Scientific Method / Science & Exploration


A special issue of Science summarizes the preliminary results of the Stardust …

Summary of the mission

The stardust probe was launched into space on February 7, 1999, by a Delta 2 rocket with the intention of getting up close and personal with a comet. Once within astronomical touching distance, the probe captured some space dust. On board, the probe had a dust flux monitor and mass spectrometer, which helped the scientists make decisions about exactly when to begin capture. The dust was captured in "aerogel," a specially engineered solid that has the same density as air—making it the least dense solid known to man. The close encounter with comet Wild 2 occurred over a period of 100 days, all while taking pictures of the comet and analyzing particles. In the middle of the encounter period the satellite passed within 240km of the comet core, with a relative velocity of some 6km/s—slow in terms of space encounters. Around the point of closest approach the aerogel was exposed to space, allowing it to capture particles. These particles are most likely to have their origins in the comet and hence be a relatively unmodified sample of the accretion disk that existed before the formation of planets and the more recent nebula. The trajectory of the spacecraft then returned it to earth on January 15, 2006.

Read on.

The history of Wild 2

Wild 2 is thought to have formed in the dusty, low density region beyond Neptune, called the Kuiper belt. Because the particles aggregated in such low temperature and density conditions, the comet does not form a hard solid mass like a meteor. The result is that the particles making up the comet are mostly unmodified by the conglomeration process and may even be unchanged since the sun's accretion disk formed. Over a period of millions of years Wild 2 moved inwards until it passed close enough to Jupiter to be slung towards the inner solar system. It's orbit continues to be influenced by Jupiter, meaning that it will be thrown out of the solar system or destroyed by a collision within about 10,000 years. This is important because it means Wild 2 passes close enough to the sun to boil off all the water but has not passed through the inner solar system long enough to alter the minerals that make up its core. It is this history that gives astronomers the confidence to claim that the dust they captured is a view into the earliest period of the solar system.

Particle capture

The comet particles were trapped by burrowing into the porous silica aerogel or by impacting on the surrounding aluminum frame. This occurred on time scales ranging from less than 1ns through to a microsecond, thus these particles can get quite hot. Simulations, experiments, and work with meteorites indicated that particles bigger than about 10 micrometers should be relatively unaffected by the slowing process, while smaller particles will be changed. Even with all this work no one really knew what would happen to the particles because the composition of the particles was largely based on guess work. To everyones relief, the predictions turned out to be fairly accurate; particles larger than 1 micrometer were largely unaffected in composition (though they may have disintegrated), while smaller particles show evidence of surface melting and even melting and mixing with the surrounding aerogel. Apart from those particles that are completely melted into the surrounding aerogel, the fractional composition of the dust particles remains unambiguously intact. More problematic are crystalline/amorphous measurements. These are important because amorphous solids are indicative of rapid cooling, while crystalline materials indicate either slow cooling or annealing. All of which can be used to understand the scale and significance of mixing in the solar accretion disk and nebula. Unfortunately, the capture process involved both rapid heating and cooling and so far it has not been possible to exclude the capture process as the source of any amorphous material discovered.

Comet formation

Comet formation is not a well understood process, although, like all scientific problems there is a current "best explanation." In this model, comets are formed from aggregates of tiny interstellar dust particles, and water ice. However, analysis of the dust particles shows that they must come from within the solar system, which directly contradicts our model. The ratio of iron to silicon is the same as that for material native to the solar system but not interstellar material. The high abundance of aluminum, calcium and chromium is in direct disagreement with the spectroscopy of interstellar dust. By analyzing the impact craters on the aluminum and the shape of the tracks in aerogel, scientists were able to determine that particles ranged from loosely bound conglomerates to compact, strong grains with sizes ranging from less than 1 micrometer to over 100 micrometer. This is in direct contrast to the comet model which predicts that they are almost entirely made from loosely bound conglomerates less than 1 micrometer in size. These results, combined with one known problem of the current comet model are probably enough to begin a significant re-write of comet formation.

Stirring the solar system

One of the biggest unknowns in our models of solar system formation is the amount of radial mixing that occurs. Essentially, we don't know how likely it is for a particle to travel from the Mercury orbit out to beyond Neptune. Several models have suggested different degrees of mixing, however, that mixing was considered to be small. Even though the analysis is incomplete, the results from Stardust indicate that radial mixing was much more significant than previously thought. The crystalline particles they found indicate that they must have been near the sun at some point so that the amorphous material could anneal and crystallize. In addition they found several minerals that are known to only form at high temperatures—temperatures experienced by particles that are closer to the sun than Mercury. This tells us that significant percentage of the particles in the outer solar system have previously been in the inner solar system. The size of the particles indicates that the mixing process must be capable of driving quite large particles into the outer reaches of the solar system. These results will allow theorists to constrain the parameter range of the solar system models, which, in turn will provide additional context for the Stardust results.

Organic chemicals

Chemicals based around carbon and hydrogen, called organic chemicals, were also found in the particle collector. Although there is a small degree of contamination from the manufacture of the aerogel, this is well characterized by the analysis of control samples. More difficult to analyze is effect of the capture process on the organic chemicals. These compounds are more fragile than the inorganic minerals discussed earlier and as a result are more likely to have undergone additional reactions during the collisional process. For instance, two discrete populations of ring carbon structures, called polyaromatic hydrocarbons (PAHs) were observed. PAHs are of importance because they are a candidate for the basic organic chemical from which life may have originated. The group that contains one and two ring PAHs are of uncertain origin; they could be from the comet or they could have been produced from the background organic chemicals in the aerogel during the collision. On the other hand, the heavy PAHs (three ring and up) are unlikely to have formed during the collision and are thought to have originated from the comet. Interestingly, the organic chemicals are rich in oxygen and nitrogen, which is not consistent with interstellar dust but is necessary for origin of life theories. In general the organic chemicals found on the comet are less stable than those found in interstellar dust particles collected from the upper atmosphere, or meteors. However, as the authors note, this may simply be due to the more extreme environment the meteors and interstellar dust particles have been exposed to.

Disagreement abounds

One of the major features of these results is how different they are from those obtained by the Deep Impact experiment. Deep Impact measured the composition of the interior of the comet by colliding a probe with the core and then measuring the infrared absorption of the ejected material. The results of Deep Impact are also inconsistent with current theories on both comet formation and solar system mixing but are substantially different from the preliminary results of Stardust, which is causing a small amount of angst. There are a few possible explanations of the differences; firstly, pictures of Wild 2 show that it was crater free, indicating that possible mineral changes due to any collisions had been lost over time. This is in contrast to Deep Impact, where the collision site aimed for a place between two craters. Secondly, the spectroscopic results were obtained by comparing the measured spectra with spectra obtained in the laboratory by an intelligent "mix and match" session. This method leaves open the possibility that the researchers have matched the spectra without accurately matching the composition—though I find that to be a bit of a stretch. Thirdly, they are two comets, formed in different regions from different material and the differences may be within the variability of the nebula itself. There is some evidence to support this last view, since the particle size distribution measurements of Wild 2 and Hale-Bopp are generally in agreement, while those from Halley and Deep Impact are also in agreement with each other.

A small amount of dust has certainly shown just how much there is to learn about our solar system.

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