April 6, 2013
Experts Puzzled By Origin Of Rare Achondrite Meteorite Discovered In 2012
redOrbit Staff & Wire Reports - Your Universe Online
A stone originally discovered last year in Southern Morocco has been identified as a rare type of meteorite known as an achondrite, though its exact origin remains a mystery, according to experts from Washington University of St. Louis (WUSTL).
Most meteorites are stony, and approximately 90 percent of those are ordinary chondrites — pieces of small, unmelted asteroids that are uniform in composition, according to WUSTL research professor Randy Korotev.
Achondrites, however, are pieces of large asteroids or planets that are at least 200 kilometers in diameter and capable of producing enough internal heat in their earliest days “to partially melt and segregate into a metal core surrounded by a rocky exterior,” Korotev said. This type of meteorite originates from the crust or mantle of these celestial bodies and comprises only five percent of the stony meteorites discovered to date.
The chemistry of the stone was analyzed by Tony Irving of the University of Washington´s Department of Earth and Space Sciences in an attempt to determine where it originally came from. Nearly half of all achondrites come from the large asteroid 4 Vesta, while others came from Mars, the moon, or other asteroids.
This particular meteorite, which has been officially designated Northwest Africa 7325 (NWA 7325), was found to have highly unusual chemistry — chemistry that was oddly similar to readings measured by NASA´s Messenger probe while observing Mercury´s surface.
“It is high in magnesium and very low in iron, which is what they´re seeing on the surface of Mercury,” Korotev, said in a statement. “But it´s got more plagioclase (an aluminum containing mineral) than they´re seeing on the surface of Mercury and it plots funny in ℠oxygen isotope space.´ It´s plotting in a region of oxygen isotope space where we´ve never had meteorite data points before — except for a few ureilites, which also have oddball chemistry.”
Some of the chemical ratios were not a match, but Irving explained that those results could have been “excavated from depth” and then sent into space following a collision that deeply damaged Mercury. However, Tim McCoy, the curator-in-charge of the Smithsonian Institution´s meteorite collection, noted that the preliminary data suggested the meteorite had crystallized from the melt 4.5 billion years ago. That would have effectively eliminated Mercury as the source of the meteorite, as the planet´s rocks should have crystallized no more than 4.2 billion years ago.
“The moon began to crystallize 4.5 years ago, but we don´t have any 4.5-billion-year-old meteorites from the moon, because all of those rocks would have been bashed to smithereens during the late heavy bombardment that pockmarked the moon with craters between 4 to 3.8 billion years ago.” Korotev said.
“The same thing would have happened on Mercury, so the question is how did this rock survive for that long? There´s no sign of it being brecciated, or busted up,” he added. “But if it´s not from Mercury,” he said, “then where is it from? That´s really the question“¦ It has very odd chemistry for a meteorite. If somebody had walked in with this chemical analysis and nothing else I would have told him that it wasn´t a meteorite, just based on the chemistry.”