Via the BBC: NASA has just released some preliminary results from the Stardust probe, which brought dust particles from Comet Wild-2 back to Earth in January. Intriguingly, it seems that a significant proportion of the material collected consists of minerals, such as olivine and other silicate minerals, which only form at temperatures >1000 C.
Back when the probe first landed I explained how examining the composition of the cometary material could help to constrain our models of planetary formation. What these results may be telling us is that the planetary nebula which eventually formed our solar system was a much more dynamic place than we have supposed up to now. From the NASA press release:
"We have found very high-temperature minerals, which supports a particular model where strong bipolar jets coming out of the early sun propelled material formed near to the sun outward to the outer reaches of the solar system," said Michael Zolensky, Stardust curator and co-investigator at NASA's Johnson Space Center, Houston. "It seems that comets are not composed entirely of volatile rich materials but rather are a mixture of materials formed at all temperature ranges, at places very near the early sun and at places very remote from it."
In the model referred to (known as the “X-wind model” – see here for some background), the “strong bipolar jets” are thought to be generated by the interaction of a rapidly spinning accretion disk with the magnetic field of the growing central protostar. This model was originally proposed to explain similar mixing of material which formed at different temperatures (and therefore locations) in asteroids, but the fact that we are seeing material from the very inner solar system being transported right out into the outer reaches where comets formed means that these jets must have been very powerful indeed.
As always, of course, there is another alternative – this material might predate the solar system. Having formed close to another, older, star, it was blown into interstellar space by a supernova, and then incorporated into the planetary nebula which eventually became our solar system. Measuring some isotopic ratios for these grains should help to distinguish between these two possibilities.