[meteorite-list] The Almahata Meteorite (Asteroid 2008 TC3)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 5 May 2010 16:00:15 -0700 (PDT)
Message-ID: <201005052300.o45N0F1E012690_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/April10/AlmahataSitta.html

Asteroid, Meteor, Meteorite
Planetary Science Research Discoveries
April 30, 2010

--- Detected in space less than a day before hitting Earth, the Almahata
Sitta meteorite from asteroid 2008 TC3 gives clues to the complex
evolution of small asteroids.

Written by Linda M. V. Martel
Hawai'i Institute of Geophysics and Planetology

Almahata Sitta is the name identifying the collection of meteorite
remnants of the first observed fall of a tracked asteroid, 2008 TC3 .
Ground-based observatories, orbiting satellites, a pilot of a commercial
airline flight, and eyewitnesses of the fireball in the Nubian Desert of
northern Sudan all observed evidence of the spectacular events on
October 6, 2008. The first meteorites were recovered two months later in
Sudan by students and staff from the University of Khartoum (Sudan) led
by Dr. Muawia Shaddad and further guided by Dr. Peter Jenniskens of the
SETI Institute and NASA Ames Research Center (Mt. View, California). A
session at the 41st Lunar and Planetary Science Conference held March
1-5, 2010 focused on ureilitic asteroids and insights from Almahata
Sitta, and forms the basis for this article. Rather than discuss the
results of each of the talks and posters presented at the conference, I
highlight what makes the impact, recovery, and characterization of the
ureilite meteorite fragments so outstanding. The complete listing of
authors and topics is available in the conference program (see reference
link below).

Reference:

    * Session at the (2010) 41st Lunar and Planetary Science
      Conference-- Ureilitic Asteroids: Insights from Almahata Sitta
      Full set of abstracts (pdf) from talks and links to posters
      <LPSC_ureiliteAbstracts.pdf>.
    * Jenniskens, P. and 34 coauthors (2009) The Impact and Recovery of
      Asteroid 2008 TC3 . /Nature,/ v. 458, doi: 10.1038/nature07920.

------------------------------------------------------------------------

The Story of Asteroid 2008 TC3

In the early morning of October 6, 2008 an asteroid close to Earth was
detected by a Catalina Sky Survey (CSS) telescope at Mount Lemmon,
Arizona. The CSS, begun in 1998, is the only near-Earth object survey
covering both Northern and Southern hemispheres and obtains about 20
gigabytes of data with each of its three telescopes per night. Observers
do near-real-time analysis of Earth-approaching objects, which was the
case when CSS observer Richard Kowalski discovered the small
(2-meter-diameter) object beyond the orbit of the Moon and moving toward
Earth at 12 kilometers per second.

The Catalina Sky Survey immediately alerted scientists at the Minor
Planet Center in Cambridge, MA and NASA's Near Earth Object Program in
Pasadena, CA. From the measured orbital parameters of the asteroid, the
scientists calculated the asteroid would hit Earth's atmosphere over the
Sudan within 19 hours. Though observatories around the world, both
professional and amateur, rapidly supplied additional positional
measurements, this asteroid posed no threat because of its small size.
It was simply expected to disintegrate and burn up in the atmosphere.
[Watch this video interview "Catalina Sky Survey Keeps an Eye on the
Skies < http://uanews.org/node/23269>" from the University of Arizona
News.]

As it entered Earth's atmosphere, the asteroid compressed and heated the
air in front of it, heating itself and releasing a tremendous amount of
light and energy. In those dark, wee hours of the morning on October 7,
2008 a KLM airlines pilot and co-pilot (who had received an alert about
the incoming asteroid from the KLM dispatcher) were flying at an
altitude of 10,700 meters over Chad and saw three or four short flashes
of light beyond the horizon. Other reports of the asteroid's entry into
the atmosphere came from U.S. satellites and infrasound signals from at
least one ground station. A brief flash was even captured by an infrared
channel on the weather satellite Meteosat-8. It disintegrated and
exploded at an altitude of 37 kilometers. A high-altitude train of
residual clouds lit up the early dawn sky.

The event was remarkable in itself because it was the first time an
asteroid was detected and tracked in space before impacting Earth's
atmosphere. Even more remarkable is the notion that pieces of this first
observed fall of a tracked asteroid could be held in the palm of your hand.

------------------------------------------------------------------------

Finding Fragments of Asteroid 2008 TC3

The first field expedition to hunt for meteorites from asteroid 2008
TC3 was organized on December 2-9, 2008 led by Dr. Muawia Shaddad
(University of Khartoum, Sudan) and further guided by Dr. Peter
Jenniskens (SETI Institute and NASA Ames Research Center, Mt. View,
California). The location was Nahr an Nil in the Nubian Desert of Sudan.
Students and staff from the university lined up, side by side along a
kilometer line, to walk in a coordinated search along the projected
ground path of the meteor. To the excitement of everyone, fifteen
meteorites were found, totaling 563 grams. The closest recognizable
location marker was the railroad's Station 6 located between Wadi Halfa
and Abu Hamed. Almahata Sitta in Arabic means Station 6, and so the
meteorite earned its name. Two additional searches organized by the
University of Khartoum recovered more fragments, for a total number of
280 weighing 3.95 kilograms. The meteorites were found over an area of
28 x 5 kilometers.

A Briefing on Ureilite Meteorites

Given that the recovered meteorites have been classified as a kind of
ureilite, we will begin with a review of this meteorite group before
looking at the specific characteristics of Almahata Sitta. Within the
class of stony meteorites called achondrites
is a group known as ureilites. The
story of how they got their name is like any other--based on the
location where the meteorite was found--but with a twist. Here is an
excerpt from the book *Meteorites and Their Parent Planets* (Cambridge
University Press, 1999) by Harry (Hap) Y. McSween, Jr.

    "On a September morning in 1886, several meteorites fell near the
    village of Novo Urei in the Krasnoslobodsk district of Russia. This
    was a particularly interesting fall for several reasons. One of the
    stones was soon recovered by local peasants, whereupon it was broken
    apart and eaten. The motivation for this rather unusual action is
    not known, but this constituted an impressive feat from a dental
    perspective, because the meteorite contained numerous small diamonds."

Since Novo-Urei, 260 more ureilites have been identified [see the entire
listing of ureilites in the Meteoritical Bulletin Database
<http://tin.er.usgs.gov/meteor/metbull.php?sea=&sfor=types&ants=&falls=&valids=&stype=contains&lrec=50&map=ge&browse=&country=All&srt=name&categ=Ureilites&mblist=All&rect=&phot=&snew=0&pnt=Normal%20table&dr=&page=1>].
Ureilites are coarse-grained ultramafic achondrites chiefly made of olivine
and pyroxene, plus tiny grains of graphite (a low-pressure form of
carbon), diamond (a high-pressure form of carbon), minor iron-nickel
metal, and other minor phases. Cosmochemists specializing in the study
of ureilites have discovered that ureilites are igneous rocks with
diverse oxygen isotopic compositions that signify incomplete melting of
their parent asteroid. The working hypothesis is that ureilites are
mantle material that may preserve a unique stage of early
differentiation of a
carbonaceous-chondrite-like parent asteroid. The diamond most likely
came from the original graphite in the rock that was converted by
high-pressure shock waves during large impact events on the parent
asteroid.

The typical ureilites have nice igneous texture with olivine more
abundant than pyroxene. But about 10% of ureilites are fragmental
breccias from near the surface of the
asteroid. They are mostly made of rock fragments from different parts of
the ureilite parent asteroid with about 1% debris from other asteroids.

------------------------------------------------------------------------

Characteristics of Almahata Sitta Meteorite

The analyses so far, and the reports from the 2010 Lunar and Planetary
Science Conference, identify Almahata Sitta as a remarkable ureilite
fragmental breccia of subrounded mineral fragments (commonly
milimeter-scale), olivine aggregates, pyroxene aggregates, and
aggregates of carbonaceous material, in a matrix of ureilitic material
[Data link <http://tin.er.usgs.gov/meteor/metbull.php?code=48915> from
the Meteoritical Bulletin]. The carbon is mostly crystalline graphite,
but diamond aggregates, up to several micrometers across, have been
reported within graphite grains. The term anomalous is applied to this
meteorite and refers to its unusual or distinctive textures, namely the
large size of carbonaceous aggregates, high porosity, and the overall
fine-grained texture of so many of the fragments. The boundaries between
clasts are commonly separated by mixtures of carbon-phase+metal+sulfide
and/or void space. The void spaces seem to be places where mineral
grains were not completely welded together and some voids have
crystalline linings. What makes Almahata Sitta even more unique is that
it contains dozens of different lithologies making it a spectacular breccia of
achondritic and chondritic materials.
[For another example of an extraordinary meteorite breccia see *PSRD*
article: Kaidun--A Meteorite with Everything but the Kitchen Sink.]

In addition to the different coarse-grained and fine-grained ureilite
meteorite fragments, researchers at LPSC also reported on the other
kinds of lithologies found in the Almahata Sitta samples. They have
found enstatite chondrites (of different kinds; for those of
you who need to know: at least EL3, EL3/4,
EL4/5, EL5, EL6), H-group ordinary chondrites, and several unique
meteorite fragments having, so far, unknown textures and mineralogies.
Researchers have also found aromatic hydrocarbons and amino acids
(formed by nonbiologic processes, such as the Fischer-Tropsch reaction.
>From the titles of the presentations at LPSC
it's obvious that this meteorite is spurring further research into the
fine details of ureilite breccias and asteroid 2008 TC3 including
mineralogy, density, thermal history, magnetism, and geologic evolution.

------------------------------------------------------------------------

Learning More About Ureilites

The tremendous interest in, and motivation behind the work on,
Almahata Sitta stems from the important fact that researchers know
exactly what asteroid these remnants came from. This event has provided
cosmochemists and astronomers new views into the composition and
structure of asteroids. For instance, although ureilite meteorites were
thought initially to be derived from a S-class asteroid, asteroid 2008
TC3 is regarded as a F-class asteroid based on visible and
near-infrared reflectance spectra. F-class objects are found mainly in
the asteroid belt at about 2.45 AU and research on the isotopic
compositions in the meteorite set a cosmic-ray exposure age of
approximately 15 million years. That number represents the length of
time asteroid 2008 TC3 was hurling through space to Earth. But where
exactly did asteroid 2008 TC3 originate? The Almahata Sitta meteorite
is so different from other ureilite breccias that researchers are trying
to figure out if it was blasted from deep within the rubble-pile parent
body or from the regolith where they think the other breccias came from.

The fascinating details of the ureilites and their parent asteroid (or
asteroids) are the subject of much current work and researchers are
making a coordinated effort to learn all they can from Almahata
Sitta/asteroid 2008 TC3 . A consortium has been organized to analyze
Almahata Sitta meteorite samples. And another group of research teams is
coordinating their analyses of the orbit, spectroscopy, and impact
trajectory of asteroid 2008 TC3 .

In addition to the session at LPSC, a topical session on the impact and
recovery of asteroid 2008 TC3 was held in October 2009 (the one-year
anniversary of the fall) at the 41st annual meeting of the Division for
Planetary Sciences of the American Astronomical Society. Researchers
will discuss more results at Meteoroids 2010--an international
conference on minor bodies in the Solar System, on May 24-28, 2010.
Still in the works are the detailed papers planned for publication in an
upcoming special issue devoted to Almahata Sitta/asteroid 2008 TC3 in
/Meteoritics and Planetary Science,/ the journal of the Meteoritical
Society, which should help satisfy your curiosity about this asteroid,
meteor, meteorite.

------------------------------------------------------------------------

ADDITIONAL RESOURCES *LINKS OPEN IN A NEW WINDOW.*

    * *PSRDpresents:* Asteroid, Meteor, Meteorite --Short Slide Summary
      <PSRD-AlmahataSitta.ppt> (with accompanying notes).

    * Goodrich, C. A., Scott, E. R. D., and Fioretti, A. M. (2004)
      Ureilitic Breccias: Clues to the Petrologic Structure and Impact
      Disruption of the Ureilite Parent Asteroid. / Chemie der Erde,/ v.
      64, p. 283-327, doi:10.1016/j.chemer.2004.08.001. [NASA ADS entry
      <http://adsabs.harvard.edu/abs/2004ChEG...64..283G>]
    * Jenniskens, P., Shaddad, M. H., Numan, D., Elsir, S., Kudoda, A.
      M., Zolensky, M. E., Le, L., Robinson, G. A., Friedrich, J. M.,
      Rumble, D., Steele, A., Chesley, S. R., Fitzsimmons, A., Duddy,
      S., Hsieh, H. H., Ramsay, G., Brown, P. G., Edwards, W. N.,
      Tagliaferri, E., Boslough, M. B., Spalding, R. E., Dantowitz, R.,
      Kozubal, M., Pravec, P., Borovicka, J., Charvat, Z., Vaubaillon,
      J., Kuiper, J., Albers, J., Bishop, J. L., Mancinelli, R. L.,
      Sandford, S. A., Milam, S. N., Nuevo, M., and Worden, S. P. (2009)
      The Impact and Recovery of Asteroid 2008 TC3 . /Nature,/ v. 458,
      doi: 10.1038/nature07920. [NASA ADS entry
      <http://adsabs.harvard.edu/abs/2009Natur.458..485J>]
    * Lunar and Planetary Science Conference (2010) Ureilitic Asteroids:
      Insights from Almahata Sitta Full set of abstracts (pdf) from
      talks and links to posters <LPSC_ureiliteAbstracts.pdf>.
    * Martel, L. M. V. (October, 2009) Kaidun--A Meteorite with
      Everything but the Kitchen Sink. /Planetary Science Research
      Discoveries./ http://www.psrd.hawaii.edu/Oct09/Kaidun_meteorite.html.
    * Meteoroids 2010--An International Conference on Minor Bodies in
      the Solar System (May 24-28, 2010)
      http://www.cora.nwra.com/Meteoroids2010/
    * NASA Solar System News: The Impact and Recovery of Asteroid 2008
      TC3 <http://www.nasa.gov/topics/solarsystem/tc3/>, with
      high-resolution images.
    * The night 2008 TC3 was Discovered <
      http://www.fullmoonphotography.net/g96_2008_10_06.htm>. A
      time-lapse sequence of photographs of the telescope and night sky:
      Catalina Sky Survey's 1.5-meter telescope at Mount Lemmon, Arizona
      by Richard Kowalski (Full Moon Photography).
Received on Wed 05 May 2010 07:00:15 PM PDT