Meteorite mystery: Chondrules formed from high-pressure collisions in early solar system

This is an artist's rendition of a sun-like star as it might have looked at one million years of age.

As a cosmochemist, the University of Chicago's Lawrence Grossman reconstructs the sequence of minerals that condensed from the solar nebula, the primordial gas cloud that eventually formed the sun and planets. 

Credit: NASA /JPL-Caltech /T. Pyle, SSC

A normally staid University of Chicago scientist has stunned many of his colleagues with his radical solution to a 135-year-old mystery in cosmochemistry.

"I'm a fairly sober guy. People didn't know what to think all of a sudden," said Lawrence Grossman, professor in geophysical sciences.

Lawrence Grossman

At issue is how numerous small, glassy spherules had become embedded within specimens of the largest class of meteorites—the chondrites. British mineralogist Henry Sorby first described these spherules, called chondrules, in 1877.

Sorby suggested that they might be "droplets of fiery rain" which somehow condensed out of the cloud of gas and dust that formed the solar system 4.5 billion years ago.

Researchers have continued to regard chondrules as liquid droplets that had been floating in space before becoming quickly cooled, but how did the liquid form? "There's a lot of data that have been puzzling to people," Grossman said.

Grossman's research reconstructs the sequence of minerals that condensed from the solar nebula, the primordial gas cloud that eventually formed the sun and planets.

He has concluded that a condensation process cannot account for chondrules. His favorite theory involves collisions between planetesimals, bodies that gravitationally coalesced early in the history of the solar system.

"That's what my colleagues found so shocking, because they had considered the idea so 'kooky,'" he said.

Cosmochemists know for sure that many types of chondrules, and probably all of them, had solid precursors. "The idea is that chondrules formed by melting these pre-existing solids," Grossman said.

One problem concerns the processes needed to obtain the high, post-condensation temperatures necessary to heat the previously condensed solid silicates into chondrule droplets.

Various astonishing but unsubstantiated origin theories have emerged. Maybe collisions between dust particles in the evolving solar system heated and melted the grains into droplets. Or maybe they formed in strikes of cosmic lightning bolts, or condensed in the atmosphere of a newly forming Jupiter.

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

Grossman and his UChicago co-author, research scientist Alexei Fedkin, published their findings in the July issue of Geochimica et Cosmochimica Acta.

More information: "Vapour saturation of sodium: Key to unlocking the origin of chondrules," by Alexei V. Fedkin and Lawrence Grossman, Geochimica et Cosmochimica Acta, Vol. 112, July 2013, pages 226-250.