Stardust's Big Surprise

Dr. Don Brownlee
Stardust Principal Investigator
July 6, 2007

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.

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.

We find spectacular silicate crystals in the comet. The presence of crystals in comets was suggested by astronomical observations but the Stardust mission results provide important new insight into their origin and history. When the presence of crystals in comets and in disks of dust orbiting other stars was first observed astronomically it was a mystery because crystals are not detected in interstellar dust (the dust between stars), the building materials that are used to form solar systems and comets. Interstellar dust is composed of glassy materials with no crystalline order. The widely accepted hypothesis for the origin of cometary crystals is that they are interstellar grains that were transformed to crystals by mild warming in the vicinity of the star that they orbit.

The comet samples collected by Stardust contain abundant crystalline minerals and in most cases it is clear that they did not form by the predicted mild heating of interstellar dust. Many are too large, and have complex mineralogical and chemical compositions that could not have formed by this process. Instead of the mild heating that astronomers envisioned the comet samples were heated during their formation to severe temperatures, temperatures high enough to melt or vaporize them. The temperatures above 1300 ¼C and the samples were white hot. This is quite remarkable because the some of the ice components of comets appear to have formed only 30 degrees above absolute zero. The comet is very odd mix of materials that formed at the highest and lowest temperatures that existed in the early solar system. Comets have been cold for billions of years but their ingredients are remarkable products of both fire and ice. Because the rocky materials in comet Wild 2 formed at such high temperatures, we believe that they formed in the hot inner regions of the young solar system and were then transported all the way to beyond the orbit of Neptune. There had been several theories that suggested that such large scale mixing might have occurred and the Stardust results appear to be proof that this large scale mixing did occur and that many of the large rocky particles formed close to the Sun. This implies that while the comets contain ices that formed at the edge of the solar system, the rocky materials that actually make up the bulk of a comet's mass actually formed in the hottest possible conditions. The inner solar system can be thought of as a factory producing rocky materials that were distributed outwards to all the bodies and regions of the solar system.

CAI Particle Found in the Stardust Collection

One of the most remarkable particles found in the Stardust collection is a particle named after the Inca Sun God Inti. Inti is collection of rock fragments that are all related in mineralogical, isotopic and chemical composition to rare components in meteorites called "Calcium Aluminum Inclusions" or CAI's for short. CAI's are the oldest materials that formed in the solar system and they contain a remarkable set of minerals that form at extremely high temperature. In addition to these same minerals, Inti also has tiny inclusions that may have been the first generation of solids to condense from hot gas in the early solar system. These include compounds of titanium, vanadium and nitrogen (TiN and VN) as well as tiny nuggets of platinum, osmium, ruthenium, tungsten and molybdenum. In certain chemical environments and at high enough temperature in the early solar system these exotic materials were the only solid materials that could survive without being vaporized.

The track the CAI particle was found in from aerogel cell #54

The comet particles returned by the Stardust mission have been a real bonanza. They do contain some stardust grains from other stars but the majority of solids are solar system materials that appear to have formed over a very broad range of solar distances and perhaps over an extended time range. Comet Wild 2 is a collection of materials that probably came from all regions of the young solar system and thus it has turned out to be wonderful "time capsule". Hundreds of scientists around the world have worked on these samples and the first results from these studies were presented in the December 15, 2006 issue of Science magazine. Having samples from the edge of the solar system has provided a fabulous way to explore the early solar system and test ideas for its origin. The samples will be explored for decades to come and we believe that they provide a wonderful new way to better understand our origins.