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TITLE: Recognizing Meteorites


There are four visual clues that a rock has an extraterrestrial origin.

[Photo: Four Rocks ]





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Clue # 1 – Fusion Crust
When a meteorite hits the atmosphere, friction melts its surface, and the liquid continuously blows away. The atmosphere gradually slows the meteorite's passage, allowing it to cool enough for the surface liquid that remains to harden into a distinctive coating called a fusion crust. As this crust shrinks during cooling, a network of tiny cracks may form on the surface.

The dark, iron-rich fusion crusts on Plantersville is unweathered. Fine cracks from shrinkage are visible in New Concord's crust.

 

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Clue # 2 – Aerodynamic Sculpting
Meteorites that don't tumble during their fall acquire a conical, aerodynamic shape as their leading surface melt and blow away. In some instances, distinctive radiating flow lines remain in the crust where molten glass was swept from front to back.

Lafayette and Grosvenor Mountains 85201 show radiating flow lines.

 

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Clue # 3 – Regmaglypts
Uneven melting during descent through the atmosphere can excavate depressions on the surfaces of some meteorites. These features are called regmaglypts.

The regular, shallow depressions on some meteorites are called regmaglypts. The elongated ones on Olivenza probably parallel the direction of air flow during the meteorite's flight. The depressions on Elephant Moraine 92029 are beautifully faceted.

 

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Clue # 4 – Metal
Meteorites generally contain at least some iron and nickel alloy. On Earth, by contrast, our oxygen-rich environment prevents iron from occurring naturally as a metal except under very rare circumstances.

Notice the different amounts of metallic iron in Farmville and Staunton. Staunton is made entirely of metal, while Farmville has shiny flecks of metal distributed throughout.




Can You Spot the Meteorites?
hree of these specimens are meteorites. The other three are Earth materials often mistaken for meteorites. Use the clues in this exhibit to figure out which is which.


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Granite pebble

Earth rock: This rock lacks a fusion crust and metal. Meteorites do not have such rounded shapes.

 

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Nuevo Mercurio

Meteorite: Notice the aerodynamic shape and black fusion crust.

 

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Homestead

Meteorite: This specimen has black fusion crust, shallow regmaglypts, and tiny metal flakes.

 

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Cobble with Desert Varnish

Earth rock: The black outer surface resembles fusion crust, but it resulted from weathering, not melting. Also, meteorites are not so flat and smooth.

 

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Henbury

Meteorite: The surface of this iron is covered with beautiful regmaglypts.

 

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Slag

Earth material: This piece of industrial furnace slag is metal. But there's no fusion crust, and meteorites rarely have bubbles.


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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits