Cosmochemistry

Sifting Stardust

Whether it's comet particles brought back by the Stardust probe or tiny fragments of flint 3.5 billion years old, in the Extraterrestrial Materials Research Laboratory (LEME),(1) dust is a hot research topic. Using NanoSIMS, an extremely high-performance ion microprobe, scientists are able to shed more light on the dust's past, thus unraveling the mysteries of the universe.

 

The eighteenth century architecture of the National Natural History Museum (MNHN) in Paris might evoke the sumptuousness of former explorers and dust-covered insect collections, but don't be fooled. Behind these walls, one of the most advanced scientific instruments has just been installed: the NanoSIMS 50. This ion microprobe can reveal the chemical and isotopic composition of particles of microscopic dust, whether of terrestrial or extraterrestrial origin. How does it work? “A beam bombards several atoms on the surface, which are further pulverized and analyzed. A map can then be outlined of regions rich in carbon, nitrogen, and oxygen over dimensions of 50 nanometers, in other words barely the size of several molecules,” explains François Robert, director of LEME. This giant contraption will stand beside another instrument, a mass spectrometer, capable of revealing the exact quantity of each element present in the sample. “We had to take it apart to fit in the NanoSIMS, but it will soon be back in action,” promises Smaïn Mostefaoui, research engineer at the Museum. Here, one can arrive with a microscopic grain of material and leave with the exact knowledge of what it contains in terms of chemical and isotopic elements, and thereby deduce its origin.

 

The Rich Trawl from Stardust

Long-awaited and coveted by teams of researchers throughout the world, microscopic particles from the tail of the Wild 2 Comet are finally being scrutinized by this instrument. Brought back to Earth in January 2006 by NASA's Stardust probe, it is part of a real treasure: a thousand or so grains larger than 10 micrometers as well as tens of thousands of smaller particles (all from the comet's tail), in addition to around a hundred interstellar grains, normally found in the space between stars and which sometimes penetrate the solar system.

This trawl of dust was collected and trapped in an extremely porous gel known as “aerogel,” when the space probe passed through the comet's tail. These mythical celestial bodies are in fact conglomerates of ice, dust, and rocks, which evaporate when they come close to the Sun and eject a diffuse atmosphere trail known as a “coma” or “tail.” The Stardust probe passed within 500 kilometers of the comet's nucleus, allowing it to sweep up the precious grains of dust which have since been shared with a few hand-picked laboratories (including seven with CNRS). Apart from the Nano-analysis Laboratory, which will perform isotopic mapping of ten or so grains, CNRS' Petrographic and Geochemical Research Center (CRPG)² in the city of Nancy, will study the isotopic composition of the oxygen in the cometary grains by means of an ion microprobe. The CRPG will also be able to determine the quantity of nitrogen and noble gases present in the grains using a sophisticated technique named “laser extraction and static mass spectrometry.” Volatile elements such as hydrogen, nitrogen, oxygen, and carbon are systematically associated in extraterrestrial matter. They are key elements for understanding the origins of both water and life on our planet and thus determining the exact role played by comets.

But in addition to this precious “trawl,” the study of the “net” itself will also provide a wealth of information: At the Space Astrophysics Institute (IAS)³ in Orsay and at the Solid State Structure and Properties Laboratory (LSPES)⁴ in Lille, the aluminum frame that held the aerogel in place will also be intensely scrutinized. Cometary grains smashed into this structure, causing small impact craters. By calculating their number and diameter, researchers are closer to understanding the size of the floating dust dragged behind these comets. Finally, to avoid losing even a tiny fragment of these precious particles, scientists will be carefully recovering dust residue at the base of the impact craters. With the University of Lille's scanning electron microscope, this initial diagnosis will, in a very short time, provide an overview of the grains' chemical composition. “Over the next few months we are going to focus on studying these grains,” explains Robert. “Then, we'll send another application to NASA for further grains. A large chunk of the history of the solar system is going to be revealed by this research.”

 

 

The Museum's Treasures

These extraterrestrial fragments are not the first ones to have found their way into the museum: Its collection of one thousand or so meteorites is one of the most impressive in the world. “Obviously, these rocks have been studied very closely, but NanoSIMS will reveal details that until now were unattainable.” For example, the famous Murchison meteorite, which fell in Australia in 1969, has been the subject of much study and has been found to contain amino acids. However, in the organic matter extracted from this meteorite, Robert and his coworkers Jean Duprat of the Nuclear Spectrometry and Mass Spectrometry Center (CSNSM)⁵ in Orsay and Jérôme Aléon of the CRPG made an interesting discovery using the CRPG's ion probe.⁶ Although the great majority of meteorites have an isotopic composition similar to that of the Sun,⁷ the grains of silicon dioxide mixed with the organic matter from the Murchison meteorite have 100 times more 17O and 18O. In order to explain this anomaly, the researchers have proposed a specific mechanism, based on the irradiation and spallation⁸ caused by the intense activity of the Sun in its infancy.

Other jewels in the collection are chondrites, which Brigitte Zanda (assistant professor at the Museum) is studying in the laboratory. These small beads included in meteorites, rich in silicates, magnesium, and iron, are the first solid objects from the solar system. And, last but not least, there is another rare specimen in the collection: the Bencubbin meteorite. As Claude Perron, CNRS researcher, explains, “it was formed from materials that have been pulverized and then re-condensed. Studying it will make it possible to reconstitute several violent episodes that took place during the infancy of the solar system.”

 

The Earth's Past

While waiting for other missions (see box) to collect cosmic grains, some terrestrial rocks could unveil an important slice of the history of the solar system: the Earth's past, its climates, and the circumstances that presided over the appearance of life. For instance, the study of organic matter contained within 3.5 billion-year-old fragments of flint has revealed one of the oldest traces of life. “With Sylvie Derenne of the ENSCP,⁹ we have identified a 'biosignature,' a long carbon chain from the alkane family, proof of the existence of life,” explains Robert. In fact, the cellular membrane is characterized by an enzymatic process that favors chains constituted of an uneven number of carbon atoms over those comprising an even number. This biosignature will be used while searching for life in terrestrial samples. The NanoSIMS could cast new light on these questions. It is now widely accepted and proven that the measurement of chemical elements and isotopes present in a minuscule grain can reveal much about the history of the cosmos. “Twenty years ago, when Marc Chaussido (CRPG) and I first thought of these methods, cosmochemistry was underrepresented in France,” remembers Robert. “No one would have bet on it. Today, there are more than fifteen laboratories working in this field and they have provided proof of its relevance and diversity of applications. In the past twenty years, France has risen to second place in the world for research in this field.”

 

Azar Khalatbari

 

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