Interstellar dust pre-dates solar system


Rebecca Pool

Wednesday, June 13, 2018 - 16:00
EDS map of glassy grains (blue) in a carbonaceous matrix, inside a interplanetary dust particle. [Captured with a FEI TitanX Berkeley Lab’s Molecular Foundry]
US-based researchers have confirmed that samples of interplanetary particles, believed to originate from comets, contain dust leftover from the initial formation of the solar system.
Using electron microscopy, secondary ion mass spectroscopy and Fourier Transform infared spectroscopy, Professor Hope Ishii from the University of Hawaii at Manoa and colleagues, characterised the particles' chemical composition to single nanometre resolution.
According to Ishii, TEM maps of element distribution revealed new detail on the α-silicate-rich 'GEMS' grains - glass embedded with metals and suphides - that form the dust particles.
Analyses indicated these grains are made up of subgrains that could only have aggregated in a different environment and prior to the formation of the comet parent body.
As Ishii points out, the subgrains were bound together by dense organic carbon.
The GEMS grains were then later 'glued together' with other cometary dust components but this time, within a distinct, lower-density organic carbon matrix.
This cometary-type interplanetary dust particle, L2005AR7, was collected by NASA stratospheric aircraft. Its porous aggregate structure is evident in this scanning electron microscope image. [Hope Ishii, University of Hawai'i]
Ishii and colleagues reckon this carbon decomposes with weak heating, suggesting that the GEMS could not have formed in the hot inner solar nebula.
Instead, the glassy grains would have formed in a cold, radiation-rich environment, such as the outer solar nebula or pre-solar molecular cloud. 
As Ishii's colleague, Jim Ciston from Berkeley Lab, highlights: “The presence of specific types of organic carbon in both the inner and outer regions of the particles suggests the formation process occurred entirely at low temperatures."
“Therefore, these interplanetary dust particles survived from the time before formation of the planetary bodies in the solar system, and provide insight into the chemistry of those ancient building blocks.,” he adds.
“Our observations suggest that these exotic grains represent surviving pre-solar interstellar dust that formed the very building blocks of planets and stars," says Ishii. "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.”
Research is published in PNAS.
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