Part of Hall of Meteorites.
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.
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.