Researchers follow the journey of a speck of dust through the newborn solar system



By Daniel Stolte, University of Arizona

June 21, 2021

Artistic illustration of the early solar system at a time when planets had not yet formed. A swirling cloud of gas and dust surrounded the young sun. The section through this so-called protoplanetary disk shows its three-dimensional structure.
Heather Roper

A team of researchers led by the University of Arizona has reconstructed the story of a speck of dust that formed when the solar system was born more than 4.5 billion years ago in unprecedented detail. The results provide insights into the fundamental processes that underlie the formation of planetary systems, many of which are still mysterious.

For the study, the team developed a new type of scaffolding that combined quantum mechanics and thermodynamics to simulate the conditions the grain was exposed to during its formation when the solar system was a swirling disk of gas and dust known as a protoplanetary Disc or solar nebula. Comparing the predictions of the model with an extremely detailed analysis of the chemical composition and crystal structure of the sample, as well as a model of the transport of matter in the solar nebula, gave indications of the grain’s journey and the environmental conditions that shaped it on its way.

The grain analyzed in the study is one of several inclusions known as calcium-aluminum-rich inclusions, or CAIs, that were discovered in a sample from the Allende meteorite that fell over the Mexican state of Chihuahua in 1969. CAIs are of particular interest because they were among the first solids to form in the solar system more than 4.5 billion years ago.

Allende meteorite

This piece of the Allende meteorite shows the typical crust of material that melted when it entered the earth’s atmosphere. The grain examined in this study was taken from a similar piece and from the depth of the specimen, where little, if any, changes would occur during the meteorite fall.
H. Raab / Wikimedia Commons

Much like stamps in a passport tell a story about a traveller’s journey and the stops along the way, the micro- and atomic-scale structures of the samples open up a record of their genesis controlled by the collective environment to which they were exposed.

“As far as we know, our paper is the first to tell a genesis that provides clues about the likely processes that have taken place on the astronomical distance scale with what we see in our sample on the atomic distance scale,” said Tom Zega, Professor at the University of Arizona Lunar and Planetary Laboratory and lead author of the article published in The Planetary Science Journal.

Zega and his team analyzed the composition of the inclusions embedded in the meteorite using state-of-the-art scanning transmission electron microscopes with atomic resolution – one in the Kuiper Materials Imaging and Characterization Facility of UArizona and its sister microscope in the Hitachi factory in Hitachinaka, Japan.

The inclusions were found to be composed primarily of mineral species known as spinel and perovskite, which are also found in rocks on Earth and are being studied as candidate materials for applications such as microelectronics and photovoltaics.

Similar types of solids are found in other types of meteorites known as carbonaceous chondrites, which are of particular interest to planetary scientists because they are known to be leftovers from the formation of the solar system and contain organic molecules, including those that provided the raw materials could for life.

A section through an Allende meteorite

A section through an Allende meteorite reveals various spherical particles known as chondrons. The irregularly shaped “island” to the left of center is a calcium-aluminum-rich inclusion, or CAI. The grain in this study was isolated from such a CAI.
Shiny Things / Wikimedia Commons

The precise analysis of the spatial arrangement of the atoms enabled the team to examine the structure of the underlying crystal structures in great detail. To the team’s surprise, some of the results contradicted current theories about the physical processes believed to be active in protoplanetary disks, causing them to dig deeper.

“Our challenge is that we don’t know which chemical pathways led to the origins of these inclusions,” said Zega. “Nature is our laboratory beaker, and this experiment took place billions of years before we existed in a completely alien environment.”

Zega said the team set out to “reverse engineer” the composition of the alien samples by designing new models that simulate complex chemical processes that the samples would undergo in a protoplanetary disk.

“Such models require a close convergence of expertise from the fields of planetary science, materials science, mineral science and microscopy, which we set out to do,” added Krishna Muralidharan, Co-author of the study and Associate Professor in the United States Department of Materials Science and Engineering.

Based on the data the authors were able to take from their samples, they concluded that the particle that formed in a region of the protoplanetary disk not far from today’s Earth then made a journey closer to the sun, where it always was got hotter. only to turn back later and continue to wash away from the young sun in cooler places. Eventually it was built into an asteroid, which later broke into pieces. Some of these pieces were caught by Earth’s gravity and fell as meteorites.

The samples for this study were taken from the inside of a meteorite and are considered primitive – that is, unaffected by environmental influences. It is believed that this primitive material has not undergone significant changes since it was formed more than 4.5 billion years ago, which is rare. It remains to be seen whether similar objects will occur in the Bennu asteroid, from which samples will be returned to Earth by the UArizona-led OSIRIS-REx mission in 2023. Until then, scientists rely on samples that fall to earth via meteorites.

Illustration of the dynamic history that the modeled particle may have experienced during the formation of the solar system

Illustration of the dynamic history that the modeled particle may have experienced during the formation of the solar system. The analysis of the micro- and atomic structures of the particle and their combination with new models that simulated complex chemical processes in the disk revealed its possible journey over many orbits around the sun (callout box and diagram on the right). Starting from the formation of the earth, the grain was transported to the inner, hotter regions and finally washed up in cooler regions.
Heather Roper / Zega et al.

“This material is our only record of what happened in the solar nebula 4.567 billion years ago,” said Venkat manga, Article co-author and Assistant Research Professor in the Arizona Department of Materials Science and Engineering. “Being able to look at the microstructure of our sample on different scales down to the length of individual atoms is like opening a book.”

The authors said studies like this could bring planetary researchers one step closer to a “grand model of planetary formation” – a detailed understanding of the material that moves around the disk, what it is made of, and how it leads to the sun and the planets.

Powerful radio telescopes such as the Atacama Large Millimeter / submillimeter Array or ALMA in Chile now enable astronomers to observe star systems as they develop, said Zega.

“Maybe at some point we can take a look at the evolving hard drives, then we can really compare our data across disciplines and answer some of these really big questions,” Zega said. “Do these dust particles form where we suspect them to be in our own solar system? Are they common to all star systems? Should we expect the pattern we see in our solar system – rocky planets near the central star and gas giants further out – in all systems?

“It’s a really interesting time to be a scientist when these areas are developing so quickly,” he added. “And it’s great to be at an institution where researchers can create transdisciplinary collaborations between leading astronomy, planetary and materials science from the same university.”

The study was jointly conducted by Fred Ciesla of the University of Chicago and Keitaro Watanabe and Hiromi Inada, both with the Nano-Technology Solution Business Group at Hitachi High-Technologies Corp. in Japan.

Funding came from NASA’s Emerging Worlds Program; NASA Origins Program; and NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network, sponsored by NASA’s Science Mission Directorate. NASA and the National Science Foundation provided funding for the instrumentation in the LPL’s Kuiper Materials Imaging and Characterization Facility.



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