Meteorite Mystery Solved: Glassy Spherules Caused by Collisions

The largest class of meteorites, called chondrites, have a few mysteries left in store for astronomers. Small, glassy spherules have been found embedded in samples of these massive meteorites, leading scientists to wonder how they actually appeared there in the first place. Now, a researcher from the University of Chicago may have the answer. He believes that these spheres, called chondrules, may be caused by collisions.

When a British mineralogist first discovered these chondrules in 1877, he believed that they might be "droplets of fiery rain" which somehow condensed out of the clouds of gas and dust that formed the solar system about 4.5 billion years ago. In fact, the theory has mainly remained the same since then; researchers believe that the glassy spheres are liquid droplets that have been floating in space before becoming quickly cooled. Yet scientists were at a loss to explain how the liquid got there in the first place.

Lawrence Grossman, the head of the study, wasn't satisfied with this theory. In order to find out how these chondrules might form, he reconstructed the sequence of minerals that condensed from the solar nebula, the primordial gas cloud that eventually formed the sun and planets. It turns out that the condensation process couldn't account for the chondrules, which means that something else must have caused their formation.

So what did cause the chondrules? Researchers know for sure that many types of chondrules had solid precursors and then formed by melting these pre-existing solids. This means that high bursts of heat would be needed. In addition, chondrules contain iron oxide, which forms at very low temperatures in the solar nebula. Fortunately, Grossman has a theory to explain this seeming contradiction.

"Impacts on icy planetesimals could have generated rapidly heated, relatively high-pressure, water-rich vapor plumes containing high concentrations of dust and droplets, environments favorable for formation of chondrules," said Grossman in a news release.

Essentially, planetisimals, consisting of metallic nickel-iron, magnesium silicates and water ice, condensed from the solar nebula, well ahead of chondrule formation. Decaying radioactive elements inside the planetesimals then provided enough heat to melt the ice. This water percolated through the planetesimals, interacted with the metal and oxidized the iron. With further heating, the magnesium silicates reformed, incorporating iron oxide in the process. When the planetesimals collided with one another, liquid droplets of iron oxide sprayed out.

"That's where your first iron oxide comes from, not from what I've been studying my whole career," said Grossman in a news release.

The findings are published in the journal Geochimica et Cosmochimica.