Dust


A synopsis by P.H. Benoit

Interplanetary dust particles (or IDPs in common useage) are extremely small (0.001 cm in diameter) grains or particles found on Earth that are most probably from outer space. Their small size poses a problem for the most common chemical and petrographic analytical techniques and thus research into IDPs is marked by the application of new analytical procedures and new technologies.

While IDPs 'fall' everywhere in the world, near the Earth's surface they are mixed with the much more common ash, soot, and terrestrial dust created by humans and by natural processes. Recognition and separation of IDPs from surface samples is a long and tedious process and fraught with uncertainty. In fact, many of the 'IDPs' collected and described at the start of the space age were terrestrial contaminants. In the 1970's, it was found that there was less contamination in the stratosphere (about 20 km above the Earth's surface). Numerous IDP's have since been collected either by high altitude ballons, or by aircraft-borne experiments. Many of the IDPs in the current collection were collected using boxes of silicone gel mounted on 'spy' planes. NASA curates one of the most comprehensive collections in the world at its Johnson Space Center facility.

There is now little question that these dust particles are 'space dust'. Their chemical compositions are strongly suggestive of extraterrestrial origin, and they have high abundances of helium, a major component of the solar wind. They also have high densities of fossil nuclear tracks, produced as heavy atoms in solar flares penetrated the mineral grains.

While their extraterrestrial origin has gained general acceptance, their relationship to meteorites, our only other extraterrestrial samples outside of lunar samples, is still very much under discussion. While some IDPs do seem to match with known meteorite classes, most seem to differ in crystal chemistry and petrography. This has lead to the suggestion that IDPs may represent material even more primitive than meteorites, most of which appear to have been subject to alteration on asteroidal parent bodies.

The ultimate source of IDPs is uncertain and a matter of considerable debate. The most likely sources are thought to be comets or asteroids. It has, however, been suggested that at least some IDPs came from other solar systems. Observations made by the Ulysses spacecraft within the last couple years lend strong support to this idea.

Research into IDPs is an active, and sometimes contentious field, as scientists attempt to probe the very earliest history of our own solar system (and perhaps the history of other solar systems). One particular study of interest is that of Society member Don Brownlee. He has proposed sending a spacecraft beyond the region of Earth's influence to collect a sample of IDPs unbiased by terrestrial contaminants or by the selection effects of Earth's atmosphere, and finally returning to Earth with the sample for analysis. His STARDUST spacecraft was selected as a Discovery mission and will be launched in February 1999 on its seven year mission, which will also sample debris from the track of comet Wild 2. The idea that some IDPs may be from other solar systems will also be explored by upcoming spacecraft missions. The STARDUST mission may collect some of these 'foreign' particles and return them to Earth and the upcoming Cassini mission, although it will not bring any samples back to Earth, will carry special instruments to detect these unusual bits of dust and even conduct compositional analysis on them.

References. As a rapidly evolving topic, there are numerous papers and a few books that describe IDPs and research into their origin. A couple of older, but still very interesting references are: Brownlee, D.E. (1985) Cosmic dust: Collection and research. Annual Review of Earth and Planetary Science, volume 13, pages 147-173. Stanford, S.A. (1987) The collection and analysis of extraterrestrial dust particles. Fundamentals of Cosmic Physics, volume 12, pages 1-73. A more recent review of possible materials from other solar systems reaching Earth is: Anders E. and Zinner E. (1995) Interstellar grains in primitive meteorites - Diamond, silicon carbide, and graphite. Meteoritics, volume 28, pages 490-514.

Photo. The Great Nebula in Orion - The Orion Molecular Cloud is approximately 500 parcsecs from Earth and contains about 500 solar masses of material in the form of gas, dust, and ices. From the cover of the July, 1996, issue of Meteoritics and Planetary Science.