First big-picture look at meteorites from before giant space collision 466 million years ago
To learn what the meteorite flux was like before the big collision event, Philipp Heck of The Field Museum in Chicago and his colleagues had to analyze meteorites that fell more than 466 million years ago. Such finds are rare, but the team was able to look at micrometeorites—tiny specks of space-rock less than 2 mm in diameter that fell to Earth, which are a little more widespread. Heck's Swedish and Russian colleagues retrieved samples of rock from an ancient seafloor exposed today in a Russian river valley that contained micrometeorites, and then dissolved the rocks in acid so that only microscopic chromite crystals remained.
"Chrome-spinels, crystals that contain the mineral chromite, remain unchanged even after hundreds of millions of years," explains Heck. "Since they were unaltered by time, we could use these spinels to see what the original parent body that produced the micrometeorites was made of."
Analysis of the chemical makeup of the spinels showed that the meteorites and micrometeorites that fell earlier than 466 million years ago are different from the ones that have fallen since. A full 34 percent of the pre-collision meteorites belong to a meteorite type called primitive achondrites; today, only 0.45 percent of the meteorites that land on Earth are this type. Other ancient micrometeorites sampled turned out to be relics from Vesta, the brightest asteroid visible from Earth, which underwent its own collision event over a billion years ago.
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