[X] My Assistant

[elakdawalla]

IIRC the reason the Deep Impact impact was so spectacular was because of the amount and fineness of the dust. The lack of any visible curtain probably tells us something pretty significant about the nature of the substrate but I'm not sure what it is. Mcaplinger, correct me if I'm wrong, but I don't think it's the presence or absence of volatiles (which weren't expected to have been present at more than a couple percent as far as I understand) but the grain size and porosity of the material at the impact site that contributes most to the size of the ejecta curtain. When Deep Impact was about to happen, there was lots of discussion of Peter Schultz's experimental work on impacts into different kinds of materials, and I think their biggest concern was that they'd impact into a cohesionless target where there hardly be an ejecta curtain at all, just sort of a backward squirt out a deeply punched transient crater. (I think that was a "compression-controlled" impact as opposed to "gravity-" or "strength-controlled."

Funny, while I was researching that last sentence I was reminded of the manner in which the Deep Impact impact was unexpected. The Society ran a contest to see who could guess closest to the actual size of the impact crater, but the Deep Impact ejecta curtain turned out to be so spectacularly large and dusty that they were never able to see the crater. So we were forced to pick at random an entrant who'd been among the many who'd predicted a size between 100 and 250 meters, which pissed a lot of people off. Guess that goes to show you that making predictions is a bad idea

--Emily

[Holder of the Tw...]

I hope the Society kept the list. In mid-February 2011, you might have another chance to make the award a second time, albeit belatedly.

[mcaplinger]

Mcaplinger, correct me if I'm wrong, but I don't think it's the presence or absence of volatiles (which weren't expected to have been present at more than a couple percent as far as I understand) but the grain size and porosity of the material at the impact site that contributes most to the size of the ejecta curtain.

The size, perhaps, but not necessarily the visibiliity. See http://lcross.arc.nasa.gov/docs/Colaprete....SS-overview.ppt page 14 for a discussion of the "vapor cloud" which contributes in some not-well-explained way to the impact visibility through excitation of OH- and H2O+.

I read http://www.lpi.usra.edu/meetings/leagilewg...pm/Bart4050.pdf but it's not clear there how volatiles were expected to contribute to plume visibility; all of the predictions were based on 1%.

At any rate, I don't claim to be especially knowledgeable on this topic and could well be wrong, but to my eye these results aren't lending a lot of credence to the already fairly poor case, prior to LCROSS, for there being a lot of ice at the lunar poles.

[nprev]

That's a pretty good point re excitation of ions; if there were a lot (& I mean a LOT) of volatiles in the impact plume, you'd expect it to light up like a neon sign once it was exposed to the solar wind.

I think the issue is whether there was a plume of any significance or not, though. Evidence thus far seems to indicate that there wasn't, and I'm not sure if the often-cited hypothetical 1% H2O content would have been a player in that at all if it does in fact exist; seems like too small a concentration to affect the material properties of the soil (if the Centaur in fact hit soil) at the impact site.

All we really seem know right now is that at that specific place where the impact occurred there was less material ejected then expected, reason unknown. The spectroscopic data should shed some light (ta-da, da!) on the ejecta's chemical composition but probably won't yield much information about its pre-impact physical properties. And when the dust finally settles (please, somebody stop me!), all the data & analysis will again only be truly relevant to that very specific point on the Moon's surface, and it might be erroneous to extrapolate the results as representative of the entire South Polar region.

[MarsIsImportant]

I just thought of something. Some say that there wasn't much ejecta because maybe LCROSS hit something very hard like a boulder. Well, the hydrogen signature is definitely there in that crater. Most believe it is water ice...perhaps only 1% of H2O in the soil. That's an assumption, right? What if the concentration is a lot greater than that?

Water ice in space is much different than here on Earth. It cannot form crystals in a vacuum. We call this form 'amorphous ice'. It is very much like hard glass only far colder. Imagine LCROSS hitting a block of hard glass that is 50 or more meters thick. It might as well as hit a solidified lava flow. Am I right?

I'm sure there would have been some damage to the block. But the forces involved in the impact would have been directed differently than expected. Wouldn't there be a lot less ejecta going upward in such a case? Wouldn't most of the forces be directed more horizontally and the ejecta would be less likely to get above the rim? I'm not an expert on impact dynamics, so help me out on this one.

[jmknapp]

LCROSS could only be considered a failure in the sense that the Michelson-Morley experiment to detect "luminiferous ether" was a failure. Rather, both experiments were executed flawlessly and disproved the existence of the hoped-for substance (in this case volatiles). So disappointment rather than failure is indeed the bottom line.

[imipak]

Imagine LCROSS hitting a block of hard glass that is 50 or more meters thick.

A 50m thick layer of ice would have been unambiguously detected by one of the many previous spectrometers and other instruments to overfly the poles.

[centsworth_II]

...executed flawlessly and disproved the existence of the hoped-for substance (in this case volatiles). So disappointment rather than failure is indeed the bottom line.

Everyone here should agree: Good science and successful experiments tell us what is there, not what we wish was there.

However, I think you're premature in saying what the experiment has proved or disproved. No visible debris cloud does not equal no volatiles. That information is found in the spectroscopic data which has yet to be reported on. What I got from the press briefing was that spectra of the flash were obtained. I don't know if those are ideal or sufficient for detecting volatiles. Also, although there was no visible cloud, I don't know if it has been stated that there was no spectrographic data collected in the aftermath of the flash from invisible products of the impact.

[MarsIsImportant]

I think we should wait for the results before making any conclusions. I've read that they obtained good data and need to process it thoroughly before making anything public.

There really is no reason to play a guessing game until we have all the information.

[Reed]

Rather, both experiments were executed flawlessly and disproved the existence of the hoped-for substance (in this case volatiles).

I don't see how a negative result from LCROSS would unambiguously disprove volatiles at the poles in general. Since we don't have any instruments that resolve down to the scale of the crater, we can't say whether we hit a representative sample. The LEND data already suggests that hydrogen concentration is variable in the shadowed craters at larger scales, and it seems reasonable to assume this would continue to smaller scales. If good spectra has been obtained from both the centaur and spacecraft impacts, that would help, but AFAIK we don't know that yet.

A positive result would tell us, yes, there is at least X percent in some places. A negative by itself doesn't tell us much, although the rest of the data might.

[JRA]

I didn't see it mentioned on the last several pages, but apparently the LRO detected an impact plume from LCROSS with its LAMP instrument and Diviner detected the impact crater. So it sounds like there was a plume created from the impact, but just not large enough to be seen from Earth I suppose. Hopefully they release the data from LAMP soon, if they haven't already

Some links.
http://lroupdate.blogspot.com/
http://www.diviner.ucla.edu/blog/?p=184

[glennwsmith]

Although I must agree with Imipak that a 50m thick layer of ice would have been unambiguously detected by now, I like the way marsisimportant is thinking in his post about the strange physics that may go on in the vacuum and cold of the moon, which physics might prevent a ejecta cloud from forming -- and precisely because it DOES contain a lot of water.

For water, as we all know, does have a lot of enigmatic qualities from a physics standpoint.

Indeed, what happened with LCROSS might be akin in terms of science history to the recent discovery of water across the sunlit surface of the moon. Yes, theoretical physicists might know that high speed protons can interact with the oxygen atoms in rock to form water, but apparently nobody had thought of applying this arcane physics to the surface of the moon to predict the presence of water.

Likewise, there may be some arcane physics -- possibly involving water! -- which accounts for the non-appearance of the ejecta cloud. In fact, Emily recently mentioned in another context that the water molecule is very sticky.

[Mogster]

I haven't noticed negativity in the UK media, some "Americans bomb the moon" headlines but that's to be expected over here.

Most of the after action reports seem to be highlighting the unknown factor, why didn't it produce visible results as expected? Which is OK imo.

[djellison]

The live coverage I heard on Radio 5 Live here in the UK was bordering on cringeworthy. The women presenter was dismissive, demanding and to be honest, offensive to the on-air expert.

[Guest_Sunspot_*]

They're like that with any thing to do with science - most BBC journalists come from the world of the arts or politics. Coverage of the '99 eclipse was appalling.