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Video Title - Interstellar & early solar system organics in Stardust samples from Comet 81P / Wild 2
By - Scott Sandford, 2009 (Co-investigator, Stardust Mission)
Extracts - 18:23 - "When we got there, we see all sorts of geo forms that are unique to this object ... not only does it NOT look like other asteroids & surface of planets (& moons, as per slide), but it doesn't look like comets either"
19:00 - Some pits on Wild 2 may look like impact craters but hey are not. The morphologies are different [ no raised crater rim, flat bottoms, kind of like comet 67P's big sinkhole ]
19:30 - There is more surface relief on comet Wild 2, compared to other comets & the reason for that is these are comets that have gone close to sun many times & so all their outer materials have been lost and you are seeing secondary surface associated with that loss, whereas in comet Wild2's case, we still have bits of original surface" [ comet 67P has gone around the sun many times & still showed a very rough surface, proving Scott's understanding as wrong ]
21:20 - Regarding jets from shadows, "the sublimation that's driving the jets is not caused by direct insulation of sunlight on to exposed ice at the surface, instead sunlight is somehow heating the surface & that heat is penetrating deeper into the nucleus and there is some underground source region where volatiles are going into the gas phase & then streaming out" [sublimation based explanations are pure pseudoscience, has never been experimentally verified. it defies both logic & physics ]
21:40 - "When we flew by the comet, most of the particles collected, came from a few of these jets & since these jets must have a deeper origin, it tells us that we are sampling not the surface of the comet but some lower portion of the nucleus that are at some depth." [ wrong understanding, evidence suggests that it is surface material being blasted off into space ]
36:54 - What are some of the things we found ?
One of things that surprised us all is what a zoo of minerals we found. If you go through the particles, you find lots of minerals like forsterite, olivine, enstatite etc. These are common minerals in normal meteorites but in Wild2 samples, these are well crystallized silicates. A lot of people thought that comets are made of interstellar stuff that just come in and hasn't been affected. All our telescopic data tells us that most of the silicates & interstellar medium are amorphous (are like glass & not nice crystals) but these Wild 2 minerals are all nice crystals. [ wrong interpretations based on telescopic observations can be corrected only by sample analysis ]
We also find iron & nickel sulfides, which are common in meteorites as well. Then we start finding things that were a little more flabbergasting. Anorthite & Diopside are minerals you do find in meteorites but these are not super abundant and these are very high temperature minerals, you don't make these at low temperatures. And then we found Osbornite (vanadium titanium nitride minerals).
38:00 - If you go to the modelers & ask them - if you took the entire solar system & vaporized it & then let it cool off and asked what condenses first, that's one of the first things that comes out (Osbornite). This suggests that the comet contains materials that didn't just drift in gently from the cold interstellar medium, get in a cold comet & stay cold till they were delivered to the Stardust collector. Instead, we have materials that were formed in the hottest regions of the solar nebula. So this idea that stuff came in from outside, got in a comet & that is what made the comet is not right.
38:39 - This complex composite particle was a shock to lot of folks. It came in from this direction & this big iron nickel sulfide grain basically acts as a blast shield & protected all the fragile stuff from being busted up in the track.
39:46 - Elements are heterogeneously distributed along the impact tracks in aerogel.
41:24 - Only a few circumstellar grains were found so far. Comet Wild 2 is not simply a collection of presolar grains.
43:34 - Not all the returned particles look primitive. Slide shows a particle with igneous history.
44:07 - A lot of this is pointing to a necessity for a lot of mixing in the solar nebula.
44:51 - Not everything got hot before it got into the comet. Some things went all the way down in & then came all the way back out but a lot of things didn't do that trip down cos if they did, they would have been destroyed. So some things did get into comets (directly) & not get any closer to the sun. [ assumes that comets formed in the outer solar system, this cannot explain active asteroids & comets in the main asteroid belt, in the inner solar system. the idea of sublimation driven activity is fundamentally wrong ]
45:27 - Organics are present, & like the silicates, they are very diverse & very heterogenous, both spatially & compositionally.
46:58 - What are these organics ? We're still fussing with that.
48:23 - Flash heating is what happens when you impact a comet particle into the aerogel. We find 2 patterns of PAHs, one of them probably present in the comet itself but may have been somewhat modified by the impact process & the other produced from flash heating by impact into the aerogel & not originally present in the comet.
50:05 - Comet Wild2 materials are very rich in oxygen & nitrogen. They have almost twice as much oxygen and nitrogen relative to carbon, compared to Murchison meteorite.
That implies these organics are fairly primitive. If you cook an organic, you tend to drive the oxygen and nitrogen out and get more and more carbon rich material, like the Murchison meteorite.
51:00 - More than 1 type of organic material is present.
One of the types is chemically complex, relatively dense & superficially looks similar to meteorite organics, except for the extra O & N. The other type is chemically simple, having low density, with very few bonds, but very rich in oxygen. The latter type is not seen in meteorites.
52:14 - Wild2 particles vary considerably but fall at the primitive end of trends for meteoritic organics.
53:06 - The organic component of the impacting particle vaporizes and the resulting gases diffuse into the aerogel far beyond the track volume & recondense into solids. So far, Wild 2 material shows no clear affinities with any specific meteorite type.
Q & A :
1:08:26 - Q: You showed tracks that were sort of 2 characteristic shapes of carrot and the turnip. Do you understand what's going on, particularly in the turnip, why it broadens out then narrows down ?
Scott : People have done some modeling and maybe in a global way people understand it but I don't think it's entirely satisfactory yet. One of the problems is if you ask people to model this whole impact thing, some of them will come back with results where they look you right in the eye and they say every single piece of this particle was heated to at least 3,000 F (1,649 C).
You know that is the case, so it's got to be that the whole impact process is a highly stochastic one (pattern that may be analyzed statistically but may not be predicted precisely), that parts of the grain give up their life to protect other parts of the grain. So far the modeling that has been done doesn't have sufficient fidelity to take that kind of effect into account and it's a little hard for us to provide them with good information, to base their models on.
You can go to the vertical gun facility here at Ames and you can shoot mineral grains in and so we have lots of data on these carrot tracks. We can take the diameter and the length of a track, from the aerogel and predict what the mass and velocity of that particle probably would be and so on, but we can't accelerate fragile things, we can't accelerate these aggregates bcos they come apart.
The closest we've ever come is at Johnson Space Center where we managed to accelerate a sample of Nestle's cocoa powder to 6 km/s and the net result was a series of bulbs. They didn't have any of these terminal tracks at the bottom because there was nothing hard mixed in there and when we measured that, we found that a lot of chemistry had happened in that stuff. We also see things that that have been in the comet Wild2 samples, that have been cooked way less than our cocoa powder.
So sometimes the organics can survive because the mineral grains are there to protect them and so far, the predictions made by the models, kind of qualitatively explain what we see but quantitatively, I'm not sure how informative they are yet.
1:10:35 - Q: Now that we're beginning to have this rich set of data about the microstructure of a comet and very primitive meteorites, is anybody putting together a self-consistent model of how you can make both the most primitive carbonaceous chondrites and a comet like Wild2, in an early solar solar system protoplanetary nebula model ?
Scott : Models of the solar nebula already existed that had predicted mixing, but nobody made a big deal out of it or even the modelers themselves, i don't think, thought about what that meant. And when Stardust came back and we got these results, suddenly everyone realized that it wasn't just a little mixing, this is mixing on all size scales. And now, the be-all and end-all (ultimate aim) of the models is to ask - once you have a model, what does it predict for further mixing and I think a lot is going to come out of that in the long run, although I don't see a whole lot of clarity yet and precisely how we will use this effect to discriminate between models. Ultimately, Stardust will only give us some clues in that because we have had this high velocity impact and there may be components we don't even know that were there before because they utterly didn't survive. There's also the issue that if we get all the grains from 1 or 2 jets, how do we know that's representative of the whole comet ?
On this mission I'm pitching on Friday, one of the things we would specifically do is dip down in and get samples at different points in the orbit and also make a point of collecting samples where we can keep them separate. So collect a sample from a jet and collect a sample from a different jet and so on and find out if it is always the same or does it vary.
Somehow you had to get some of these things really hot and get them back out and get them mixed into these aggregates and then get them in the comets, so that gives you really big constraints on how early that had to happen. And at the same time, you have got to have stuff that comes in from outside and never gets hot even slightly and then they got to get together at the right time and place to all end up in the comet.
And then just to make it more complicated, this is the one that really puzzles me, how do you have the interstellar isotopic anomalies be reasonably common and have the circumstellar grains be reasonably uncommon, because naively, you would expect the circumstellar grains to be the cores on which ices are condensed, on which you made the interstellar organics. So why aren't they traveling together and if they are traveling together, and then they get separated at a later date, how come the fragile organics survive & not the robust circumstellar grains ? So there has to be some serious clues about the timing of things in that & I still haven't quite got my head wrapped around that one.
1:13:37 - Q: You had mentioned that that one particle, osbornite, was the result of a high temperature event, may be a star explosion. Would a thermal event like an asteroid impacting a planet or a volcano be a sufficiently hot to create that ?
Scott : I'm not a super expert at mineralogy but I think the I think the answer to that is no. Probably the osbornite does not come from an outflow of a star, it probably is associated with our solar nebula. As far as i know, the isotopes in that grain are completely solar, that they measured.
If we took the whole solar system, with everything in it - the sun, planets, asteroids, comets - and vaporized it & put in a box and then you let the let the temperature in the oven drop, the gas will cool, cool, cool and at some point something will start to condense and the things that will condense the most are refractory minerals and this osbornite is one of those.
And so, in a gas that's roughly of solar composition, osbornite would be one of the first things to form but if the gas doesn't have solar system abundances & has higher proportion of oxygen or whatever, osbornite may not form & something else will form first. So, in an impact you might get vapor that's at this temperature but the actual composition of the gas would be very unlikely to give you osbornite as a condensate.
In addition, in something like an explosion like that, things have to condense really fast whereas we see things that are crystalline, which implies they condense a little more slowly. So i won't say no to that possibility but i think that the main thinking right now is that probably we're looking at grains that must have formed out of a solar nebula, way in close to the Sun where things were basically getting completely torched and then that material somehow had an opportunity to cool by mixing outward or getting screened by something else or whatever and then that gas, as it started to cool off, first formed osbornite and later it started to form less refractory minerals and as it cools more and more, you got more and more stuff come out, but that osbornite was one of the very first things. I think that's the general thinking that most people are running with at the moment.
1:16:00 - Q: You're saying there is chemistry going on when they're impacting into the aerogel. So how much of the results you getting from the organics do you believe to be actually indicative of what's actually out there on the comet versus stuff that's going on, and the second final question is, given you're seeing a lot of PAHs, unless you know that they are original, how do you know they're not chemistry and that some of the other lighter elements like oxygen, hydrogen haven't actually combined with some of the more refractory minerals, generated some of those PAHs ?
Scott : Well I mean this is a this is an issue and it's going to be an ongoing one, forever basically. And my answer has to be a little complex. I think when you see a material that's present and it's a fairly robust material, capable of surviving a certain amount of heating or whatever, you have to be suspicious that this could have been formed by let's say the impact but if you see a material which is extremely volatile and would not have survived any kind of heating then it's unlikely that you made it by heating.
So there's a couple possibilities there. One is that all of those existed in the comet or the other is that maybe you had some of this more volatile stuff on the comet and some of the stuff you see down here at the heated end was all made you know by the impact process.
So it would be more problematic if everything we saw was always a refractory organic. Because then you would say well you know every one of these atoms and molecules could have been rejumbled during the impact collection but the fact that you see these isotopic anomalies and some of these isotopic anomalies are in these very volatile materials suggests at least some of the stuff is definitely from the comet.
And there's also an issue of a matter of degree. Maybe some stuff got changed a little. Then how would you recognize that ? So this is a problem that we will just have to struggle with all along and this is also one of the prime motivations for me going to headquarters and saying that on the next mission, if I can get material collected at 1 m/s and i can get a 1,000 times as much stuff, then I won't have to worry about these problems anymore and I'll know the answer for sure. There's no doubt that at least some of the organics we see, have to have been processed to some degree. I mean it's just impossible that it's all pristine.
And also, we have a strong reason to suspect that at least in some cases like these really lightweight PAHs, where there's no other organics around, that those may be completely secondary products and have nothing to do with the comet.
But in the case of the really volatile stuff that is largely unaffected, even if you heat those modestly, either they would have been altered to more robust materials or they would have isotopically started to equilibrate with everything around them. So at least some of this stuff did a remarkably good job of surviving the whole whole trip.
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Title - Looks like a comet but feels like an asteroid? That’s wild !
By - The Conversation, 2013
Extracts - In 2006 the NASA Stardust mission returned to Earth with the first samples collected directly from a comet. What scientists discovered when they analyzed the samples was not what they were expecting. The results challenged many commonly-held assumptions about the composition of comets and asteroids, as well as the formation of the solar system as we know it today. Before this mission, scientists had always characterized comets and asteroids as completely different.
Asteroids ... are thought to have formed in the inner solar system, close to the sun. They contain rock minerals that formed in high temperatures, early on in solar system history. Their orbits are generally circular.
Comets, on the other hand, are less-frequent visitors from the outer solar system. They are often called dirty snowballs and are believed to act as time capsules from the deep freeze of the solar system, containing the primary dust and gases from which everything we can touch and smell began. Comets glow and produce a tail of gas and dust when they pass through the inner solar system because the sun’s heat causes frozen gases within them to sublime. Their orbit is different to that of asteroids, being generally more elliptical.
The Stardust mission collected grains from the tail (this is wrong, its coma actually) of comet 81P/Wild2. Analysis of these grains revealed them to be high-temperature rock minerals, indistinguishable from those commonly found in asteroids, their presence indicated an inner solar system origin.
The big question is: how can a comet contain minerals formed in the inner solar system when it is supposed to have been formed some 30AU away from the sun, where AU is the distance from the Earth to the sun, and its surroundings are as cold as -200 C ?
This question has thrown space scientists into disarray, showing that classic solar system formation models that have been relied on for many years do not square with the evidence in these samples. Unfortunately only one comet has been sampled directly of the trillion or so that are estimated to exist.
The current thinking is that the 2 extremes of composition do exist. Some asteroids are true asteroids – they contain only material formed in the inner solar system and no water ice. Conversely some comets are true comets – they contain only outer Solar System material.
But the samples obtained from 81P/Wild2 and remote sensing of other solar system objects, highlights that solar system models must account for a continuum of compositions from asteroid to comet.
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By - Edward Drobyshevski, 2008
Extracts - Findings of Deep Impact mission (comet 9P/Tempel 1) and Stardust mission (comet 81P/Wild2) are at odds with the traditional condensation-sublimation paradigm treating comets as a dead product of accretion of matter that had condensed in the outer cold fringes of the Solar system. Actually, this paradigm had earlier been incapable of explaining the origin of outbursts and fragmentation of cometary nuclei, jet outflow of matter from the nucleus and the appearance of radicals and ions close to the nucleus etc., without violation of the conservation laws and invoking additional hypotheses.
It has suddenly turned out that a cometary nucleus is not a friable lump of snow or loose rubble pile of 100-200 m sized dirty ice ‘grains’, which are ready to break up under the tidal action of the Jupiter into thousands of re-assembling pieces.
The nucleus surfaces demonstrate regions of different morphology and geological processing, layering, numerous large-scale structures, including ones resembling impact craters, etc. Such rather vast coherent formations cannot be created in absence of significant gravity or strength of material binding all parts of nucleus together. The comet ices contain rocky particles that had been acted upon by high temperatures.
The presence of particulate products of high-temperature condensation and metamorphism has caused a certain consternation and bewilderment, which ended in invoking new and contradictory hypotheses assuming strong mixing in the preplanetary disk and/or ballistic ejection of particles out of the intra-mercurian zone into Neptune’s zone and beyond it (which would require velocities above 60 km/s !). Strange as this might seem, the latter point follows directly from our concepts of the Jupiter-Sun system as the limiting case of a binary star, and of other planets as a by-product, albeit unavoidable, of its early evolution. This is what can be called the close binary cosmogony (CBC) of the Solar System. The presence of minerals forming at high temperatures (1,127-1,727 C) forces one to invoke the close-binary cosmogony of the Solar system, which 3 decades ago had predicted the existence of such a cloud.
Turbulent mixing sharply shortens the disk lifetime, an argument that, when applied to standard approaches, creates serious difficulties for the formation of giant planets.
Short-period and long-period comets are an inevitable endpoint of evolution of various groups in the great number of small (Moon- and Pluto-like) planets that had formed, just as large planets, in the proto-Jupiter. Short-period comets are produced in global explosions of icy mantles of Galilean satellites and of Titan, that are saturated by products of ice electrolysis, and long-period comets, in rare collisions of trans-Neptunian dwarf planets and, further on, of their fragments.
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Title - Comet Wild 2: A window into the birth of the solar system ?
By - Hawaii.edu article, 2015
Extracts - "So, now we ask the question: Does the fine-grained dust from comet Wild 2 represent a diverse sampling of many inner-solar-system objects that were transported to the outer solar system, or in fact, the starting raw materials of the solar system ?” — Ryan Ogliore, main author of the paper
Processing of material in the inner solar system should alter the abundance of circumstellar grains and volatile elements in the fine-grained dust. “If the fine-grained material is enriched in circumstellar grains and not depleted in volatiles, we can say with certainty that we are looking at primitive solar system dust,” said Ogliore. “If circumstellar grains are not over-abundant compared to meteorites, and volatiles are depleted, we can say with certainty that we are looking at a very diverse sample of fine-grained inner solar system material in the comet.”
Reflecting on the complex life history of comet Wild 2’s constituent material, Ogliore added, “The comet’s nucleus today is made up of small rocks and ice, separated by fractions of an inch, that originally formed billions of miles apart.
Some rocks have seen temperatures above 1,371 C, but adjacent ice has been kept close to absolute zero for billions of years. Every tiny grain we look at has its own fascinating story to tell.
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