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marsbug
This seems like the relevant place to post this (could be wrong): Water plumes from Europa? Apologies if it's already been up. The link to the Science article at the bottom doesn't work for me, does anyone have a working link to the original? Cheers.
Hungry4info
Here's a link to the paper.
http://hubblesite.org/pubinfo/pdf/2013/55/pdf.pdf
JRehling
Apparently, Europa joins Enceladus in having active water plumes observable in the present.

http://www.sciencemag.org/content/early/20...science.1247051

This might suggest tailoring exploration plans, already in progress, to focus on the study of these plumes, if they are continuous enough to be active when a putative mission could observe them in situ.
marsbug
QUOTE (Hungry4info @ Dec 12 2013, 05:08 PM) *

Thank you!


Moderator note: Two topics merged. Quote added at the top of this post to make its context clear.
JRehling
The modeled density is comparable to that of plumes at Enceladus. Campaigns to observe plumes over Europa visually failed, which may indicate simply the transience of such activity, although the schedule of the transience becomes extremely interesting. If there is outgassing at some point during every (or nearly every) orbit, then any jovian orbiter with Europa flybys could observe the plume in situ, if the tour is designed appropriately. If the outgassing is rarer, or unpredictable, then that makes in situ observation a challenge.

An extraordinarily interesting possibility here is that the source of Europa's plumes may involve water in contact with a sub-surface ocean floor, which could make the chemistry arbitrarily complex, whereas some models of Enceladus's plumes indicate that the source may be surrounded by more ice on all sides, which limits the possible chemistry.

The lens-melt model of Europa's ice argues that the evidence of surface/melt-through contact occurs between lakes of water which are not in direct contact with the ocean, but exist between the ocean and the top of the ice. This would be of greater interest if the ice that melted to form those lakes had been part of the ocean previously (especially if they were in contact recently).

The proposals for the Europa Clipper mission include a neutral mass spectrometer making flybys of <1000 km to sample the Europa atmosphere. The new discovery, and follow ups, may suggest a different trajectory, but that instrument is already part of the package. The proposals suggest a launch of 2021 or 2022, although that seems tentative.

The possibility of a free-return trajectory sample return bringing some of these plumes to Earth is extremely exciting. A sample return from Europa's surface would be very challenging. The free-return option is much, much more modest in terms of delta-v.
Bjorn Jonsson
I have now taken a quick look at the article. An enhanced emission was detected near 90 west longitude. Here is a quick and dirty orthographic render of Europa's southern hemisphere:

Click to view attachment

If I understand the article correctly the most likely location for the plume source is near longitude 90W in the far south. As the render above shows, most of the terrain in the area of interest isn't very well imaged although there is a narrow swath of good images near 90W.

There are some Europa diagrams near the top in this interesting blog entry at the Planetary Society website but the diagrams are rather fuzzy and it's not completely obvious to me what they are showing or where - possibly the location of a likely source region.

It will be interesting to see how this affects the JUICE plans. Plumes like this one should be easily detectable by JUICE - the problem is if they are transient. As currently planned, the closest approach occurs approximately over Thrace and Thera Macula (in the image above, the two dark spots near bottom at 180W).
nprev
I think this finding dramatically strengthens the case for what would essentially be Galileo Mk 2. It's quite possible that ALL the major Jovian satellites experience transient activity.

Europa is of particular interest, of course, but a mission with broader objectives would be much more likely to fly.
0101Morpheus
I think only Europa is the only moon with a thin enough crust to sustain geysers though. But who knows? Maybe Ganymede had geysers billions of years ago.

I can't wait until JUICE is launched. Absolutely one of the most exciting planetary missions of the next decade.
rlorenz
QUOTE (JRehling @ Dec 12 2013, 04:27 PM) *
An extraordinarily interesting possibility here is that the source of Europa's plumes may involve water in contact with a sub-surface ocean floor, which could make the chemistry arbitrarily complex, whereas some models of Enceladus's plumes indicate that the source may be surrounded by more ice on all sides, which limits the possible chemistry.


But for the chemistry to be interesting, there must be available carbon. There is little evidence of any on Europa, and models of the protojovian nebula have it too hot to incorporate much. So yeah, if sulphates are your thing, then Europa is demonstrably great. Prebiotic chemistry may be a very different matter.
elakdawalla
Wouldn't infalling comets deliver carbon to Europa as they do to other places in the solar system?
nprev
I wonder about that. Certainly the Jovian system gets more than its share of comet & asteroid impacts...what have we seen, two or three since the early 1990s, now that we know what we look for?

I don't see why Europa could not have obtained abundant carbon from these infalls. We seemingly don't have any way to assess its impact history any further back than 10 million years or so, which is not surprising for what seems to be a dynamic surface environment.

I look forward to any sort of exploration of this moon. There are a LOT of questions to be answered, and doubtless we'll have even more later.
Juramike
QUOTE (rlorenz @ Dec 12 2013, 07:38 PM) *
So yeah, if sulphates are your thing, then Europa is demonstrably great. Prebiotic chemistry may be a very different matter.


Which is one of the very reasons Europa might be so interesting. At absolute worst it might all be hydrothermal vents and energy, with no organics.
Compare/contrast with Titan: abuncha cool-o organics, but no obvious hydrothermal system to throw in energy. (proposed in one model, but not yet observed.)

And sulfur/sulfates iron(II)/iron(III) are excellent start points. Anywhere there is a chemical disequilibrium is exciting.
marsbug
A habitable environment without life would be very useful as a comparison to habitable environments with life. Not as headline and budget friendly perhaps, but scientifically just as good.
vjkane
Looking at smaller missions than ESA's JUICE or the proposed Europa Clipper, there have been at least two proposals to explore Enceladus with Discovery-class missions. One, JET would image the tiger stripes with a high resolution thermal instrument and sample the plume chemistry with a duplicate of the Rosetta mass spectrometer (much more capable than Cassini's spectrometer). The other would return samples collected in aerogel a la Stardust. I suspect that the concepts could be easily adapted for Europa with the added advantage of being able to use solar power instead of an MMRTG.

Does anyone know whether JUICE would be lucky enough to encounter Europa at its apojove when the plumes (if confirmed and if persistent) are likely to be active?
elakdawalla
QUOTE (Bjorn Jonsson @ Dec 12 2013, 02:02 PM) *
I have now taken a quick look at the article. An enhanced emission was detected near 90 west longitude. Here is a quick and dirty orthographic render of Europa's southern hemisphere:
Click to view attachment
If I understand the article correctly the most likely location for the plume source is near longitude 90W in the far south. As the render above shows, most of the terrain in the area of interest isn't very well imaged although there is a narrow swath of good images near 90W.

There are some Europa diagrams near the top in this interesting blog entry at the Planetary Society website but the diagrams are rather fuzzy and it's not completely obvious to me ...

Thanks for making that orthographic image. Here is a higher-resolution version of the graphic that I put in that blog entry. It shows a model for stresses along cracks near the south pole when Europa is at apoapsis. Can you identify which specific images contain these cracks?
Click to view attachment
JRehling
There's certainly some evidence of carbon in/on Europa's icy crust in the form of CO2.

http://onlinelibrary.wiley.com/doi/10.1029...031748/abstract

Carbon is relatively rare as a bulk constituent of Earth's crust (<1%). It's even rarer in the oceans (0.0028%), but that doesn't prohibit, some (to say the least) interesting organic chemistry in the Earth's oceans.

Understanding of the non-H2O composition of Europa's surface is pretty crude at present, definitely not accurate to earthlike levels of carbon abundance, further complicated by the fact that the immediate surface which is visible in IR spectroscopy may be different than the near-subsurface. Enceladus, for one, shows a difference between surface composition and plume composition with more non-H2O stuff in the plumes than on the surface.

So I'd say on the issue of Europa and carbon, we have reason to believe that at least some is present. As far as larger quantities go, we have more of an absence of evidence than an evidence of absence.

I think the real interest in the plume discovery is not about what it says about the nature of Europa as much as what it says about the explorability of Europa. It's an extremely challenging target for a lander, much more so if it were necessary to work to get into the subsurface, and borderline impossible for a lander + sample return. A sample return from the plume via a free-return trajectory is potentially quite affordable, if there's adequate assurance that it would arrive while a plume is active. If the science from a Jupiter-orbiting mission is promising, I think the free-return sample becomes a very high priority mission.
nprev
Thing is, it seems as if the plumes are rapidly chemically decomposing upon emission (I'm gonna guess that this is due to the Jovian radiation environment). If the water's already quickly dissociating into atomic oxygen & hydrogen from same, how likely would it be for more complex compounds to survive long enough to be sampled?
MarcF
Emily, do you mean that Astypalaea and Thasus Lineae could be still active ?
JRehling
The duration of exposure to local radiation would be a function of how close to Europa's surface the sample is taken. The material is ejected at about 700 m/s, so it ought to be possible to sample it in less than a minute after it was ejected. How short that duration could be made is a question for the engineers. 10 seconds? 5? The lower the flyby, the tighter the margins of error. As the duration is made shorter, the extent of decomposition would be reduced; that's a question for the (organic) chemists. Certainly some organic molecules are extremely durable. I'd be surprised if 10-30 seconds of radiation would obliterate every interesting molecule, if there are any.

The (very preliminary) proposal for the Europa Ice Clipper mission (not to be confused with the current Europa Clipper mission in development) was for a flyby of 50 km. 20+ years later, we might be able to do a lot better than that. It may be desirable to have a leading element, perhaps on the same launch, to verify the presence and location of plumes right before the collector arrives, and to send back a homing signal as a sort of Jupiter-Europa GPS for the collector.
Bjorn Jonsson
QUOTE (elakdawalla @ Dec 13 2013, 04:19 PM) *
Thanks for making that orthographic image. Here is a higher-resolution version of the graphic that I put in that blog entry. It shows a model for stresses along cracks near the south pole when Europa is at apoapsis. Can you identify which specific images contain these cracks?
Click to view attachment

Not unexpectedly, the imaging coverage is somewhat limited. There is a very big map available from the USGS together with an image footprint chart (europa_simp.pdf):

http://astrogeology.usgs.gov/search/detail...obal-Mosaic/cub

I can't find an image footprint chart for the south polar region at the USGS website now but here is the relevant part from one I downloaded back in 2001 so there might be changes (not big though). The numbers are the same as in europa_simp.pdf. The longitudes were added by me:

Click to view attachment

The northernmost latitude is probably ~55S (I can't find an exact number anywhere). Much of the terrain in the red ellipse has been imaged at 1.5-2 km/pixel resolution. The main/only exception is what's labeled as 13 in europa_simp.pdf mentioned above. These are images from orbit 17, observation id 17ESREGMAP01. These images have a resolution of ~200 m/pixel.

More interestingly, observation 17ESSTRSLP01 has images of Astypalaea Linea which is within the red ellipse and has high tensile stress. They have a resolution of ~40 m/pixel and these images are not in the USGS map (their resolution really is too high for a 500 m/pixel map). Photojournal images:

http://photojournal.jpl.nasa.gov/catalog/PIA02960
http://photojournal.jpl.nasa.gov/catalog/PIA01645

Interestingly, the terrain in the red ellipse is not in view when the plume was visible - at that time the sub-observer longitude was ~90. If the activity is within the red ellipse it means the plume is seen poking above the limb from behind. I don't know if it's of significance or not but Thrace and Thera Macula aren't very far from the red ellipse. If I have understood everything correctly, as currently planned they are a major focus of interest for JUICE near closest approach during its two close Europa flybys. These two maculae have been considered to be among the most likely places on Europa to be currently active (chaos formation in progress) although I don't remember the exact details.
Bjorn Jonsson
Here is a quick (i.e. seams might be visible somewhere) mosaic of Astypalaea Linea, the strike slip fault that apparently might be one of the suspects here. This is from the 17ESSTRSLP01 observation. North is approximately up.

Click to view attachment
elakdawalla
The cross-cutting relationships through here are just maddening! smile.gif
Bjorn Jonsson
Yes, this is interesting terrain. In hindsight it would probably have been more accurate for me in the post above to say that this general area is a suspect, rather than only Astypalaea Linea. There are small craters visible at various locations in most of the mosaic and interestingly, some of these occur in clusters, e.g. near (2150,4650) and (500,1800) in the mosaic. But there's also terrain that seems completely devoid of craters. A good example is in the upper left corner of the mosaic
rlorenz
QUOTE (elakdawalla @ Dec 12 2013, 06:39 PM) *
Wouldn't infalling comets deliver carbon to Europa as they do to other places in the solar system?


Of course, the amount of organics delivered by impactors is not zero. But likely less than Mars (where e.g. the
Viking mass spec failed to find them*) since modeling by the late Betty Pierazzo showed that the bulk of material
re-escapes Europa because of the high impact velocity - see http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1656.pdf

Of course, these inconvenient considerations will be quietly ignored in all the hoo-ha clamouring for a Europa mission.

It will be interesting to see if the observations hold up - is this Europa's ALH84001 moment, or is this going to be like
the methane on Mars.....?


(*Europaphiles have made much of oxidants in the ice as 'energy sources' - these same oxidants will mop up the organics in the ice too, rather analogously to Mars)
vexgizmo
The area of interest is along 180 lon; the authors say 55 and 75 south lat. This is near the boundary between the Galileo E14 global color imaging and G7 global image. Completely coincidentally, the image that shows the area best in one image is the one released the other day, with the comet crash story:
http://photojournal.jpl.nasa.gov/catalog/PIA17658
That two-frame mosaic is from E17, obtained for global shape.
Ted Stryk has a pretty version here: http://planetimages.blogspot.com/2012/05/e...other-take.html

The Europa Clipper currently has 10 flybys at high southern latitudes, and the study team is examining what it would take to do a targetted campaign.

Click to view attachment
Click to view attachment
vexgizmo
And from the USGS global map (colorized).
Click to view attachment
marsbug
QUOTE (rlorenz @ Dec 14 2013, 04:32 PM) *
Of course, the amount of organics delivered by impactors is not zero. But likely less than Mars (where e.g. the
Viking mass spec failed to find them*) since modeling by the late Betty Pierazzo showed that the bulk of material
re-escapes Europa because of the high impact velocity - see http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1656.pdf

Of course, these inconvenient considerations will be quietly ignored in all the hoo-ha clamouring for a Europa mission.

It will be interesting to see if the observations hold up - is this Europa's ALH84001 moment, or is this going to be like
the methane on Mars.....?


(*Europaphiles have made much of oxidants in the ice as 'energy sources' - these same oxidants will mop up the organics in the ice too, rather analogously to Mars)


Considering the abundance of icy objects that may harbour subsurface water in the solar system (and therefore other star systems) I think a mission to definitively confirm/deny the presence of an ocean, characterise it as much as possible, and investigate its chemistry (if possible) has scientific merit aside from just the hunt for space squid beneath the ice.

That said, the target doesn't need to be Europa, and all else being equal Europa may not even be in the top five icy targets of interest for such information gathering*. However all things are not equal, especially in times of constrained budgets. I would wait and see if this 'plume' is an old faithful or a bathtub bubble before I begin worrying that resources spent on a putative Europa mission might be getting taken from more deserving causes......

* I wonder if there's any chance that, post DAWN's arrival there, even Ceres might climb higher? Odder things have happened, and there is some evidence for a water plume over Ceres pole too.....
JRehling
One of the tricky factors in our limited Europa coverage is the enormous importance of sun angle. The surface is very rough and low sun angle creates shadows along linear features which appear dark. This is easy to confuse with the albedo differences that pertain to composition (dark -> higher non-H2O constituents).

One of the hypotheses for the darker (low albedo, not shadow) linear features is that emission of subsurface liquids which are "dirty" deposit along open fissures, spraying the dark material to the sides. If this is correct, then the active plumes may exist exactly where linear features with low albedo are most prominent. There are so many unknowns in the above, I couldn't begin to estimate how likely this is to be true, but at least if one is beginning to consider possibilities, that seems like the possibility to start with.

We're not going to get better maps of Europa until a spacecraft sends them back. Theoretical work on the location of stresses is a nice start, but they depend on unknown and (given only the data we have) unknowable parameters concerning the structure of the icy shell. I don't see any way to pin down the origin of the plumes until observations can be made in situ. That's assuming, in fact, that the plumes and their sources are even persistent over a period of years. Maybe 2023's plumes (if any) will be different than the ones observed so far.
scalbers
In these blog comments in a Planetary Society post by Leigh Fletcher, I note that Paul Fieseler comments that he thinks he could have seen something in the Galileo data. Perhaps worth following up on?

http://www.planetary.org/blogs/guest-blogs...-of-europa.html
ngunn
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.

scalbers
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
Bjorn Jonsson
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.
machi
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.
tedstryk
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.
machi
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).
JRehling
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.
Bjorn Jonsson
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:

Click to view attachment

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.
ngunn
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.
nprev
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.
JRehling
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.
vexgizmo
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."
JRehling
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.
rlorenz
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.

dvandorn
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
Gerald
I'd think, that the carbon-nitrogen triple bond should be rather stable.
A paper about CN-photochemistry.
rlorenz
QUOTE (dvandorn @ Dec 31 2013, 01:00 PM) *
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?


I think the answer is that all bonds are weak compared with the energies of the intense trapped particle environment around Jupiter. Thus
the observed abundances reflect not relative bond strength but the fact that a free carbon atom is most likely to encounter a free oxygen radical rather than another carbon.
dvandorn
So, observed species abundances reflect the end result of a process of emission of some carbon species, the dissociation of those species in the radiation environment, and finally the recombination of some species (as allowed by the environment) and the emplacement of these recombined species on the surface?

Just trying to wrap my mind around the most likely process. Seems to me that modern science offers a lot of theoretical concepts that are weak on the actual processes you have to have to get to the current observed conditions, which is why I'm always harping on the process side of things.

-the other Doug
Gerald
Some species will escape, and therefore thin out selected chemical elements at the surface.
(Hydrogen (besides helium) is the first element going lost by photolysis of water, leaving oxygen.)

Things will be rather complicated in detail. Here a paper which tries to simplify the processes for the Martian atmosphere, detailed photochemical reactions in Appendix A (p. 22ff).
The more chemical elements and transport processes have to be considered the more complicated.

Comet impacts may replace some of the lost surface material.
JRehling
Other Doug,
I think it's useful to consider the relative rates of processes on the surface of a world. We have few visible craters on Earth because erosion and tectonics both work faster than impact cratering.
On Europa, the effects of radiation work fastest, although those obviously alter only the immediate surface.
Recycling of crustal material (faults, occasional melt-through) is next fastest. Then the flux of major impactors comes after that.

I've read enough about Europa to know that there are certainly unknowns and seemingly unknowable unknowns (to paraphrase Donald Rumsfeld), or at least internal parameters that cannot be deduced unless we get hard data about the interior of a kind we don't have, or someone comes up with new and clever ways of interpreting the clues we have. It is generally the case that the interior properties of worlds are hard to pin down without a tremendous amount of in situ seismographic, etc, data of a kind we have only for Earth and to a lesser extent the Moon and a far lesser extent Mars.
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