Enceladus Plume Search, Nov. 27 Options

[edstrick]

The Voyager estimates of masses and densities for some of the moons were very marginal. The actual dataset used was the tracking data from 2 flybys, plus the rather noisy Pioneer 11 flyby a year before Voyager 1. (Pioneer was very close to Solar Conjunction and the signals were going through the solar corona.)

Added to the spacecraft tracking (and position location in images) was the historical record of optical observations of the moons. Precision is low, but the long time base gives moons time to show subtle orbital interactions due to resonances. Without checking old xeroxes, I think Dione's mass was rather well determined from it's resonant orbital mechanics effect on Enceladus.

The larger inner moons masses were relatively well determined, the smaller ones were more marginally measured. Cut a moon's diameter in half and it's mass is 1/8'th the original. Iapetus, Hyperion and Phoebe were so far out they interact little and they were all far from Voyager's trajectories.

I don't know how the final pre-Cassini mass estimates compared with analyses from 15-20 years earlier, but in all of those cases, to some extent, the error bars on mass estimates are educated guesses, based on adding random noise to models to judge sensativities for each case, and based on in part very educated guesses on the level of systematic errors in the data.

Cassini's long orbital tour gives more ability to separate weak gravitational effects on the trajectory and better separate different satellites masses, giving on it's own a better set of data for moon mass estimates. And for any satellite, a really close flyby gives the first chance for a really accurate mass, and for all but the outer sats, a chance to crudely estimate the difference between the oblate and tidally elongated shape of the satellite and of it's gravity field. A moon that never differentiated and has "rock" and "ice" uniformly distributed from core to surface will have a gravity field that matches it's shape. A moon that fully melted and has all the "rock" as a well defined core will have it's physical shape more flattened and stretched than the gravity field, as the deeply buried rocky core's gravity will be nearly spherical. Local gravity anomalies, if found, are "icing on the cake", datawise.

But the main point here is that searching for any deep meaning in differences between pre-Cassini moon masses and the new results is an effort in futility.

[jmknapp]

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's why I included the caveat "for such calculations," meaning for escape velocity. Tidal effects are higher order.

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.

The orbital speed of Enceladus relative to Saturn is 12,640 m/sec. Seems like 240 m/sec (less than 2% of the total) is a reasonable amount needed to "de-orbit" from Enceladus, so to speak.

[jmknapp]

But the main point here is that searching for any deep meaning in differences between pre-Cassini moon masses and the new results is an effort in futility.

To me the most interesting aspect is that the 1994 peer-reviewed Icarus paper (link) quoted an *upper bound* of 1.17 g/cm^3 for Enceladus and used sweeping language like "we now know that..." based on presumably scientific methods. So while physics itself may not be changing the quality and reliability of analysis varies quite a bit.

[ugordan]

The orbital speed of Enceladus relative to Saturn is 12,640 m/sec. Seems like 240 m/sec (less than 2% of the total) is a reasonable amount needed to "de-orbit" from Enceladus, so to speak.

Again, what I'm stating is that it's very likely even a much smaller delta-V is sufficient to escape from Enceladus. I don't know how much smaller, but my gut feeling tells me it out to be a significant reduction. Not and order of magnitude, but 2 or 3 times smaller might seem reasonable.
I might program a simple simulation of particle trajectories of several different speed ranges, but I'm way too lazy to do that at the moment.
Especially if it turns out someone else already did just that...

EDIT: A crude method might be using Orbiter and setting your location on the Enceladus' south pole and applying a short burn upward and seeing where it gets you. Of course, you'd need to change the current best mass estimate in the config file first. I'll play around with it a bit later, I'm currently at work...

[JRehling]

No one has mentioned this yet: A gas might escape from a body without the plumes themselves exceeding escape velocity. Stand atop the LEM and throw a child's balloon of earth air *downward*, and it will pop, and the gas will eventually all escape, despite the downward initial motion.

H2O should simply diffuse away from the source on Enceladus and escape according to exponential decay. Of course, in this case, there is apparently also considerable upward velocity (or the diffusion would be more wide than high), but don't discount gas diffusion as part of the package.

[ugordan]

Gaseous diffusion you're talking about has everything to do with the temperature of the gas. Temperature is a statistical figure describing the average kinetic energy of all the molecules. This translates into an average speed for molecules of certain mass. The average velocity can be much less than the moon's escape velocity, but there will always be a "tail" in the high end of the energy distribution where a small number of molecules will have a speed in excess of the moon's escape velocity. These molecules can escape the moon. Thus the exponential decay, the hotter the gas, the faster the decay is because more molecules have speeds above escape velocity. Statistically, there will be molecules escaping even for very low average energies (temperatures), but it will take a longer time.
I wasn't saying the plumes need to be directed upwards for the vapor to escape, it might as well be warm ice sublimating in all directions. The observed plumes, however, seem to be pretty well collimated.
Whether we're talking about water molecules or ice particles, there exists a lower bound on their speed that will allow them to escape into space. In any case, the theoretical 240 m/s escape figure holds for every direction, though I believe the mechanism I was talking about (Saturn helping out) benefits from upwardly directed plumes.

[tasp]

Would solar UV ionize the gas? Then Saturn's magnetic field would 'sweep' gasses from Enceladus.

Although weak at Saturn's distance from sun, Poynting-Robertson effects (hope I spelled it right) would also be operative on small particles and would accelerate material from Enceladus.

Is the E-ring continuous around Saturn? Or does it 'parse' in the vicinity of Enceladus. (I'm thinking of horse shoe orbits for the E-ring particles)

[ugordan]

Would solar UV ionize the gas?  Then Saturn's magnetic field would 'sweep' gasses from Enceladus.

AFAIK, it certainly would. I think the observation of a massive increase in oxygen atoms prior to Cassini SOI indicates that water is efficiently broken down by UV rays. Still, that mechanism is probably too slow to immediately help with stripping away of the plumes.

Although weak at Saturn's distance from sun, Poynting-Robertson effects (hope I spelled it right) would also be operative on small particles and would accelerate material from Enceladus.

Is that the one with thermal radiation being re-emitted unevenly? I believe that's an even slower effect, especially since water ice particles don't absorb very much light and hence heat up very little.

[jmknapp]

Would solar UV ionize the gas?  Then Saturn's magnetic field would 'sweep' gasses from Enceladus.

Although weak at Saturn's distance from sun, Poynting-Robertson effects (hope I spelled it right) would also be operative on small particles and would accelerate material from Enceladus.

Is the E-ring continuous around Saturn? Or does it 'parse' in the vicinity of Enceladus.  (I'm thinking of horse shoe orbits for the E-ring particles)

I believe the thinking is that particles from the solar wind, or Saturn wind, would sweep away gas particles. But seems like macroscopic ice particles would be a bit more of a problem?

Jason recently posted two excellent links to Dr. Esposito's talk back on August 30. Here is one slide:



see: slides from Esposito talk (PDF)

The accompanying transcript says:

"I’m moving to my next slide, which is about the first clues that Cassini had of something more exciting happening on Enceladus. On the 17th of February 2005 we had a close fly-by of Enceladus, and Cassini’s magnetometer detected a bending of the magnetic field around Enceladus. Now, this may seem a minor or maybe even a negligible discovery, but the magnetic field, like an electromagnet, or the earth’s magnetic field, is excluded by conductors. It looks as though there was a conducting layer surrounding the moon Enceladus, which the investigators hypothesized may have been a result of ions formed by sunlight falling on the atmosphere of Enceladus, if it had one. So the idea was that there might be an atmosphere around Enceladus and light from the sun would be removing electrons from some of the atoms in the atmosphere, making them conductors, and those conductors would exclude the magnetic field from the region around Enceladus. This was a very exciting indirect indication that Enceladus might have an atmosphere, and the project decided to reinvest some of its resources and to re-plan a later fly-by of Enceladus, which was scheduled for July of this year, to fly very, very close to the moon, to fly within 175 kilometers of the surface of Enceladus."

...

"I’m moving to my next slide, which is a demonstration of the effect that was seen by the magnetometer, and what you’re seeing here are these yellow arrows and lines and all that yellow stuff is truly invisible, it’s a representation of the Van Allen belts of Saturn, which are trapped by the magnetic field. As the moon, Enceladus, which you can see on the right, moves around Saturn and moves through that magnetic environment, mostly without any trouble, except if there might be some cloud near Enceladus."

"Here are the artists’ drawings, and you can see on the diagram how the lines of magnetic force have changed their location. The lines of magnetic force are excluded by the neutrals in the cloud, which is shown here near the southern pole of Enceladus; that’s a point we didn’t know yet, so that already in February we had some indications that Enceladus was unusual and the project used some of its capability to readjust the mission to aim for a close fly-by of the moon in the month of July. So, Cassini was redirected to fly within 179 kilometers of Enceladus on the 14th of July 2005."

[jmknapp]

EDIT: A crude method might be using Orbiter and setting your location on the Enceladus' south pole and applying a short burn upward and seeing where it gets you. Of course, you'd need to change the current best mass estimate in the config file first. I'll play around with it a bit later...

I'd be intersted to know how that comes out, but I tried a little simulation myself, modeling the three-body situation of Enceladus, Saturn, and a small particle ejected from the south pole. I used three different initial velocities: 100, 200 and 300 m/sec:





So the escape velocity models out as between 200-300 m/sec as expected.

[volcanopele]

Press Release: NASA's Cassini Images Reveal Spectacular Evidence of an Active Moon
http://ciclops.org/view.php?id=1714

4-frame movie showing plumes:
http://ciclops.org/view.php?id=1702

[jmknapp]

Exploring this ice sublimation theory some more...

If ice is subliming in sufficient quantities to create powerful jets sending water vapor and/or particles into space at hundreds of meters per second, it seem that the pressure at the vents must be very high, well over atmospheric pressure on Earth?

Also, the temperature of the jets must be over 273K, no?

That translates to a lot of heat, which would tend to melt the sides of the tiger stripes where the vents issue.

The Cassini team has released a color image of the area:



Dr. Esposito of the Cassini team referred to the bluish tinge of the tiger stripes in the above image:

The parallel stripes were unexpected from earlier images and show two things; they show that there’s been a recent, we don’t know exactly how recent, many millions and even billions of years, but a recent event which has covered up the southern region of Enceladus and that has stretched the surface so that it’s broken to form these fissures, which we see as tiger stripes.  The color of those features is a bluish hue, and that bluish hue says that the ice that’s in the fissures, in those tiger stripes, that the ice there is relatively young and may be fresh, at least geologically fresh.  So with this information, there is geologic evidence of recent resurfacing and splitting of the icy surface of Enceladus.

Could the hot plumes of water vapor be melting the tiger stripes and entraining atomized water droplets in the stream? Moreover, the plumes would quickly cool and wouldn't that tend to make the water vapor and/or droplets condense as ice or even snow?

Maybe the plumes act as enormous snow machines:



Ski Enceladus!

[edstrick]

"...the 1994 peer-reviewed Icarus paper ..."

The analysis predicting the moon's mass based on it's tidally-deformed shape was probably correct (Peter Thomas is a damn competent researcher), but based on a false assumption. We don't know what that assumption is yet.

The amount of tidal deformation varies with the distribution of mass in the moon. The amount is different if the "rock" in the moon is uniformly mixed with the ices, compared with all the rock forming a core and all the ices forming a mantle. That sort of variation shows up in the difference between their two estimated densities: 1.12 vs 1.00. Clearly, that's not enough to explain the discrepancy.

Another possibility is that Enceladus is not hydrostatically relaxed. There may be an ongoing convection pattern, maybe rising at the south pole and descending at the north (probably more complicated, but..), resulting in dynamically supported topography. The Tharsis bulge on Mars may be supported by a still active, though very slow and sluggish, monster convective plume in Mars' mantle.

What *is* important is that unless there was a booboo in the calculations (can't rule that out), the discrepancy is TELLING us something. The measured shape of the gravity field, compared with the density and more accurate global shape measurements will probably provide enough additional information to blow some candidate explanations out of the (ammonia)water, and make one a "prefered solution".

[ugordan]

I'd be intersted to know how that comes out, but I tried a little simulation myself, modeling the three-body situation of Enceladus, Saturn, and a small particle ejected from the south pole. I used three different initial velocities: 100, 200 and 300 m/sec:

So the escape velocity models out as between 200-300 m/sec as expected.

I played with Orbiter a bit and it turns out you were right. Ejecting from the pole and perpendicular to the orbital plane really doesn't change distance from Saturn considerably so Saturnian gravity more or less remains cancelled out. The situation becomes interesting when you're ejecting the "particle" inwards towards Saturn or outwards. In the case of 100 m/s I quickly winded up back on the surface, but in the case of 200 m/s there was a period of normal deceleration due to Enceladus' gravity and then, some hundreds of km above the surface, the speed started to pick up again and it was clear I wasn't going back down.

In short, I probably underestimated the strength of Enceladus' gravity, though it's obvious things get interesting well below nominal escape velocity if the emission direction is "right". The plumes are however neatly emitted from the pole so this effect really doesn't come into play.

[jmknapp]

The situation becomes interesting when you're ejecting the "particle" inwards towards Saturn or outwards. In the case of 100 m/s I quickly winded up back on the surface, but in the case of 200 m/s there was a period of normal deceleration due to Enceladus' gravity and then, some hundreds of km above the surface, the speed started to pick up again and it was clear I wasn't going back down.

Interesting... just checked & I get the same result. Away from Saturn the escape velocity comes out as about 190 m/s and towards Saturn about 205 m/s.

A space.com article yesterday, Active Moon of Saturn Excites Astronomers , has an interesting bit:

It's unclear what causes the geologic activity, but scientists think it's due to internal heating caused by radioactivity or tides.

But Enceladus is tidally locked to Saturn & so would not experience tides? Or maybe it might... there's a little wobble longitudinally, about 1 degree in extent every Enceladus day, based on current SPICE models. Would that little wobble be enough to generate heating?

The space.com article referred to a talk yesterday given at the American Geophysical Union conference in San Francisco. A list of the planetary science talks being given today includes the following:

Calculation of Loss Rate from Saturn's E Ring

* Leisner, J S (jleisner@ess.ucla.edu) , IGPP/UCLA, Box 951567, Los Angeles, CA 90095
Russell, C T (ctrussell@igpp.ucla.edu) , IGPP/UCLA, Box 951567, Los Angeles, CA 90095
Dougherty, M K (m.dougherty@imperial.ac.uk) , Imperial College, London, Blackett Lab Space & Atmos Physics, London, SW7 2AZ United Kingdom
Blanco-Cano, X (xbc@geofisica.unam.mx) , UNAM, Ciudad Universitaria Instituto Geofisica, Coyoacan, DF 4510 Mexico
Strangeway, R J (strange@igpp.ucla.edu) , IGPP/UCLA, Box 951567, Los Angeles, CA 90095
Strangeway, R J (strange@igpp.ucla.edu) , Imperial College, London, Blackett Lab Space & Atmos Physics, London, SW7 2AZ United Kingdom
Bertucci, C (c.bertucci@imperial.ac.uk) , Imperial College, London, Blackett Lab Space & Atmos Physics, London, SW7 2AZ United Kingdom
Bertucci, C (c.bertucci@imperial.ac.uk) , UNAM, Ciudad Universitaria Instituto Geofisica, Coyoacan, DF 4510 Mexico


Ionization--whether through solar photons, impacting particles, or charge exchange--is a significant loss process in the neutral E ring. When these particles are ionized, they are energized by the electric field associated with the corotating magnetized plasma. If the pick-Up energy is great enough, the particles generate ion cyclotron waves with a magnetic field amplitude that is determined by the energy of the pick-Up ions. These waves, with frequencies near the local water-group gyrofrequencies, were first seen in the E ring by the Pioneer 11 and Voyager 1 magnetometers, but it is Cassini's large coverage of radial distance, local time, and latitude within the E ring that allows us to use this spacecraft's magnetometer to conduct a comprehensive study of these waves. Using a simple plasma profile and modeling the E ring as a mass source in the equatorial plane, we use the ion cyclotron waves to calculate the ionization rate through the ring. We find that between 3.5 and 7 Rs, the E ring loses a total of 6e^26 neutrals, or 18 kilograms of H2O, per second. This total is about one fourth of the amount expected from previous models and the radial erosion rate agrees qualitatively with E ring density profiles, except at 4.6 Rs. Using the ion cyclotron waves, our calculations show a discontinuity in the erosion rate at that equatorial distance and all latitudes. This discontinuity can not be explained with the assumed smoothly varying plasma profiles or production rates.

Assuming that the E ring is in steady state, it must be picking up at least 18 kg/sec from the vents?

Also:

Saturn's E ring as seen by Cassini

* Kempf, S (sascha.Kempf@mpi-hd.mpg.de) , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Beckmann, U , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Burton, M , Jet Propulsion Laboratory, Oak Grove Drive, Pasadena, CA United States
Helfert, S , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Helfert, S , Helfert Informatik, Bits Road, Mannheim, 60000 Germany
Srama, R , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Moragas-Klostermeyer, G , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Roy, M , Jet Propulsion Laboratory, Oak Grove Drive, Pasadena, CA United States
Gruen, E , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Gruen, E , HIGP Univ. Honolulu, Honolulu, Honolulu, Hawaii United States


The Cosmic Dust Analyzer (CDA) onboard the Cassini mission measures the properties of micron sized dust particles in the environment of Saturn. Since its arrival at Saturn in July 2004, the CDA detector recorded many thousand dust impacts within the E ring of Saturn. Data analysis revealed enhanced dust densities until a radial distance of at least 16 Saturn radii. The dust densities and the mass distribution of the ring particles is investigated. Furthermore, compositional measurements indicate a clear dominance of water ice particles in the size range between 0.5 and 2 micro meter. Dust charge measurements are in agreement with Cassini plasma measurements. The discovery of the extended E ring changes the former understanding and modeling based on remote sensing observations. The dust rate measurements during the Enceladus flyby on July 14, 2005, support a strong active dust source on the southern hemisphere of Enceladus.