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Water plumes over Europa
JRehling
post Dec 13 2013, 06:09 PM
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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.
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nprev
post Dec 13 2013, 07:29 PM
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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?


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MarcF
post Dec 13 2013, 07:46 PM
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Emily, do you mean that Astypalaea and Thasus Lineae could be still active ?
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JRehling
post Dec 13 2013, 07:54 PM
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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.
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Bjorn Jonsson
post Dec 13 2013, 08:44 PM
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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?
[attachment=31645:europa_stresses.png]

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:

Attached Image


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.
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elakdawalla
post Dec 13 2013, 10:01 PM
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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.


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Bjorn Jonsson
post Dec 14 2013, 01:18 AM
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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.

Attached Image
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elakdawalla
post Dec 14 2013, 01:35 AM
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The cross-cutting relationships through here are just maddening! smile.gif


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Bjorn Jonsson
post Dec 14 2013, 01:48 AM
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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
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rlorenz
post Dec 14 2013, 04:32 PM
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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)
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vexgizmo
post Dec 15 2013, 03:26 PM
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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.

Attached Image

Attached Image
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vexgizmo
post Dec 15 2013, 03:30 PM
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And from the USGS global map (colorized).
Attached Image
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marsbug
post Dec 15 2013, 04:12 PM
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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.....


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JRehling
post Dec 15 2013, 07:25 PM
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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.
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scalbers
post Dec 15 2013, 08:10 PM
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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


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