Deep Impact mission reveals comet's icy cargo

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Everything I need to know I learned through Googling.

Thank you, ToSeek, That is a much better conclusion, it is based upon Deep Impact data, and I hope he is willing to explain this to O'Hearn and Sunshine, who don't seem to get it.

Even this characterization is troubling: We did not see the impact crater, and the density is based upon an analysis of the hyperbolic return of dust particles to the surface. The error bars on this analysis are at 50%, but I think they could even be wider, since static effects are not know and the initial velocities of the returning particles are inferred. The surface looks like it is cratered, which means there would be surface resilience, strength, a stable morphology.

Finally, to accumulate and hold all of that surface dust implies much greater mass and density - what is holding all that dust? It should escape with each and every impact - just as dust excapes from the moon. This is a very small body.

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jwj

It's ok not to know. We should try harder to find out.

Of course, Tempel-1 deep inpact is only a sample of 'one'. But there seems to be enough evidence that perhaps comets are not 'dirty iceballs' anymore, in fact have little water, and are more like dirty 'dust balls' instead.

I've long had a notion (if any had followed my past posts) that comets are little solar region 'vacuum cleaners', gathering molecules and dust on their way out into the colder regions, and letting them loose again into the inner regions of the solar system. At this time, I'm not sure we really know what comet tails are made up of, but they should turn out as de-pressurized dust particles from comets in the hotter inner regions; in the outer colder regions, the process should be opposite, where they gather particles; hence, they don't shrink into nothing over time, but remain replenished. This is not the same as now theorized, that comets are remnants of the early solar system's formation; rather, they are cosmic entities in their own right. Will comets turn into asteroids eventually? Hard to say, but probably not.

BTW, this is perhaps more ATM for now, but as more data comes in from the outer solar system, we should get a better fix on why comets do what they do, and are fluffy dust balls rather than ice balls. (However, for dust to be more attracted on the comet's mass in outer regions might mean something that had been debated on ATM, that G is greater out there than here, but that's a line of reasoning which is not crickey to discuss here.) One way to confirm whether or not comets are distant solar system scrapers is to tally what loose dust and molecules constitute 'empty' space out by the Kuiper belt and beyond.

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Credibility is simply incredible... sometimes even to me.
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That's not necessarily true. A powdery surface can indeed show cratering effect due to a lack of destinctive erosion systems during and after an impact. The dust and ice is ejected and sent on a ballistic trajectory. Those particles with less velocity will, of course, fall back down to the surface...weak gravity or not. Also the initial point of impact could cause a "ripple" of molten material to form due to the powdery surface quickly melting and refreezing...much in the way a crater stiking bedrock can form ripples of molten material inside the blast crater that quickly freeze into place.

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"The bread's hollowed out --- the veggies go on forever --- and --- oh my God! --- it's full of meat!" - Maksutov

I don't think that its time to put Whipple to rest. We smacked one comet that has been in the inner solar system for who knows how long. It would have been interesting if we could've smacked a comet that had just come in from the Kuiper Belt. Hale-Bopp or Hyakutake would have been ideal. Unfortunatly comets such as those are rare and don't give us enough lead time to get a mission ready.

Be great if we could get another DI mission ready, and put it on standby for the next great comet.

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I'm not a scientist, but I play one on the internet.
http://www.rrac.org

I don't agree with you assertion that Whipple's comet model has any value at all. It is clearly shown that there is not enough ice to produce whatever kind of jets (by orders of magnitude). There is now evidence from several comets (Borelly, dry as a bone, Wild 2, with sharp relief and steep cliffs, and now Tempel 1) that show us that ices are a no-show, jets are extremely narrow and can emit from the "darkside". This means that the whole idea of solar irradiation of comets giving rise to outgassing and jet formation is void.

Whipple is out of the picture, the ices have now gone "underground" (subsurface), and please don't tell me you can get the ices heated from the outside through layers of dust. It just doesn't add up. Admit it, we need a new comet model.

Cheers.

Not so, dust won't give you steep cliffs, it will give you gentle slopes, and what impacts do you think occur on comets? If they are impacted, it will be micro-sized material. Comets are tiny, and don't have any gravity to speak of. They are just not a target for impacts, unless guided by electronics of course
.

Cheers.

Or randomness. Remember how long those comets have been out there. Those impacts probably only happen a billion or so years after its birth...plenty of stuff still floating around the early solar system. Given that these things were cutting in and out of the orbital plain I wouldn't be surprised if they were impacted quite readily with small particles. That would certainly explain its dust coating.

** Edited -- crappy spelling

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"The bread's hollowed out --- the veggies go on forever --- and --- oh my God! --- it's full of meat!" - Maksutov

Agreed...but I still think the nucleus is more dense than O'Hearn's team. The density is based, in part, upon particle size, and Stardust gives us reason to believe the particle size estimates may be too small. It is EXTREMELY difficult to accurately estimate particles size on the bases of backscattered light.

Remember, the Whipple model was just an educated guess before we had ANY real data. NONE of the real date point in the direction of an icey dirt ball, and certainly not a dirty ice ball.

This is exactly why evidence must be studied objectively, and without preconceptions as to what it means. I had an undergrad professor who insisted on calling the 'Nobel elements', inert gases; even though it was obvious that they react, both naturally and in man-induced synthesis. It was what he had been taught and what he taught for forty years, but it was wrong.

It is ok for a theory that has been taught forever to be wrong - that should be the keystone difference between a scientific theory, and a religious tenent. It has taken NASA six months to admit the Whipple model is now dead. Comets are still very fascinating objects - But Fred would not be defending a dirty iceball with less than 0.01% surface moisture. Fred knew he was mortal, and an educated guess is always trumped by observational evidence to the contrary. The icey dirtball is just as lame. The jets - high in moisture - clearly do not represent the same class of sample ejected by Deep Impact.

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jwj

It's ok not to know. We should try harder to find out.

Hold on inert or "nobel" gases are called so because normal conditions don't allow them to react very readily to other compounds...they have to be forced into doing so; either through a special natural process or a man-made initiation. It wasn't that you were taught wrong, its that inert elements are inert most of the time.

So why does a comet have to have a high surface moisture content? Can't voids in a dust covering allow different jets with different compositions depending on the distribution of the compounds within the core? As I see it we <i>are</i> looking at a different class of comet; one that's drier than outer solar system specimens. Whipple may have not been completely correct, but it certainly wasn't wrong. They definately aren't as dry as an asteroid.

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"The bread's hollowed out --- the veggies go on forever --- and --- oh my God! --- it's full of meat!" - Maksutov

Remember that ice was central to Whipple's model, specifically for the production of jets, the new data show that it doesn't work this way. Whipple's model is wrong, it's that simple (btw Borelly was aslo dry as a bone, so Tempel 1 is not a one-off comet).

Now to get to a new model, whatever the mechanism, I think jets are the key feature to be explained. The "Whipplers" are now resorting to subsurface pockets of ices/volatiles, but that leaves the big question how those pockets get heated to produce jets. I want to point out that jets are produced far from the Sun as well, and remember those jets are very narrow and high speed. Just solar heat isn't going to work.

Cheers.

Forgot to mention that asteroids are thought to contain ices too, so your assertion that comets are not as dry as asteroids is unfounded.

As an aside, there is a news item about a binary asteroid (Patroclus) that seems to resemble a comet (based on it's calculated density), so possibly asteroids and comets have much more in common. Since this is the only binary asteroid found to date in the Trojan family of asteroids in Jupiter's orbit, I'm curious what Jerry has to say about this density calculation.

Cheers.

All we have looked at are comets that have spent a lot of time in the inner solar system. Even Halleys Comet made numerous passes through the inner solar system. Look at Enceladus, its outgassing as a comet would, but its a moon of Saturn. We would certainly call it a comet if it passed through our realm of space.

Until we can examine comets from the Kuiper Belt or Oort Cloud, I don't think the Whipple model can totally be put to rest. What we found at Temple 1 seems to me to be consistant with an almost burnt out dirty snowball, its mostly dirt now.

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I'm not a scientist, but I play one on the internet.
http://www.rrac.org

Whipple was trying to characterize the comets we have seen, not what might be out there. Otherwise, I agree - There is no reason that a family of comets cannot exist that more closely resemble the icy moons of Saturn.

Check out this image of a spec of something in Stardusts Aerogel:

http://www.berkeley.edu/news/media/r...stardust.shtml

Notice the Sonic coning, caused by the high speed penetration of the particle, and now think about this:

Aerogel has a void volume of >99%. According to NASA, Tempel 1 has a void volume of 90%. Both aerogel and the dust raised from Tempel 1 by Deep Impact are silicates. If Deep Impact entered a body with 90% void volume, most of the energy would be dispersed in the same way it is in aero gel: Outwards, and downward, relative to the point of entry. This is true of fiberglass, kevlar, pumis, steel wool, styrofoam, goose down, wool, and the head on a pint of Guiness. In any subtrate with high void volume, momentum and force are absorbed outward and downward from the point of impact.

Deep Impact's ejecta emerged in the same direction in which the probe impacted. Since there was little water and other volatiles in the ejecta, there should have been very little gas pressure to disipate within the nucleus of the comet. If Tempel 1 has the structure NASA proposes, there should have been little, if any dust emerging from the hole. The model proposed by NASA is STILL fatally flawed.

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jwj

It's ok not to know. We should try harder to find out.

Interesting you mention Enceladus, it only makes it worse for the dirty snowball model; if the mechanism responsible for the jets on that moon are comparable with the mechanism that form cometary jets, sunlight is not the source. So, whatever the ice content of the body in question, the results so far, silicates and jets, even from what you describe as a "burnt out dirty snowball", indicate that Whipple's model is wrong on the key points.

We might as well start afresh. Starting with the first important question: how can we get water molecules (or OH-ions) from a surface that contains almost exclusively silicates or dust, without resorting to hidden pockets of ice?

Cheers.

Upon further reflection, when deep impact slammed into the comet, it is possible to get a reflection from each and every layer - this may be why Sunshine is talking about layering in the nucleus of Temple 1. But layering also requires different strata with different densities. It does not seem likely to me that anything other than a surface layer of dust would produce dust. In other words, the deeper layers should break up in chunks - they must have varying densities to continue to expell ejecta, and the ejecta must vary layer by layer.

We don't see that: We see dust, dust that in my opinion, was much more likely the results of a surface (transitional) wave moving rapidly across the surface of a very hard object.

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jwj

It's ok not to know. We should try harder to find out.

The old "iron covering", right? Clang, clang, clang went the comet, eh?

It would be nice if you could "show us the evidence" instead we get...

...your thinly veiled implication that NASA doesn't know what it is talking about...once again...

I sure would like to know exactly how it benefits NASA to be so "stubbornly" wrong about this...


edited to add...IMO...comets having hard "shells" is an ATM idea...

The dust is very fine, is it likely that the dust from the impact could support a surface pockmarked with craters, depressions and scarps as seen in the images? Also what about comet Wild 2's surface features, could those be formed in a vast layer of dust?

Cheers.