Time to start a thread about a far more interesting jovian moon

Passing probe to study 'crop circles' on Europa
17:38 27 February 2007
NewScientist.com news service
Kelly Young

As mentioned in the general NH at Jupiter thread, the first Europa image is down.

http://pluto.jhuapl.edu/gallery/missionPho...s/022807_2.html

Thanks, Gordan. Now we're getting somewhere!

And this is the viewing geometry:

Click to view attachment

I'm a bit frustrated that these are probably the best images we'll be seeing of the Galileans until the 2020s or later (except possibly for occasional images from huge future Earth-based telescopes). Having seen Cassini at Saturn as compared to Galileo at Jupiter makes me wish there was a 'Galileo 2' or something comparable.

No guff.

I had the same kind of wistful thought when I saw that Europa image--the last close-up of my favorite satellite for a long time...

Probably. However, crude as they might be, I wonder if JunoCam or the imagers on NASA's proposed Solar Probe, which is baselined for a 2015 Jupiter gravity assist, might return something useful.

It's a longshot but maybe one of the emerging planetary spacecraft nations (India or China) will choose the Jupiter system as a target for a flagship. If not, perhaps Russia's newly reborn planetary program will take on a mission that goes further than Mars orbit.

Thanks!

This is highly complementary with the geometry Voyager 1 had of Europa, eg:

http://photojournal.jpl.nasa.gov/catalog/PIA00016

Maybe an aspiring astrocartographer could normalize the NH image's color to the Voyager one and then make a cylindrical map from this pair. Oh, to have free time!

Several "Oh woe is me" posts in this thread about how we'll all be dead before anything ever happens in space ever again have been deleted. Seriously guys, can you not keep anything on topic?

The NH snap of Europa has similar geometry to the best full-disk image taken by Galileo:

http://photojournal.jpl.nasa.gov/catalog/PIA00502

Most features visible in the NH image are visible here.

i know there is much more to come when NH turns to face us, but are we expecting to learn anything from the europa images or will it largely be just a few new views in B&W? the images of Io so far seem pretty distant, and that works for Io as it is so active and plumes can be seen from quite a distance. but what might we discover at europa from such a quick pass?

Hmm...Actually, that's a very good question, MG.

Do we obtain any good silhouette views of Europa with low illumination angles at all from NH for a plume search? If there are any, I'm sure they'll be several orders of magnitude less spectacular than Io (or Enceladus, for that matter)...still definitely worth a look, though, if we can get it, particularly because NH is uniquely equipped to observe dim objects....

See slides 17-20 of the presentation here:
http://www.boulder.swri.edu/~spencer/nhjupitersupport/

Atmosphere and surface composition...

that seems pretty meaty for a quick swing. most exciting is the remote possibility we may see some emissions at europa as we did at enceladus. that would be fantastic. if that did happen, wouldn’t that almost insure the odds of a flagship mission to europa?

New Horizons is also supposed to map the intriguing arcuate depressions (or "crop circles") by making near terminator imaging. Will the resolution really be high enough to see them ?

Thinking about Europa pictures, I remember that I saw (a long time ago) some small pictures taken by Voyager 2 showing Europa being eclipsed by Jupiter's own shadow (a rare Voyager 2 "Kodak moment").
Has anybody an idea where these pictures could be found ?
(Sorry, this question is not directly linked to this thread).

Marc.

That would be exciting indeed, but remote is definitely the word. Just eyeballing the trajectory with respect to Europa seems to indicate that there probably weren't any good angles for such observations...plus any putative Europan geysers would in all likelihood be pretty tiny.

Of course, I'd be delighted to be completely wrong, here!

I speculate that Europa simply cannot be emitting anything more than a tiny fraction of the mass that Enceladus is emitting (if any at all), or else the Voyagers and Galileo would have observed the ring/torus of particles that such outgassing would create. Assuming, of course, that geysers on Europa would propel particles fast enough to escape its gravity and form a ring in the first place...

But, I mean, really -- the smoke ring created by Enceladus is noticeable enough that, if there was such a ring at Europa's orbit, it would have been discovered by now (as, for example, the sulphur torus at Io's orbit was discovered fairly early on).

-the other Doug

From earth based observations? Or the Voyager flybys?

Voyager flybys, as I recall. I'm just thinking that any really energetic geysers on Europa would eject enough water (ice crystals) to create an observable torus. I grant you, the sulphur torus is very energetically enriched, constantly... but also as I recall, many of the probes that have gone to or past Jupiter have had rather sensitive spectrometers that would have detected even a very tenuous water ice torus in Europa's orbit.

I'd think that such detection would be possible from Earthbound telescopes, too, at this point -- but I could be wrong.



-the other Doug

Somebody do the arithmetic...

What is the escape veolcity at Enceladus...?

What is the estimated plume velocity at zero altitude...?

What is the escape velocity at Europa...

Surface escape speed at Enceladus ~ 239 m/sec.
Surface escape speed at Europa ~ 2026 m/sec.
(Quick and dirty escape velocity calculations\spreadsheet here )

I don't believe that we have much in the way of accurate data on the velocity profile of the contents of the plumes at the moment but my understanding is that the observed density profile strongly suggests that the majority of the material is slower than the Enceladan (osian? usian? Yankee?) escape velocity (e.g the concusion from this paper - Understanding the escape of water from Enceladus by Burger et al) .

i'm confused… the way you wrote that sounds like you are comparing plumes on enceladus to those on europa. i would love someone to tell me that i somehow missed the discovery of emissions at europa

I'd say Earth based in 1972 as discussed at the following URL:

http://www.phim.unibe.ch/pig/io.htm#Detect...ion%20near%20Io

Actually, IIRC, the Io torus, Saturn's E-ring, and Titan's hydrogen torus were all discovered from ground-based observations back in the 70s. Io's position along its orbit was found to have an effect on Jupiter's radio emissions, and this was the first hint that something was going on there. The E-ring was found visually, and I think that Titan's hydrogen was also found via radio astronomy, but don't know the details.

The interesting implication here is that planetary satellites in equatorial orbits with 'outgassing' surfaces tend to leave rather obvious signatures/artifacts along their orbits. (I wouldn't expect to see anything in Triton's orbit; Triton itself only stays there out of sheer inertia!) This probably does not bode well for any significant surface activity on Europa.

Speak not too soon--and credit Cassini researchers with the discovery:

Energetic neutral atoms from a trans-Europa gas torus at Jupiter
B. H. Mauk, D. G. Mitchell, S. M. Krimigis, E. C. Roelof and C. P. Paranicas
Nature 421, 920-922 (27 February 2003)

Here we report the analysis of ... a torus of emission residing just outside the orbit of Jupiter's satellite Europa. The trans-Europa component shows that, unexpectedly, Europa generates a gas cloud comparable in gas content to that associated with the volcanic moon Io. The quantity of gas found indicates that Europa has a much greater impact than hitherto believed on the structure of, and the energy flow within, Jupiter's magnetosphere.

There is a nice summary of this article here:
Johns Hopkins Applied Physics Lab Researchers Discover Massive Gas Cloud Around Jupiter
http://www.jhuapl.edu/newscenter/pressrele...2003/030227.htm

Apologies - I was just responding to Edstrick's question regarding escape velocities. No one has found any plumes on Europa as far as I'm aware. The thread started on the back of speculation that NH might be able to image a dust\gas\debris torus if one existed * - the escape velocity point indicates that the Enceladan mechanism would probably be incapable of achieving escape velocity for any material on Europa even in the (unlikely) event that it also existed there. this was all entirely speculative, at least from my POV.

Edited to add: I see that vexgismo has pointed out that Cassini has already found just such a torus. Well explaining that is going to be fun.

Ho, ho!!! VERY interesting, Vexgizmo, thank you! So far in our experience, where there's a torus, there's a "fire"....

EDIT: Here's a thought. Enceladus blasts out through a few cracks near its south pole (okay, maybe I could have phrased that better... )...but Europa is cracked pretty much all over, with some concentration in the equatorial regions, it seems. Maybe we need to be looking for an overall 'haze' of outgassing instead of discrete plumes...in any case, we need some backlit imagery. Galileo apparently acquired some of this, but I'm not very good at retreiving things from thr NASA PDS...would be worth the effort of some of the imagesmiths here to take another look at this data!

Well they seem pretty certain that Jovian environmental radiation is sufficient to explain what has been detected.

A significant percentage of such atoms will (eventually) escape from Europa even with its fairly low surface temperatures - H2 would have an rms molecular speed of around 1400m/sec and O2 around 350m/sec in that environment - that definitely puts H2 firmly in the escaping from the moon (eventually) camp, not so sure about the Oxygen molecule. Non molecular H and O atoms both fall into the escaping set too.

Mmm...yeah, probably. Still, I remember that Io's torus was once explained as a result of radiational "sputtering" of sodium atoms from its surface. I question both the replenishment rate & longevity of Europa's torus via this similar mechanism, although H/H2 certainly can escape much more readily than Na.

On comparing Enceladus and Io:
The torus of particles in Io's orbit does not come directly from the volcanoes; it comes from "atmospheric sputtering." The volcanoes contribute to Io's tenuous atmosphere; which is "stripped" by the magnetospheric radiation (atmospheric sputtering). Surface sputtering, the ejection of molecules following ion impact mentioned above, might also contribute.

To see an Enceladus-like plume, we'll need to look close to the surface.

There's now 7 separate Europa observations up on the SOC page; by my count we're waiting for 5 more, most of them taken after the 7 shown here. Here's an animation of the 7 we've got (click to animate). I tried to set them all to have roughly similar histograms. Many of the observations have two different exposures, 5 and 10 milliseconds, to look at terminator topography. I tried to merge some detail along the terminator from the 10-millisecond exposures with the 5-millisecond exposures, but after a bit of futzing with it I wasn't achieving anything that looked much better than the 5-millisecond exposures by themselves.
Click to view attachment

--Emily

Very cool, Emily. Going off your idea, I've created a movie of the Io images uploaded thus far.

Any sign of the great circle topography the NH folks were looking for?

Alright, who are you and what did you do with the real volcanopele.

Seeing the images so far, I kind of have trouble imagining any topography (especially as subtle as the "crop circles") will be easy to make out.
Looking forward to be proven wrong.

I did find the terminator to be a little less perfect-shaped than I would expect for the solar system's smoothest moon, but I think that the interesting stuff will be buried in the fine details of the pixel values in the non-JPEGged images.

--Emily

I think we'll need to subtract out the albedo features (which we can do using high-sun Galileo images of the same regions) in order to see the topography...

John.

Regarding that, just what is the S/N ratio you achieved with Europa, seeing how low exposures were needed and Europa being a pretty high albedo body?

This image is a comparison between a NH image of Europa (not necessarily the best!) and a Galileo image reduced to the same scale and contrast. Three parallel troughs near the terminator (ringed in the right-hand image) are parts of one of the 'crop circles'. You can see how difficult they will be to isolate visually. But I would say they should be relatively easy to spot when Galileo albedo images are subtracted.

Phil

Click to view attachment

New image of Io and Europa next to each other in MVIC image!

http://pluto.jhuapl.edu/gallery/missionPho...ges/040207.html

Beautiful!


What an honor!

And a shout out to Hendric too in the comments, this is the first of the Hendric Moments.

Nice. I hope you are getting lots of copies to show folks. Magical.

Kudos to hendric and kudos to the new horizon team for making this possible.

Ah man, I didn't do anything except run a few spreadsheets. It sure is a damn fine image! Kudos to the NH team!

John sent me a copy of this last week, and it was excrutiating not to talk about it! And now I can change out my avatar!

Let me just say that with the 5 minutes of fame from Emily's story, and the 10 minutes from this one, I am now Space Enthusiast Richard Hendricks forever!

Major major kudos to Alan, John and Space Enthusiast Richard Hendricks...don't know if I should be, but I'm sort of proud of that image.

I've given Space Enthusiast Richard Hendricks a new forum title

DOug

Well, it probably wouldn't be there without your efforts in launching the board.

I think it's an inspiration in that, inasmuch as engineering this missions is a devilishly complex matter at lots of stages, some worthwhile results can come from an easily accessible level of knowledge. It's a nasty engineering process that doesn't recognize the difference and be open to the second. That's the message behind a lot of the ESA-directed complaints on this board, and of the downtrodden throughout history. The cry of the unrecognized talent saying, "Hey, I can do this!"

It's a great image for that reason and for the reasons the retina sees immediately. It's got a little of that Apollo 8 earthrise feel for me.

Yes, thanks to Doug, UMSF and Hendric in particular for making this possible! The LORRI image that goes along with this should be equally spectacular, of course, though I don't think we get that down for a while. Especially cool for me personally is the fact that the picture was taken on my 50th birthday.

Thanks too to Emily for her nice writeup on her blog.

This is such a great community we have here- I'm proud to be a part of it.

John.

Happy birthday, john_s. A significant milestone, but your peak is yet to come. Keep going strong!

I myself have been printing copies of that picture to show to my friends. It is very beautiful in my opinion.