Samples of interplanetary particles — collected from the upper atmosphere of Earth and believed to originate from comets — contain presolar dust from variable interstellar environments, a group of researchers from the University of Hawaii at Manoa and elsewhere has discovered.
The initial solids from which the Solar System formed consisted almost entirely of amorphous silicate, carbon and ices. This dust was mostly destroyed and reworked by processes that led to the formation of planets.
Surviving samples of presolar dust are most likely to be preserved in comets — small, cold bodies that formed in the outer Solar Nebula.
In a relatively obscure class of interplanetary dust particles believed to originate from comets, there are tiny glassy grains called glass embedded with metal and sulfides (GEMS) — typically only tens to hundreds of nanometers in diameter.
Using transmission electron microscopy, Dr. Hope Ishii and co-authors made maps of the element distributions and discovered that GEMS are made up of subgrains that aggregated together in a different environment and prior to the formation of the comet parent body.
This aggregate is encapsulated by carbon of a different type than the carbon that forms a matrix gluing together GEMS and other components of cometary dust.
The types of carbon that rims the subgrains and that forms the matrix in these particles decomposes with even weak heating, suggesting that GEMS could not have formed in the hot inner Solar Nebula, and instead formed in a cold, radiation-rich environment, such as the outer Solar Nebula or presolar molecular cloud.
“Our observations suggest that GEMS represent surviving presolar interstellar dust that formed the very building blocks of planets and stars,” Dr. Ishii said.
“If we have at our fingertips the starting materials of planet formation from 4.6 billion years ago, that is thrilling and makes possible a deeper understanding of the processes that formed and have since altered them.”
In the future, Dr. Ishii and co-authors plan to search the interiors of additional comet dust particles, especially those that were well-protected during their passage through the Earth’s atmosphere, to increase understanding of the distribution of carbon within GEMS and the size distributions of GEMS subgrains.
The findings were published this week online in the Proceedings of the National Academy of Sciences.
Hope A. Ishii et al. Multiple generations of grain aggregation in different environments preceded solar system body formation. PNAS, published online June 11, 2018; doi: 10.1073/pnas.1720167115