Re: [meteorite-list] Questions about accretion. Part 2 UAE, Shock wave distribution proto Solar System
Great postings Elton. They take the whole discussion to a far greater level and I fo one applaud you for it. I like to think there are others that appreciate it and thin this is what this list should be about. As an addition to what you say I will say the following. The short half life of Al26 (yes, I believe it is 720 thousand years) is a really good indication that differentiation took place very quickly. Al26 would have been present in large quantities (1 part in 10^5 of aluminium atoms) and would provide a large source of energy. Info from encyc of meteorites). It's short half life limits the differentiation period to less than 10Ma, borne out by the majority meteorite samples we have). There is, of course the issue of homogeneity amongst the pre/proto solar nebula. Even distribution of isotopes around the nebula used for dating the solar system is assumed rather than confirmed. Personally, I don't think it makes much difference. The sphericity of the Oort cloud versus the disk of the solar system is likely a density of matter issue. Beyond 60AU, the material is likely to be too thinly spread in the early solar system to form into a proper disk (a factor that would also induce heating in the inner region thoug I don't know how much and it'd be more significant closer in). There is also the issue of the E-M effect produced during the T-Tauri phase. I adored the idea you made (I've never heard it before) of it resisting differentiation. I think you're right and it may be a contributing factor to the size of planetary bodies. Only when gravity can overcome such an effect can differentiation occur. We know that T-Tauri stars eject material out through their poles. Maybe as much as 0.0001 solar masses may re-accrrete to the disk (+/- an order of magnitude). As it does so, huge EM effects will take place. We know it happens but we don't know how or why or the effect it has. Personally, I think it's great that we have found out so much but still have so much to know and I love being able to chew it over here. Rob --- On Wed, 4/8/09, Mr EMan mstrema...@yahoo.com wrote: From: Mr EMan mstrema...@yahoo.com Subject: Re: [meteorite-list] Questions about accretion. Part 2 UAE, Shock wave distribution proto Solar System To: Meteorites USA e...@meteoritesusa.com, meteorite-list@meteoritecentral.com Date: Wednesday, April 8, 2009, 3:33 AM There was a question regarding the sorting of elements and why for example common chondrules had more iron than did Carbonaceous chondrites. The reason for the difference also includes why we use isotope ratios to determine from where a parent body probably formed within the solar system. Sometime in early solar system development there was a sustained and or repeated strong solar wind or mini-nova, or perhaps our own ancestral sun's predecessor nearby supernova, or other cosmic water hose(?) that sweep through the swirling matter in the proto-solar disk, significantly sorting it out by elemental and molecular weights. Heavier particles weren't pushed out as far as the lighter ones. Thus we have heavy to light sorting of particles/ elements/ molecules/ solids/ gases etc from the inner rocky planets at one end to the giant gas planets beyond the asteroid belt and all way out to the Ort cloud. The sorting was not perfect but did rearrange the mixtures of elements locally. Conservation of angular momentum must have broken down at some level such that the Oort Cloud is theorized to be more or less spherical while planetary masses tend to lie close to the plane of the ecliptic. (This glitch influences measured elemental ratios of our known solar system and just mentioned for those paying attention) Thus before significant planetary accretion(first 3-5 million years?) we experienced a cycle of sorting that left zones of like particles to be accreted. This sorting also locally affected the ratios of the individual isotopes of elements from a concept we know as the Universal Abundance of the Elements.(UAE) (The UAE says that based on human measurements the mass of the universe is concentrated in the first 20 elements which incidentally were the main elements associated with living processes). When the local Solar system abundance of the UAE was disturbed, distribution of isotope ratios were also skewed in the local solar system. Ergo oxygen isotope studies in meteorites tell us what relative distance/radius a parent body formed away from the sun. On Earth the ratios for Oxygen: O18(Tritium)-O17(Deuterium)-O16 is something like 18O / 16O = 2005.20 ±0.43 ppm (a ratio of 1 part per approximately 498.7 parts) 17O / 16O = 379.9 ±1.6 ppm (a ratio of 1 part per approximately 2632 parts) This ratio signature is specific to an origin in the Earth Moon distance and there is a different one for Mars, the asteroid belt, Jupiter, Saturn and carbonaceous chondrites etc. Complications
Re: [meteorite-list] Questions about accretion.
Thanks Rob! Great response. That pretty much sums it up for me and answers just about everything I was curious about in that email. You mentioned... ..If the rock is big enough, (which provides enough radioactive material to generate the heat AND enough lying over the middle to prevent the heat escaping, the body will melt... How big is big enough? Eric Rob McCafferty wrote: Hi Eric You are correct in thinking that electrostatics causes the initial clumping. The early sun would have been extremely energetic and X-ray and UV radiation would produce electro static charging of small particles. Once they begin to clump to a sufficient size, they will attract particles through gravity. The dynamics are as follows An object with radius R will naturally sweep up any object within its radius (pi*R^2) but gravity will draw material from a greater distance S inside and outside its orbital path S=(R^2 + 2GMR/V^2)^1/2 M mass of body, V initial closing velocity of body and impactor Initially, you are correct, everything begins as a big clump of mixed material. Whether an iron core is formed will depend on the size of the initial clump of stuff. Heat is generated by radioactivity of short lived isotopes such as Al26. If the rock is big enough, (which provides enough radioactive material to generate the heat AND enough lying over the middle to prevent the heat escaping, the body will melt. Once this begins, the iron will migrate to the core as rock and iron don't mix. Iron, being denser, will sink. Accretion to differentiation is a very rapid affair, just a few million years. The almost identical ages of all asteroidal meteorites tends to confirm this. My understanding is that this leads to the different classes of achondrites. These have been properly melted and lose their chondrules. The widmanstatten patterns in irons comes from the rocky material insulating the iron/nickel core allowing it to cool very slowly. Parent bodies forming in different orbits are likely to have differing constituents according the condensation model, hence different achondrite types. Chondrites may have come from smaller initial parent bodies, ones that weren't big enough to generate enough heat to fully melt. Higher petrographic types of chondrite (4-6) are samples that are progressively closer to the core and were heated more in bodies that were not properly differentiated. Petrographic type 3 are essentially the same material as the early solar system, mostly unaltered by heat, likely from near the surface of undifferentiated bodies. I don't see that all parent bodies would necessarily need 3-6 petrographic types. Small parent bodies may not reach the higher grades in the middle as they never got hot enough. Grade 6 seems to be the limit. If the parent body grew any bigger then it would melt producing a differentiated parent body. I think petrographic type goes to 7 but I don't think any are actually given this grade (though I think it was NWA3133 that may have been discussed as a possible). It is likley that H, L and LL meteorites come from different parent bodies possibly from different regions in the protosolar nebula. The relative rarity of petrographic type 3 ordinary chondrites may be due to them being removed first and subsequently removed from the system many aeons ago. Carbonaceous Chondrites are a whole different kettle of fish but I think I've said quite enough for now. I hope I've not made any glaring errors but if I have someone will put me right. Rob Mc -- Regards, Eric Wichman Meteorites USA http://www.meteoritesusa.com 904-236-5394 __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Questions about accretion.
Just a smigen bigger than not enough? - Original Message - From: Meteorites USA e...@meteoritesusa.com To: rob_mccaffe...@yahoo.com; meteorite-list@meteoritecentral.com Sent: Tuesday, April 07, 2009 12:40 PM Subject: Re: [meteorite-list] Questions about accretion. Thanks Rob! Great response. That pretty much sums it up for me and answers just about everything I was curious about in that email. You mentioned... ..If the rock is big enough, (which provides enough radioactive material to generate the heat AND enough lying over the middle to prevent the heat escaping, the body will melt... How big is big enough? Eric Rob McCafferty wrote: Hi Eric You are correct in thinking that electrostatics causes the initial clumping. The early sun would have been extremely energetic and X-ray and UV radiation would produce electro static charging of small particles. Once they begin to clump to a sufficient size, they will attract particles through gravity. The dynamics are as follows An object with radius R will naturally sweep up any object within its radius (pi*R^2) but gravity will draw material from a greater distance S inside and outside its orbital path S=(R^2 + 2GMR/V^2)^1/2 M mass of body, V initial closing velocity of body and impactor Initially, you are correct, everything begins as a big clump of mixed material. Whether an iron core is formed will depend on the size of the initial clump of stuff. Heat is generated by radioactivity of short lived isotopes such as Al26. If the rock is big enough, (which provides enough radioactive material to generate the heat AND enough lying over the middle to prevent the heat escaping, the body will melt. Once this begins, the iron will migrate to the core as rock and iron don't mix. Iron, being denser, will sink. Accretion to differentiation is a very rapid affair, just a few million years. The almost identical ages of all asteroidal meteorites tends to confirm this. My understanding is that this leads to the different classes of achondrites. These have been properly melted and lose their chondrules. The widmanstatten patterns in irons comes from the rocky material insulating the iron/nickel core allowing it to cool very slowly. Parent bodies forming in different orbits are likely to have differing constituents according the condensation model, hence different achondrite types. Chondrites may have come from smaller initial parent bodies, ones that weren't big enough to generate enough heat to fully melt. Higher petrographic types of chondrite (4-6) are samples that are progressively closer to the core and were heated more in bodies that were not properly differentiated. Petrographic type 3 are essentially the same material as the early solar system, mostly unaltered by heat, likely from near the surface of undifferentiated bodies. I don't see that all parent bodies would necessarily need 3-6 petrographic types. Small parent bodies may not reach the higher grades in the middle as they never got hot enough. Grade 6 seems to be the limit. If the parent body grew any bigger then it would melt producing a differentiated parent body. I think petrographic type goes to 7 but I don't think any are actually given this grade (though I think it was NWA3133 that may have been discussed as a possible). It is likley that H, L and LL meteorites come from different parent bodies possibly from different regions in the protosolar nebula. The relative rarity of petrographic type 3 ordinary chondrites may be due to them being removed first and subsequently removed from the system many aeons ago. Carbonaceous Chondrites are a whole different kettle of fish but I think I've said quite enough for now. I hope I've not made any glaring errors but if I have someone will put me right. Rob Mc -- Regards, Eric Wichman Meteorites USA http://www.meteoritesusa.com 904-236-5394 __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Questions about accretion.
According to O. Richard Norton's Encyclopedia of Meteorites 2002, 100-200km (abstract page for chapter 9) Rob --- On Tue, 4/7/09, Meteorites USA e...@meteoritesusa.com wrote: From: Meteorites USA e...@meteoritesusa.com Subject: Re: [meteorite-list] Questions about accretion. To: rob_mccaffe...@yahoo.com, meteorite-list@meteoritecentral.com meteorite-list@meteoritecentral.com Date: Tuesday, April 7, 2009, 5:40 PM Thanks Rob! Great response. That pretty much sums it up for me and answers just about everything I was curious about in that email. You mentioned... ..If the rock is big enough, (which provides enough radioactive material to generate the heat AND enough lying over the middle to prevent the heat escaping, the body will melt... How big is big enough? Eric Rob McCafferty wrote: Hi Eric You are correct in thinking that electrostatics causes the initial clumping. The early sun would have been extremely energetic and X-ray and UV radiation would produce electro static charging of small particles. Once they begin to clump to a sufficient size, they will attract particles through gravity. The dynamics are as follows An object with radius R will naturally sweep up any object within its radius (pi*R^2) but gravity will draw material from a greater distance S inside and outside its orbital path S=(R^2 + 2GMR/V^2)^1/2 M mass of body, V initial closing velocity of body and impactor Initially, you are correct, everything begins as a big clump of mixed material. Whether an iron core is formed will depend on the size of the initial clump of stuff. Heat is generated by radioactivity of short lived isotopes such as Al26. If the rock is big enough, (which provides enough radioactive material to generate the heat AND enough lying over the middle to prevent the heat escaping, the body will melt. Once this begins, the iron will migrate to the core as rock and iron don't mix. Iron, being denser, will sink. Accretion to differentiation is a very rapid affair, just a few million years. The almost identical ages of all asteroidal meteorites tends to confirm this. My understanding is that this leads to the different classes of achondrites. These have been properly melted and lose their chondrules. The widmanstatten patterns in irons comes from the rocky material insulating the iron/nickel core allowing it to cool very slowly. Parent bodies forming in different orbits are likely to have differing constituents according the condensation model, hence different achondrite types. Chondrites may have come from smaller initial parent bodies, ones that weren't big enough to generate enough heat to fully melt. Higher petrographic types of chondrite (4-6) are samples that are progressively closer to the core and were heated more in bodies that were not properly differentiated. Petrographic type 3 are essentially the same material as the early solar system, mostly unaltered by heat, likely from near the surface of undifferentiated bodies. I don't see that all parent bodies would necessarily need 3-6 petrographic types. Small parent bodies may not reach the higher grades in the middle as they never got hot enough. Grade 6 seems to be the limit. If the parent body grew any bigger then it would melt producing a differentiated parent body. I think petrographic type goes to 7 but I don't think any are actually given this grade (though I think it was NWA3133 that may have been discussed as a possible). It is likley that H, L and LL meteorites come from different parent bodies possibly from different regions in the protosolar nebula. The relative rarity of petrographic type 3 ordinary chondrites may be due to them being removed first and subsequently removed from the system many aeons ago. Carbonaceous Chondrites are a whole different kettle of fish but I think I've said quite enough for now. I hope I've not made any glaring errors but if I have someone will put me right. Rob Mc -- Regards, Eric Wichman Meteorites USA http://www.meteoritesusa.com 904-236-5394 __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Questions about accretion. Part 1 Aluminium 26, and Asteroid ages
My ISP continues to lose much of my email else send them in huge batches. Some additional points to what was discussed thus far: Iron migration to the core of a heat building/holding sized body is a buoyancy issue and gravity driven so long as the iron remains molten. Accretion probably had an electrostatic component which may be an anti accretion force, there was some covalent molecular bonding but as strange as it seems the primary attractant has to be gravity yes molecule to molecule-- chondrule to chondrule. Chondrule formation is a whole other treatise not covered here. After accretion: Aluminum 26 is a radioactive isotope with half life of .73(?)million years which decays to Magnesium 26. The bulk occurrence of Al26 in the early solar system had to be ejected from a solar fission furnace. When we find magnesium within a crystal matrix where aluminum should be, we know it started out as an atom of Al26. The heat of that Al26 decay is widely believed to be the driver for differentiating in asteroids accreted from chondrules and non-chondrule particles. Except for the planetary meteorites and Impact Melt Breccias(IMB) all original common chondrite to achondrite parent body conversion appears to have taken place in the approximate 15-20 Million years starting with the formation of the current solar system. The first 5 million being the time when accretion was ongoing. There are two theories of H Chondrite parent body formation. Both include zones. One is that there were multiple H class parents of different sizes yielding different petrological classes. The other is that there were but one or very few H parent bodies and what started off as H3 and melted from heat distributed inside to out. As the heat source ran lower and lower, the chondrite cake was left partially uncooked resulting in an onion layer set of zones with H3 on the surface and H7/achondrite toward the center(yep with an iron core) Either way, there is a successive fall off of formation/cool-off ages in H Class formation ages and that is to be expected. H3 chondrite zones/bodies ran out of heat earlier than H5s so fewer chondrules were melted (thermally metamorphosed). As a class, H3s zones congealed a bit earlier than the other H4,H5,H6 zones. Because Al26 was more or less uniformly distributed, we may infer that H3s either came from smaller bodies which were barely large enough to hold some heat but not large enough to let the full melting cycle run to achondrite sizes. And/or They come from the crustal regions of a substantial sized asteroid. Either way they were liberated in a major disruption that exposed them down to their cores. From Widmanstatten studies we know that the cooling at the metallic core was a very slow rate of a a couple to a few tens of degrees per million years. I am sure somewhere someone has cross referenced these rates to improve on what we believe we know about asteroid formation ages. For more reading: http://www.psrd.hawaii.edu/Sept02/Al26clock.html (See the last chart on the above link for asteroid/meteoroid formation ages) http://www.thefreelibrary.com/Aluminum+emerges+as+early+timekeeper-a018639626 Elton Note that Formation age, Cosmic Ray Exposure age(CRE) are not the same. The formation age of meteoric material may or not be the same age as when it was liberated/ejected from the parent body depending if the shock was sufficient to reset the atomic clocks. __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Questions about accretion. Part 2 UAE, Shock wave distribution proto Solar System
There was a question regarding the sorting of elements and why for example common chondrules had more iron than did Carbonaceous chondrites. The reason for the difference also includes why we use isotope ratios to determine from where a parent body probably formed within the solar system. Sometime in early solar system development there was a sustained and or repeated strong solar wind or mini-nova, or perhaps our own ancestral sun's predecessor nearby supernova, or other cosmic water hose(?) that sweep through the swirling matter in the proto-solar disk, significantly sorting it out by elemental and molecular weights. Heavier particles weren't pushed out as far as the lighter ones. Thus we have heavy to light sorting of particles/ elements/ molecules/ solids/ gases etc from the inner rocky planets at one end to the giant gas planets beyond the asteroid belt and all way out to the Ort cloud. The sorting was not perfect but did rearrange the mixtures of elements locally. Conservation of angular momentum must have broken down at some level such that the Oort Cloud is theorized to be more or less spherical while planetary masses tend to lie close to the plane of the ecliptic. (This glitch influences measured elemental ratios of our known solar system and just mentioned for those paying attention) Thus before significant planetary accretion(first 3-5 million years?) we experienced a cycle of sorting that left zones of like particles to be accreted. This sorting also locally affected the ratios of the individual isotopes of elements from a concept we know as the Universal Abundance of the Elements.(UAE) (The UAE says that based on human measurements the mass of the universe is concentrated in the first 20 elements which incidentally were the main elements associated with living processes). When the local Solar system abundance of the UAE was disturbed, distribution of isotope ratios were also skewed in the local solar system. Ergo oxygen isotope studies in meteorites tell us what relative distance/radius a parent body formed away from the sun. On Earth the ratios for Oxygen: O18(Tritium)-O17(Deuterium)-O16 is something like 18O / 16O = 2005.20 ±0.43 ppm (a ratio of 1 part per approximately 498.7 parts) 17O / 16O = 379.9 ±1.6 ppm (a ratio of 1 part per approximately 2632 parts) This ratio signature is specific to an origin in the Earth Moon distance and there is a different one for Mars, the asteroid belt, Jupiter, Saturn and carbonaceous chondrites etc. Complications to this gradient include the amount of oxygen returned to earth via comets in what was known as the great bombardment-- back skewing the post shockwave sorting in the early sweep out. Ok we are at the end almost. O18 being two neutrons heavier takes more latent energy to vaporize and results in a slight concentration of its ratio in seawater depending on how much extra energy is around. The colder the climate the more O18 gets left behind in seawater and available for building carbonate seashells. The higher the temperature trends the more gets evaporated and a portion of that gets preserved in paleo-ice cores. Thus ratios differ in sequestrations such as in coral reefs and sea shells. This characteristic makes O18 content in ancient ice cores and fossil shells equivalent to a paleo thermometer. Long way around answering why some classes of meteorites have more iron in them than others. Elton __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Questions about accretion.
Hi Eric You are correct in thinking that electrostatics causes the initial clumping. The early sun would have been extremely energetic and X-ray and UV radiation would produce electro static charging of small particles. Once they begin to clump to a sufficient size, they will attract particles through gravity. The dynamics are as follows An object with radius R will naturally sweep up any object within its radius (pi*R^2) but gravity will draw material from a greater distance S inside and outside its orbital path S=(R^2 + 2GMR/V^2)^1/2 M mass of body, V initial closing velocity of body and impactor Initially, you are correct, everything begins as a big clump of mixed material. Whether an iron core is formed will depend on the size of the initial clump of stuff. Heat is generated by radioactivity of short lived isotopes such as Al26. If the rock is big enough, (which provides enough radioactive material to generate the heat AND enough lying over the middle to prevent the heat escaping, the body will melt. Once this begins, the iron will migrate to the core as rock and iron don't mix. Iron, being denser, will sink. Accretion to differentiation is a very rapid affair, just a few million years. The almost identical ages of all asteroidal meteorites tends to confirm this. My understanding is that this leads to the different classes of achondrites. These have been properly melted and lose their chondrules. The widmanstatten patterns in irons comes from the rocky material insulating the iron/nickel core allowing it to cool very slowly. Parent bodies forming in different orbits are likely to have differing constituents according the condensation model, hence different achondrite types. Chondrites may have come from smaller initial parent bodies, ones that weren't big enough to generate enough heat to fully melt. Higher petrographic types of chondrite (4-6) are samples that are progressively closer to the core and were heated more in bodies that were not properly differentiated. Petrographic type 3 are essentially the same material as the early solar system, mostly unaltered by heat, likely from near the surface of undifferentiated bodies. I don't see that all parent bodies would necessarily need 3-6 petrographic types. Small parent bodies may not reach the higher grades in the middle as they never got hot enough. Grade 6 seems to be the limit. If the parent body grew any bigger then it would melt producing a differentiated parent body. I think petrographic type goes to 7 but I don't think any are actually given this grade (though I think it was NWA3133 that may have been discussed as a possible). It is likley that H, L and LL meteorites come from different parent bodies possibly from different regions in the protosolar nebula. The relative rarity of petrographic type 3 ordinary chondrites may be due to them being removed first and subsequently removed from the system many aeons ago. Carbonaceous Chondrites are a whole different kettle of fish but I think I've said quite enough for now. I hope I've not made any glaring errors but if I have someone will put me right. Rob Mc __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
[meteorite-list] Questions about accretion.
Hi all, I love asking questions because I learn cool stuff! ;) How bout these... How long does the formation of meteoroid bodies and larger asteroids take? How does the iron migrate to the core? Do all large asteroids consist of an iron core surrounded by lighter materials further towards the asteroids surface? I understand the basic process of accretion, however I'm still a bit perplexed as to how the iron condenses into such a solid structure at a large asteroids center. Is this due in part to impacts with other meteoroidal (is that a word?) and asteroidal bodies, compacting the mineral structures into denser and denser materials toward the core? I'm familiar with how much force an impact can have when two larger bodies collide. But maybe I'm going in the wrong direction with this. If a meteoroid is a small part of a larger asteroid, wouldn't all asteroids once have been meteoroids by definition during their formation within solar nebulae? -- Regards, Eric Wichman Meteorites USA http://www.meteoritesusa.com 904-236-5394 __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Questions about accretion.
How long does the formation of meteoroid bodies and larger asteroids take? I really don't know, but gonna throw out a guess. I'm assuming that in the beginning of star and planet formation, there is a lot of dust around. I recall an experiment aboard one of the Shuttles or space station where a lot of fine material such as talcum powder was floating around weightless in a container. I guess there was amazement about how this material was clumping very fast due to electrostatic charges. Based on that scenario, I'd have to guess that we can expect to see fist sized clumps in about a month maybe? I'd imagine eventually gravity itself will have to get into the picture as well. Overall, I wouldn't think it would take too many years for asteroid sized bodies to form...as long as there are a lot of raw material available. How does the iron migrate to the core? Again I don't really know, but will throw out a guess for someone to work me over with. :O) I'm assuming that the iron will have to melt in order for this differentiation to occur. I guess there will also have to be a minimum sized asteroid in order for iron to melt so it can migrate. Okay...what could melt the iron then? Things that comes to mind is the heat from radioactive elements; Heat from compression; heat generated if the asteroid is in a strong magnetic field around the sun (like the moon Io around Jupiter); and heat from impacts as well. then it becomes sorta like gold in a pan...the heavies at the bottom or middle and lighter material on top...but in this case without the melting. Do all large asteroids consist of an iron core surrounded by lighter materials further towards the asteroids surface? My guess...if there was some internal melting, I'd say yes. If a meteoroid is a small part of a larger asteroid, wouldn't all asteroids once have been meteoroids by definition during their formation within solar nebulae? I'd say yes to those that formed from dust. But if a solar nebula is the remnants of previous stars that went supernova, I would imagine there could be a fair amount of asteroids left over from that explosion as well. I don't really know. If that was right, I'd expect to hear about a few meteorites that were older than our solar system...unless our solar system formation began very fast after it's source of material from a supernova occurred showing a near similar age. GeoZay **Feeling the pinch at the grocery store? Make dinner for $10 or less. (http://food.aol.com/frugal-feasts?ncid=emlcntusfood0001) __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Questions about accretion.
Thanks for the responses thus far... I've studied lots of material and scientific papers on accretion, but still have some questions. The gravity explanation is great, but it's a little vague. I want to know what causes it I guess at the molecular level. What physical forces and interactions cause the iron to migrate into such a solid mass at the core? If gravity alone were the case, why is it we have H and L chondrites at all? Everything would be one big clump of mixed material. Has the iron not had a chance yet to migrate out of this layer of rock to the center of the asteroid? I know H and L chondrites are meteoroids that have broken off the parent bodies but my question is simply, had they not been blasted off the main body, how long would it take and in what manner would the iron have migrated from these layers of rock to the core? Iron doesn't just move through stone without some sort of catalyst or outside force does it? Gravity itself is not sufficient to move iron through a stone matrix no matter how much time passes is it? If there are no impacts or outside forces acting upon the body how does the iron loose itself from the grasp of the stone matrix to move through toward the core? Impacts? At the beginning of the formation of a meteoroid is it electrostatic attraction that causes it to get larger? At what size does it produce it's own gravity? Or does it? How does and asteroid become so dense? If asteroids are super dense, and comets are loosely bound material and gases, would that mean that asteroids are dead comets? Wow! I know that a lot of questions. sorry... ;) Eric __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Questions about accretion.
Hi Eric, I'll take a stab at a few of your questions: How long does the formation of meteoroid bodies and larger asteroids take? This is not an easy question, as there were many processes at work during the early solar system -- some constructive (gravitational/electrostatic clumping), some destructive (high velocity impacts between clumps), and the time it would take to form, say, a 100-km sized body would depend on the initial quantity of dust in the pre-solar nebula. I don't know how long planetary scientists believe it took to form 1-km-sized bodies, but it was at least hundreds of thousands of years, probably longer. But when do you start the clock? When what became the solar system was just a molecular cloud, when the protostar formed, or tens of millions of years later when the protostar transitioned from T-Tauri stage to main sequence burning?) Whichever you choose, once you have asteroids a kilometer or so in size, barring collision with other such bodies they would continue to accrete at a rate of centimeters per year. So it would still take more than a million years to grow from 1-km to 100-km size. How does the iron migrate to the core? Through the combination of porosity, heat and gravity. If you start with a glass of finely crushed ice and let it melt, the water doesn't stay put in the ice matrix -- it settles to the bottom (since water is denser than ice). Do all large asteroids consist of an iron core surrounded by lighter materials further towards the asteroids surface? Yes, beyond a certain size nearly all should. One way to create an exception might be to have a large, already-differentiated asteroid get impacted by a smaller one in such a way that its iron core remains intact, but a portion of the outer rocky shell is blown off. Any large fragments of the original differentiated asteroid would then be depleted in iron/nickel. --Rob __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Questions about accretion.
- Original Message - From: Meteorites USA e...@meteoritesusa.com To: geo...@aol.com; meteorite-list@meteoritecentral.com Sent: Sunday, April 05, 2009 3:52 PM Subject: Re: [meteorite-list] Questions about accretion. Thanks for the responses thus far... I've studied lots of material and scientific papers on accretion, but still have some questions. The gravity explanation is great, but it's a little vague. I want to know what causes it I guess at the molecular level. What physical forces and interactions cause the iron to migrate into such a solid mass at the core? If gravity alone were the case, why is it we have H and L chondrites at all? Everything would be one big clump of mixed material. Has the iron not had a chance yet to migrate out of this layer of rock to the center of the asteroid? I know H and L chondrites are meteoroids that have broken off the parent bodies but my question is simply, had they not been blasted off the main body, how long would it take and in what manner would the iron have migrated from these layers of rock to the core? Iron doesn't just move through stone without some sort of catalyst or outside force does it? Gravity itself is not sufficient to move iron through a stone matrix no matter how much time passes is it? If there are no impacts or outside forces acting upon the body how does the iron loose itself from the grasp of the stone matrix to move through toward the core? Impacts? At the beginning of the formation of a meteoroid is it electrostatic attraction that causes it to get larger? At what size does it produce it's own gravity? Or does it? How does and asteroid become so dense? If asteroids are super dense, and comets are loosely bound material and gases, would that mean that asteroids are dead comets? Wow! I know that a lot of questions. sorry... ;) Eric __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Questions about accretion.
Field Guide to Meteors and Meteorites, Norton, Page 36. There are two models that could describe the interior of a chondritic asteroid parent body. The origional body is accreted as it orbits in the protoplanetary disk. The result is a homogeneous body with its mineral components evenly distributed throughout the interior. Internal heating by the short-lived radioisotope Aluminum 26 provides the energy to heat the interior from the deep core of the body to the near surface. Thermal metamorphism slowly heats the interior to a petrographic type 6 at the core. The heat makes its way through the body, slowly converting various regions of the interior to different petrographic types from type 6 to type 3. The result is a layered structure something like an onion's interior, thus, the onion shell model. enjoy, [Erik] Date: Sun, 5 Apr 2009 12:52:46 -0700 From: e...@meteoritesusa.com To: geo...@aol.com; meteorite-list@meteoritecentral.com Subject: Re: [meteorite-list] Questions about accretion. Thanks for the responses thus far... I've studied lots of material and scientific papers on accretion, but still have some questions. The gravity explanation is great, but it's a little vague. I want to know what causes it I guess at the molecular level. What physical forces and interactions cause the iron to migrate into such a solid mass at the core? If gravity alone were the case, why is it we have H and L chondrites at all? Everything would be one big clump of mixed material. Has the iron not had a chance yet to migrate out of this layer of rock to the center of the asteroid? I know H and L chondrites are meteoroids that have broken off the parent bodies but my question is simply, had they not been blasted off the main body, how long would it take and in what manner would the iron have migrated from these layers of rock to the core? Iron doesn't just move through stone without some sort of catalyst or outside force does it? Gravity itself is not sufficient to move iron through a stone matrix no matter how much time passes is it? If there are no impacts or outside forces acting upon the body how does the iron loose itself from the grasp of the stone matrix to move through toward the core? Impacts? At the beginning of the formation of a meteoroid is it electrostatic attraction that causes it to get larger? At what size does it produce it's own gravity? Or does it? How does and asteroid become so dense? If asteroids are super dense, and comets are loosely bound material and gases, would that mean that asteroids are dead comets? Wow! I know that a lot of questions. sorry... ;) Eric __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
