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NASA Briefs Preliminary Plume Findings from Moon Mission
Beauford
post Nov 13 2009, 10:38 PM
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QUOTE (ngunn @ Nov 13 2009, 04:23 PM) *
Before the specific mention of methane as a 'possibility' there was a comment about having found substance(s) that would evaporate at temperatures just 20-30 degrees warmer than ambient ground temperatures of -220C or -230C, possibly sublimed off a wider area around the crater by falling warm debris. Methane has a boiling point in this range - but are there other candidates?


There are quite a few that might move around by sublimation. Among the more interesting (and likely?) are formaldehyde, acetaldehyde, and ethylene oxide. ...the beginnings of a toolbox for organic chemists.
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SFJCody
post Nov 13 2009, 10:55 PM
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QUOTE (ngunn @ Nov 13 2009, 10:23 PM) *
Before the specific mention of methane as a 'possibility' there was a comment about having found substance(s) that would evaporate at temperatures just 20-30 degrees warmer than ambient ground temperatures of -220C or -230C


They certainly seemed to hint at there being a lot of highly volatile materials down there. Makes me wonder whether we could get brief, regular bursts of cometary style activity by putting a huge reflective surface in lunar polar orbit. It would make boring old Luna look a bit like Enceladus! (entirely different mechanism, of course)
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stevesliva
post Nov 13 2009, 11:23 PM
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QUOTE (Holder of the Two Leashes @ Nov 13 2009, 12:21 PM) *
Infrared detection of water vapor and ice, ultraviolet detection of hydroxyl (OH). Lower limit of about 100 kg of water was detected. "Twelve buckets full".


phew. That settles that, I hope. Glad it didn't end up the dry hole that was feared.

Any comments on the plume size versus any sort of modeling at this point?
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Fran Ontanaya
post Nov 13 2009, 11:38 PM
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They said the plume was magnitude 8 and had a narrow, vertical and less dense part, which was only visible for a few seconds, and a wider, more dense part that lingered for a longer time. Since the Centaur created a nice crater, they speculated that the ice wouldn't be a solid surface. Overall it fit their models, it just didn't display much over the crater rim.

The 100kg figure would be based in what LCROSS could see from the plume, not the whole plume.

They said also that the analysis from the telescopes was going to take a longer time because their signal vs background wasn't so strong and, if I understood it right, that they have seen changes in the surrounding terrain due to the materials coming from the impact.


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elakdawalla
post Nov 13 2009, 11:46 PM
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Re: what else other than water was in the plume: Colaprete said that he compared the spectra to those from Centaurs and Trojans and found a lot of similarities. There is probably a kitchen sinkful of crud at the poles that has accumulated there through billions of years of comets and asteroids bringing volatiles in that move around the lunar surface & get trapped at the poles. Centaurs & Trojans include water, CO2, CH4, SO2, methanol, ethanol, more complex organics; some of these are certainly present; but which ones and at what abundances are going to take lots and lots of modeling and comparisons among instruments and there may not be a unique model solution. He showed a model solution that was a pretty good fit, just to show that they are capable of fitting spectra, but he declined to indicate which species were in that model.


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Gsnorgathon
post Nov 14 2009, 12:46 AM
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Any idea what mass fraction that 100kg represents? Based on previous findings I'd guess "around 1%," but I'd love to know how much more (or less) it is.
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nprev
post Nov 14 2009, 01:11 AM
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Rats. Swear I saw an estimate for the dimensions of Centaur Crater somewhere, but can't find it now. Anybody got that?

BTW, check out the Google homepage; a nice little surprise just popped up there within the last hour. smile.gif


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tanjent
post Nov 14 2009, 03:07 AM
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Colaprete mentioned that at the time the target was switched from Cabeus A to Cabeus proper, all the Earth-based observers knew that their prospects for seeing the plume would be much diminished. They could have made that clearer to the general public at the time - it would have avoided the disillusionment when we saw those visuals with very little going on.

Regarding the effect of the water on the cost calculus of lunar exploration (I hope this is permitted...if not, by all means kill it) a useful metric would be to estimate the number of vehicle loads of infrastructure that would be needed to get the variable cost of rendering the local water down to the point where it is no more expensive than bringing water from Earth. If it were a matter of just filling plastic bags with volatiles and dragging them to a sunny slope, you could probably be in position to do that with virtually no heavy equipment. But the implication of twelve buckets from a thirty-meter crater would be that you'd have to set up a major factory to just take an occasional bath. In this case the scientific fascination with identifying all the volatiles, while it fully justifies the present mission, is not likely to translate into any speedup of the exploration timetable. Perhaps some beverage company will be interested to bring a little of the stuff back to Earth and bottle it? http://www.theonion.com/content/node/30505 .
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elakdawalla
post Nov 14 2009, 03:54 AM
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The Centaur crater is 20 or 30 meters in diameter.


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nprev
post Nov 14 2009, 04:43 AM
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Thanks, Emily.

Well, here's my (probably wrong) WAG. Assuming that the crater's only 5m deep & an average soil density of 3.5 g/cc, I get something like 2300 m^3 of material excavated for 8050 tonnes of mass.

They're inferring 100 kg of water from that, but of course that can't be interpreted as a full scan of all exhumed material.

All this says to me is that there's some water there. The means employed for detection really can't determine fine percentages of composition, IMO.

EDIT: Forgot to mention that I assumed crater diameter as 30m.


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ngunn
post Nov 14 2009, 09:03 AM
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QUOTE (Gsnorgathon @ Nov 14 2009, 12:46 AM) *
Any idea what mass fraction that 100kg represents?


I did a rough estimate and came up with one part in 40000 of the excavated material.
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nprev
post Nov 14 2009, 09:41 AM
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Actually, thinking about this some more, I don't know how that 100kg H2O figure could have been derived unless it was with respect to the mass of the plume itself, which has to be less than the total mass excavated due to sidescatter, oblique ejecta trajectories, etc.

If that's true, how then did they estimate the mass of the plume since it appeared to differ so significantly in many respects from the pre-impact models?


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Fran Ontanaya
post Nov 14 2009, 01:02 PM
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It could be from the strength of the spectral signal against the background.

They said 100kg is the estimated lower limit in the field of view. Many volatiles were quickly vaporized and expanded away.


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centsworth_II
post Nov 14 2009, 05:41 PM
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QUOTE (Fran Ontanaya @ Nov 14 2009, 08:02 AM) *
It could be from the strength of the spectral signal against the background.
Right. I thought the 100kg was an estimate of what was actually seen in a narrow field of view and that they are not ready to estimate what was in the entire plume or in the entire excavated area.
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remcook
post Nov 14 2009, 05:43 PM
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My guess for the additional fitted gases in their plot ( http://www.nasa.gov/images/content/402265m...sults9_full.jpg )
(They look like gases... for instance, the big 1.5 and 2 micron water ice features [ http://www.tng.iac.es/news/2001/02/05/tran...g-trans-nep.gif ] do not seem to be fitted):
CO2 at 2-2.1 micron:
http://vpl.astro.washington.edu/spectra/co...agesmicrons.htm
ethane? or some other hydrocarbon at 1.7 micron
http://vpl.astro.washington.edu/spectra/c2...agesmicrons.htm
http://vpl.astro.washington.edu/spectra/ch...agesmicrons.htm
http://vpl.astro.washington.edu/spectra/c3...agesmicrons.htm
...

edit - added link to NASA plot
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