The sky is falling... or is it?

The 1992 'Peekskill' meteorite and its point of
impact in Peekskill, New York. Credit: "Pierre ThomasWhy is it that meteorites are brought to me for identification in clusters? I don't mean that people bring clusters of meteorites-but it seems I get calls and visits from possessors of unknown rock samples, hopeful that they are of extraterrestrial origin, in bursts. This time I got two inquiries in two days!

The first thing I tell people is that I'm not a meteorite expert, but that I have a contact who is. This rarely discourages them from wanting to bring their rocks in for a look.

The first sample was brought in by a family who said they collected the chunk of iron from Lake Tahoe. This one actually looked promising to my mostly untrained eye: a fist-sized chuck of magnetic metal, with pits and holes and an overall melted look. I took some pictures to send off to our regional expert and told the family I'd call them to let them know what he said. The response to the pictures was pretty certain: it wasn't a meteorite, but a chunk of metallic slag. I was told that this is a common mistake; that often bits of slag from old foundries or other sources are taken for meteorites.

The second sample brought to me didn't really strike me as a meteorite, by appearance. It was metallic, but not magnetic; it was pretty heavy for its size; it didn't have any obvious signs of melting, and no real pits or holes-other than one, deep, tunnel-like hole the width of a finger. It didn't appear jagged or shrapnel-like, as fragments from an exploding metallic meteorite often do. Finally, it had wide, flat facets that looked much more like the result of natural rock cleaving as pieces of Earth's crust break apart.

I went ahead and performed a density measurement on the sample. It was pretty heavy, so our sensitive balance scales wouldn't handle the load. Instead, I resorted to our "learn your weight on other planets" scale-the one that tells you how much you would weigh on the Moon, Mars, and other planets, in addition to your Earth weight. (I found this scale useful when I had a package to mail and needed to know the weight; by selecting the Moon weight of the package, I would pay only one-sixth the normal Earth rate!)

The double-fist-sized sample was 11.3 pounds, which converted to 5126 grams. Then, I selected a graduated beaker from our lab, filled it with water and submerged the sample. Reading the difference in water level with and without the sample, I measured a volume displacement of 750 cc. So, the density-mass divided by volume-turned out to be about 6.83 grams/cc. That's twice the typical density of silicate-type rocks (stone), and fairly close to that of pure iron.

I sent the owner off with my appraisal that the rock didn't present the appearance of a meteorite, and though the density was in neighborhood of that of iron, the appearance (black, inside and out) and non-magnetic nature suggested some other metal or metal-stone mixture. As always, I encouraged him to seek an expert appraisal.

Let's face it, all rocks found on Earth are ultimately of extraterrestrial origin-though what we regard as Earth rock has been on Earth for many billions of years, and shaped, reshaped, and metamorphosed by eons of weathering and geological activity. Meteorites, then, are only the newcomers....

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