[meteorite-list] Asteroid Or Comet Sutters Mill

From: aerubin at ucla.edu <aerubin_at_meteoritecentral.com>
Date: Wed, 02 May 2012 19:54:53 -0700
Message-ID: <20120502195453.1375958udzkwblvh_at_mail.ucla.edu>

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 <meteoritemike at 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
>
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>
>
>
> On 5/2/12, lebofsky at lpl.arizona.edu <lebofsky at 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 fairly
>>> porous
>>> and had a fair amount of water, present either as ice or in
>>> phyllosilicates.
>>> Stochastic impacts on this asteroid caused fracturing in some regions
>>> more
>>> than others and during subsequent aqueous alteration (probably caused by
>>> impact mobilization of water), the more fractured regions retained more
>>> water and became more altered.
>>>
>>>
>>> Alan Rubin
>>> Institute of Geophysics and Planetary Physics
>>> University of California
>>> 3845 Slichter Hall
>>> 603 Charles Young Dr. E
>>> Los Angeles, CA 90095-1567
>>> phone: 310-825-3202
>>> e-mail: aerubin at ucla.edu
>>> website: http://cosmochemists.igpp.ucla.edu/Rubin.html
>>>
>>>
>>> ----- Original Message -----
>>> From: "Matson, Robert D." <ROBERT.D.MATSON at saic.com>
>>> To: "meteorite-list" <meteorite-list at meteoritecentral.com>
>>> Sent: Wednesday, May 02, 2012 2:16 PM
>>> Subject: Re: [meteorite-list] Asteroid Or Comet Sutters Mill
>>>
>>>
>>>> Hi Paul,
>>>>
>>>> Probably not a misquote -- Dr. Jenniskens is interested in deciphering
>>>> the
>>>> nature of the original asteroid (meteoroid) body that produced the
>>>> meteorites. The original body was large enough that it may not have been
>>>> a monolithic body; as with 2008 TC3 (Almahata Sitta), the pre-encounter
>>>> body may have been a rubble pile, consisting of more than just CM2
>>>> material. In any case, I don't think the parent body (or bodies) for CM2
>>>> is cometary. Would be interested in hearing Dr. Rubin's theory on the
>>>> nature of the CM2 parent. --Rob
>>>>
>>>> -----Original Message-----
>>>> From: meteorite-list-bounces at meteoritecentral.com
>>>> [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of Paul
>>>> Gessler
>>>> Sent: Wednesday, May 02, 2012 2:08 PM
>>>> To: meteorite-list
>>>> Subject: [meteorite-list] Asteroid Or Comet Sutters Mill
>>>>
>>>> In the LA times article it reads in part:
>>>>
>>>> We want to learn about this asteroid," said Peter Jenniskens, an
>>>> astronomer and senior research scientist at the Carl Sagan Center at the
>>>> SETI (Search for Extraterrestrial Intelligence) Institute and the NASA
>>>> Lunar Science Institute. "This is scientific gold."
>>>>
>>>> I hope/probably they miss quoted him?
>>>>
>>>> I vote comet
>>>>
>>>> Paul G
>>>>
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>>
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Received on Wed 02 May 2012 10:54:53 PM PDT