Where's the discussion on Stardust data?

CalliArcale · Mar 22, 2006

<blockquote>In reply to:<hr />Despite all the tap dancing, there's no getting around the fact that the kind of porous object scientist suggest comets are would fracture, shatter or spin wildly upon significant impact. <hr /></blockquote> I don't think that's the inescapable fact you present it as. All it really means is that the craters we see on comets weren't caused by objects the size of the Chiczulub impactor. They were lower-energy impacts. Most likely, if struck by larger and/or faster moving objects, they would indeed shatter. It is certainly possible that this has happened on a number of comets -- possibly even the ones we've observed. There's no guarantee that they've always been the size they are now, after all. <blockquote>In reply to:<hr />Dating by crater count is inane<hr /></blockquote> It's not inane; it's just not precise. In many cases, it's the only option available, which means it's better than nothing. Scientists are well aware that it hinges upon a lot of assumptions about which little is known. It is therefore inevitable that as they refine those assumptions, the estimated ages will change as well. <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

bonzelite · Mar 22, 2006

maybe meet half-way and ponder that the surface of the comet was worked over by something else besides a tried and true impactor(s). that's all i ask. consider, as well, the coma may be a fragment of something. several comets that visit Sol may have originated from the same body. and they may have been very molten when they scattered. like lava rocks. i will consider impactors if you consider another means of surfacing. we may never fully agree, but we both don't really know what they are all the same! <img src="/images/icons/wink.gif" />

CalliArcale · Mar 22, 2006

No problem. I'd expect many processes to be involved at different times. <img src="/images/icons/wink.gif" /> <blockquote>In reply to:<hr />and they may have been very molten when they scattered. like lava rocks. <hr /></blockquote> Not to get too far from comets, but at least one larger body appears to have been liquified at some point in its past. Tethys. (Enormous impacts have very obviously been a major factor in the Saturn system, so Saturnian moons are a good place to study how this works.) It has a huge crater on one side, called Odysseus. But the crater is not sharply defined at all. It's smooth and relatively faint, and conforms very much to the curvature of Tethys. It is popularly thought that it was struck by an object just a little bit too small and/or slow to disrupt Tethys, but big enough to nearly completely liquify the tiny world. Thus, Odysseus has a very fluid look to it, even though it has long since solidified. Large objects made of liquid seem to end up very smooth. I'm not sure if comets are too small to do that, but I would think they would not be as pockmarked as a pumice stone; it is large enough for gravity to have an effect in smoothing out some of that. But not all of it. Certainly they're not big enough to become spherical, but the small bodies which have been studied more closely (especially 433 Eros) show signs of gravitational affects (things rolling downhill, mainly), so I would think that some voids and pockmarks would collapse under their own weight, while others would have enough structural strength to stay open. One thing that points in favor of a truly catastrophic impact in the past of Wild-2 is that such an impact could possibly provide the heat neccesary to produce olivine. At least, I think it could. Does anybody know? Or would such an event be too transitory to produce minerals such as olivine? <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

bonzelite · Mar 22, 2006

JohnClarke can speak to olivine quite well. John? you around?

yevaud · Mar 22, 2006

Calli: I'll mull it over, look at all of them, and get back to you on this this evening. *Sorry - was at work all day* <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

dmjspace · Mar 22, 2006

Yevaud said: Comet densities have been estimated (and in some cases, actually determined via observational data) at between 1.0 gram per c^-3 - 2.0 gram per c^-3. That indicates the presence of significant volatiles. Determining the density of a comet coma is not the same thing as determining the density of its nucleus. The density of comet nuclei is unknown. Even the figures from comets we've supposedly measured are highly controversial. Or, as the science team member in your LINEAR link said, "Our opinion about the low density is tentative and controversial." The fact that Tempel 1 was unchanged after it was slammed with a probe strongly suggests it is a solid body. The Deep Impact team did indeed infer a nucleus density from the transient fallout. However, the team used many debatable assumptions, and if Deep Impact merely kicked up a lot of regolithic dust and hardly touched the nucleus, all estimates of density pertain to the regolith, not the comet itself. I posted this before, but here's a summary of why the Deep Impact team's estimates are dubious. Comments in brackets are marked with EPH proponent Tom Van Flandern's initials ("tvf") : Comets. Deep Impact results. http://www.sciencemag.org/sciencexpress/recent.shtml. Three papers with mission results in Science Express, with publication in the regular magazine to follow: (1) “Deep Impact Observations by OSIRIS Onboard the Rosetta Spacecraft”, H.U. Keller et al. The dust to water mass ratio is much larger than 1. [tvf: Comets are more “dirt” than “snowball”.] (2) “Deep Impact: Observations from a Worldwide Earth-Based Campaign “, K.J. Meech et al. There was new post-impact material compositionally different from that seen pre-impact. The dust-to-gas mass ratio in the ejecta was much larger than pre-impact. The new activity

yevaud · Mar 22, 2006

Lot to cover here. Most of which will take a while, so I'll initially answer the last first: No, I'm not suggesting that at all. If you want to discuss the EPH in it's entirety, I don't see a problem with it on the one hand; however, I don't see how that can be done in this thread, without overriding the Stardust discussion. Therefore, I am suggesting a seperate thread where EPH can be discussed. Else, the Stardust debate will be co-opted into an EPH debate. They should be somewhat seperate, as they will diverge widely before each is "concluded," such as it were. More later. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

yevaud · Mar 22, 2006

Determining the density of a comet coma is not the same thing as determining the density of its nucleus. The density of comet nuclei is unknown. Perhaps and perhaps not. How large a nucleus are we speaking of? Over a certain mass, it would tend to predominate when overall density was determined, would it not? So if there, it may not be terribly large. I must mention that I am not antithetical to the idea of a Comet being formed around a nucleus. Certainly this is what I was taught with respect to the formation of raindrops, hail and snow. If the preliminary findings of the mission are correct (“Layered like an onion”), then this wouldn’t be a terrible stretch. Even the figures from comets we've supposedly measured are highly controversial. Or, as the science team member in your LINEAR link said, "Our opinion about the low density is tentative and controversial." Of course. But that would equally make the assertation as to the nucleus being present and of a much higher density just as speculative, wouldn’t it? The fact that Tempel 1 was unchanged after it was slammed with a probe strongly suggests it is a solid body. Doubtful. Again, we’re not talking a tiny body – we’re discussing a very large object indeed (a radius of 3 km). The Deep Impact team did indeed infer a nucleus density from the transient fallout. However, the team used many debatable assumptions, and if Deep Impact merely kicked up a lot of regolithic dust and hardly touched the nucleus, all estimates of density pertain to the regolith, not the comet itself. True. However, a point not mentioned is that the Stardust platform must have been at least nominally affected by whatever gravitation Tempel-1 possesses, which would provide some important information as to the total mass of the Comet. I imagine this is being parsed as we speak. There's no doubt that water resides in comets and/or th <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

bushuser · Mar 23, 2006

Am I covering old ground with this? Comet Wild-2 may be a frozen rock now, but we cannot assume the orbits of most comets remain stable over eons. Could the heat signature in the Stardust debris reflects a time when perihelion was much closer to our sun?

yevaud · Mar 23, 2006

Most certainly. There's no telling what events took place leading to it's current orbit and composition, except theory, observation, and - more and more in the future, I hope - direct sampling. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

michaelmozina · Mar 23, 2006

<blockquote>In reply to:<hr />Yevaud, they report that of 134 specks looked at so far, 1 in 4 is a mineral formed under high heat but say nothing of whether the remainder are volitiles. Maybe they were just too excited about the "fire" objects to state if they had confirmation of "ice" objects.<hr /></blockquote> I would like to ask a layman's question here that relates back to something that colesakick touched upon earlier. Are there any confirmed frozen volatiles in these Stardust specks? If so, what is their overall chemical composition? I may be significantly jumping the gun at this point to expect such answers this early. While we do know some foresterite/olivine particles are present, what do we know about the rest of the samples at this point? That one photo of a volcanic meteorite fragment speaks volumes about the density issue IMO. It's a bit premature IMO to "assume" that a reasonably light average density must equate to a high number of volitiles present. I'd sure like to see some figures about the percentage of volatiles in these samples before simply assuming that the remainder of the samples are composed of volatiles. <div class="Discussion_UserSignature"> It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. - Kristian Birkeland </div>

CalliArcale · Mar 23, 2006

I doubt they've analyzed all of them yet; they were soliciting volunteers to pore over images of the collector grid looking for specimens, so I think they're expecting it to be a very laborious process. They probably only had the press release for these because they were so exciting. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

yevaud · Mar 23, 2006

I'd think that most volatiles would be gone, wrt the samples collected. The collision energies from capturing the fragments would likely have "cooked" them right off. Posibly some trace amounts? Hard to say as yet. Since the focus is density, it's known (so far) that the overall density of this Comet is very low, it's almost a given that there are numerous volatiles present: methane, ammonia, hydrogen, oxygen, and so on. In actuality, I think the debate is all about the proportion of volatiles versus denser material. Which, of course, then leads off into the somewhat seperate topic of how Comets were formed in the first event. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

JonClarke · Mar 23, 2006

And how they evolve. Multiple aphelios by a comet when it has been slung into a short period orbit will have a major impact on its bulk composition, especially the other layers. Jon <div class="Discussion_UserSignature"> Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars Arthur Clarke </div>

michaelmozina · Mar 24, 2006

<blockquote>In reply to:<hr />I'd think that most volatiles would be gone, wrt the samples collected. The collision energies from capturing the fragments would likely have "cooked" them right off. Posibly some trace amounts? Hard to say as yet.<hr /></blockquote> I hadn't really considered the "cooking off" aspect, and I'm not sure how the instrument is "sealed" afterwards, nor unsealed at the other end. There are certainly a lot of complications to consider. <img src="/images/icons/smile.gif" /> <blockquote>In reply to:<hr />Since the focus is density, it's known (so far) that the overall density of this Comet is very low, it's almost a given that there are numerous volatiles present: methane, ammonia, hydrogen, oxygen, and so on.<hr /></blockquote> The term "almost" in that sentence makes me a bit nervous, particularly since some meteorites we have run accross have been composed of relatively light configurations of heavier elements like the photograph from earlier in this thread. <blockquote>In reply to:<hr /> In actuality, I think the debate is all about the proportion of volatiles versus denser material.<hr /></blockquote> I agree. <blockquote>In reply to:<hr />Which, of course, then leads off into the somewhat seperate topic of how Comets were formed in the first event. <hr /></blockquote> Indeed. I guess these samples won't ultimately resolve that question, and I hadn't really even considered the notion of the light elements "cooking off" based on the collision with materials in the collector. That seems to make the determination of percentages quite a bit more difficult than I originally imagined, and I didn't think it would be an easy job in the first place. <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. - Kristian Birkeland </div>

dmjspace · Mar 28, 2006

Yevaud said: How large a nucleus are we speaking of? Over a certain mass, it would tend to predominate when overall density was determined, would it not? This brings up an interesting point. According to the EPH, the overall density of a cometary system (nuclei plus coma) should actually be far lower than dirty snowball projections. Unfortunately, the accuracy of the data relies on us knowing what we're measuring. I think the Deep Impact team may be making a big mistake by assuming it's measuring the composition of the nucleus, when it could simply be measuring the surface dust kicked up by the impact. This is one of the pitfalls of science, of course. If your assumptions are in error, your conclusions will be just as faulty. Precise, but not accurate, as they say. Given its repeated failure to predict fundamental characteristics of comets such as albedo, satellite frequency or the presence of more "evolved" materials, there is little reason to trust the dirty snowball model as a working hypothesis. It's probably time to consider some of the other models on the table, such as the EPH or EU. But that would equally make the assertation as to the nucleus being present and of a much higher density just as speculative, wouldn’t it? The difference is that the EPH has consistently predicted the things we observe: low albedo (caused by explosive "charring"), a relatively high frequency of satellites, the presence of salts and other compounds formed at high temperatures or in the presence of water, etc. The dirty snowball model, on the other hand, never expected these findings. I agree that no comet density is known with a high degree of certainty, however. I wish the authors who keep insisting density is known would admit as much. Doubtful. Again, we’re not talking a tiny body – we’

yevaud · Mar 28, 2006

Speaking of, you may find this interesting - further muddying the waters, so to speak: Clandestine Comets <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

bonzelite · Mar 28, 2006

excerpt from the article: The new study underscores the increasingly hazy distinction between comets and asteroids. "There are different definitions of comet used by different people at different times," Jewitt told New Scientist. Activated asteroids Asteroid expert Richard Binzel at MIT questions the need for the new classification. "I prefer to think of them as activated asteroids," he told New Scientist. "It's no surprise if some asteroids have some water content, particularly in the outer asteroid belt." He says volatiles have been measured to make up about 10% of some carbonaceous meteorites that are thought to come from the region. Jewitt says potentially tens of thousands of main belt objects contain ice and have simply not been observed during their active period. In order to be seen spewing dust, the objects would have to have been hit by a meteor size boulder within the last thousand years or so, he adds. you already know what i'm going to say, so i'm not saying anything. i don't need to.

yevaud · Mar 28, 2006

Er, no I don't. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

bonzelite · Mar 28, 2006

The new study underscores the increasingly hazy distinction between comets and asteroids./i>

yevaud · Mar 28, 2006

Have you been reading this thread, and all that have preceded it? This is something that is almost precisely half-way between what Dmj has asserted, and what I have. Is there some sort of problem with that? Please elaborate. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

bonzelite · Mar 28, 2006

no problem. no elaboration needed. we can all sleep better <img src="/images/icons/wink.gif" />

yevaud · Mar 28, 2006

Good news as well, for anyone interested in this topic. The DAWN mission, which was to have visited the Asteroids Vesta and Ceres, and was unfortunately cancelled, has been reinstated. Dawn Mission Reinstated <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

dmjspace · Mar 28, 2006

Yevaud said: Dawn Mission Reinstated Yep. I was happy to see that too. Getting more data on asteroids will surely help us determine if there really is a difference between comets and asteroids. Let's not lose sight of the fact that any observation that blurs the line between comets and asteroids is a "win" for the EPH, which has maintained since its inception that they are essentially the same entity. The dirty snowball model, on the other hand, has historically insisted that the two are very, very different. In the sometimes sneaky way in which scientific progress proceeds, one model often morphs into a competing model without explicit acknowledgment that that's happening. The gradual progression from dirty snowball to snowy dirtball to, eventually (and soon), icy dirtball, is one such example. There will likely be no credit given to the EPH, even though it has stood without significant modification for several years now, and despite the fact that certain prominent astronomers are well aware of its successful predictions. (The most striking example: Van Flandern's bet with Don Yeomans, currently with the Deep Impact team, that asteroids and comets would have satellites far more frequently than standard models predicted. Van Flandern was vindicated in convincing fashion, and Yeomans made good on the bet, which I believe was a mere six pack. In hindsight, Tom should've bet the house on that one, but he's too much of a gentleman...) I don't know enough about the Stardust mission to guess whether it will be able to estimate cosmic ray exposures or otherwise provide timetables. The EPH suggests the breakup event was recent. The solar nebula model says formation happened billions of years ago. The solar nebula model's new requirement for ancient "radiogenic heating" should produce far different predictions than the EPH, which suggests a recent, rapid explosive event.

dmjspace · Mar 28, 2006

Though it's hard to find explicit "dirty snowball" predictions, we read from the Stardust web site a vague description of what scientists expected from Wild 2: Since Wild 2 has passed the Sun only a few times, it still has most of its dust and gases and it is relatively pristine condition. This is important because comets are made up of material left over from the solar nebula after the planets were formed. Unlike the planets, most comets have not changed very much since the formation of the solar system. Therefore, comets may hold the key to understanding the early development of the solar system. Comet Wild 2 should contain much of this ancient material, making it an ideal choice for study. What they found, however, was very much the opposite of pristine, primordial, unevolved material. Instead, they found what looks like a chunk of a former highly evolved (i.e. heated and thus differentiated) body. The best candidate for said body? A planet. There is nothing speculative about this. We know what planets are made of and, basically, how they must have formed. As we read on the Stardust site *after* samples of Wild 2 were examined: "Remarkably enough, we have found fire and ice," said Donald Brownlee, Stardust principal investigator and professor of astronomy at the University of Washington in Seattle. The returned samples show high-temperature materials from the coldest part of our solar system. Comets, they said, may not be as simple as the clouds of ice, dust and gases they were thought to comprise. They may be diverse with complex and varied histories. Wild 2 seems to be an example of that complexity. The material like that in the green Hawaiian beach sand is called olivine. Its presence in the comet's dust trail was a surprise. "It seems that comets are … a mixture of materials formed at all temperatures, at places very near

Where's the discussion on Stardust data?

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