Tuesday, August 09, 2005

Enceladus, plumes, and the CDA

Bruce Moomaw has a pretty length post over at the Unmanned Spaceflight.com forum covering some confusion regarding the Cosmic Dust Analyzer (CDA) results from the Enceladus flyby last month. The confusion results from whether the particles seen by the CDA instrument were in fact from impact-generated dust, or from a south polar plume. Early analysis suggested that the particles came from impact-generated dust rather than the south polar "plume" since the dust appeared to be uniform over the surface while the atmosphere/plume is not. Emily Lakdawalla, from the Planetary Society, posted an article a week or so ago describing the results, using information from the press release and an interview with Linda Spilker, the Cassini Deputy Project Scientist. In the article, Lakdawalla quoted Spilker as saying that the dust seen by the CDA instrument (the dust near Enceladus and the E-ring) were different from the water vapor seen by UVIS, INMS, and MAG, and that the south polar vents were not the source of the E-ring. However, as Bruce mentioned in a comment here, if the E-ring was generated solely by impacts on Enceladus, wouldn't other moons like Mimas have the same sort of impact-generated ring co-orbiting with the satellite.. So Bruce did a little digging and talked with Spilker. Turns out that Lakdawalla quoted her incorrectly. The difference between the CDA dust and the atmosphere seen by UVIS et al. is that the CDA saw water particles and UVIS et al. saw water vapor, nothing more.

In addition, Bruce dug up a new tidbit. Previously, the CDA team has been reporting that the dust was uniform across the surface (leading to the conclusion that the particles didn't have a localized source). Even if the particles were formed by condensing water vapor from the vent, there would be some sort of asymmetry in the CDA data biased to the south polar region. Turns out that the CDA may have seen an asymmetry in the amount of dust with respect to location over Enceladus. This is a very intriguing development and may indicate that the particles near Enceladus and perhaps the E-ring itself were formed from condensing particles from vents in the south polar region.


Blogger Mossbury said...

Apropos my original comments re origin of the e-ring. I was initially sceptical that sputtering was the cause as I read that even under best circumstances,sputtering probably would not produce enough vapour factoring in the expected e ring lifetime. Also, I read that the particles were spherical, of similar size as if condensed from a gas cloud. I am feeling vindicated, although that might change! Also, regarding my original comment that Bruce picked up on and concurred with, if the e-ring is due to sputtering why does mimas not have one. I have not heard any explanation. Is there an informal one circulating amongst the scientists? I would love to know. Steve Taylor

8/09/2005 11:52:00 AM  
Blogger Jason said...

I've heard 5 different explanations (some saying they ARE NOT condensed water vapor particles), others saying they saw an asymmetry.

I think what we need to do is stand back, let the CDA team finish their analysis of their data and see what shakes out. I understand that we want the answer now, but I don't think this is a problem that will be solved today, tomorrow, or next week.

8/09/2005 12:07:00 PM  
Anonymous Jerry said...

All the same, the candor of the team members, and others on this blog is refreshing. Please do not curtail this public dialogue because some of it is wrong.

The surprising data clearly demonstrate many preconceptions are wrong. This is science, not religion. We get to be wrong.

Thanks for sharing.


8/09/2005 12:52:00 PM  
Blogger Mossbury said...

At risk of pre-empting detailed analysis, is the Cassini science team expecting to get a more accurate estimate of the temperature of the surmised hottest part of Enceladus, namely the centre of the tiger stripes? Going slightly off immediate topic, I am absolutely spellbound by these images. You would imagine they are from some 3000 kilometer moon. Steve Taylor.

8/09/2005 01:22:00 PM  
Anonymous Emily Lakdawalla said...

Yes, it does now appear that I misunderstood the comments that Linda made to me about the difference between the E ring particles and the vapor vented from Enceladus -- I thought she was speaking of a compositional difference, but in fact she was referring to a different physical state, ice-particulate vs. molecular water. She told me that CDA is issuing a press release that should clarify a bunch of this soon -- but that the release is being hung up because HQ doesn't want robotic mission news to overlap with the Shuttle coverage. Grr. Hopefully that roadblock is just about out of the way now.

8/09/2005 03:43:00 PM  
Blogger Bruce Moomaw said...

Yes, my apologies again for jumping to the conclusion that you had actually seriously misquoted Dr. Spilker -- or simply concocted her statement: "So the vents are not the source of the E Ring." She's confirmed to me that she actually did tell you that, because it was the Cassini science team's own initial conclusion immediately after the flyby -- a conclusion which they have now wholly retracted (although they're still not yet willing to state definitively that the vents ARE the source of the E Ring).

I think we both got hornswoggled by the fact that the Cassini team jumped to that conclusion on ridiculously inadequate evidence -- and so you concluded that they had to have genuine chemical evidence for their conclusion, while I concluded that you'd just totally misquoted Spilker (a point that she didn't clear up at all during our first conversation).

8/09/2005 05:16:00 PM  

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