Stardust's Big Surprise
by Dr. Don Brownlee, Principal Investigator
A common expectation among scientists was that the comet samples returned by the Stardust Mission would in fact be composed of stardust, tiny grains that formed around other stars. Many expected that much of the solid matter collected from comet Wild 2 (pronounced Vilt 2) would be aggregates similar to “dirt clods” assembled from tiny grains of glass, minerals and carbon. It was imagined the individual components would be ancient samples of the same material that we see in the night sky as the dark band in the middle of the Milky Way, the edge-on view of our Galaxy. For a variety of reasons it was expected that the individual grains composing the dirt clod structures would be only about a third of a micrometer across or about 0.3 % of the width of a human hair.
The hypothesis that tiny stardust grains would dominate comet dust came from the knowledge that comets formed in the coldest, most distant region of the young solar system. In this cold place the initial building blocks of the solar system, interstellar dust and gas, might survive without modification. When we first looked at the tracks of comet dust captured in silica aerogel, it was clear that they were not solely “dirt clods” of sub-micrometer components. Most of the capture tracks, formed as particles slowed to a stop, were deep and were shaped like carrots. The production of deep tracks requires relatively large strong particles and even when we first opened the capsule we could see with our unaided eyes rather large particles at the ends of some of the tracks. We immediately suspected that the comet contained a sizeable amount of solid material that is much larger than interstellar grains. If the comet dust had been made of the expected dirt-clods composed of tiny stardust grains, they would have produced holes in the aerogel that looked more like shallow bowls than carrots because tiny components stop quickly and cannot travel far in aerogel. Some of the bigger particles found at the ends of the carrot-shaped tracks are a million times more massive than typical stardust grains.
CAI Particle found in the Stardust collection. Courtesy: NASA
When we started pulling these particles out and examining them in electron microscopes and other instruments, we found even more surprises. First of all we found evidence that the standard astronomical predictions for the origin of dust in comets, or at least the ones in this comet, appear to be incorrect. While we did find stardust grains in the cometary materials, they appear to be only a minor component, at least in the particles larger than a micrometer that were well preserved during high speed capture. This judgment is based on the concept that the isotopic composition of stardust should be different from that of typical solar system materials. This is the way that rare stardust grains have been identified in meteorites and interplanetary dust. Like in meteorites most of the components from the comet have isotopic compositions similar to Earth and are of solar system origin.