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
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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 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: aeru...@ucla.edu
website: http://cosmochemists.igpp.ucla.edu/Rubin.html
----- Original Message -----
From: "Matson, Robert D." <robert.d.mat...@saic.com>
To: "meteorite-list" <meteorite-list@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-boun...@meteoritecentral.com
[mailto:meteorite-list-boun...@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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