Meteorite Mystery 135 Years in the Making Solved

Scientists have provided a solution to a 135-year-old mystery in cosmochemistry, saying chondrules may have formed from high-pressure collisions in the early Solar System.

Chondrites are the largest class of meteorites, and scientists have wondered how numerous small, glassy spherules had become embedded within them. British mineralogist Henry Sorby first described these spherules in 1877 and suggested they may be “droplets of fiery rain” that somehow condensed out of the cloud of gas and dust that formed the Solar System 4.5 billion years ago.

However, Lawrence Grossman, professor in geophysical sciences at the University of Chicago, and colleagues have conducted research that better defines this 135-year-old thought.

The scientists reconstructed the sequence of materials that condenses from the solar nebula and concluded that a condensation process cannot account for chondrules. Grossman believes that chondrules derived from collisions between planetesimals, or bodies that gravitationally coalesced early in the history of the Solar System.

Cosmochemists believe that many types of chondrules had solid precursors. One problem is that high temperatures are needed to turn the condensed solid silicates into chondrule droplets. Another problem is that chondrules contain iron oxide. Iron can only enter the crystal structures of magnesium silicates when it is oxidized, and this is another process that requires very high temperatures.

“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,” Grossman said.

During experiments, the scientists discovered a tiny pinch of sodium in the cores of the olivine cyrstals embedded within the chondrules. When olivine crystalizes form a liquid of chondrule composition at temperatures of 3,140 degrees Fahrenheit, most sodium remains in the liquid form if it doesn’t evaporate. The researchers found no more than 10 percent of the sodium ever evaporated from the solidifying chondrules.

Grossman and colleagues calculated the conditions required to prevent any degree of evaporation. They created calculations in terms of total pressure and dust enrichment in the solar nebula of gas and dust and determined that this couldn’t take place in the solar nebula. This led Grossman to the theory of planetesimal impacts.

July issue of Geochimica et Cosmochimica Acta.

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