MANTLE: JEWELS FROM SPACE
THESE STUNNING METEORITES COME FROM A REGION BETWEEN THE CRUST AND CORE OF A DIFFERENTIATED ASTEROID.
WHY THE STRANGE MIXTURE?
When newly formed planetary bodies heat and melt, iron and rock-forming minerals separate into a core and mantle. These materials separate like oil and water. So why is iron mixed with the silicate mineral olivine in these stony-iron meteorites?
Perhaps chunks of an olivine mantle fell into an outer core, where churning molten metal tore them apart. Or perhaps the iron and olivine never completely separated. No one really knows. But whatever caused the mixing, the stony-iron meteorites known as pallasites provide a snapshot of this dynamic process at the moment when the mixture solidified.
Esquel meteorite
© AMNH/Jackie Beckett
LOOK CLOSELY
Esquel shows liquid metal surging into and breaking apart a cluster of silicate crystals. These dazzling crystals are made of olivine, a mineral type that includes the gemstone peridot.
ROUND VS. ANGULAR CRYSTALS
Most pallasites, including Springwater, have rounded olivine crystals. But in Huckitta (right),
Huckitta meteorite
© AMNH/Jackie Beckett
A MISLEADING MIXTURE
Mincy resembles the other meteorites in this case because it is a mixture of stone and iron. But it is actually a mesosiderite, a completely different kind of meteorite. Unlike pallasites, which come from a region near the core, mesosiderites are a mixture of iron from the core and rock from the crust. These materials cooled at very different rates and thus could not have hardened together, so they likely were mixed by a powerful impact.
GEMS FROM SPACE
Pallasites, with their combination of gleaming metal and translucent olivine crystals, are among the most rare and beautiful of all meteorites. Less than one percent of all known meteorites are pallasites.
WHERE ARE THE "MISSING" OLIVINE METEORITES?
Why have no meteorites been found from an asteroid's olivine mantle? Here are a few theories:
Theory 1
No pieces of an asteroid mantle happened to land on Earth. But considering that thousands of meteorites provide numerous samples of crusts and cores, it is hard to explain why not one single piece of mantle would have reached us.
Theory 2
After pieces of the crust were knocked off, the olivine mantles eroded away, pounded into dust by numerous collisions over billions of years. But other meteorites made of material just as fragile as olivine have managed to reach Earth.
Theory 3
A thick layer of olivine simply does not exist inside asteroids. Perhaps the olivine in differentiated asteroids did not fully separate from the crust. Or perhaps metal from the core remained mixed with olivine, and the bulk of the mantle is composed of the mixture found in pallasites.
This mystery is likely to remain unsolved until a spacecraft lands on an asteroid and analyzes its interior.
More than two-thirds of Earth's volume consists of a mantle of mostly olivine composition. Earth and the asteroids formed from similar materials—yet no meteorites from the olivine mantle of a differentiated asteroid have ever been found. This absence is one of the great meteorite mysteries.
