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New Horizons at Europa
nprev
post Mar 3 2007, 03:01 PM
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QUOTE (mgrodzki @ Mar 3 2007, 05:42 AM) *
most exciting is the remote possibility we may see some emissions at europa as we did at enceladus.


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! smile.gif


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dvandorn
post Mar 4 2007, 02:17 AM
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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


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Decepticon
post Mar 4 2007, 04:23 AM
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QUOTE
the sulphur torus at Io's orbit was discovered fairly early on


From earth based observations? Or the Voyager flybys?
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dvandorn
post Mar 4 2007, 05:29 AM
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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.

smile.gif

-the other Doug


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edstrick
post Mar 4 2007, 08:40 AM
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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...
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helvick
post Mar 4 2007, 10:11 AM
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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) .
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mgrodzki
post Mar 4 2007, 02:29 PM
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QUOTE (helvick @ Mar 4 2007, 05:11 AM) *
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


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 biggrin.gif


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scalbers
post Mar 4 2007, 03:33 PM
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QUOTE (Decepticon @ Mar 4 2007, 04:23 AM) *
From earth based observations? Or the Voyager flybys?


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


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nprev
post Mar 4 2007, 03:40 PM
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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. sad.gif


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vexgizmo
post Mar 4 2007, 05:26 PM
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QUOTE (nprev @ Mar 4 2007, 08:40 AM) *
The interesting implication here is that planetary satellites in equatorial orbits with 'outgassing' surfaces tend to leave rather obvious signatures/artifacts along their orbits. This probably does not bode well for any significant surface activity on Europa. sad.gif

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
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helvick
post Mar 4 2007, 07:24 PM
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QUOTE (mgrodzki @ Mar 4 2007, 02:29 PM) *
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 biggrin.gif

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. smile.gif
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nprev
post Mar 4 2007, 07:30 PM
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Ho, ho!!! VERY interesting, Vexgizmo, thank you! So far in our experience, where there's a torus, there's a "fire".... rolleyes.gif

EDIT: Here's a thought. Enceladus blasts out through a few cracks near its south pole (okay, maybe I could have phrased that better... rolleyes.gif )...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!


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helvick
post Mar 4 2007, 09:23 PM
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Well they seem pretty certain that Jovian environmental radiation is sufficient to explain what has been detected.
QUOTE
Stretching millions of miles around Jupiter, the donut-shaped cloud, known as a "torus," is believed to result from the uncommonly severe bombardment of ion radiation that Jupiter sends toward Europa. That radiation damages Europa's surface, kicking up and pulling apart water-ice molecules and dispersing them along Europa's orbit into a neutral-gas torus with a mass of about 60,000 tons.

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.
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nprev
post Mar 4 2007, 09:51 PM
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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.


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tim cassidy
post Mar 5 2007, 05:00 PM
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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.
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