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The Lorton meteorite. Photo by Linda Welzenbach.
On Monday, January 18, 2010, a large fireball was reported by viewers in the greater Washington, DC, USA, metropolitan area. Reports from as far away as New Jersey describe a large greenish fireball moving roughly south and east between 5:30 and 5:45 PM. Reports from Virginia include sounds of a large detonation and smoke trails. Within minutes of the sightings, one stone was recovered from within the Williamsburg Square Family Practice in Lorton, VA. The doctors, who were in the office at the time of the fall, found the meteorite in three large pieces embedded into the carpet tile and concrete floor of an exam room. The doctors describe the sound of the impact as bookshelves crashing to the ground.
The meteorite is approximately 8 cm x 5 cm x 5 cm, and is roughly rectangular, with a matte black fusion crust. Fusion crust is a quenched glass, made from the meteorite itself, which forms from frictional heating during its atmospheric passage causing minute amounts of the rock to melt. The fireball is a combination of this same molten rock streaming off the heated surface and the superheated atmosphere surrounding it. Once the meteorite loses its cosmic velocity and slows to the point where the friction no longer heats the surface, the remaining melted rock freezes or "fuses" to the remaining stone, forming a glassy crust. For typical asteroidal materials, this "crust" is always thin (a few mm), smooth, without holes, and matte black.
The interior of the meteorite is light gray in color. To the naked eye, the Lorton meteorite contains thin dark veins, minute metal grains, a few small breccia clasts and even rarer relict chondrules. The meteorite was chemically analyzed in the Department of Mineral Sciences by Dr. Cari Corrigan and Linda Welzenbach. Classification of all ordinary chondrite meteorites is based primarily on the overall iron content derived from the composition of olivine and pyroxene. We also assign a number which corresponds to the degree of metamorphism experienced by the meteorite, the latter of which is merely a microscopic appraisal of chondrule preservation. Individual grains of olivine are reported as a range of fayalite compositional values, and pyroxene as ferrosilite values, both of which pertain to iron content. The lowest iron group is the LL chondrites, followed by L chondrites, with the highest iron group designated as the H chondrites.
The Lorton meteorite is classified as an L6 chondrite, which reflects low iron content, and a high metamorphic grade. The number 6 corresponds to the presence of rare chondrules, which when observed, show only diffuse boundaries or internal structure, and some degree of recrystallization, where crystals share boundaries that form 120-degree triple junctions. While chondrites make up nearly 90% of all meteorites classified, only a fraction are recovered as freshly fallen objects, and are highly valued because of the lack of terrestrial contamination and weathering. This meteorite fall is the first in the Washington DC area, and also the first in nearly 100 years—the closest are from Richmond, VA in 1828; St. Mary’s County, MD, in 1919; and Sharps, VA, in 1921. For now, the total mass recovered is 329.7 grams.
Why do we care about this meteorite? When we have a large number of one type of meteorite, we can compare and contrast the chemistry and textures within them to identify patterns and/or subtle differences that help us form a more complete picture. Another important thing about newly fallen meteorites is that we can examine the cosmogenic nuclides in them (short-lived elements that are generated by the sun) to learn more about a meteorite’s journey through the inner Solar System (between the asteroid belt and Earth). Each new fall has the potential to provide new clues to the earliest history and formation of the solar system.
Meteorite impact hole in roof of the Williamsburg Family Practice, Lorton, VA. Photo by Sarah Timm.
Click to enlarge. Google Earth composite image showing radar returns from six different Doppler radars. Radar returns (blue, yellow, red and grayscale appear to define a curved path of falling debris from the Lorton meteor (red curve). A second, possible string of falling debris is seen north of that line (dotted yellow line) which may indicate material generated by a separate detonation event during the meteor's flight. The size of meteoritic material that produced these radar returns is probably quite small. While this precludes recovery of the material, it illustrates the ability of Doppler weather radar imagery to investigate the fall behavior of small material which would otherwise be difficult or impossible to study. Note that radar returns are comparatively sparse around the meteorite recovered at the doctor's office (lower left), which is consistent with the smaller number of radar targets available with larger, but fewer, falling masses. Graphic and text courtesy of Dr. Marc D. Fries, Planetary Science Institute (Fries@psi.edu)
Images of the Lorton Meteorite. Photos by Chip Clark, Smithsonian Institution.
Both of these photos are by Chip Clark of the Smithsonian Institution. These are close up images of the largest piece of the meteorite. The more “close up” image shows iron metal (center) and a relict barred olivine chondrule just to the right (slightly striped looking grain). This piece is about 3 inches across 7.5 cm).
Thin section contains a relict barred olivine chondrule (~1mm across). It has diffuse boundaries (visible along top and upper right) and you can see (lower left) that the olivine grains have been broken up. This has been magnified 20 times. Photo by Linda Welzenbach.
Thin section image shows three relict chondrules (see if you can pick them out!) Black areas are iron metal in the thin section. Thin sections are very thin slices (about the thickness of a human hair or approximately 30 microns) that are glued to glass microscope slides. Photo by Linda Welzenbach.
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