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

[Guest_BruceMoomaw_*]

The question that comes to my mind is:

If that's the case, then maybe Enceladus started out a lot bigger and has been losing mass -- and size -- for billions of years.  Otherwise, you'd have to think that the activity we're seeing now is relatively rare, and we're lucky to be seeing it...

-the other Doug

This just might explain why Enceladus has the highest density -- and thus the biggest rocky core relative to its size -- of any of the smaller Saturnian moons.

[jmknapp]

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:

"If the mass of a satellite is unknown, but the satellite is differentiated and has a deep mantle of known composition, then we show that measurement of the shape alone can lead to a determination of the satellite's mass, mean density, and moment of inertia. Application of this method to Enceladus, assuming that the satellite has a deep mantle of water-ice of density 0.93 g/cu cm, gives the result that the mean density of the satellite is 1.00 +/- 0.03 g/cu cm. This result fills the one remaining gap in our knowledge of the structure of the Saturnian satellite system.We now know the mean densities of all the primary Saturnian satellites 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.

[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.

[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?