Amalthea And Company Options

[tedstryk]

I had a thought. Is it possible that there was once a fifth large moon in the Jovian system - maybe not another Galilean, but at least a Rhea or Enceladus? And perhaps it was ripped apart by a collison with a kuiperoid in Jovian orbit. The pieces of it, mixed with the fragments of the impactor, may have never been able to reassemble given the tidal environment, but instead formed the rings and the little group of inner moons. If the kuiperoid was much larger than the satellite it hit - but coming at a high enough relative speed to be shattered anyway, that would explain the lack of rocky materials. Any thoughts on this?

[edstrick]

I divide outer planet moons into "inner gravel", "regular moons", and "outer gravel".
Post-Voyager interpretations were that the tiny inner satellites of the giant planets could have been fragmented and reaccreted many times, and that even the mid-sized inner satellites at Saturn, Mimas through maybe Dione, could have been fragmented and reaccreted, particularly in the first hundreds of millions of years of the solar system.

[tedstryk]

I divide outer planet moons into "inner gravel", "regular moons", and "outer gravel".
Post-Voyager interpretations were that the tiny inner satellites of the giant planets could have been fragmented and reaccreted many times, and that even the mid-sized inner satellites at Saturn, Mimas through maybe Dione, could have been fragmented and reaccreted, particularly in the first hundreds of millions of years of the solar system.

I just wonder if Amathea, Adrastea, Thebe, and Metis have a common parent (or parents, if it was a primordial moon and an impactor).

[edstrick]

Exactly!

[Guest_BruceMoomaw_*]

Big problem with that theory: Amalthea seems to be mostly ice (its density is so low that it apparently can't even be a "rubble pile" made of loosely clumped rock fragments), whereas Galileo proved beyond doubt that Metis and Adrastea are the source of Jupiter's ring particles -- which are made of silicate rock. I think we may have several separately captured objects here, with Amalthea being a KBO (like Phoebe) while Metis and Adrastea are captured asteroids. (God knows about Thebe.) Anyway, this is yet another example of the Solar System's seemingly endless ability to throw totally unexpected surprises at us.

Is it possible that some of the objects captured by Jupiter migrated in close enough to the planet that the gravitational fields of the Galilean moons could start ricocheting them around the system -- so that they all ended up either being flung completely away from Jupiter (or into very distant orbits) or crashing into the Galileans, except for a very few that finally ended up in a safe haven very close to the planet? If so, we have another analogy between the Jupiter system and the Solar System, since the giant planets did the same thing to most of the debris in the outer Solar System.

[Bob Shaw]

I suspect that the orbital dynamics of small moons and large moons in the Jovian system are currently all about resonances, and that - a la shepherd moons at Saturn - there are some strange dances with an almost indefinite lifespan. However, the orbital 'pumping' of the debris (a better word than 'ricochet', I think) in the early Jovian system could well have ejected material. And yes, that seems *so* similar to the inner Solar System, doesn't it?

[Gsnorgathon]

... with Amalthea being a KBO ...

I wonder what sort of ring Amalthea would make if it broke up?

[tedstryk]

"Big problem with that theory: Amalthea seems to be mostly ice (its density is so low that it apparently can't even be a "rubble pile" made of loosely clumped rock fragments), whereas Galileo proved beyond doubt that Metis and Adrastea are the source of Jupiter's ring particles -- which are made of silicate rock. "

Yes, but it is also possible that Amalthea and Thebe are from different parts of the primordial object than Metis and Adrastea. This would take more number crunching than I am capapble of, but perhaps a Kuiperoid came smashing into a primordial enceladus-size satellite that was differentiated with a rocky core. It is possible that the remains of the core were not evenly distributed in the remains that no doubt would have made a spectacular set of rings, and thus the current satellites are rebuilt remnants. It would be neat if in a high resolution image of Amalthea, we could see a chunk that showed a portion of the surface of a primordial world. I hope that Juno's orbit will allow for close enough serendipitous passes by these four worlds to get better mass figures (other than for Amalthea, for which we have good numbers). Other remote sensing compositional studies may be limited, as these moons may be covered with too much Io-junk (perhaps this is what is sputtering off Metis and Adrastea into the rings - and we know there are particles related to Amathea either sputtering off or related to its acreation, based on the star-scanner data).
Here are the best images of the moons, mixed with color data. I tried doing the super-resolution thing with Thebe. The sets of images that looked like they could be stacked were too distant to be useful - they still weren't good enough to compare with the best regular images, except for the e4 set. And it is well matched up with the E26 high resolution image. So well matched up that with a little refiguring, I was able to overlay it on the E26 data with a good result. I also sampled other Amathea color sets, painting pixels from the various regions covered in the E26 data into a mask to make a color dataset for the E26 b and w data. Amalthea is the best imaged of the bunch. Although the one "highres" view of Thebe has a resolution of 2.0 km/pixel compared to 2.4 km/pixel, smaller detail can actually be discerned in the Amalthea image, because the Thebe image is very noisy, ruining the fine detail (and causing a false smooth appearance in processed images). I used mutispectral data to get a rough color for Metis (which was all that could be done because such sets were so distant) and overlay it on the E26 data.

[dvandorn]

Ah, yes -- I remember from the Voyager releases the mention of the "pizza-colored" Amalthea. I hadn't been aware that Metis and Thebe were also tinted orange-red, though.

Am I right in my recollection that the red color comes from sulphur sputtered off of Io (the same source as the sulphurous torus that accompanies Io in its orbit)?

-the other Doug

[tedstryk]

Ah, yes -- I remember from the Voyager releases the mention of the "pizza-colored" Amalthea.  I hadn't been aware that Metis and Thebe were also tinted orange-red, though.

Am I right in my recollection that the red color comes from sulphur sputtered off of Io (the same source as the sulphurous torus that accompanies Io in its orbit)?

-the other Doug

I based my color calibration on the fact that looking at response in each filter and exposure, and comparing it to Amalthea, it is strikingly similar for the other inner moons. There is no data good enough to tell if the other moons have varied areas like Amalthea's white spots (There is one data set for Thebe that might show a large patch if it were there, but it shows nothing of the sort).

[4th rock from th...]

Very very nice color images of these little moons!
Good work!

[tedstryk]

Thanks. I have also put together a montage of all views of Amalthea (in the case of multispectral sets I simply selected the best frame to use). It shows this little moonlet from all angles it has been photographed, showing how truely irregular it is (this set includes both Voyagers and Galileo).

[Decepticon]

Wow! Thats a keeper!

[Guest_BruceMoomaw_*]

It is. And those light-colored patches make SO much more sense now that we know they're exposed ice.

[Guest_BruceMoomaw_*]

I hadn't even noticed that Ted's color image of Thebe clearly shows a similar white patch on it. I just finished an E-mail suggesting that the reason we aren't seeing Jovian rings made out of material blasted off Amalthea or Thebe (like those formed from Metis and Adrastea) is that both moons are ice, and high-speed infalling meteoroid impacts on ice tend not just to shatter it but to completely vaporize it.

Galileo's star sensor was used during its close Amalthea flyby to look for local debris blasted off it -- and, sure enough, it clearly saw several nearby objects up to several dozen meters across. But such detached ice chunks have a much shorter lifetime than the chunks of silicate rock blasted off Metis and Adrastea. Amalthea must have undergone one hell of a sandblasting during the time when the remaining debris ring close to the early Jupiter braked it into a close circular orbit -- it must have been much bigger before then -- and I wouldn't be surprised if Thebe isn't a piece broken off Amalthea during that period.