April 9, 2004 — For the first time, researchers have identified organic material in interplanetary dust particles (IDPs), gathered from the Earth's stratosphere, that was made before the birth of our Solar System. The material was identified on the basis of its carbon isotopic composition, which is different from the carbon found on Earth and in other parts of the Solar System. Isotopes are variations of elements that differ from each other in the number of neutrons they have, making them similar chemically but different physically.
Christine Floss, Ph.D., senior research scientist in Earth and Planetary Sciences and Physics at Washington University in St. Louis, said that the organic material in the IDP she and her colleagues analyzed probably was formed in molecular clouds in the interstellar medium before the formation of the Solar System. The isotopic anomalies are produced by chemical fractionation at the very low temperatures found in these molecular clouds.
"Our findings are proof that there is presolar organic material coming into the Solar System yet today," Floss said. "This material has been preserved for more than 4.5 billion years, which is the age of the Solar System. It's amazing that it has survived for so long."
The finding helps in understanding the Solar System's formation and the origin of organic matter on Earth. The work was published in the Feb. 27, 2004 issue of Science, and was supported by NASA grants.
Over the past 20 years, researchers have found isotopic anomalies in nitrogen and hydrogen from IDPs but never before in carbon. Floss said one of the reasons for this was the limitations of earlier instruments. She and her colleagues used a new type of ion microprobe called the NanoSIMS, which enables researchers to analyze particles at much greater spatial resolution and higher sensitivity than before.Until recently, ion probes could only measure the average properties of an IDP.
In 2000, with help from NASA and the National Science Foundation, the University bought the first commercially available NanoSIMS. Made by Cameca in Paris, the NanoSIMS can resolve particles as small as 100 nanometers in diameter. A hundred thousand such particles side-by-side would make a centimeter. Typical sub-grains in IDPs range from 100 nanometers to 500 nanometers.
"The question has always been: Why don't we see any unusual carbon isotopic compositions?" Floss said. "The thinking was if the nitrogen and hydrogen isotopic anomalies are formed in the same regions of space, it was logical to expect unusual carbon isotopic compositions as well. One school of thought was that there were different fractionation processes with carbon in opposite directions, that cancelled out any anomalies produced. Another possibility was that the nitrogen and hydrogen might have been produced in phases that weren't originally organic - that the organic material itself was formed in the solar system and basically inherited the hydrogen and nitrogen isotopic compositions from some precursor material. But our isotopic analysis shows that the organic material was formed before the Solar System existed and was later incorporated into the IDP." Floss and Frank Stadermann, Ph.D., Washington University senior research scientist in Physics, worked with colleagues at Lawrence Livermore National Laboratory in drawing their conclusions.
"A lot of IDPs come from comets," Floss said. "It makes sense that organic material would be preserved in a very cold environment, such as where comets form at the edge of the Solar System. For something to stay this pristine and primitive, one can assume that it came from that kind of environment."
Floss said it's estimated that, over a million years, about a centimeter of carbonaceous material comes in the form of such cosmic dust and a significant amount of that material may be presolar in origin.
Floss said that her work builds on the pioneering work of the late Robert Walker, Ph.D., professor of Physics at Washington University. Walker was instrumental in the acquisition of the NanoSIMS and in the 1980s made landmark studies verifying the extraterrestrial origin of such stratospheric dust particles.
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