[meteorite-list] Meteorite Shower in Park Forest, Illinois

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Aug 27 15:31:33 2004
Message-ID: <200408271931.MAA22987_at_zagami.jpl.nasa.gov>


Meteorite Shower in Park Forest, Illinois
Planetary Science Research Discoveries
August 26, 2004

--- An L5 chondrite strewnfield is centered at Park Forest, Illinois, a
southern suburb of Chicago.

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

Steven Simon (University of Chicago) and seven colleagues from the
University of Chicago, the Planetary Studies Foundation, Harper College,
Pacific Northwest National Lab, and the Field Museum in Chicago have
classified the meteorite fragments that fell on Chicago's southern
suburbs on the night of March 26, 2003. Described as ".. the most
densely populated region to be hit by a meteorite shower in modern
times," the village of Park Forest is at the center of the strewnfield
and fortuitously also happens to be home to the Simon family, who
answered scores of phone calls from neighboring meteorite finders. No
injuries were reported though plenty of roofs, windows, walls, and cars
were hit, and the police department took individual fusion-crusted
fragments into custody as evidence.

Its chemical and mineralogical compositions establish the Park Forest
meteorite as an L5 chondrite, one of the most primitive groups of known
meteorites. It is a strongly shocked monomict breccia (a term applied to a
breccia made of one kind of rock) with light-colored clasts in a very
dark matrix. The team measured cosmic radionuclides in Park Forest and
found nearly zero 56Co and high 60Co, values that indicate a large
preatmospheric mass. They estimate the meteoroid was at least 900
kilograms and possibly as large as 7000 kilograms before it broke apart
in the atmosphere, of which only about 30 kilograms of fragments have
been recovered.


Simon, S. B., Grossman, L., Clayton, R. N., Mayeda, T. K., Schwade, J.
R., Sipiera, P. P., Wacker, J. F., and Wadhwa, M. (2004) The fall,
recovery, and classification of the Park Forest meteorite, Meteoritics &
Planetary Science, v. 39, p. 625-634.


Asteroids Don't Usually Drop on Neighborhoods

Most meteorites are samples of asteroids and most are recovered from one
of the world's cold or hot deserts (eg. Antarctica, north Africa, Oman)
a long time after striking ground. [See, for example, PSRD article
Searching Antarctic Ice for Meteorites
<http://www.psrd.hawaii.edu/Feb02/meteoriteSearch.html>.] Witnessed
falls, on the other hand, have a better chance of being recovered and
transferred quickly into the laboratory for analyses while still in
relatively fresh, uncontaminated condition. About 33% of the meteorites
in the world's collections are witnessed falls, but not necessarily in
heavily populated areas. Allende (Mexico, 1969), Murchison (Australia,
1969), and Tagish Lake (British Columbia, Canada, 2000) are examples of
observed falls that have greatly expanded our understanding of the solar
system's formation. [See PSRD article Tagish Lake -- A Meteorite from
the Far Reaches of the Asteroid Belt

The Park Forest meteorite is one of the most recent observed falls in
the United States. And it happened to land in the midst of a group of
highly talented and busy cosmochemists--men and women who specialize in
the study of meteorites. Talk about a lucky break! The newly established
Chicago Center for Cosmochemistry
is one example of how the cosmochemistry communities at the University
of Chicago, Argonne National Laboratory, and the Field Museum have
combined their expertise and facilities to study solar system science.

The fireball approached from the southwest and was visible from parts of
Illinois, Indiana, Michigan, and Missouri. The subsequent meteorite
shower hit the southern suburbs of Chicago at approximately 11:50 p.m.
on March 26, 2003. Hundreds of meteorite fragments ranging from a few
grams to 5.26 kilograms were recovered from the elongate strewnfield
(approximately 8 x 3 kilometers), which spread by strong westerly winds
over residential neighborhoods and a forest preserve (see the map
below). The smallest pieces were deflected the furthest eastward and the
largest pieces, carrying more momentum, were deflected the least.

strewnfield map

The center of the strewnfield is Park Forest, about 40 kilometers south
of Chicago. "I don't know of any other time when a meteoriticist was in
the middle of a strewnfield," quipped co-author Lawrence Grossman
(University of Chicago) referring to colleague Steven Simon's residence,
which is marked with a star on the map shown above. The colored dots
show the locations where meteorites were recovered. The colors represent
different sizes of individual stones. A total of three kilograms are now
in the meteorite collection at the Field Museum in Chicago, including
the type specimen, a 545-gram fragment recovered from the Park Forest
fire station (see photos below).

Hole in the roof and meteorite that caused it.


meteorite damage in room


Classification of the Park Forest Meteorite

Park Forest is a breccia with light gray clasts in a very dark matrix,
referred to as the light and dark lithologies, respectively. Most of the
recovered meteorite fragments, especially the larger ones, are dominated
by the light lithology. Olivine ((Mg, Fe)2)SiO4), pyroxene ((Ca, Mg,
Fe)2Si2O6), metal (FeNi), sulfide (FeS) and chromite (FeCr2O4) grains
are present in both lithologies, though only the dark lithology has a
pervasive network of fine sulfide veins, which are opaque. They prevent
light from penetrating into the rock, giving it a very dark appearance.
Both lithologies contain chondrules with diameters ranging from ~600
microns to 1.3 millimeters. Olivine chondrules are the most abundant
followed by olivine-free, pyroxene chondrules.

A fragment of the Park Forest meteorite This 232-gram sample of the Park
Forest meteorite shows the light-colored angular clasts in the dark
matrix. This piece was recovered the night of the shower and was used to
measure cosmogenic radionuclides. The small divisions on the scale bar
are millimeters.

Average olivine, low-Ca pyroxene, and bulk oxygen-isotopic compositions
show that Park Forest is an L chondrite (see plots below). The
researchers further classified the meteorite as petrologic type 5 based
on their observations that ferromagnesian minerals are well equilibrated
(they are the same composition everywhere in the rock), the chondrules
are easily recognized, and maskelynite (a glass formed from plagioclase
feldspar by high shock pressures) is relatively fine grained (mostly
less than or equal to 50 microns across).

plot of olivine and pyroxene compositions O-isopotic compositions

This plot shows olivine and pyroxene compositions in the light and dark
lithologies of Park Forest compared with typical ranges in ordinary
chondrites. Higher values reflect higher concentrations of iron in each
mineral. Park Forest plots in the L chondrite field. Oxygen isotopic
compositions of the light (PF Light) and dark (PD Dark) lithologies in
Park Forest are similar to each other and to the average of L chondrites
(L fall avg).

The conversion of plagioclase to maskelynite in Park Forest is one
indication that it was strongly shocked. Other shock features in the
meteorite include mosaicism and planar deformation in olivine,
undulatory extinction in pyroxene, and glassy veins. Simon and his
colleagues classify it as shock stage S5. [See PSRD article Asteroid
Heating: A Shocking View
for a good summary of the shock stages in chondrites, including movies
of shocked and unshocked olivine grains rotated in polarized light using
a petrographic microscope.]

About half the meteorites that fall today are L chondrites and many of
them were shocked 465?15 million years ago, presumably when the parent
asteroid body was smashed to pieces in a huge collision. Though the age
of the Park Forest meteorite is yet unknown, its shock stage of S5 is
consistent with the idea that it derived from the break-up of the L
chondrite parent body. Some scientists consider the Flora family of
S-type asteroids to be more remnants of the L chondrite parent body.


How Big was the Meteoroid?

One way of estimating the probable size range of a meteoroid before its
breakup in the atmosphere (preatmospheric mass) is by measuring the
concentrations (also called activities) of cosmogenic radionuclides in
the meteorite. These are radioactive isotopes formed as a result of
exposure to high-energy cosmic rays in space. Some of these, such as
56Co, can only form at the surface of a body, while others, such as
60Co, require some shielding and increase with depth in the parent body.
If an object is too small, the cosmic rays will just pass through it and
not make 60Co.

Simon and colleagues used a fragment of the Park Forest meteorite
recovered the night of the shower (pictured above) and began their
measurements less than 72 hours after the fall. They found very low
(essentially zero) 56Co activity and high 60Co activity, consistent with
a large preatmospheric mass for Park Forest. When they compared these
observed radionuclide activities with calculated production rates in
chondrites they determined that the sample they measured came from a
depth of about 40 centimeters and could have been from an object that
was at least 900 kilograms and possibly as large as 7000 kilograms.
Other ways of estimating the original size of the object, based on work
by Peter Brown of the University of Western Ontario and his colleagues
of the energy released (seismic, light, sound) give an even higher range
of 8000-11000 kilograms. The hundreds of fragments of the Park Forest
meteorite that have been officially documented total about 18 kilograms.
The authors estimate that at least 30 kilograms in all were probably
recovered. Assuming that some of the fragments simply burned up during
atmospheric entry, that still leaves plenty of extraterrestrial material
out there to be found.


meteorite collected from Park Forest roadway



Additional meteorite photograph from Astronomy Picture of the Day
<http://antwrp.gsfc.nasa.gov/apod/ap030506.html>, May 6, 2003.

Brearley, A. J. and Jones, R. H. (1998) Chondritic meteorites, in
Planetary Materials, J. J.Papike editor, Mineralogical Society of
America, Washington D. C., p. 3-1 - 3-398.

Brown, P., Pack D., Edwards W. N., ReVelle, D. O., Yoo, B. B., Spalding
R. E., and Tagliaferri, E. (2004) The orbit, atmspheric dynaimics and
initial mass of the Park Forest meteorite, Meteoritics & Planetary
Science, in press.

Clayton, R. N., Mayeda, T. K., Goswami, J. N., and Olsen, E. J. (1991)
Oxygen isotope studies of ordinary chondrites, Geochimica et
Cosmochimica Acta, v. 55, p. 2317-2337.

Historical meteorite falls
12-page pdf file, Lesson 15 in Exploring Meteorite Mysteries: Teacher's
Guide with Activities, NASA EG-1997-08-104-HQ.

Martel, L. M. V. (2002) Searching Antarctic ice for meteorites.
Planetary Science Research Discoveries.

Mittlefehldt, D. W. (2002) Tagish Lake--a meteorite from the far reaches
of the asteroid belt. Planetary Science Research Discoveries.

Park Forest fireball website
<http://aquarid.physics.uwo.ca/~pbrown/Videos/park_forest.htm> with
video clips and more information from Peter Brown, University of Western
Ontario. (Wait for the videos to load.)

Simon, S. B., Grossman, L., Clayton, R. N., Mayeda, T. K., Schwade, J.
R., Sipiera, P. P., Wacker, J. F., and Wadhwa, M. (2004) The fall,
recovery, and classification of the Park Forest meteorite, Meteoritics &
Planetary Science, v. 39, p. 625-634.

St?ffler, D., Keil, K., and Scott, E. R. D. (1991) Shock metamorphism of
ordinary chondrites. Geochimica et Cosmochimica Acta, v. 55, p. 3845-3867.
Received on Fri 27 Aug 2004 03:31:29 PM PDT

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