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Extraterrestrial Messages: What Meteorites Can Tell Us About the Early Solar System

Jamie Elsila
Department of Chemistry
Stanford University
March 2002

How many movies have you watched where meteorites crash into Earth, creating untold disaster? While such catastrophes are products of the filmmakers' imaginations, it is true that Earth is hit by 40,000 tons of extraterrestrial material each year. That's the equivalent of 500 baseballs bombarding our planet every minute. But rather than heralding disaster, this material--most of it in small particles--arrives carrying some of the oldest information about our solar system. In my astrochemical research, I analyze meteorites to determine the presence and distribution of certain carbon-containing compounds. This research provides information about the creation and history of meteorites and will improve our understanding of the early solar system, letting us look back over 4 billion years to the very formation of the solar system.

Meteorites are chunks of larger parent bodies such as asteroids or planets. These parent bodies emerged as the solar system formed out of a large cloud of dust and gas called a planetary nebula. Small particles stuck together to form larger ones, and these larger grains gradually accumulated to form asteroids and planets like Earth. Different areas of the nebula were exposed to different conditions, including strong and rapid heating that melted the particles. Then, as the molten particles cooled, they formed spherical objects called chondrules. These became part of the forming parent bodies, where they were embedded in a non-heated material known as matrix. Meteorites today show a mix of matrix, chondrules, and other features reflecting different formation histories and conditions.

Meteorites also contain a variety of chemical compounds that were either present in the nebula during meteorite formation, or were later produced by reactions carried out within the meteorite itself. One important class of meteoritic compounds is the polycyclic aromatic hydrocarbons (PAHs). PAHs make up approximately 20% of all carbon in the galaxy. Because of their abundance, PAHs are frequently studied by scientists wishing to understand the distribution of extraterrestrial carbon. Knowing where PAHs are found gives us insight into the environments and conditions present in different extraterrestrial locations, and studying PAHs in meteorites tells us about the conditions present when the meteorites formed.

In my research, I determine the amount of PAHs contained in different types of meteoritic features, such as matrix and chondrules. This provides information about each feature's formation and history. I analyze PAHs with a technique/instrument called mL2MS. In mL2MS, one laser vaporizes a very small part of the material being analyzed. A second laser then selectively picks out the PAHs, allowing them to be detected by another part of the instrument. The mL2MS is very sensitive to PAHs and can analyze extremely small samples, which is important when working with rare meteoritic materials.

To analyze a meteorite, we first cut a sample about a quarter-inch across from a larger chunk. We photograph this piece at high magnification (120x) to produce a detailed picture of the surface of our small sample on which we can distinguish different features such as matrix (typically dark) and chondrules (typically white). Next, this piece goes inside the mL2MS, where a camera shows us the location of our laser on the sample's surface. Because the mL2MS gets complete PAH data from very small spots, we can take over a hundred shots in a straight line across the quarter-inch-wide sample, and repeat this for different areas of the sample. We then use the mL2MS data, which tells us about the presence and abundance of PAHs at each shot location, with the photograph to create a map that shows the PAH distribution on the sample's surface. We use this map to determine the amount of PAHs present in each meteoritic feature.

The first meteorites we've studied show some interesting results. PAHs are present in the matrix material, but are not detectable in the chondrules and other features. PAHs are also distributed fairly uniformly throughout most of the matrix, with higher amounts appearing as rims around some of the chondrules. We've examined several different PAHs; all show the same distribution, and the relative amounts of each PAH are the same in different parts of the sample. From this information, we infer that the PAHs were formed before becoming part of the meteorite, rather than being produced through reactions once the meteorite was created.

These results imply that a source of PAHs was present before or during solar system formation, giving us new information about that distant time. The meteorites bombarding Earth are bringing us messages from our earliest history and shedding light on a time long past.