Astro Bob A look at celestial happenings in the Northland and beyond Bob King View my profile Click here to view previous posts or additional resources

Sky cheese from Wisconsin

My 2 gram part slice of the recent Wisconsin meteorite fall. It's composed of many small fragments and sports a bit of melted crust along the lower right side edge. It measures one-inch across. Photo: Bob King

My little sliver of the Wisconsin meteorite arrived in yesterday mail in the middle of a freak May snowstorm. It's a part-slice of the so-called "Kerchner Stone" found by Joe Kerchner of Illinois. Joe spent many days and hundreds of hours searching fields and pastures until he found the beauty. At 332 grams (11.7 ozs.) it's the largest stone found so far from the fireball that split the sky over southwestern Wisconsin on April 14. Since Joe cut it up, the Kerchner stone has fallen out of first place as the largest one around. That honor currently belongs to a 300g fragment found by Marvin Kilgore of Arizona.

The fireball of April 14 lit up the sky over Madison, Wisconsin. Credit: UW-Madison

As of May 4, 37 people had found at least 71 stones weighing a total of 3262 grams or just over seven pounds.The smallest stone weighs just 3.2 grams. Some were picked up by landowners but most by meteorite hunters flying and driving in from all over the country. Despite rains and planted fields, people continue to hunt for space rocks in the Livingston-Mifflin-Mineral Point area and occasional new stones are reported. As for what the meteorite fall will be named, there's been no word yet from the Meteoritical Society, the organization in charge of naming. Meteorites are named for a nearby town or geographic feature where they were found.

There are currently 38,818 classified and named meteorites either seen to fall or discovered in deserts and elsewhere after the fact. Another 11,953 are provisional and in the process of being analyzed and named. Only about 1080 meteorites are witnessed falls. The U.S. has recorded more witnessed falls than any other country with over 140, followed by India with 127 and France with 63.

Scientists classify meteorites in very general categories as irons, stony-irons and stones. Within each of those large divisions there are many fine subdivisions. Over 80% of meteorites that fall are common chondrites (KON-drites), stony meteorites composed of silicates (minerals like olivine and pyroxene) and bright flecks of iron-nickel alloy. Most chondrites contain small spheres called chondrules which were some of the first solid materials to form from the gas and dust clouds of the early solar system 4.6 billion years ago. Only 5% of falls are iron meteorites but they're more likely to stand out from other rocks because of their unusual appearance and hefty weights.

The Wisconsin fall hasn't been classified yet but it appears to be a brecciated stony chondrite. Breccia (BRECH-chuh) is a rock composed of broken and angular mineral or rock fragments cemented together. There's no doubt that something hit the parent asteroid of the meteorite and broke it into fragments in the distant past. Last month one of those liberated fragments hit Earth's atmosphere and fragmented even further into individual stones.

 A rock's tale never ends. Holding a meteorite is to become a character in a story that began eons ago.

Posted by: rking@duluthnews.com on 5/8/2010 at 9:11 AM | Comments (0) | Permalink | Edit

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