[meteorite-list] Tagish Lake -- A Meteorite from the Far Reaches of the Asteroid Belt (Part 1 of 2)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:53:34 2004
Message-ID: <200212130149.RAA07517_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Dec02/TagishLake.html

Tagish Lake -- A Meteorite from the Far Reaches of the Asteroid Belt
Planetary Science Research Discoveries
December 12, 2002

     --- A new type of primitive meteorite with much to tell us
     about the formation of the solar system.

Written by David W. Mittlefehldt
NASA, Johnson Space Center

Pieces of a 56-metric-ton meteorite rained down over a wide area of Canada
on January 18, 2000. Many pieces landed on the frozen Tagish Lake, allowing
scientists to recover numerous samples, and giving the meteorite its name.
Studies show that the meteorite is intermediate in composition between the
two most primitive groups of chondrite meteorites, CI and CM carbonaceous
chondrites. Observations of its trajectory allowed scientists to calculate
its path through the solar system. The calculations show that it hails from
the outer asteroid belt, in a place where dark, carbon- and water-rich
asteroids reside. Tagish Lake is a new type of primitive meteorite that will
surely shed light on how the solar system formed.

     Reference:

     Mittlefehldt, David W. (2002) Geochemistry of the ungrouped
     carbonaceous chondrite Tagish Lake, the anomalous CM chondrite Bells,
     and comparison with CI and CM chondrites, Meteoritics and Planetary
     Science, v. 37, p. 703-712.

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

Arctic Fireball

Every year, the Earth is pelted by debris that occupies the void between the
planets. This debris is mostly dust- to sand-sized rocky bits shed by comets
or asteroids. You, dear web-surfer, may have witnessed the recent
spectacular Leonid meteor shower. This occurs when the Earth plows through
dust expelled from comet Temple-Tuttle as it travels through the inner solar
system -- in 2002 we went through a particularly concentrated dust tail,
resulting in the excellent Leonid display. However, these grains burn-up in
the upper atmosphere and don't reach the ground as meteorites. A rarer event
is a bright fireball streaking across the sky. These sometimes result in the
recovery of meteorites that can be studied by scientists to uncover the
secrets of the solar system. A recent example of this is the fall of the
Peekskill meteorite in 1992. This spectacular fireball was witnessed by
thousands as it streaked northeast across the skies of the Mid-Atlantic
States, before smashing into the trunk of a parked car in Peekskill, New
York. The Peekskill meteorite is of a type known as an ordinary chondrite.
As the name implies, this is a very common type of meteorite. It is still an
important object for scientific study, but because it is a common type of
meteorite, it adds only incrementally to our understanding of the formation
of the solar system.

A much rarer event is the very bright fireball that promises an especially
large meteorite, and it is a very rare event indeed when this promised
behemoth turns out to be an unusual type of meteorite. Such an event
occurred at local dawn on January 18, 2000 over the skies of Alaska, the
Northwest and Yukon territories, and British Columbia. This was the fall of
the Tagish Lake meteorite. Entering the atmosphere over the Canadian Arctic,
the fireball was detected by down-looking US satellites tasked to look for
such mundane man-made events as ICBM launches. Traveling south-southeast,
the exceptionally bright fireball was witnessed by numerous observers. Why
they were out at dawn on a cold winter's day is simply beyond my
comprehension.

The Tagish Lake fireball was detected by a wide variety of sensors. Infrared
and optical sensors on the satellites detected the light output of the
fireball, while infrasound detectors and seismographs picked up the airwaves
generated by the meteor. From these data, Peter Brown of the University of
Western Ontario and his colleagues were able to estimate the size of the
object that hit the Earth's atmosphere, and how much energy was released
during its passage. They calculated that the rock initially was about 4
meters in diameter with a mass of about 56 metric tons. The meteor was
fragmented several times between 50 and 30 kilometers altitude while plowing
through the atmosphere, and heavily ablated until terminal velocity was
reached at an altitude of about 28 km. Brown and colleagues estimate that at
this point, only about 1.3 metric tons of rock remained -- 97% of the rock
had simply burned-up in the atmosphere! All told, the fireball released
about 1.7 kilotons of energy. This is about 1/12 the energy of the first
atomic bomb.

The witnesses' accounts of the fireball allowed searchers to home in on the
likely region where meteorites may have impacted the ground. In this case,
"ground" is a bit of a misnomer. The region is mountainous and heavily
forested. Fortunately, the Tagish Lake meteorite fell in deep winter in northern
British Columbia. Most of the recovered meteorites were found on the ice of
frozen Tagish Lake, even though there must be many hundreds of fragments
scattered over the mountains. The largest single fragment massed 159 grams.

 Right: The meteorite recovery team used a
  chain saw to cut through the lake ice to
    collect meteorite fragments trapped in
                                  the ice.

                                  The largest Tagish Lake stone, originally
                                  massing 159 grams. It has been broken open
                                  to show the gross texture of the
 [piece of Tagish Lake meteorite] meteorite. The right side of the stone
                                  shows the fusion crust formed by melting
                                  of the surface by friction with the
                                  atmosphere as it fell. The cube is 1 cm
                                  across.

Only about 0.1% of the estimated 1.3 metric tons of surviving rock was ever
recovered. This is only 0.002% of the estimated mass of rock that entered
the atmosphere! Had the meteor arrived in July, it is almost certain we
would never have found any of the fragments that reached the surface.

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

A Meteorite's Orbit

Eye witnesses to the fireball were able to provide scientists with
sufficiently detailed accounts to permit an orbit to be calculated for the
meteor. The calculation done by Peter Brown and his colleagues shows that
the pre-entry orbit of Tagish Lake extended all the way from the outer
reaches of the asteroid belt to just inside the orbit of the Earth.

        [diagram of orbits and asteroid belt]
        Orbital diagram for the inner solar system showing the
        pre-entry orbit of Tagish Lake, and the asteroid belt. The
        planets are shown in their positions at the time of fall at
        16:43 UT on January 18, 2000.

Scientists know that most meteorites originate on asteroids, but including
Tagish Lake, only five meteorites have known pre-entry orbits. Peter Brown
and friends noted that the orbit of Tagish Lake extended to that part of the
asteroid belt where asteroids classified by astronomers as C, P and D types
predominate. These asteroids are know to have hydrated silicates
(water-bearing minerals) on their surfaces, and because of their dark color,
are suspected to be rich in carbon compounds. These are also characteristics
of the most primitive meteorite types known -- the carbonaceous chondrites,
especially the types classified as CI and CM. This was the first clue that
Tagish Lake might be a primitive type of chondrite.

However, too much could be read into the significance of the orbit. Getting
a meteoroid from the asteroid belt to the Earth is a two-stage process.
First, an impact on the parent asteroid is needed to "liberate" bits of rock
and boost them off the asteroid into their own orbits. Then the meteoroids
have to have their orbits altered. This is most commonly done through
gravitational interactions with massive Jupiter. The gravitational tug by
Jupiter can, over time, change the paths of meteoroids and asteroids that
are in certain orbits, sending some of them into the inner regions of the
solar system. Thus, the pre-entry orbit of a meteor may not accurately
reflect the location of its parent asteroid. This is where the work by
Takahiro Hiroi of Brown University and his colleagues came into play. These
scientists measured the spectrum of light reflected by samples of the Tagish
Lake meteorite, and compared these results to telescopic spectra of various
types of asteroids. They found a good match with the D-type asteroids of the
outer regions of the asteroid belt, suggesting that Tagish Lake may be a
sample of this type of asteroid.

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

Enter the Meteoriticists

At this point, the meteoriticists got into the game. Samples of Tagish Lake
were distributed to several researchers for mineralogical, petrological and
chemical characterization. Studies led by Mike Zolensky at NASA's Johnson
Space Center provided a detailed mineralogical and petrological look at
Tagish Lake. And what a surprise this meteorite turned out to be! Although
it does show some similarities to the two most primitive carbonaceous
chondrite types (the CI and CM chondrites) it is nevertheless quite distinct
from either of them. In order to better understand the significance of
Tagish Lake, let's digress a bit and talk about chondrite petrology in
general, and what it tells us.

Chondrites are firstly classified by composition, and then by their texture
and mineralogy -- that is, their petrology. The different chemical
classifications are given letter designations (for example, CI, CM), while
the petrologic divisions within the chemical classes are give numerical
designations 1 through 7. Now, this may seem odd, but the texturally most
primitive chondrites are type 3. These are composed of chondrules,
fine-grained matrix, metal and sulfide. Chondrules are mm-sized spherical
rocky beads that were formed by partial to complete melting of clumps of
silicate mineral dust in solar nebula -- I like to call these clumps
"nebular dust-bunnies." Chondrite types 4 through 7 were formed by
increasing dry heating of primitive material resulting in increasing amounts
of recrystallization of it. In type 7, the chondrules are no longer
recognizable and all fine-grained matrix material has been destroyed. They
retain their original minerals, but the original texture cannot be
discerned. These chondrites have the texture of high-grade metamorphic rocks
found on Earth.

Types 2 and 1 chondrites are formed by increasing degrees of low temperature
alteration in the presence of water, with type 1 chondrites being the most
altered. Here, the original textures are quite often well preserved, but the
original high temperature silicate minerals have been replaced by low
temperature silicate minerals containing water, carbonate minerals, plus
other phases. In many ways, these chondrites are similar in texture to
sea-floor rocks on Earth that have been altered by interaction with ocean
water circulating through them. The original texture is quite often clearly
preserved even though all the original minerals have been replaced. This is
known as pseudomorphic replacement.

One final bit of information will help us understand the significance of
Tagish Lake. Although meteoriticists know that chondrules were formed by
melting pre-existing nebular dust-bunnies, we really don't know what heat
source accomplished this. We do have several hypotheses, however, and all of
them have one thing in common -- they should all be more effective closer to
the sun than farther away. Thus, we expect that chondrules would have been
more abundant close in by the early sun, and unmelted dust-bunnies farther
out.
Received on Thu 12 Dec 2002 08:49:06 PM PST