French scientists said they have reproduced the structure of primitive interplanetary dust in the laboratory. Reporting in an upcoming issue of Astronomy & Astrophysics, the team said they have produced a compound called GEMS – for glass with embedded metal and sulfides – a major component of interplanetary dust.
Understanding the origins of the particles – samples of which recently were returned to Earth by NASA's Stardust spacecraft – is considered one of the major objectives of planetary science.
Team leader C. Davoisne and colleagues attempted to reproduce GEMS, which are only a few nanometers (billionths of a meter) in size and composed of a silicate glass that includes small, rounded grains of iron/nickel and metal sulfide. A small fraction of GEMS (less than 5 percent) floating in interplanetary space are thought to have formed before the solar system, and therefore are interstellar in origin. This mix of GEMS types has made it difficult, however, for scientists to sort out their origin and formation process.
Team members at two French laboratories – the Laboratoire de Structure et Propriétés de l'Etat Solide in Lille and the Institut d'Astrophysique Spatiale in Orsay – heated various amorphous samples of olivine – which is composed of manganese, iron oxide and silicon – under high vacuum and at temperatures ranging from 500 degrees to 750 degrees Celsius. After the heating, samples showed microstructures that closely resemble GEMS, with rounded iron nanograins embedded in a silicate glass.
The team said this is the first time a GEMS-like structure has been reproduced in the lab. Testing results show that the iron-oxide component undergoes a chemical reaction known as reduction, in which the iron gains electrons and releases its oxygen. Therefore the heated samples contained practically no iron in the silicate glass.
This is why dust observed around evolved stars and in comets is composed mainly of magnesium-rich silicates and apparently lacking in iron, the team said, because iron in metallic spherules becomes totally undetectable by the usual remote spectroscopic techniques. They added that the work could provide an important insight into the composition of interstellar grains as well.
The chemical process that produces GEMS could be quite common, both in the solar system and around other stars. The scientists said they are now awaiting the analysis of the grains collected by Stardust to find out whether some GEMS truly originated in the interstellar medium.
