4.6-billion-year old meteorite suggests much dustier origin for Earth

Washington, June 20 : The analysis of a 4.6 billion-year-old meteorite that fell in India in 1940 has revealed higher than expected levels of sodium, suggesting that the dust clouds from which the building blocks of the Earth and neighboring planets formed were much denser than previously supposed.

Conel Alexander and Fred Ciesla of the Carnegie Institution's Department of Terrestrial Magnetism, with colleagues from the US Geological Survey and 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 degree Celsius (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.

"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," he added.

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.

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.

ANI