Re: [meteorite-list] [2] Asteroid Or Comet Sutters Mill
Not guilty! It was 1.2 AU away from the Earth at the time--nice try! There are, however other C-type NEOs that are around and could keep our supply up. By the way, 13 of the 20 largest asteroids in the Main Belt are either C-type or B-type and I think that all of them have the spectral signature of hydrated silicates and make up something like 45% of the mass of the entire asteroid belt (Ceres itself is 1/3 of the entire mass of the asteroid belt). So lots of potential sources for CM meteorites, though I think that 10 Hygiea is the best candidate at the moment (spectrum, location, etc.). Larry Or NEO (175706) 1996 FG3 ! (MPOD 24 Oct 2011) Kindest wishes Doug -Original Message- From: lebofsky lebof...@lpl.arizona.edu To: aerubin aeru...@ucla.edu Cc: meteorite-list meteorite-list@meteoritecentral.com Sent: Wed, May 2, 2012 11:47 pm Subject: Re: [meteorite-list] Asteroid Or Comet Sutters Mill Hi Again: One other place that seems to have abundant CM-like material, the surface of Vesta. There are dark areas on Vesta that seem to be composed of carbonaceous chondritic material (based again on albedo and spectrum). I do not know all of the details (missed some of the papers at the Lunar and Planetary Science Conference), but my impression is that the dark material did have the spectral signature of material altered by water, implying that Vesta has been hit over time by C-class asteroids. This is consistent with what Alan is saying about clasts in howardites (which a thought to come from Vesta). To answer your question, Mike, once you alter (hydrate) the silicate material and make a phyllosilicate, it is not that easy to get rid of the water (need temperatures that are in the hundreds of degrees centigrade. You just needed temperatures low enough when the asteroids formed for water to condense out, probably the middle of the present asteroid belt. Larry CM chondrites are also ubiquitous. The most abundant foreign component of the lunar soil is chemically similar to CM chondrites. If i recall, many fireballs also seem to be CM like, although other list members would be better able to address this point. More CM chondrites would be in our meteorite collections if they weren't so friable. There are also many CM clasts in meteorite breccias, both ordinary chondrite regolith breccias like Abbott, Plainview, Dimmitt and Fayetteville, and howardites such as Kapoeta. This ubiquity mandates a reliable local source, i.e., not a comet but an asteroid. Some of the clasts in ordinary chondrites are unshocked, meaning that they came in at low relative velocities, also very un-comet like. As the asteroid guys say, the CM chondrites are probably from some types of C asteroids located at the outer reaches of the main belt; at those places ambient temperatures are low and volatiles are more likely to remain on the parent body. That is why CMs contain about 9 wt.% water (within phyllosilicates) and CI chondrites contain appreciably more. Quoting Michael Gilmer meteoritem...@gmail.com: Hi List, This is great stuff. Thanks to Alan and Larry for enlightening us on this. There has been some talk of the volatiles content of CM meteorites. So, is it safe to assume that CM meteorites also originate from the darker outer reaches of the asteroid belt where Tagish Lake hails from? Meteorites rich in volatiles presumably come from that region where solar effects are minimized? Best regards, MikeG -- --- Galactic Stone Ironworks - MikeG Web: http://www.galactic-stone.com Facebook: http://www.facebook.com/galacticstone Twitter: http://twitter.com/GalacticStone RSS: http://www.galactic-stone.com/rss/126516 --- On 5/2/12, lebof...@lpl.arizona.edu lebof...@lpl.arizona.edu wrote: Hi Alan: I would agree with you on the consensus that CMs would appear to come from asteroids. Based on spectra and albedo, CM meteorites look like C-class (and possibly several other low-albedo classes) asteroids (very common in the Main Belt). These are asteroid that have surface compositions showing that they have been exposed to liquid water, phyllosilicates. There is no (or little) evidence that comets have had interiors warm enough to melt ice and create the water necessary to form phyllosilicates. Larry I guess I've been goaded into responding. First, at this point we don't know if the meteorite is a CM chondrite or not. No meteorite researcher has completed an analysis of it yet (perhaps tomorrow or Friday) and I have not seen a piece. But, on the more general question of CM chondrites, most researchers believe that the carbonaceous chondrites all are derived from asteroids. There is more or less a continuum in properties across the chondrite groups; it is difficult to imagine that they are from different
Re: [meteorite-list] [2] Asteroid Or Comet Sutters Mill
Or NEO (175706) 1996 FG3 ! (MPOD 24 Oct 2011) Kindest wishes Doug -Original Message- From: lebofsky lebof...@lpl.arizona.edu To: aerubin aeru...@ucla.edu Cc: meteorite-list meteorite-list@meteoritecentral.com Sent: Wed, May 2, 2012 11:47 pm Subject: Re: [meteorite-list] Asteroid Or Comet Sutters Mill Hi Again: One other place that seems to have abundant CM-like material, the surface of Vesta. There are dark areas on Vesta that seem to be composed of carbonaceous chondritic material (based again on albedo and spectrum). I do not know all of the details (missed some of the papers at the Lunar and Planetary Science Conference), but my impression is that the dark material did have the spectral signature of material altered by water, implying that Vesta has been hit over time by C-class asteroids. This is consistent with what Alan is saying about clasts in howardites (which a thought to come from Vesta). To answer your question, Mike, once you alter (hydrate) the silicate material and make a phyllosilicate, it is not that easy to get rid of the water (need temperatures that are in the hundreds of degrees centigrade. You just needed temperatures low enough when the asteroids formed for water to condense out, probably the middle of the present asteroid belt. Larry CM chondrites are also ubiquitous. The most abundant foreign component of the lunar soil is chemically similar to CM chondrites. If i recall, many fireballs also seem to be CM like, although other list members would be better able to address this point. More CM chondrites would be in our meteorite collections if they weren't so friable. There are also many CM clasts in meteorite breccias, both ordinary chondrite regolith breccias like Abbott, Plainview, Dimmitt and Fayetteville, and howardites such as Kapoeta. This ubiquity mandates a reliable local source, i.e., not a comet but an asteroid. Some of the clasts in ordinary chondrites are unshocked, meaning that they came in at low relative velocities, also very un-comet like. As the asteroid guys say, the CM chondrites are probably from some types of C asteroids located at the outer reaches of the main belt; at those places ambient temperatures are low and volatiles are more likely to remain on the parent body. That is why CMs contain about 9 wt.% water (within phyllosilicates) and CI chondrites contain appreciably more. Quoting Michael Gilmer meteoritem...@gmail.com: Hi List, This is great stuff. Thanks to Alan and Larry for enlightening us on this. There has been some talk of the volatiles content of CM meteorites. So, is it safe to assume that CM meteorites also originate from the darker outer reaches of the asteroid belt where Tagish Lake hails from? Meteorites rich in volatiles presumably come from that region where solar effects are minimized? Best regards, MikeG -- --- Galactic Stone Ironworks - MikeG Web: http://www.galactic-stone.com Facebook: http://www.facebook.com/galacticstone Twitter: http://twitter.com/GalacticStone RSS: http://www.galactic-stone.com/rss/126516 --- On 5/2/12, lebof...@lpl.arizona.edu lebof...@lpl.arizona.edu wrote: Hi Alan: I would agree with you on the consensus that CMs would appear to come from asteroids. Based on spectra and albedo, CM meteorites look like C-class (and possibly several other low-albedo classes) asteroids (very common in the Main Belt). These are asteroid that have surface compositions showing that they have been exposed to liquid water, phyllosilicates. There is no (or little) evidence that comets have had interiors warm enough to melt ice and create the water necessary to form phyllosilicates. Larry I guess I've been goaded into responding. First, at this point we don't know if the meteorite is a CM chondrite or not. No meteorite researcher has completed an analysis of it yet (perhaps tomorrow or Friday) and I have not seen a piece. But, on the more general question of CM chondrites, most researchers believe that the carbonaceous chondrites all are derived from asteroids. There is more or less a continuum in properties across the chondrite groups; it is difficult to imagine that they are from different classes of parent bodies, i.e., asteroids vs. comets. All chondrite groups (except CI) contain chondrules, CAIs, matrix, metal and sulfide although the abundances of these phases can vary a lot among the groups. Even CI chondrites contain a few olivine and pyroxene grains that seem to be chondrule fragments, a few refractory mineral grains that seem to be CAI fragments, and even one reported intact CAI. Furthermore, the isolated olivine and pyroxene grains in CI chondrites have the same olivine Fa vs. CaO distribution as in CM chondrites suggesting that they are from a similar source. I think that the CM chondrites are from an asteroid that was
