnian equinox, perhaps photos of Enceladus' shadow on other moons will tell us more too. And Enceladus passing throught the shadows of other moons might help too. All you really need is Saturn really. Would be interesting to see what the plumes look like when Enceladus is eclipsed by Saturn.
4) Any chance of a radio occultation of Cassini's transmissions by the plumes? Maybe in the extended mission, though I can think of better measurements to pull off.

[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.