06Feb2013

The First-Ever Meteorite from Mercury?

Wow, that’s what I call green! Green, glassy fusion crust coats one side of Ralew’s new meteorite. This is the largest of the 35 fragments, weighing just over 100g. Cube at right is 1 cm across. Click for larger version. Credit: Stefan Ralew

 In April 2012 Stefan Ralew, a meteorite collector from Berlin, found himself staring at a spread of 35 green meteorite fragments for sale by a dealer in Morocco. It looked puzling.

“It was offered as a Martian (meteorite) but for me it was simply far too green,” said Ralew. Moroccan meteorite always keep an eye out for green rocks in the belief that they’re of Martian origin. Sometimes however they turn out to be nothing more than Earth rocks. Since this one was expensive, Ralew would have normally declined, but he noticed that the pieces had fusion crust, that frothy, typically dark coating of melted rock that forms when a meteorite is heated during its fall through the atmosphere.

Stefan Ralew Credit: Mirko Graul

“It was a big risk because of the high price,” said Ralew, but he sealed the deal and mailed off a piece to Dr. Tony Irving at the University of Washington, well-known for his expertise in meteorites from other planets.

After chemical analysis, Irving discovered that Ralew’s green rock was a completely new type of achrondrite (ay-KON-drite), a class of igneous meteorite that forms deep within the crust of larger asteroids and planet-sized bodies. In fact, Ralew’s green meteorite shared similarities with the planet Mercury, making it a one-of-a-kind.

Many of the more familiar achondrites that scientists and meteorite hunters have picked up here on Earth were blasted from the surface of Vesta by meteorite and asteroid impacts. Still others have been liberated from the moon and Mars. They drift through space until swept up by the ceaseless Earth. Scientists have done the math and arrived at the conclusion that meteorites from Mercury impacts should also by lying around in the deserts of the world, preserved by arid air and lack of rain. But no one had definitely identified a rock from Mercury until the green meteorite entered the scene.

A closeup of a polished, cut face of NWA 7325 shows striking green crystals of chromium diopside (a silicate mineral with chromium) and gray crystals of plagioclase, a rock also common in Earth’s crust. Click for larger version. There are a total of 345 grams (about 12 ounces) mostly in small fragments. Credit: Stephan Ralew

Other classes of achondrites called aubrites and angrites were once believed to have originated on the innermost planet, but further research points to their home on a yet-unknown asteroid or planet.

Mercury photographed by MESSENGER. The planet’s crust lacks iron and is pockmarked by countless craters. One of these impacts possibly sent NWA 7325 our way. Credit: NASA

Stefan’s meteorite, now classified as NWA 7325 (NWA=Northwest Africa, its find location), is a near-match for rocks examined from orbit by Mercury MESSENGER space probe. NWA 7325 is rich in magnesium, calcium and a silicate material laced with chromium that lends it an emerald sparkle, but it lacks iron. And that’s the key. Surface rocks on Mercury are likewise igneous and depleted in iron.

The match isn’t perfect. NWA 7325 has more calcium than it should and lacks the silicate mineral enstatite (common on Mercury), but that doesn’t worry scientists too much. Because the rock was excavated from deeper down in the crust, it would be expected to have its own unique qualities.

Mars meteorites show evidence of shock from impact in their crystal structures, and the same would be expected for rocks delivered to us from Mercury. Plagioclase, a very common mineral in Earth’s crust, and found in abundance in NWA 7325, has been completely melted, likely due to shock from the impact that sent it flying from the planet long ago.

Bubbly fusion crust on another fragment of Stefan’s meteorite. Click for larger version. Credit: Stefan Ralew

While the evidence points to a Mercury origin, we won’t really know for certain whether Ralew’s rock originated from the innermost planet until further studies are done. Scientists are still working to determinewhen those gorgeous green crystals formed as well as how long the rock coasted through space before arriving on Earth.

“Ultimately, only a sample return from Mercury may provide an answer,” wrote Irving in his group’s recent report on NWA 7325. In the meantime, Stefan’s meteorite stands as one of the most singular finds to date. It couldn’t have happened to a better guy. Ralew has a been a great friend of meteorite collectors and the scientific community for years. Source: Astro Bob

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