Stardust: A Mission With Many Scientific Surprises
Dr. Don Brownlee
Stardust Principal Investigator
October 29, 2009
The primary goal of the Stardust mission was to collect samples of a comet and return them to Earth for laboratory analysis. Comets are ancient bodies of frozen ice and dust that formed beyond the orbit of the most distant planet. They were expected to contain materials that the solar system formed from, preserved in ice for billions of years. When the international team of 200 scientists began examination of the returned particles, we found that the particles were indeed ancient building blocks of the solar system but the nature and origin of the particles was quite unexpected. Before the mission, there were very good reasons to believe that we knew what comets would be made of and there was a general expectation was that the particles collected from comet Wild 2 would be mainly be dust that formed around other stars, dust that was older than the Sun. Such particles are called stardust or pre-solar grains and this was the main reason why the mission was named Stardust.
What we found was remarkable! Instead of rocky materials that formed around previous generations of stars we found that most of the comet's rocky matter formed inside our solar system at extremely high temperature. In great contrast to its ice, our comet's rocky material had formed under white-hot conditions. Even though we confirmed Comets are ancient bodies with an abundance of ice, some of which formed a few tens of degrees above absolute zero at the edge of the solar system, we now know that comets are really a mix of materials made by conditions of both "fire and ice". Comet ice formed in cold regions beyond the planet Neptune but the rocks, probably the bulk of any comet's mass, formed much closer to the Sun in regions hot enough to evaporate bricks. The materials that we collected from comet Wild 2 do contain pre-solar "stardust" grains, identified on the basis of their unusual isotopic composition, but these grains are very, very rare.
Among the high temperature materials some are already well known components of primitive meteorites; rocks from asteroids that formed between Mars and Jupiter. These include odd rounded particles called chondrules and white irregular particles known as Calcium Aluminum Inclusions (CAIs). Chondrules are the dominant material in many primitive meteorites and they are rounded droplets of rocks that melted and then quickly cooled as they orbited the Sun. CAIs are much rarer than chondrules and are distinguished by their unusual chemical and isotopic composition. They are also the oldest solar system materials and are composed of exotic minerals that form at the very high temperature.
It was very exciting to find that pieces of CAIs and chondrules in the comet and the scientific implications of this are profound. When we first presented the discovery of comet CAIs at the annual Lunar and Planetary Science conference, just three months after Stardust landed, you could see jaws drop in the room crowded with 600 scientists. It was just phenomenal to discover something this profound, right in the beginning of the analysis program. The discovery of chondrules and CAIs proves that matter abundantly formed in the inner solar system was somehow transported to the edge of the young solar system where comets formed. There are some theories that suggest that CAI's formed just a few radii from the surface of the Sun, 4.567 billion years ago. The finding that inner solar system materials, formed at very high temperature, were transported all the way to the edge of the Solar System to the region where Pluto is one of the major scientific findings of Stardust. In other words, instead of being dominated by particles formed around other stars, our comet's rocks were predominantly formed close to the Sun. Thus, these comet sample studies have provided a direct look at the nature and origin of the building blocks of planets, materials that were sprayed all over the young solar system and must have been incorporated into all planets and moons.
Stardust also had variety of other surprises. One of the most unexpected was the 2009 discovery of the amino acid glycine by a team of scientists from the Goddard Space Flight center. While perhaps not totally unexpected that a comet would contain amino acids it was unexpected that this molecule could be detected in the tiny particles that were collected at such high speed (six times the speed of a rifle bullet!). It was quite a technical triumph to develop the methods that made the detection possible and incorporated the use of isotopic composition to prove the glycine was not a contaminant from our own planet. The significance of this discovery is that comets must have delivered at least one amino acid to our planet before it had life. Because most stars have comets it suggests that all Earth-like planets obtain important pre-biotic molecules from space.
Another surprise from the 2004 comet flyby came when we flew through the dust escaping the comet. It had been expected that the impact rate of particles on the spacecraft would increase with time, reach a peak, and then decline as the comet nucleus disappeared "in the rear view mirror". Instead, the rate of impact rate changed in spurts, probably caused by entering and exiting "jets" of dust flowing off the nucleus and also the breakup of "cometary dirt clods" as they drifted away from the nucleus and lost ice that had served as glue to hold them together.
But the biggest surprise discovered during the flyby came with the comet images (72 taken during the pass). The camera team, led by JPL's longtime comet expert, Ray Newburn, had expected that the comet would be a rather bland object looking somewhat like a black potato. What we saw, even in the very first picture sent back, was quite dramatic. We saw kilometer-sized deep holes bounded by vertical and even overhanging cliffs; flat topped hills surrounded by cliffs; spiky pinnacles hundreds of meters tall, pointed skyward: in addition to the numerous jets of dust and gas escaping into space. Two of the dust jets came from the comet's night side, a region that was expected to be inactive because if its lack of heating by sunlight. What we did not see in the images were impact craters, such those found on the Moon, Mars and practically every other surface exposed to space. The lack of impact craters indicates the surface is new, the old cratered surface is gone. The astounding thing is that the surface of Wild 2 is very different from the surfaces of any other asteroids and comets that have been imaged by spacecraft. It is much rougher, much more dramatic and it clearly is not the bland body that we expected it to be.