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Water plumes over Europa
ngunn
post Dec 15 2013, 09:02 PM
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I see only two references to Galileo in that article:
And the Galileo spacecraft discovered a weak 'induced' magnetic field, caused by the interaction of Jupiter's magnetosphere with a highly-conductive layer beneath the crust, most likely the liquid ocean.
and
Galileo didn't really cover the poles during its 11 passes of Europa, so it doesn't really help us here.

Nothing about plumes there, so what am I missing? Maybe there is relevant comment in his blog, just not in that particular article?? I'd really like to know if there is indeed something worth following up.

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scalbers
post Dec 15 2013, 09:05 PM
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True any plumes aren't mentioned in the blog article, just in a comment by someone who posted they may have seen something in the Galileo data. We might check with Paul Fiesler further about his comment. I would speculate that maybe some more distant high phase SSI images could show something and they may have yet to be examined in sufficient detail. UVS would be another instrument to check.

Below is his quote from the Planetary Society blog comment...

................................................................................

Paul Fieseler: 12/12/2013 08:05 CST

Oh my. I think that I may have seen a plume in Galileo data from one of the later Europa flybys, and I didn't realize what I was seeing until now....

................................................................................

With SSI, this image catalogue mentions some high-phase and plume search observations:

http://lasp.colorado.edu/JUPITER/CH15/EuropaGLLSSITable.pdf


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Bjorn Jonsson
post Dec 15 2013, 10:00 PM
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I think plumes are extremely unlikely to be present in the Galileo SSI data. The images aren't that many and I'm sure every image (including the low-res and/or high-phase ones) have been carefully analyzed by lots of people. If there are any possible hints of plumes they would be highly ambiguous.
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machi
post Dec 16 2013, 01:37 AM
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I looked at Galileo images of Europa and I found only one interesting image.
It's c0484888253 from E19 flyby. It has very low compression and it shows something like haze in the northern polar region of Europa (>70N latitude).
I compared this image with another ones from same flyby and no other images is pointing to the same direction and not surprisingly they show nothing.
One of them is for comparison in this brightness enhanced version.
Attached thumbnail(s)
Attached Image
 


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tedstryk
post Dec 16 2013, 01:40 AM
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Fieseler is the one who was behind the star tracker discovery of objects around Amalthea. I wonder if it is in the engineering data from that instrument.

QUOTE (machi @ Dec 16 2013, 01:37 AM) *
I looked at Galileo images of Europa and I found only one interesting image.


I noticed that while I was putting that mosaic together and assumed it to be a double exposure of some kind.


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machi
post Dec 16 2013, 01:47 AM
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Yes, I think that's the most plausible explanation. But it's interesting that it's only in this image with so short exposition (6.25 ms).


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JRehling
post Dec 16 2013, 05:47 PM
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I notice that the putative haze in the haze image is brighter on the left side, where the surface of Europa is also brighter. That reinforces the double exposure hypothesis.

It seems exceptionally unlikely that such a tenuous plume could be visible with the Sun behind Galileo in an image with the contrast set to show detail on the surface of Europa, which has an albedo of 0.7.
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Bjorn Jonsson
post Dec 16 2013, 08:37 PM
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I agree, looking closely this looks like a double exposure. In addition to being brighter on the left side it's also parallel to the horizon. The exposure, viewing geometry and lighting geometry also isn't particularly favorable for detecting plumes. Here is a schematic view showing the context for image 484888253 at 30 times Galileo's field of view:

Attached Image


The small gray box at center shows the camera's field of view.

QUOTE (machi @ Dec 16 2013, 01:47 AM) *
...But it's interesting that it's only in this image with so short exposition (6.25 ms).

In one way this image is 'special': It's the last one in the sequence of 400x400 pixel short exposure image. A 4 minute pause followed before the next sequence of images started (800x800 pixel images). I don't know the details of how Galileo's camera was operated so I don't know if this is of significance - doubt it though.
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ngunn
post Dec 16 2013, 09:24 PM
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QUOTE (Bjorn Jonsson @ Dec 16 2013, 08:37 PM) *
I agree, looking closely this looks like a double exposure.


I agree too. In fact I think individual features in the ghost image can be matched with features on the limb. The displacement between the two is approximately parallel to the top of the image. It's just a fainter copy of the main image shifted to the left. How that might arise I have no idea.
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nprev
post Dec 16 2013, 10:46 PM
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I kinda said this earlier, but it it possible that the radiation environment dissociates plumes into atomic constituents almost immediately (well, at least in too short a time frame to catch the molecular effluent in action using the imaging systems that have been close enough in the Jovian system to date)?

Given the apparent volume of the ground-based plume image & even if the activity is highly sporadic it sure seems like we should have seen something visible long before. Io vents mostly sulfur compounds; lots heavier than hydrogen & oxygen, and with colorful allotropic states to boot.

Not asserting this as a theory, just throwin' it out there for consideration.


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JRehling
post Dec 17 2013, 11:44 PM
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I'm curious as to what would be the half-life of an H2O molecule in a plume over Europa, given the dissociation that radiation can cause.

This paper:
http://people.virginia.edu/~rej/papers09/Paranicas4003.pdf

Derives for the surface, an "average integrated column production rate of H2O2 in icy regions" of ~5 × 10^10 H2O2/cm^2/s.

The recent report is of 7 tonnes of H2O per second, which spends about 1200 seconds between ejection and falling back onto the surface. That means 2x10^29 molecules of water being ejected per second, so at any time there should be about 3x10^32 molecules of water in the plume.

The plume has an area of about 2x10^25 cm^2, so radiation hitting that area of surface would radiolyse about 1e^36 molecules of water per second, quite a bit more than what is in the plume. But the final factor to take into account is how much of the radiation would actually hit any molecules in the plume, which would seem to be a slim minority since it's mainly empty space, whereas all charged particles hitting the surface will hit some molecule very soon.

Given 3x10^-10 for the span of a water molecule, the plume has a total cross section of about 2.5x10^13 m^2, which means it's only about 10^-8 of the plume, so the radiolysis rate should be more like 1x10-28 molecules per second. The plume has 30,000 times that number of molecules in the air for 1200 seconds, so it looks like the amount radiolysed should be several percent, but less than half. And, as I noted earlier, the effect would be proportionately less closer to the plume's origin.

This sort of computation merits peer review if anyone's interested.
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vexgizmo
post Dec 18 2013, 02:02 PM
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QUOTE (ngunn @ Dec 16 2013, 03:24 PM) *
I agree too. In fact I think individual features in the ghost image can be matched with features on the limb. The displacement between the two is approximately parallel to the top of the image. It's just a fainter copy of the main image shifted to the left. How that might arise I have no idea.


Indeed!

Phillips, C.B., A.S. McEwen, G.V. Hoppa, S.A. Fagents, R. Greeley, J.E. Klemaszewski, R.T. Pappalardo, K.K. Klaasen, and H.H. Breneman. The search for current geologic activity on Europa. J. Geophys. Res., 105, 22,579-22,598, 2000.

"An interesting side note is that one of the images taken in
another imaging sequence on orbit E19 had what appeared upon
initial inspection to be a limb haze just off the bright limb of
Europa. frame s0484R88253 is shown in its raw, unprocessed
(just contrast-enhanced) form in Figure 2a, and a cutout of just
the limb, with a hard stretch, is shown in Figure 2b. The
potential limb haze is visible in figures 2a and 2b as a bright
feature paralleling the limb ~100 km above the surface, at a
brightness level -7% of the average surface brightness. There
was originally much guarded excitement when this image was
received on the ground, but the fact that the "haze" brightness
seemed to exactly parallel the limb brightness, and that the
"haze" was not visible in immediately adjacent images, led the
Galileo engineering team to search tor another possibility. The
match between haze and limb brightness patterns suggests the
possibility of a double image or "ghost image." This possibility
is demonstrated in Figure 2c, which shows a simulated ghost
image constructed by offsetting and adding a dimmer version of
the actual image in 2b, shifted 16 pixels to the left.

"Examination of the imaging sequence and the operation of the
SSI camera itself reveals a likely cause. The image was taken in
the Al8 camera mode, which has a fast frame time and thus does
not reset the charge-coupled device (CCD) detector by
performing a full light flood and erasure cycle in between
exposures [Klaasen eta!., 1997]. This mode also has a reset of
the shutter blades 0.2 seconds before the exposure begins. In all
other imaging modes, the light flood and erasure take place
between the shutter reset and the exposure, but since this
particular mode has no light flood, this docs not occur. The
location and brightness of the offset "ghost image" in frame
s0484888253 are consistent with a small light leak equivalent to
about 0.5 ms of exposure during the shutter reset stage of image
aquisition, which occurred during a slew from the position of the
previous image to this position. The direction and speed of the
slew are consistent with the position of the "ghost image.'' The
light leak would not be noticeable unless the many conditions or
this image were met, namely, the platform slewed from one
position to the next; the exposure time was short enough that the
slight light leak was visible next to the full image; and the image
contained a high-contrast feature (the limb) against which the
ghost image is obvious. The last two frames of the first swath of
plume search images (s04R4889846 and s0484889849) also show
a ghost image of the limb that is consistent with the shutter reset
light leak theory. Only four other images taken during the
Galileo orbital mission have the characteristics necessary (camera
mode, high-contrast boundary, short shutter time, platform
slewing) to detect ghost images produced during the shutter reset;
of these, ghost artifacts consistent with this theory were detected
in three of them."
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JRehling
post Dec 30 2013, 07:10 PM
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Coincidentally, I'd already asked for the very nice "Europa" book from Arizona State, edited by vexgizmo and others, as a Christmas present before this discovery (I'd previously read some of it in the form of PDFs available online) and with it now in hand, one of the first things I looked for was information about carbon on Europa.

CO2 is one of the detected components and the estimated abundance is 360 ppm in certain locales. There are many interesting points to make about that:

1) Trace amounts of CO2 should not be stable on Europa's surface for long time scales, and it's obvious that Europa's surface is nearly uncratered, and thus has been reworked through processes much faster than impacts could emplace new exogenous carbon, so whatever is present should be in some sense coming from below, even if its origin was previously through impact.
2) Spectroscopy only tells us about the immediate surface, so we don't have any source of information regarding carbon below the first cm or so, but point (1) gives us some reason to believe that the subsurface abundance would be the same or greater below the immediate surface.
3) IR spectroscopy gives us a potentially incomplete account even of the surface composition, much less the subsurface. The total carbon inventory can be greater than that indicated by CO2.
4) Even given the CO2 figure alone, that implies an abundance of carbon of about 100 ppm by mass.
5) The Earth's oceans have a carbon abundance of 28 ppm. The Earth's crust has a carbon abundance of somewhere between 200 and 1800 ppm (Wikipedia offers many sources).

At the very least, the presence of carbon looks promising.

I wonder about the prospects of using occultations to study the composition of plumes from Earth. The most common kind of occultation study, generally, is to capitalize on circumstances when the body in question (here, Europa's plume, much smaller, unfortunately, than Europa itself) happens to pass in front of a star. A far more common event would be when Europa passes in front of one of the other Galileans, a technique which has been used to study the Galileans for decades, although the signal-to-noise ratio of such work might be lacking given all the light coming from the two moons as opposed to the tiny amount filtered through the plume. Occultations of Io by Europa have been used to pinpoint Io's volcanic activity, and perhaps Io could return the favor in the near future.

Pluto has been observed occulting stars every 5-10 years or so, and Europa's plume (if active) ought to do so a bit more often than that. That could be the next chance to upgrade our information, likely sooner than any in situ measurement.
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rlorenz
post Dec 31 2013, 05:47 PM
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QUOTE (JRehling @ Dec 30 2013, 02:10 PM) *
CO2 is one of the detected components and the estimated abundance is 360 ppm in certain locales.


It is an interesting coincidence that the abundance in Europa's ice - a pretty secure detection, judging from the literature -is the same as in the Earth's atmosphere when the observation was made.
But this is the only carbon species detected - no carbon-carbon or carbon-nitrogen bonds, despite all the garish red Europa pictures one sees in the popular literature. My point was just because Europa might have water vapor plumes like Enceladus, doesnt mean they are 'rich' plumes like Enceladus' ones.

QUOTE
using occultations to study the composition of plumes from Earth.


Indeed, I'd immediately had the same thought (UV stellar occultations seen by Cassini were among the first probes of Enceladus' plumes). I think it should be possible for Europa from the ground I expect the possibilities from JUICE are being looked at, IIRC it has a UV instrument involving investigators on the HST Europa plume paper.

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dvandorn
post Dec 31 2013, 06:00 PM
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I guess I'll be the one who asks -- how much more or less susceptible to degeneration by the radiation environment are carbon-carbon and carbon-nitrogen bonds when compared to carbon-oxygen bonds?

In other words, since the environment rapidly dissociates a lot of molecular bonds, can the lack of more interesting carbon bonds than C-O be attributable to it?

-the other Doug


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