[meteorite-list] Chelyabinsk Meteorite Sheds Light on Dinosaur Extinction Mystery

From: Vishnu Reddy <reddy_at_meteoritecentral.com>
Date: Wed, 16 Jul 2014 15:50:59 -0700
Message-ID: <7FFBB67B-A851-4209-A144-C1E5641D7B3E_at_psi.edu>

Hi Mike

The original link between Baptistina Family and the K/T impactor was proposed based on the composition of the "fossil" meteorite that was discovered in the K/T layer by Frank Kyte at UCLA.
Here is a quote from his 1998 paper.

"The fossil meteorite from DSDP Hole 576 appears to be from (1) a chondritic meteorite with (2) significant amounts of metal and sulphide (4-8%), (3) large inclusions [larger than 200 um] of mafic minerals that also contained metal, and (4) 30-60% fine-grained matrix. The known meteorite groups that best fit these criteria could be the CV, CO, and CR carbonaceous chondrites."

Bottke et al. proposed the link between K/T impactor and Baptistina family based on dynamical evidence and also the colors (very rudimentary analog for composition). Baptistina asteroid family seems to have lower albedo and weaker absorption bands similar to what one would expect for a carbonaceous asteroid.

We looked at several members of Baptistina asteroid family and got their near-IR spectra to constrain their surface composition. What we noted was that Baptistina family asteroid spectra looked very similar to the background Flora family but were subdued by some unknown darkening material. The mineralogy of Baptistina suggested that they were similar to LL chondrites just like the Floras. We also looked for OH/H2O absorption bands in Baptistina asteroid family and found none. Our rationale there was if some of them were mixtures of LL chondrite material and carbonaceous then they could show such a feature. We see these OH/H2O bands in some of the carbonaceous meteorites under the right laboratory conditions on Earth. We found no such evidence and so ruled out the possibility of the darkening agent being a carbonaceous impactor like we see on Vesta. There is no evidence from LL chondrites for widespread carbonaceous xenoliths like we see in howardites. So there is also not much support from the meteoritical
side. Shock darkening and impact melt that we see on Chelyabinsk seems to be the most logical way to explain the spectral properties observed on Baptistina family.

The take away message would be that if BAF is the source of the K/T impactor then K/T impactor is not carbonaceous contrary to what Kyte reports. A more logical conclusion would be that Baptistina Asteroid Family had nothing to do with the K/T impactor in the first place and the compositional link between the K/T impactor and BAF asteroids is not valid in light of what we see in Chelyabinsk. So the original hypothesis that K/T impactor might be carbonaceous remains.

I hope that clears the air.

Vishnu Reddy

On Jul 16, 2014, at 3:23 PM, Galactic Stone & Ironworks via Meteorite-list <meteorite-list at meteoritecentral.com> wrote:

> This is an interesting theory. But, how does Chelyabinsk completely
> rule out a carbonaceous KT impactor? Until we recover an extant
> sample of the KT impactor, the question is still unanswered. Yes,
> there are dark meteorites that are not carbon-rich. But how does this
> fact rule out a carbonaceous (or any) impactor for the KT impact? Am
> I missing something?
> Best regards,
> MikeG
> --
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> On 7/16/14, Ron Baalke via Meteorite-list
> <meteorite-list at meteoritecentral.com> wrote:
>> FROM:
>> Alan Fischer
>> Public Information Officer
>> Planetary Science Institute
>> 520-382-0411
>> 520-622-6300
>> fischer at psi.edu
>> Russian Meteorite Sheds Light on Dinosaur Extinction Mystery
>> July 16, 2014, Tucson, Ariz. -- A long-standing debate about the source of
>> the asteroid that impacted the Earth and caused the extinction of the
>> dinosaurs has been put to rest thanks to the Chelyabinsk meteorite that
>> disintegrated over Russia in February 2013, a new paper published in the
>> journal Icarus shows.
>> Astronomers have debated whether the dinosaur killer was linked to the
>> breakup of a large asteroid forming the Baptistina Asteroid Family (BAF)
>> beyond Mars, some of which ended up on Earth-crossing orbits. The asteroid
>> impacting Earth is thought to have been dark and carbonaceous. The BAF
>> hypothesis was bolstered by them being dark and with a spectral shape
>> similar to carbonaceous meteorites.
>> Analysis of the Chelyabinsk meteorite shows that shock produced during
>> catastrophic disruption of a large asteroid can darken otherwise bright
>> silicate material. Shock darkening was first reported by Dan Britt (now at
>> the University of Central Florida) in the early 1990s. The Chelyabinsk
>> meteorite has both bright unshocked and dark shocked material. However, the
>> details of the spectra of the dark Chelyabinsk material closely reproduces
>> spectral signatures seen with members of the Baptistina Asteroid Family,
>> said Planetary Science Institute Research Scientist Vishnu Reddy, lead
>> author of "Chelyabinsk meteorite explains unusual spectral properties of
>> Baptistina Asteroid Family that appears in Icarus.
>> "Shock and impact melt can make bright asteroids dark, Reddy said. "In
>> other words, not all dark asteroids are rich in carbon as once thought."
>> The latest measurements rule out the possibility for the Baptistina family
>> being the source of the K/T impactor, he added.
>> 'The link between the K/T impacator, thought to be carbonaceous, and BAF,
>> has been proved invalid," Reddy said.
>> Chelyabinsk provided a great opportunity to see the mixture of shocked and
>> unshocked material in a single meteorite, Reddy said while cautioning that
>> no clear evidence exists that the Russian meteorite itself came from the
>> Baptistina family.
>> "The new finding has implications for hazards from Near-Earth Objects and
>> for mining asteroids for space-based resources," Reddy said. "A potential
>> target identified as primitive and rich in volatiles/organics and carbon
>> based on its spectral colors could in fact be just shocked material with
>> entirely different composition."
>> PSI researchers David P. O'Brien and Lucille Le Corre were among the
>> co-authors on the paper.
>> This research work was supported by grants from NASA's Planetary Mission
>> Data Analysis Program, NEOO Program and Planetary Geology and Geophysics
>> Program.
>> Vishnu Reddy
>> Senior Scientist
>> 808-342-8932
>> reddy at psi.edu
>> Mark V. Sykes
>> Director
>> 520-622-6300
>> sykes at psi.edu
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Received on Wed 16 Jul 2014 06:50:59 PM PDT

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