How to tell if a rock fell from outer space.

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Jan. 4 2007 5:57 PM

Is This Thing a Meteorite?

How to tell if a rock fell from outer space.

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A golf-ball-sized metallic object crashed through the roof of a New Jersey home and lodged in a wall on Tuesday evening. Officials at the FAA said it was not material from an aircraft, and geologists will test the object to determine if it's a meteorite. How can you tell if a rock fell from outer space?

First, look at it. A freshly fallen meteorite will have a smooth coating of black or dark brown fusion crust. The coating forms as it enters the Earth's atmosphere, when the outer layer of rock begins to melt. This can result in thumbprintlike indentations (called regmaglypts) on the surface of the meteorite; the subsequent cooling-off often produces a set of cracks in the fusion crust. (Experts say the object found in New Jersey looks like it might have a fusion crust.)

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Next, pick it up. Meteorites are denser than regular rocks and feel heavier than they look. The New Jersey object reportedly weighs as much as a can of soup, even though it's the size of a golf ball. You can also try running a magnet over the object—most kinds of meteorites will attract it.

Then, send it to a lab. Geologists can run more sophisticated tests on the object, depending on what kind of meteorite they think it is. Generally, there are two kinds of meteorites—rocky and iron. Rocky meteorites are more common, and most contain chondrules—tiny spheres of formerly molten silicate minerals. A test using X-ray diffraction can help to identify these minerals in a mysterious object. Some materials—like quartz—aren't likely to turn up in a rocky meteorite.

If the geologists think they've got a metallic meteorite, they might test for high levels of iron, along with other metals like nickel, platinum, and gold. (If any man-made alloys turn up you can be sure it's a rock from Earth.) A Widmanstätten pattern test, which involves polishing the meteorite then etching it with acid, may reveal a crosshatching of nickel and iron patterns that is unique to meteorites. A mass spectrometer is used to determine isotope ratios in the meteorite—which may differ from those in a terrestrial rock.

Not everything that looks like a meteorite turns out to be the real deal. Objects often mistaken for meteorites include bits of space debris that melted as they re-entered the Earth's atmosphere, as well as a range of objects like deformed industrial grinding balls and plain old chunks of sedimentary rock.

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Explainerthanks Haldan Cohn and Nelson Shaffer of Indiana University *, and Carlton Pryor of Rutgers University.

Correction, Jan. 18: The article originally listed two sources from the University of Indiana. They are from Indiana University. Return to the corrected sentence.

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