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Enceladus Plume Search, Nov. 27
David
post Dec 19 2005, 07:01 PM
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I notice that on the Verbiscer map there are two albedo "spikes" (areas of higher albedo extending from the Enceladian antarctic northwards), at about 90° W and 140° W, which the simulation doesn't appear to account for -- at least, not as aggressively as it does some of the other "spikes". There's another high-albedo region on the other hemisphere, about 280°-340°, which is poorly filled in by these trajectories.

I wonder if these gaps could be accounted for by varying the initial velocity of the particles, or by assuming additional particle sources?
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JRehling
post Dec 19 2005, 07:44 PM
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To be part of the spoilsport / peer review on another end: How meaningful is that albedo map? It looks like some dark streaks are relief features -- if ALL of the dark features are albedo features, even ones that were not well resolved, then resurfacing is not a good account of the correlation.

Icy particle snow-out should only correlate with the albedo features if the background albedo of Enceladus is a bit darker due to compositional differences. (Or, less plausibly, due to relief differences, with the bright areas being smothered in a relief-hiding blanket of snow.)

My second note of spoilsportdom would be the general observation that many worlds with tidal heating show patterns with 180-degree periodicity of one sort or another: Europa and Miranda are examples. The correspondence shown in your map would be less amazing if we start with the notion that what's happening on Enceladus would be southern-centered activity of 180-degree periodicity. Namely, it would not be that unlikely that a set of rift complexes would alter the albedo at 30 & 210, and that a snow-out pattern would happen by coincidence to overlie it. It's not LIKELY that it would happen, but the probability of the null hypothesis might not be down to the standard of publication.

Overall, these criticisms add up to the possibility that the albedo of Enceladus (like Dione) is brighter where relief features exist, and that this pattern happens to spatially correlate acausally with your work. That mere possibility is not something that would make your work unworthy of publication -- it's just a possible explanation.

Spoilsporting over... good work!
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Bjorn Jonsson
post Dec 19 2005, 11:07 PM
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It might be interesting to check what happens if the plume sources are moved towards the equator, especially if you want to 'pretend' you don't know where the plume sources are loacted (since the images showing the plumes will not appear on the PDS until late next year). Heck, I might even try this myself now that I have the source code if I can find any time for this.

Interestingly, some of the brightness variations in the albedo map seem to be simply due to a leading/trailing hemisphere asymmetry. However, I wonder if I'm overlooking something elementary when looking at the albedo map because its left edge is much darker than its right edge and the edges are at 180° longitude ??
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tallbear
post Dec 20 2005, 07:34 AM
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QUOTE (jmknapp @ Dec 16 2005, 12:00 PM)
BTW, here's an animation of the incoming Enceladus flyby, Dec. 24:

Enceladus Dec. 24 inbound (2.2MB AVI)

The yellow dot shows where the ORS platform is pointed, based on the SPICE CK kernel. Not sure what the scanning is all about.
*



That observation is a CIRS Thermal mapping of Enceladus using FP3... The masimum phase angled is aobut 105 deg and the sub s/c lat/lon is basically 0/115 during the observation.... while ISS probably has some rider frames that's not the best geometry for viewing small forward scattering plume particles.
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jmknapp
post Dec 20 2005, 12:17 PM
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QUOTE (David @ Dec 19 2005, 03:01 PM)
I wonder if these gaps could be accounted for by varying the initial velocity of the particles, or by assuming additional particle sources?
*


For direct fallout in the north polar area I think it would have to be the latter (additional particle sources) or lower particle elevations (the sim used 75-90 degrees). The sim already uses particle speeds in excess of the escape velocity. Jason did mention that particles from the E-ring are re-impacting Enceladus, so "escaped" I guess is a relative term. Maybe such secondary fallout accounts for the areas you mentioned? I saw an analysis of ejecta from saturnian satellites (caused by meteor impacts) where 80% of the ejecta eventually re-impacted the moon in a long-term sim.

QUOTE (Bjorn Jonsson @ Dec 19 2005, 07:07 PM)
However, I wonder if I'm overlooking something elementary when looking at the albedo map because its left edge is much darker than its right edge and the edges are at 180° longitude ??
*


I wondered about the same thing. I don't have an explanation as to why the 180 degree meridians on the left and right of the albedo map don't match up.

QUOTE (JRehling @ Dec 19 2005, 03:44 PM)
Icy particle snow-out should only correlate with the albedo features if the background albedo of Enceladus is a bit darker due to compositional differences.
*


Yeah, "dark" and Enceladus don't realy seem to go together, do they? Here's an interesting quote from the 1994 Verbiscer/Veverka paper (emphasis added):

QUOTE
The extremely high albedo of this object precludes the existence of impurities in the layer into which light penetrates (Veverka and Thomas 1986, Buratti 1988). The experimental work of Clark (1981) showed that a fractional coverage of montmorillonite grains of frost of only 0.2% resulted in a decrease from 0.98 to 0.90 in the reflection spectrum at 0.65 um. Verbiscer et al (1990) used areal and intimate mixtures to model the surface of icy outer solar system satellites and concluded that the frost itself must be strongly backscattering, unlike any possible terrestrial analogs (Verbiscer and Veverka 1990). The low-temperature, low-gravity conditions on the surface of Enceladus may preserve intricate internal textures of frost grains which produce the remarkable backscattering behavior.

Enceladus and Mimas both have brighter trailing hemispheres, whereas the rest of the major saturnian satellites have brighter leading hemispheres. This effect may be due to the intercations between Mimas and Enceladus and Saturn's diffuse E-ring (Buratti 1988). In addition the southern regions on both satellites are slightly brighter than those at high northern lattitudes.

...

More observations of Enceladus are needed in order to characterize the properties of the surface of this icy satellite. The upcoming Cassini mission to the saturnian system will provide complete spatial and spectral coverage and resolutions will far exceed those of Voyager.


QUOTE (JRehling @ Dec 19 2005, 03:44 PM)
My second note of spoilsportdom would be the general observation that many worlds with tidal heating show patterns with 180-degree periodicity of one sort or another: Europa and Miranda are examples. The correspondence shown in your map would be less amazing if we start with the notion that what's happening on Enceladus would be southern-centered activity of 180-degree periodicity. Namely, it would not be that unlikely that a set of rift complexes would alter the albedo at 30 & 210, and that a snow-out pattern would happen by coincidence to overlie it. It's not LIKELY that it would happen, but the probability of the null hypothesis might not be down to the standard of publication.
*


Could be... would love to see a Cassini-derived albedo map to verify. In lieu of that, maybe I could map the distribution of particles differently (say, with an "isobar" type map) that would show the correspondence better. Seems like the fit in the 210W area is quite good.


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jmknapp
post Dec 21 2005, 12:10 PM
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A couple of questions about the albedo map were raised:

QUOTE (Bjorn Jonsson @ Dec 19 2005, 07:07 PM)
Interestingly, some of the brightness variations in the albedo map seem to be simply due to a leading/trailing hemisphere asymmetry. However, I wonder if I'm overlooking something elementary when looking at the albedo map because its left edge is much darker than its right edge and the edges are at 180° longitude ??
*


and...

QUOTE (JRehling @ Dec 19 2005, 03:44 PM)
To be part of the spoilsport / peer review on another end: How meaningful is that albedo map? It looks like some dark streaks are relief features -- if ALL of the dark features are albedo features, even ones that were not well resolved, then resurfacing is not a good account of the correlation.
*


Dr. Verbiscer responds in an email, where she refers to a pseudocolor version of the map given in the 1994 paper:



QUOTE (Anne Verbiscer @ Dec. 20 2005, 11:14 AM)
If you look at the pseudo-color
map (fig 7) in the 1994 paper, the lower left corner is completely black.
There are no Voyager data which cover this region of Enceladus, so it was
left black.  (The caption probably should have included some indication that
this region has no data.)  If you look just north of there, the green region
matches pretty well with the green region on the right, but further north,
it is quite true that the blue/green on the left does not match the yellow
on the right.

My explanation for those mismatches comes from how the map was produced
from 3 Voyager 1 images and 6 Voyager 2 images.  Voyager 1 obtained only
low resolution views of the leading hemisphere and southern latitudes.
Voyager 2 provided the higher resolution images from the northern
latitudes of the trailing hemisphere.  If I remember correctly, the
leading hemisphere portion of the map came predominantly from a single
Voyager 1 image, centered at longitude 98 deg.  Since the entire leading
hemisphere was visible in this image, it was fairly low resolution and
more importantly, one "edge" was along longitude 180 deg.


She goes on to point out that the photometric model breaks down a bit near the limbs of images.

She continues:

QUOTE (Anne Verbiscer @ Dec. 20 2005, 11:14 AM)
The root of the problem here is the photometric model.  It was done
with virtually no near-opposition (low phase angle) data and no high
phase angle data (nothing higher than 43.5 deg).  Obviously, we have
higher phase angle data now from Cassini, but they are in the PDS
(or still the ISS team hands) awaiting the same modeling smile.gif.  [...]
Cassini will provide the high phase angle observations;
all of the low phase angle observations come from HST and last
January's ground-based observation campaign (when *true* opposition
was reached for the saturnian satellites.... if you were sitting on
on of them looking back at the Earth, you would have seen Earth transit
the Sun.)

One last comment about the albedo map... yes, some of
the dark regions do correspond to topography, as shadows get reproduced
as dark regions, not as they would appear at opposition (zero deg.
phase angle).  Cassini should help out with this problem.


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jmknapp
post Dec 29 2005, 02:56 AM
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Possible plume sighting coming up next month--favorable geometry, except that distance is greater than the Nov. 27th observation. Here is the predicted view:



The solar phase angle will be around 156 degrees. Here's the science plan entry:

BEGIN_TIME: 190898765.184441 (2006 JAN 18 23:25:00 UTC)

END_TIME: 190900565.184442 (2006 JAN 18 23:55:00 UTC)

POINTING_AGREEMENT: S_N_ER_5

PRIMARY_POINTING: UVIS_FUV to Enceladus

REQUEST_ID: ISS_020EN_166W154PH001_PRIME

REQUEST_TITLE: Enceladus spectrophotometry/ phase coverage

REQ_DESCRIPTION: Turn -Y to Enceladus; Do 1x1 mosaic 3 clear + 4 NAC color + 9 NAC polarizer + 4 WAC polarizer filters; turn to WP


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alan
post Jan 20 2006, 01:05 AM
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Check this out.
http://saturn.jpl.nasa.gov/multimedia/imag...7/N00048536.jpg
Is that a curved plume or an internal reflection?
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jmknapp
post Jan 20 2006, 01:33 AM
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QUOTE (alan @ Jan 19 2006, 09:05 PM)
Check this out.
http://saturn.jpl.nasa.gov/multimedia/imag...7/N00048536.jpg
Is that a curved plume or an internal reflection?
*


Wow... right out of the south pole. If real, the curvature might be used to derive the speed?




There's a wispy curved trail off to the right maybe too. The curvature looks about the same as the disk, which is possibly suspect.


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Ames
post Jan 20 2006, 12:59 PM
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Attached Image


Not sure, but could be an arcing Jet. But as we have seen before, image artifacts are difficult to prove/refute unless it we have another image from a different angle/range.

Also a nice diffuse background glow is visable


Nick
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jmknapp
post Jan 20 2006, 01:31 PM
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QUOTE (Ames @ Jan 20 2006, 08:59 AM)
Not sure, but could be an arcing Jet. But as we have seen before, image artifacts are difficult to prove/refute unless it we have another image from a different angle/range.

Also a nice diffuse background glow is visable
*


Here's an overlay of the image and a simulated Enceladus disk:




Maybe the arc is a ghost image of the crescent.

As for other images, here's one that was less overexposed, along with a histogram-stretched version:




Looks like there might be something in the general direction, although more straight in this image.


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Guest_Myran_*
post Jan 20 2006, 10:40 PM
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I think they nailed it, its right at the south pole where it should be. Agreeing with jmknapp about the arc it might be one artifact, but I tend to think that the diffuse part are real outgassing.
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jmknapp
post Feb 3 2006, 01:08 PM
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Interesting abstract for the Lunar & Planetary conference:

Distribution of Icy Particles Across Enceladus' Surface

The abstract describes results from the Visual and infrared Mapping Spectrometer which allowed determination of the sizes of the icy particles covering Enceladus, and the distribution of same. The authors produced a graphic showing the distribution, which matches pretty well (not perfectly) with what I got for the predicted ballistic emplacement of particles from the tiger stripes. Here's their graphic along with my predicted distribution, oriented the same way:



The abstract states:

QUOTE
Our measurements show that the particle size of water ice increases toward younger regions with the largest ones in “fresh” surface material. The smallest particles were generally found in old more or less densely cratered plains and the larger ones in younger tectonically resurfaced areas (e.g. the sulci = ridged and grooved bands). The largest particles (>0.02mm) are concentrated in the so called “tiger stripes” of the south polar area. ... Our findings support the results of [1,7] with amorphous water ice being concentrated in older terrain due to the long-term exposure to incoming radiation, and crystalline water ice in the vicinity of the younger resurfaced regions, esp. the South Pole. Amorphization usually goes along with the destruction of water ice particles, resulting in the decrease of mean particle size.


But given a possible correlation between larger particle sizes and ballistic emplacement from the tiger stripes, might an alternate explanation be that the particles ejected from the tiger stripes that end up ballistically emplaced are larger, and the small particles are a more uniform dust coming from, say the e-ring? Perhaps the largest particles don't have the velocity of the smaller ones, and end up close to their source.


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volcanopele
post Feb 3 2006, 07:42 PM
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QUOTE (jmknapp @ Feb 3 2006, 06:08 AM)
But given a possible correlation between larger particle sizes and ballistic emplacement from the tiger stripes, might an alternate explanation be that the particles ejected from the tiger stripes that end up ballistically emplaced are larger, and the small particles are a more uniform dust coming from, say the e-ring? Perhaps the largest particles don't have the velocity of the smaller ones, and end up close to their source.
*

hmm, I don't see THAT good of a correlation. The correlation with surface age appears to me to be quite robust, particularly in the other figure from that abstract, which clearly shows larger grains in Diyar Planitia and fracture belts in the anti-Saturnian hemisphere and smaller grains in the cratered regions of that hemisphere, indicating a correlation between grain size and surface age.

Can you remind us again what the colors represent in your graphic?


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jmknapp
post Feb 3 2006, 08:51 PM
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QUOTE (volcanopele @ Feb 3 2006, 03:42 PM)
Can you remind us again what the colors represent in your graphic?
*


It codes the distribution from the leftmost three tiger stripes. For example, here's the fallout pattern from just the leftmost one:



Or all three as one color:



I suppose it would still be correlated with the younger surfaces, as the particle fallout would be a resurfacing.


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