New chemical clues to Earth-like planet formation

Sodium is volatile. It easily burns and boils and diffuses. Meteorites are hardy, and the type known as chondrites are also primitive, dating back to the very early solar system. Chondrites contain a high density of so-called chondrules—roughly millimeter-sized spheres like the one shown here in polarized light—that were flash-melted at temperatures around 2000 K and subsequently cooled and incorporated into a meteorite’s parent object, typically an asteroid. The heating mechanism is unknown but could involve shocks or lightning. Mostly made of silicate minerals such as olivine and pyroxene and of the metals iron and nickel, chondrules are expected to be deficient in volatile elements like sodium. But researchers at the Carnegie Institution of Washington, the US Geological Survey, and the American Museum of Natural History say it isn’t so. Using electron microprobe spectroscopy, they studied 26 chondrules from the Semarkona meteorite that fell in India in 1940 and found significant sodium throughout. The only way that could happen, they say, is if the chondrules formed as closed systems at densities in the solar nebula (the disk of gas and dust from which the planets formed) that were far higher than previously thought. That way, the cooling droplets would be crowded together in an area of saturated sodium vapor. The required ambient densities range from 10 to hundreds of grams per cubic meter, far exceeding the standard assumption of 0.1 g/m³ or less. At the much higher densities, astronomically tiny regions just a few thousand kilometers across can collapse under their own gravity. Thus chondrule formation seems to be intimately linked to planetesimal formation, the first step in making planets like Earth. (C. M. O’D. Alexander et al., Science 320 , 1617, 2008 http://dx.doi.org/10.1126/science.1156561 .)