Water plumes over Europa Options

[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]

Here's a link to the paper.
http://hubblesite.org/pubinfo/pdf/2013/55/pdf.pdf

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:




If I understand the article correctly the most likely location for the plume source is near longitude 90°W 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 90°W.

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 180°W).

[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]

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]

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]

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:


If I understand the article correctly the most likely location for the plume source is near longitude 90°W 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 90°W.

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?

[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]

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?

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:



The northernmost latitude is probably ~55°S (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.

[elakdawalla]

I wrote an article two years ago about Britney Schmidt's work on Thrace and Thera maculae possibly being the site of current geologic activity on Europa. That's probably why JUICE is so interested in those spots.

[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.

[elakdawalla]

The cross-cutting relationships through here are just maddening!

[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]

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.

[vexgizmo]

And from the USGS global map (colorized).

[marsbug]

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.

Attached thumbnail(s)

 

[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.

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:



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

...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]

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]

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]

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.

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]

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.

[JRehling]

The options for future Europa missions have to highlight a sample return that flies through the plumes, collects material, and returns to Earth. One of the big problems with this architecture is the inconsistent nature of the plumes. A free trajectory return from Earth back to Earth by way of Jupiter has the least possible delta-v, but is also completely inflexible to midcourse adjustment, and it could miss the active period of the plumes.

So an intriguing alternative has to be a mission which enters Jupiter orbit and parks, waiting for the plumes to become active, possibly performing the observations that determine when the plumes are active. To be practical, this would mean something like a Galileo-style elliptical orbit with apojove well outside the orbit of Callisto, and perijove well outside the orbit of Europa. When the determination had been made that the plumes were active and likely to remain so, it could perform a single pass over Europa's surface, perform the collection, and then exit Jupiter orbit for a return to Earth. This would entail more delta-v than the free return trajectory, but avoids the reliance on a single timeframe for sample collection, and still entails vastly less delta-v than a lander-based sample return. It would also offer opportunities for flyby science of Callisto and Ganymede and long-range science of Jupiter, Io, and Europa.

I think something like this has to become a major candidate for a future mission.

[Explorer1]

The issue with parking directly in Europa orbit for any lengthy period is dealing with the high radiation environment and its effect on electronics, correct? Because staying near would take a lot of guesswork out of detecting and intercepting plumes.

[vjkane]

If the plumes are shown to be consistent and persistent (big ifs), then gathering the samples from a hyperbolic solar orbit (i.e., not enter Jovian orbit) would result in a very high speed encounter. That would tend to break apart the molecules of interest. The slowest encounters would come from most closely matching Europa's orbit but that would also break the highest radiation exposures.

[JRehling]

A craft would have limited lifetime in Europa orbit. Moreover, that is extremely expensive in terms of delta-v to get into Europa orbit, then back out of it, then out of Jupiter's gravity well and back to Earth. Jupiter' gravity is the great obstacle here, not Europa's. What would make a mission like this work is only diving through Jupiter's gravity well, and getting that energy back on the way out.

The cheapest option would be to avoid Jupiter orbit at all, but that requires that the plumes are active at the time of a rendezvous predetermined years in advance. If we had the notion of the plumes performing on schedule regularly, that would be the best option. Otherwise, Jupiter orbit is essential. Europa orbit would kill the craft with radiation if it had to wait a long time for the plumes. So a high Jupiter orbit is the most flexible.

[vjkane]

A craft would have limited lifetime in Europa orbit. Moreover, that is extremely expensive in terms of delta-v to get into Europa orbit, then back out of it, then out of Jupiter's gravity well and back to Earth.

I wasn't suggesting being in Europa orbit, just that as the orbit around Jupiter becomes closer to one that matches Europa's orbit, the encounter speed goes down. What the best tradeoff would be between a slow enough encounter to preserve complex molecules and getting out of Jovian orbit, I don't know. The proposed Enceladus LIFE mission, which would perform an Enceladus sample return a la Stardust (and many of the principals were part of the Stardust team), planned for a ~6 km/sec encounter. This would preserve large molecules but destroy any intact organisms.

Being in Jovian orbit allows multiple encounters to build up the sample. However, if the plumes (if they persist) occur only at a certain point in Europa's orbit, it may be difficult to match the Jovian orbit to encounter Europa at that point within a reasonable time.

[JRehling]

For a mission that sits in a circular parking orbit around Jupiter and then shifts to a Hohmann transfer orbit that intersects Europa's orbit, the relative speeds at the encounter are a function of the initial orbital radius. Here are the relative encounter speeds for three different parking orbital radii:

Ganymede-to-Europa: 1.9 km/s
Callisto-to-Europa: 5.2 km/s
2*Callisto-to-Europa: 11.2 km/s

I'm not including the additional acceleration which would take place due to Europa's gravity. This would be about 2.0 km/s added in each case.

So in the Ganymede case, we could have collection take place at about 3.9 km/s. Stardust, for comparison's sake, encountered Wild 2 at 6.1 km/s.

The penalties of having the parking orbit closer are to increase delta-v on arrival/departure to/from the jovian system and to increase radiation exposure.

The radiation at Ganymede is very roughly 2% that at Europa (depends on which spectrum of charged particle energy we're talking about), so a survival time for craft systems on the order of years should be possible at that distance. This would also allow for Ganymede gravity assists that could be provide some of the delta-v for, e.g., departing the Jupiter system.

[MarcF]

Any chance that the Gemini Planet Finder instrument could detect the plume ?

http://www.gemini.edu/images/pio/News/2014...4_01/Europa.jpg

[ugordan]

I'm skeptical since GPI operates in infrared and the plumes are going to be virtually invisible in backward scattered light.

[Bjorn Jonsson]

Before a sample return mission (possibly somewhat Stardust-like) to sample the plumes is flown it is absolutely necessary to know whether the plumes are continuously active or not. If they are not it needs to be determined whether they are always active when Europa is at a specific point in its orbit. This information is necessary for deciding the mission architecture. A spacecraft like JUICE or Europa Clipper should be able to determine this. Waiting for almost 20 years until this is resolved isn't very fun though so maybe this could be determined sooner from ground based observations.

[vjkane]

For a mission that sits in a circular parking orbit around Jupiter and then shifts to a Hohmann transfer orbit that intersects Europa's orbit, the relative speeds at the encounter are a function of the initial orbital radius...

John - Thanks for doing the calculations. The good news is that after the capture, Ganymede flybys can be used to crank the orbit back up to lessen the delta V of the burn to return to Earth.

Do you know how that compares to the flyby speeds from a Jovian flyby?

[JRehling]

As mentioned earlier, opportunities when Europa occults a background object and its light passes through the plumes (or fails to do so) could give us information about the presence and composition.

Pluto has occulted stars every few years, and Europa moves against the background of stars much faster (about 20x) than Pluto does, which should provide a lot more opportunities. The downside is: Europa provides a lot more background light than Pluto does, and Jupiter is likely a factor for some observations, so a favorable signal-to-noise ratio might requires a brighter star being occulted.

As I also mentioned, Io could possibly serve as the occulted body, although that doubles the background noise of the observation.

Clearly, we will need to characterize the frequency and perhaps periodicity of the plumes before committing to any exploration. They may be very cooperative, like the Old Faithful geyser, or extremely unpredictable, like most terrestrial volcanoes.

[Paolo]

I am too lazy to go through the references, so I ask: would the plumes be detectable by the Japanese SPRINT-A UV space telescope?

[MarcF]

I am probably very naive, but I thought, if we cannot detect the plume itself, may be it could be possible to detect the hotspot where it originates from (like for Enceladus).
Do we have instruments able to detect this ?
(Again I'm thinking about GPI, since I'm so amazed by the resolution of its pictures).
Regards,
Marc.

[Greenish]

According to this document among others, Gaia will drastically increase the number of stars available for predictable occultations of solar system objects, as well as the precision resulting from those measurements. Perhaps in addition to small bodies (where if I'm reading it right, at 20-50km size the number of opportunities could go from 0.1 to 20+ events/object/year and the orbit precision could improve 100x), this will help for things like this plume study... but of course the useful occultation frequency will very much depend on how bright the star needs to be, and I suppose its spectral type, too, for this work.

[ugordan]

I am probably very naive, but I thought, if we cannot detect the plume itself, may be it could be possible to detect the hotspot where it originates from (like for Enceladus).

For Enceladus, Cassini CIRS had the advantage of being fairly close to the moon and the detector footprint was relatively small compared to the tiger stripes. For ground-based observations, any localized hotspots would be averaged over hundreds of kilometers of "regular" cold terrain.

[MarcF]

Thanks for the answer Ugordan. So we will have to wait 2 decades to know for sure.
Marc.

[vjkane]

We will know long before two decades whether or not the Hubble observations can be repeated. I suspect that we'll also get more sensitive stellar occulations as outlined above.

Then in the late 2020s...

Before the plume announcement, one of the goals for the JUICE mission was to use the UVS and cameras to look for plumes using long distance remote observations. (I.e, watch Europa throughout its orbit.) If plume sources have been discovered through any means, then the spacecraft instruments will be nearly ideal. The mass spectrometer is sensitive to AMUs near a thousand (recalling from memory). The radar unit could measure the subsurface structure around the vents. The only key instrument missing would be a thermal imager to image whatever the equivalent of Enceladus' tiger stripes turn out to be at Europa.

One other limitation of the JUICE mission is that its Europa flybys are planned for a single position in Europa's orbit. The plumes may not be active or have reduced activity at this location and the sources might be in darkness.

[JRehling]

One note on the geometry of the plumes: They, like Enceladus's plumes, seem to be associated with the south pole. Unlike the saturnian system, the jovian system has essentially no seasons, so it's conceivable that the plume source, if its sunken into the terrain, might never (or rarely) have a line of sight to Earth and/or the Sun.

On the other hand, most of the plumes themselves would never (or rarely) be in Europa's shadow.

If the sources have hotspots, it is not inconceivable that they could be observed and pinpointed from Earth, but the devil's in the details: While any IR thermal analysis would of course lack the spatial resolution to pinpoint the plumes, a temperature sufficiently higher than the background could radiate IR at wavelengths that should be at nearly zero emission from Europa's baseline. This works just fine with Io's hotspots. Moreover, occultation of Europa by Io or Ganymede could allow some extremely precise pinpointing of the source, when the hotspot passes behind the other moon.

However, it seems unlikely to me that Europa's hotspots, if any, would be nearly hot enough to allow this observation to work. The temperature difference between Europa's ambient surface and any hotspots is probably less than 300K°, probably far less than that, as well as being very limited in scale.

[vexgizmo]

Back in December, there was discussion of the available imaging data for the reported plumes region. A blog entry has just been published by Lorenz Roth describing the discovery, along with a nice orthographic view of the identified source region. http://solarsystem.nasa.gov/europa/blogs_l...roth_plumes.cfm

[Phil Stooke]

I had a vague recollection that Voyager images of possible Europa plumes were published long ago, but most people didn't really accept them.

I managed to find the references, and now they make very interesting reading.

Sky & Telescope, Jan. 1983, p. 15: Allan Cook (Harvard-Smithsonian) - in a post-flyby Voyager 1 image looking back at a crescent Europa, a bright patch at the southern cusp and a faint plume 100-150 km high. Not seen in other images.

JGR Oct. 1982, Intriligator and Miller: Pioneer 10 may have encountered a plasma cloud emanating from Europa in 1973.

IAU Colloquium 77 (Natural Satellites), Brown U. Preprint #A546 (July 1983, p. 22): A. F. Cook (Hertzberg Inst. and Harvard-Smithsonian) plus Paul Helfenstein (then at Brown): six Voyager 2 images taken during approach show a diffuse plume, not seen in similar Ganymede images. Best view is image 0391J2-002.


Phil

[Ian R]

IAU Colloquium 77 (Natural Satellites), Brown U. Preprint #A546 (July 1983, p. 22): A. F. Cook (Hertzberg Inst. and Harvard-Smithsonian) plus Paul Helfenstein (then at Brown): six Voyager 2 images taken during approach show a diffuse plume, not seen in similar Ganymede images. Best view is image 0391J2-002.

Image 0391J2-002, aka c2060558, does show a 'plume', but it's likely a residual artifact of the reseaux dot removal process; none of the raw images in this sequence appear to show any plumes, tenuous or otherwise:

c2060558 raw:

[Ian R]

Sky & Telescope, Jan. 1983, p. 15: Allan Cook (Harvard-Smithsonian) - in a post-flyby Voyager 1 image looking back at a crescent Europa, a bright patch at the southern cusp and a faint plume 100-150 km high. Not seen in other images.

This post-flyby Voyager image of Europa got me excited:

http://pds-rings.seti.org/browse/VGISS_5xx...649524_full.jpg

Until I realized it was just an unfortunately situated speck of dust on the camera lens:

http://pds-rings.seti.org/browse/VGISS_5xx...649518_full.jpg

[Phil Stooke]

Not so fast! I was summarizing. The version of that image printed in the reference shows the plume area, much bigger than a reseau removal artifact (and we can see where the reseaux are, it's not there) - I'm not saying the Voyager observations are correct, but they are not disproven by this quick look.


Phil

[Ian R]

I'm glad to hear that Phil. Makes me wonder how they managed to tease out a putative plume out of this dataset.

[JohnVV]

Makes me wonder how they managed to tease out a putative plume out of this dataset.

the plume
isis is lousy at Reseaus removal so i left them in

c2060558.imq > voy2isis > voycal > isis2raw ( omin set to 0.0000 )
convert the 32 bit raw to a 32 bit tiff
normalize ( in 32 bit float to 0 to 255 )

screen shot of the normal image

then rescaled

false color


there is something at the top of the image , but it looks like "camera shake" from the spacecraft movement
unfortunately "spiceinit" is not working well on this small of a image
--- edit ---
the area at the top IS from the spacecraft movement

there is a "bright spot" in between two reseaus BUT that is the DUST mentioned a few posts up

------------- edit --------------
from a few more images the "plume " center is at -65 x 183
so the above 3 images that is in the lower left

a few more images
( same processing as above)

so for image "c2062524.imq" the "plume" would be in the lower left


that ? might ? be something or it might be image shake from the spacecraft movement


For those that can not see the thumbnails
well no thumbnails but a link to a folder on my picasa albums
full size only
https://picasaweb.google.com/10269590129139...941/EuropaPlume
g+ ( b**s** that is even more useless than FB )
https://plus.google.com/photos/102695901291...4849?banner=pwa

[tedstryk]

Tempting, but I've been trying to search a plume out, and I've been tempted to try to see something centered around the 8 o'clock point in this six-image stack. The result looks a lot like yours but is not in the same place.

Attached thumbnail(s)

 

[ollopa]

Bob Pappalardo and an assembled cast of the usual suspects addressed this directly in JGR in 1999. "Does Europa have a subsurface ocean?" (Circa Page 31):

our analysis shows that this is a false double-exposure rather than a real phenomenon, probably resulting from incomplete closing of the camera shutter blades in combination with an imaging mode that does not include a preexposure erasure of the CCD array.

I'm not saying the Voyager observations are correct, but they are not disproven by this quick look.


Phil

[vexgizmo]

Bob Pappalardo and an assembled cast of the usual suspects addressed this directly in JGR in 1999. "Does Europa have a subsurface ocean?" (Circa Page 31):

our analysis shows that this is a false double-exposure rather than a real phenomenon, probably resulting from incomplete closing of the camera shutter blades in combination with an imaging mode that does not include a preexposure erasure of the CCD array.

That was in response to the Galileo double-exposure false-plume. There are other statements in that paper about the Cook et al "plume":

The best Voyager evidence for the presence of anomalously fresh frost on Europa's surface is from the controversial, 143° crescent "plume" image of Cook et al. [1982, 1983] (Figure 17). In addition to showing an off-limb feature that was inter¬preted as a volcanic event [Cook et al., 1982, 1983], the image shows a conspicuous bright spot on the surface at 34°, 337°. Helfenstein and Cook [1984] measured the photometric con¬trast of the bright feature relative to surrounding terrains and compared it with that measured of the same geographic region viewed at 13° phase. They found that, relative to the surrounding features, the brightness of the anomalous spot increased as the phase angle increased from 13° to 143° phase by more than seven standard deviations above the average surface change. Although Helfenstein and Cook [1984] interpreted this brightness change to be due to active emplacement of surface materials, a more conservative interpretation would be that the feature represents relatively transparent frost that was deposited on Eu¬ropa's surface in recent geological history [cf. Verbiscer et al., 1990; Verbiscer and Veverka, 1990; Verbiscer and Helfenstein, 1998]. McEwen [1986a] suspected that this bright region was not actually anomalous compared to other bright regions on Europa which had not been seen at high phase angles by Voyager. Indeed, in low-phase global-scale Galileo images, this area appears similar to other bright plains regions.

Figure 17. Unprocessed Voyager 2 clear filter image (FDS 20767.37) at a resolution of ~44 km/pxl and phase angle of 143°, argued by Cook et al. [1982, 1983] to show an active plume along Europa's bright limb. Stretched inset image shows detail of the bright limb and putative plume, and an unusually bright area on the surface. The "plume" feature, with a signal level of just 5 DN, is not observed in subsequent images. Its location in the corner of the Voyager vidicon image, where noise and distortion are most severe, suggests that it is a camera artifact.

Attached thumbnail(s)

 

[TheAnt]

Despite the result from the Hubble telescope, a check of old data by Cassini failed to reveal any plumes at Europa.

[Vultur]

Despite the result from the Hubble telescope, a check of old data by Cassini failed to reveal any plumes at Europa.

That looks like it's only for one brief period (the flyby), though, right? So that doesn't really cast doubt on the Hubble results since it's not for the same time -- does it?

[tedstryk]

I've reviewed virtually every image from Galileo, Voyager, Pioneer, Cassini, and New Horizons. Nothing except for a Voyager image that depends on one pixel, and that is almost certainly noise.

[vjkane]

That looks like it's only for one brief period (the flyby), though, right? So that doesn't really cast doubt on the Hubble results since it's not for the same time -- does it?

I only read the press accounts on this paper, but with that in mind: If plumes are a regular occurrence, then there should be enhanced plasma or gas cloud centered on Europa's orbit. This study used Cassini's observations to look for such a cloud and didn't find it. However, at least one other group doesn't think that the Cassini measurements would be conclusive and reportedly are working on a rebuttal.

I've been following this subject since the first announcement. One thing to keep in mind is that the Hubble measurements were at the limit of detection, and every subsequent analysis has emphasized that the plumes might not have existed. If they did exist, it is possible that the occur infrequently. In an analogy that I've read from the science teams that have looked at this, each of Io's volcanoes erupts infrequently. Because Io is so extremely heated by Jupiter's gravitational tug, there's usually some volcano going off. Even so, there are periods of much greater and much less volcanic activity. By this reasoning, the lesser gravitational tug on Europa might create plumes only in limited areas and there might be months or years or more between eruptions.

It may be that we'll have to wait for JUICE to arrive to get definitive answers. The one solid recommendation from the Europa Clipper Science Definition Team is that a mission oriented towards Europa plumes is very premature to consider, but that the Clipper mission should carry a suite of instruments that could search for and study plumes (much like the broad range of instruments on Cassini has allowed detailed studies of the totally unexpected Enceladus plumes).

[nprev]

I wouldn't characterize Enceladus' plumes as 'completely unexpected', though; the moon's association with the E-ring was strongly inferred prior to Cassini, and in hindsight it's about as robust an emission torus as anyone could hope for. Something was clearly going on there but the mechanism was not known.

By analogy, any eruptive activity on Europa must be both extremely transient and volumetrically small given the absence of anything remotely comparable to the E-ring. I wonder if the isolated Hubble observation could be best explained as the aftermath of an impact?

[TheAnt]

That looks like it's only for one brief period (the flyby), though, right? So that doesn't really cast doubt on the Hubble results since it's not for the same time -- does it?

Yes its ofc data only for the time of the flyby, even so Cassini did not look for any active plumes but for any resulting cloud and the latter would hang around a lot longer.

@Vjkane: I am firmly with nprev here, the plumes of Enceladus were actively searched after, and it took quite a while before we got that first backlit image showing them clearly. That something have been going on at Encladus have been known at least since the data from the Voyagers did show something were going on at this moon.
A rebuttal is a good sign, hardly anyhing in the scientific world is accepted before someone else have poked your hypothesis hard with a pointy stick. =)

[Explorer1]

If it was an impact that caused the plumes, would JUICE or Europa Clipper see any evidence? Considering that its effects were visible from Earth, there would have to be a rather large crater, right? Obviously only speculation at this point...

[katodomo]

According to its Yellow Book (page 26), JUICE will have a resolution of 500-1000 m/pixel for regional features and under 50 m/pixel for selected local features during the flybys.

[nprev]

Might not have been a particularly large impactor; I'd guess something like a 10m rock. Depending on the inbound trajectory the relative velocity might've been quite high that close to Jupiter so a lot of water vapor could result from a hit & probably the impactor would vaporize as well. Has the data been examined for spectroscopic peaks in typical meteoritic elements like carbon, silicon, iron, etc. in addition to water?

[rlorenz]

@Vjkane: I am firmly with nprev here, the plumes of Enceladus were actively searched after, and it took quite a while before we got that first backlit image showing them clearly. That something have been going on at Encladus have been known at least since the data from the Voyagers did show something were going on at this moon.

Just to enlarge on this, the Cassini Phase A report in 1988 clearly shows the degree of interest, declaring Iapetus and Enceladus the two icy satellites of particular interest, noting (section 3.4.2)
'The strange appearance of Enceladus surface and the location of the ephemeral E-ring may be a coincidence, but there is a strong probability that they are connected in some manner, possibly through current eruptive activity on Enceladus'

[Bjorn Jonsson]

If it was an impact that caused the plumes, would JUICE or Europa Clipper see any evidence? Considering that its effects were visible from Earth, there would have to be a rather large crater, right? Obviously only speculation at this point...

A new and relatively small crater could only be detected if it formed in the terrain that was well imaged by Galileo.

Plumes similar to Enceladus' plumes would be ~70 km high when scaled to Europa's gravity so they wouldn't be too difficult to image from a Jupiter-orbiting spacecraft. JUICE should have no problems imaging them from e.g. Ganymede's orbit if they are near the limb but one possible problem is that any plumes might be smaller than this and they could be episodic. For imaging you want high-phase imaging and possibly a violet or UV filter. There aren't that many good high-phase images of Europa but as Ted indicated, any plumes can be ruled out in all of them except maybe for one Voyager image where noise is by far the most likely reason.

[TheAnt]

Just to enlarge on this, the Cassini Phase A report in 1988 clearly shows the degree of interest, declaring Iapetus and Enceladus the two icy satellites of particular interest, noting (section 3.4.2)
'The strange appearance of Enceladus surface and the location of the ephemeral E-ring may be a coincidence, but there is a strong probability that they are connected in some manner, possibly through current eruptive activity on Enceladus'

Thank you for pinpointing that one rlorenz.
And yes carefully phrased as it should be. =)
It also serves as a good example that some researchers get one aha moment prior to a mission or a flyby since their minds start to deal with whatever might turn up in that area of space. And it is also a good thing for planning the observations (we've seen some such also for Pluto now which might or not turn out correct when New Horizons get a closer look). =)

[vjkane]

Just to enlarge on this, the Cassini Phase A report in 1988 clearly shows the degree of interest, declaring Iapetus and Enceladus the two icy satellites of particular interest, noting (section 3.4.2)
'The strange appearance of Enceladus surface and the location of the ephemeral E-ring may be a coincidence, but there is a strong probability that they are connected in some manner, possibly through current eruptive activity on Enceladus'

Yeah, I blew it on this one. I was focused on the contrast in level of activity of Enceladus (which I think (corrections welcome!) was not expected) with the possible level at Europa. I remember being at JPL for one of the Voyager flybys when I believe I first heard the idea that Enceladus might have some volcanic idea was discussed.

[nprev]

Nobody's keeping score, man.

If I could pick one theme for exploration of the Solar System to date it would be "expect the unexpected".

[TheAnt]

Not plumes, but another matter about Europa, signs of plate tectonics though the word that comes to my mind is ice shear. Regardless of what term to use, this explains surface features of Europa.

[Fran Ontanaya]

No confirmation for plumes yet:

http://www.scientificamerican.com/article/...rs-are-missing/

[Hungry4info]

NASA to Hold Media Call on Evidence of Surprising Activity on Europa
http://www.nasa.gov/press-release/nasa-to-...vity-on-europa/

NASA will host a teleconference at 2 p.m. EDT Monday, Sept. 26, to present new findings from images captured by the agency’s Hubble Space Telescope of Jupiter’s icy moon, Europa.

Astronomers will present results from a unique Europa observing campaign that resulted in surprising evidence of activity that may be related to the presence of a subsurface ocean on Europa.

[JRehling]

NASA to Hold Media Call on Evidence of Surprising Activity on Europa

The headline has intriguing hints. Watery plumes were seen in the past, but appeared to be intermittent. If they had another burst of activity comparable to those seen in the past, it would not be "surprising." So what is up?

It is also hinted that the observations were made during a unique observational opportunity. Europa doesn't have seasons, so the only kinds of unique opportunities that I can think of would be eclipses and transits (not that unusual) and stellar occultations (unusual). The latter would allow, probably, better observations of composition than could be made from reflected sunlight.

Jupiter and Ganymede were predicted to occult a 7th-magnitude star on April 12, but Europa was not. Perhaps the plume did? Europa was slightly north of the Jupiter-Ganymede line of sight on that date, so that's what I'm betting on: That the plume was detected at a significant distance from Europa, perhaps even several Europa radii away.

However the evidence was gained, what could be unusual? The plumes could perhaps have been much more active than in the previous observations, occurring in more places other than the south polar region, or be much larger, or perhaps the composition reveals something more than what was seen last time. If it was that April 12 occultation, then the size of the plume would be part of the answer: It would be monstrous compared to the previous observations.

That's all the detective work I can muster until Monday.

[B Bernatchez]

Water plumes detected by Hubble: http://www.nasa.gov/press-release/nasa-s-h...ers-moon-europa

[Phil Stooke]

Hot from Twitter:

" Emily Lakdawalla Retweeted


Tanya Harrison ‏@tanyaofmars · 26m26 minutes ago

Britney Schmidt notes that the idea of plumes from #Europa was proposed during Galileo, but none found at the time. (1/2)"


Actually the first suggestion was based on a Voyager image of Europa - not widely believed at the time, but it was suggested.

----------------
Title: Active Venting of Europa?: Analysis of a Transient Bright Surface Feature
Authors: Helfenstein, S.-P. & Cook, A. F.
Journal: LUNAR AND PLANETARY SCIENCE XV, P. 354-355. Abstract.

Phil
(Hi Tanya!)

[Brian Burns]

Actually the first suggestion was based on a Voyager image of Europa - not widely believed at the time, but it was suggested.
----------------
Title: Active Venting of Europa?: Analysis of a Transient Bright Surface Feature
Authors: Helfenstein, S.-P. & Cook, A. F.
Journal: LUNAR AND PLANETARY SCIENCE XV, P. 354-355. Abstract.

That's really interesting - from the paper here (from 1984) - http://adsabs.harvard.edu/full/1984LPI....15..354H - they talk about the Voyager 2 image 20767.37 -



I can sort of make out a diffuse plume, but not a bright spot - do I have the right image? If so, needs enhancing! (this is the crummy jpeg preview version, and North is downwards)



From http://pds-rings-tools.seti.org/opus/#/pla...2_ISS_2076737_N

[JohnVV]

grabbed the image "c2076737.imq"



false color image

BUT the maybe spot is also aligned with the BRIGHT spot
i would call that optic scatter