Dustier than Expected



When looking at olivine crystals in a meteorite dating from the formation of our solar system, Denton Ebel, Associate Curator in Physical Sciences, Earth and Planetary Sciences at the American Museum of Natural History, and his colleagues found something surprising: sodium. This discovery has huge implications for the composition of the early inner solar system from which planets formed and suggests that the density of solid material must have been much higher than previously thought. 

Data for this project was culled from the Semarkona meteorite found in India over sixty years ago, and the results have put this meteorite on the June 20th cover of Science-a fact that thrills Ebel since meteorites don't often land there. This particular meteorite is a chondrite that contains fine millimeter “sized spherules called chondrules. It is within these chondrules--the glass and mineral crystals formed by flash heating of early solar-system dust at 2,000ÂC (3,600ÂF)--that the unexpected sodium was found. 

The high temperatures required to form chondrules would normally vaporize sodium. But since this element was found, Ebel and colleagues estimate that the dust cloud was much denser than previously thought-at least a hundred times more dense at ten grams per cubic meter. More closely packed material suggests that molten droplets that form the chondrules were crowded close enough for the sodium vapor to reach a saturation point and not evaporate as the magma droplets cooled.

"Interesting results tend to come from this is the kind of collaboration between cosmochemists, astrophysicists and people that study meteorites in gory detail," said Ebel. "We have yet to incorporate this model into the context of a full-scale astrophysical disks such as the one from which our planet formed…but we're working on it."

Mordecai-Mark Mac Low, Curator-in-Charge of the Department of Astrophysics at the Museum, is working on it. "There is a long-standing question of how to form planetesimals-the building blocks for planets-from cosmic dust in a protoplanetary disk," explains Mac Low. "This new research stands as empirical evidence in support of my group's proposal (Nature) for how planetesimal formation proceeds through clumps of boulders dense enough to be bound by their own gravity."

Other scientists involved in this research are Conel Alexander (first author) and Fred Ciesla of the Carnegie Institution's Department of Terrestrial Magnetism and Jeffrey Grossman of the U.S. Geological Survey. The research was supported by the Carnegie Institution, the NASA Origins of Solar Systems Program, the NASA Cosmochemistry Program, and the NASA Astrobiology Institute.

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