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Amalthea And Company
tedstryk
post Jun 24 2005, 03:42 AM
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It is hard to tell if it is a true white streak a la Amalthea, or an illumination effect. Here is a color image from the E4 set without draping it on the E26, which shows the limited quality of the data.

Here is the complete set of Voyager and Galileo angles on Thebe. The only Voyager view in the set is a view where Thebe is slightly more than a point source. The other 11 are from Galileo. I selected one image from each multispectral set.


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tedstryk
post Jun 24 2005, 01:29 PM
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While I am at it, here is the set for Metis.



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Buck Galaxy
post Jul 10 2005, 03:05 AM
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So the problem as I recall is if Amalthea is ice it couldn't have formed with Jupiter because it's too close. Thus it must have been captured, but it's in a perfect orbit like something that did form there.

Just a hypothetical, could the low density readings from Amalthea be explained by it being hollow?
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JRehling
post Jul 10 2005, 08:16 PM
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QUOTE (Buck Galaxy @ Jul 9 2005, 08:05 PM)
So the problem as I recall is if Amalthea is ice it couldn't have formed with Jupiter because it's too close.  Thus it must have been captured, but it's in a perfect orbit like something that did form there. 

Just a hypothetical, could the low density readings from Amalthea be explained by it being hollow?
*


There's just about zero chance that it would have a large totally vacant cavity with solid walls around it, but it is quite feasible that a low-mass body could have lots of empty chambers inside. For example, imagine if you glued a bunch of roundish boulders together -- there would be lots of space between the boulders.

My guess for Amalthea is that it was captured because it happened to suffer a large, velocity-robbing impact while it happened to be flying by Jupiter on a near-miss solar orbit. That would likely cause an elliptical orbit, which could be rounded by tidal forces and interactions with Io. My personal theory is that impacts between small bodies when they happen to be close enough to a giant planet to be captured was a major mode of satellite capture, and is what is responsible for all the outer families of satellites in similar orbits around Saturn and (especially) Jupiter.
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Guest_BruceMoomaw_*
post Jul 10 2005, 09:55 PM
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Regardless of whether captured satellites are captured by gas drag or impact with already-orbiting satellites (one piece in "Icarus" on the orbits of Jupiter's irregular satellites says that there's orbital-mechanics evidence that BOTH possibilities have operated for different captured moons), it's not even necessary that Amalthea was initially captured during a very close flyby of Jupiter. It could easily have been captured into a more distant orbit that nevertheless took it through the orbits of some of the four Galilean moons, of which it then made close flybys that lowered its periapsis -- after which it survived long enough without hitting one of them that tidal forces (or friction with the dense gas/dust cloud still orbiting close to the early Jupiter) circularized its orbit. An awful lot of such objects might have been captured that way in Jupiter's early days, with most of them either finally crashing into one of the Galilean moons or into Jupiter itself, or being catapulted away from Jupiter and back out into solar orbit again -- and only a few finally finding a safe inner harbor like Amalthea. (If Thebe turns out to be ice, it's quite possible that it's a piece that got broken off Amalthea during this process, either by impact or by friction. I imagine Amalthea was much bigger at its start.)

This process is also plausible as the original origin for Saturn's rings -- with the captured object or objects getting so utterly fragmented in this case that it could never reassemble itself into a rubble pile.

As for Amalthea being hollow: well, in one of Arthur C. Clarke's earliest stories it turned out to be a gigantic abandoned alien spaceship, but the idea seems somewhat unlikely. (Poor Clarke -- the Solar System has turned out to be SO much less spectacular than he hopefully predicted.)
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Guest_BruceMoomaw_*
post Jul 10 2005, 09:58 PM
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By the way, the original theory was that Amalthea's low density was due to the fact that it was a loose, gravitationally clumped "rubble pile" -- which, in retrospect, is perfectly logical given the number of impacts it's undergone from infalling meteoroids. (Galileo's star sensor confirmed that some fairly large objects are being knocked off it right now and orbiting immediately nearby.)

However, the final figure for its density has turned out to be so low that it must be both a rubble pile AND largely icy -- just as with all of Saturn's moons closer than Mimas.
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edstrick
post Jul 11 2005, 05:40 AM
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Something must have *added* ice to form at least a small accretion disk around Jupiter long enough after planet formation that the sub-brown-dwarf heat from the young planet didn't evaporate the ice. There's a story there and possibly a very surprising one that only sample return from the inner "gravel" moonlets will really tell. Don't wait up for it!

I'd expected Amalthea and co. to be particularly interesting because they ought to be high temperature material from protojupiter's last accretion disk, heated by the young planet, but still presumably rather unaltered since accreting into moons.

It's unlikely the low density reading from the Galileo flyby is wrong, but I'm uneasy due to what seems the improbability of the result.
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Guest_BruceMoomaw_*
post Jul 11 2005, 11:58 AM
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Again, my operating assumption is that Amalthea did not accrete out of material from the circum-Jovian disk at all -- it's a captured KBO, like Phoebe but in an entirely different type of orbit (and I just described how it could have ended up there).
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Guest_BruceMoomaw_*
post Jul 11 2005, 12:00 PM
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Make that "a captured KBO or giant-planet zone planetesimal" -- in either case it could have been largely ice. The same applies to Phoebe.
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JRehling
post Jul 11 2005, 03:35 PM
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QUOTE (edstrick @ Jul 10 2005, 10:40 PM)
Something must have *added* ice to form at least a small accretion disk around Jupiter long enough after planet formation that the sub-brown-dwarf heat from the young planet didn't evaporate the ice.  There's a story there and possibly a very surprising one that only sample return from the inner "gravel" moonlets will really tell.  Don't wait up for it!
*


LOL.

A mission I'd like to see would be a sort of Genesis / Deep Impact hybrid that smashes impactors into various airless worlds, and collects fragments on a tennis racket - like aerogel. The real bang/buck potential would be to get multiple samples from one mission. For example, if an impactor was utilized at each of Jupiter's 5 biggest moons, with a sample taken from each. (I'm not sure if the orbital resonances allow the precise combo we wish, although I think scoring 4/5 should be possible.) That would be a heck of a lot of science return per delta-v. I think for missions of this type to take place is not a matter of if but when... maybe Mercury or Europa will be first.
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volcanopele
post Jul 11 2005, 05:30 PM
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QUOTE (JRehling @ Jul 11 2005, 08:35 AM)
LOL.

A mission I'd like to see would be a sort of Genesis / Deep Impact hybrid that smashes impactors into various airless worlds, and collects fragments on a tennis racket - like aerogel. The real bang/buck potential would be to get multiple samples from one mission. For example, if an impactor was utilized at each of Jupiter's 5 biggest moons, with a sample taken from each. (I'm not sure if the orbital resonances allow the precise combo we wish, although I think scoring 4/5 should be possible.) That would be a heck of a lot of science return per delta-v. I think for missions of this type to take place is not a matter of if but when... maybe Mercury or Europa will be first.
*

Smash and grab missions have their own problems, depending on the world. I can particularly see it as being problematic for Io. Io is predominantly covered in sulfur or sulfur compounds. While that maybe interesting to study the sulfur chemistry, the real question a sample return mission from Io (or any surface mission) needs to answer is to find out what the composition of the lava is. This help immensely in determining internal structure and patera formation. However, getting such a sample would require precise targeting of the impactor, it has to land within a known lava flow, which would require relatively recent reconnaissance (or an autonav capability for it to fly toward a 2 micron hotspot, then steer away from it at the last possible moment so it lands in fresh, but not still hot lava).


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DEChengst
post Jul 11 2005, 08:50 PM
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QUOTE (JRehling @ Jul 11 2005, 05:35 PM)
A mission I'd like to see would be a sort of Genesis / Deep Impact hybrid that smashes impactors into various airless worlds, and collects fragments on a tennis racket - like aerogel. (...) I think for missions of this type to take place is not a matter of if but when...  Mercury or Europa will be first.
*



I guess it would be quite hard on big worlds like Mercury or Europa to throw the debris high enough to capture it with the aerogel probe.


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tty
post Jul 11 2005, 09:10 PM
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QUOTE (JRehling @ Jul 11 2005, 05:35 PM)
LOL.
A mission I'd like to see would be a sort of Genesis / Deep Impact hybrid that smashes impactors into various airless worlds, and collects fragments on a tennis racket - like aerogel.


The obvious place for this would be Phobos. Phobos must have collected bits and pieces from every major impact on Mars and some minor ones. It would be the ideal place to get samples from all over the planet in one go (plus material from Phobos itself of course).
Deimos would work too and is not as deep in Marsī gravity well, but it would take rather larger impacts for the pieces to go that far, so the mixture would probably be a great deal leaner.

tty
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JRehling
post Jul 11 2005, 09:10 PM
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QUOTE (DEChengst @ Jul 11 2005, 01:50 PM)
I guess it would be quite hard on big worlds like Mercury or Europa to throw the debris high enough to capture it with the aerogel probe.
*


It would certainly be hard to get a concentration of material at a great height, even if an enormous blast took place. The key would be to get a lower flyby to compensate -- only engineering constraints prevent arbitrarily close flybys. I can imagine an autonomous flyby craft that is aimed initially to "hit" a tangent point on the sphere to achieve a nice pinpoint flyover, perhaps subkilometer.

Note that on an airless world, a "perfect" rubber ball dropped from height "h" would bounce back up to height "h" *regardless* of the local gravity. And a similar principle would apply to impact debris. An impactor would hit, eg, Mercury a lot faster than an impactor would hit Tempel 1, precisely because of the same gravity that would make the upflying stuff slow down more. In effect, you add escape velocity to the pre-encounter relative velocity. With these inelastic collisions, you certainly would get less "bounce" on a larger world than a smaller one, but the real issue is being close enough to get a nice dense (but not craft-smashing!) cloud of debris at the flyby time/place.

The ability of autonomous navigation to espy the right aimpoint in time to complete the manuevers would seem to depend upon ever-improving engineering parameters (mainly, computational speed). So I think we'll see it happen.
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Phil Stooke
post Jul 11 2005, 09:17 PM
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The Discovery mission Aladdin was designed to do just this sort of smash and grab at Phobos. I'd still like to see it.

Phil


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