[meteorite-list] Chemical Clues Point to Dust Origin for Earth-like Planets (Semarkona Meteorite)

[Ron Baalke]

Carnegie Institution

Contact:
Conel Alexander, 202-478-8478

Monday, June 23, 2008

Chemical clues point to dusty origin for Earth-like planets

Washington, D.C. -- Higher than expected levels of sodium found in a 4.6
billion-year-old meteorite suggest that the dust clouds from which the
building blocks of the Earth and neighboring planets formed were much denser
than previously supposed. The study, by scientists from the Carnegie
Institution, American Museum of Natural History, and U.S. Geological Survey,
is published in the June 20 issue of Science.

Conel Alexander and Fred Ciesla of the Carnegie Institution's Department of
Terrestrial Magnetism, with colleagues Jeffrey Grossman of the U.S.
Geological Survey and Denton Ebel of the American Museum of Natural History,
analyzed the sodium content of grains in objects called "chondrules" from
the Semarkona meteorite, which fell in India in 1940. The Semarkona
meteorite, like all other chondrule-bearing meteorites (known as
chondrites), dates from the early stages of the solar system's formation.
Unlike most others, however, its constituents have been relatively unaltered
by heat and chemical changes over the more than four billion years since its
origin, making it an important window into the early history of the solar
system.

Chondrules, which make up 20 to 80% of the volume of chondrites, are round,
roughly millimeter-sized objects made of glass and crystals. Chondrules are
thought to have formed by flash heating of dust in the primordial solar
system. From the types of minerals found in chondrules, scientists have
determined that they formed at temperatures of up to nearly 2,000 C (3600
F). The source of this high heat, which would have affected vast areas of
dust, is unknown. The heat would also be expected to have boiled off many of
the volatile chemical elements, such as sodium, leaving the chondrules
depleted in these elements.

But the chemical analyses by the research team found that the Semarkona
chondrules had surprisingly high sodium abundances when they formed,
indicating that sodium was not driven off. Rather, it remained at nearly
constant levels during chondrule formation.

"Chondrules formed as molten droplets produced by what was probably one of
the most energetic processes that operated in the early solar system," says
Alexander. "You would expect all the sodium to evaporate and be lost from
the chondrules under such conditions. Instead, the sodium was retained. The
chondrules stayed as effectively closed systems throughout the heating and
melting."

The researchers determined that in order for the molten droplets that formed
the chondrules to remain as closed systems and retain constant levels of
sodium, the initial dust cloud must have been far denser than previously
supposed. "If the droplets were crowded close enough together, then the
sodium vapor in the spaces in between would reach a saturation point," says
Alexander, "and that would prevent further evaporation."

To achieve this condition, the density of dust in the chondrule-forming
regions of the early solar system must have been at least about 10 grams per
cubic meter, and possibly much more. This is at least 100 times the
densities considered by previous models of chondrule formation, which had
assumed at most densities of only about 0.1 grams per cubic meter, and
normally considerably less. At densities of 10 grams per cubic meter or
more, regions only a few thousand kilometers across, small by astronomical
standards, could collapse under their own gravity to make objects that would
be 10s of kilometers across.

"What's notable about this result is that it raises the possibility that the
formation of chondrules in these high-density regions was linked to the
formation of kilometer-sized objects called planetesimals, which were the
first stage in building Earth-like planets," says Alexander. "It will also
help narrow down the possibilities for the cause of the heating that
produced the chondrules, as well as the sizes of the regions where they
formed. Heating chondrules to their peak temperatures and then quickly
cooling them down when they are present at such high densities is a
significant challenge for any mechanism proposed to explain chondrule
origin. These tiny objects still have a lot to tell us about how our solar
system took shape."

This research was supported by the Carnegie Institution, the NASA Origins of
Solar Systems Program, and the NASA Cosmochemistry Program.

IMAGE CAPTION:
[http://www.ciw.edu/sites/www.ciw.edu/files/news/1ac91edbig.jpg (52KB)]
False color micrograph of a chondrule from the Semarkona meteorite. Red
indicates olivine crystals, blue and green are glass. High-density dust
prevented sodium in the glass from evaporating during chondrule formation,
despite high temperatures. Scale bar = 0.5 mm.