NEWS | Oct. 1, 2014

NRL Scientists Discover Out of this World Particles on NASA Stardust Collector

By Donna McKinney

U.S. Naval Research Laboratory (NRL) research physicist Dr. Rhonda Stroud and materials engineer Dr. Nabil Bassim are part of an international preliminary examination team, led by Dr. Andrew Westphal at the University of California at Berkeley, that has spent the last six years analyzing samples from the NASA Stardust Interstellar Dust Collector in order to identify captured interstellar particles. The team findings are summarized in an article in the August 15, 2014 issue of Science, and an accompanying series of papers in Meteoritics and Planetary Science.

This research effort, Dr. Stroud explains, is the first time particles from the local contemporary interstellar medium have been analyzed in the laboratory. It gives us new insight into the environment surrounding our solar system, she says, and the types of solid materials that are building blocks for the formation of new solar systems. A total of seven particles that most probably originated outside the solar system were identified in the aerogel and Al foil collection materials. The foils analysis team, led by Dr. Stroud, identified four tiny craters, a few thousandths of the width of a human hair in size, which contained residue consistent with the impact of interstellar particles.

The dust particles that Dr. Stroud and co-workers studied were captured by the Interstellar Dust Collector on NASA's Stardust spacecraft that launched in 1999, the first solid sample return mission since the Apollo era. For 195 days in 2000 and 2002, the spacecraft collected particles from the stream of dust flowing across the solar system from the local interstellar medium. The Stardust spacecraft returned to earth in 2006 and NASA then distributed the samples from both the cometary and interstellar dust collectors to scientists for further study.

The full team led by Dr. Westphal, analyzed both aerogel and foil collector samples. The researchers were able to distinguish the interstellar dust candidates from debris because of the particles' elemental composition and on the basis trajectory modeling.

Prior to this study of the Stardust samples, researchers derived their understanding of interstellar dust primarily from astronomical observations of the interstellar medium, including optical properties of the interstellar dust and remote spectroscopy of the gas composition, and from in situ measurements by the dust analyzers on the Cassini, Ulysses, and Galileo spacecraft. Researchers depended heavily on models for their understanding of the properties of the particles, including size distribution, density, and composition.

With these samples of the interstellar dust in hand, researchers can now make direct, laboratory measurements of these particles; this provides an independent test of the assumptions on which the interpretation of spectroscopy and in situ dust measurements rest. The research team is addressing important questions, including: Is there one dominant dust phase, and if so, what is its composition? Is the dominant structure crystalline or amorphous? Is iron present in metal, oxide, carbide, and/or sulfide phases? Are the particles dense or fluffy? Is there evidence for particle mantles of either organic or silicate-like composition?

At this point the team has determined that crystalline and multiple Fe-bearing phases are present in some of the particles. As a consequence, no single current model can explain all of the interstellar grains analyzed. Although the laboratory measurements and modeling indicate interstellar origin is most likely, an interplanetary origin for one or more of the particles is possible. Future work will include oxygen isotope measurements to look for a definitive signature of origin outside the solar system. Looking forward, Dr. Stroud plans to continue the search for more interstellar impact craters on the Stardust Al foils. There over 100 foils and many aerogel tiles that still need to be imaged and searched, so the discovery of additional interstellar particles is likely.

Dr. Stroud and her NRL colleague, Dr. Nabil Bassim, work in NRL's Materials Science and Technology Division, Materials and Sensors Branch.