A Grain of Dust’s Journey Through Newborn Solar System

A research team led by the University of Arizona has reconstructed in unprecedented detail the history of a dust grain that formed during the birth of the solar system more than 4.5 billion years ago.

How UArizona tracks this grain of dust

The findings provide insights into the founding processes that underly the formation of planetary systems, many of which are still covered in mystery. For this study, the team developed a new framework that combines quantum mechanics and thermodynamics, to simulate the conditions to which the dust grain was exposed during its formation.

This exposure consists of when the solar system was a mass of spinning gas and dust known as a protoplanetary disk or solar nebula. Comparing the predictions from the model to a very sophisticated analysis of the sample’s chemical makeup and crystal structure, along with a model of how the matter was transported in the solar nebula, revealed clues about the grain’s journey and the environmental conditions that shaped it along the way.

The grain analysed in this study is one of several inclusions, known as calcium-aluminum rich inclusions, or CAIs, discovered in a sample from the Allende meteorite, which fell over Chihuahua, Mexico in 1969. CAIs are notable as they are believed to be some of the first solids that formed in the solar system more than 4.5 billion years ago.

These different samples’ micro and atomic-scale structures reveal a record of their formation histories, which were controlled by the collective environments to which they were exposed. Tom Zega, a professor in the University of Arizona’s Lunar and Planetary Laboratory and the first author of the paper, published in the Planetary Science Journal, spoke on this matter.

He said, “As far as we know, our paper is the first to tell an origin story that offers clues about the likely processes that happened at the scale of astronomical distances with what we see in our sample at the scale of atomic distances”. Zega and his team analysed the composition of the inclusion within the meteorite using atomic-resolution scanning transmission electron microscopes.

One at UArizona’s Kuiper Materials Imaging and Characterisation Facility, and its sister microscope located at the Hitachi factory in Hitachinaka, Japan. The inclusions consisted mainly of types of minerals known as spinel and perovskite, also found in rocks on Earth and are being studied as candidate materials for applications such as microelectronics and photovoltaics.

Similar kinds of solids occur in other types of meteorites known as carbonaceous chondrites, which are particularly interesting to planetary scientists as they are known to be leftovers from the formation of the solar system and contain organic molecules, including those that may have provided the raw materials for life. Analysing carefully the spatial arrangement of atoms allowed the team to study the makeup of the underlying crystal structures in great detail.

Some of the results were at odds with current theories on the physical processes believed to be active inside protoplanetary disks, inciting a deeper dive. Zega said “Our challenge is that we don’t know what chemical pathways led to the origins of these inclusions”, he also said, “Nature is our lab beaker, and that experiment took place billions of years before we existed, in a completely alien environment.”

He also said the team is trying to reverse engineer the makeup of these samples by designing new models that simulated complex chemical processes, which the sample would be subjected to inside a protoplanetary disk. Krishna Muralidharan, study co-author said, “Such models require an intimate convergence of expertise spanning the fields of planetary science, materials science, mineral science and microscopy, which was what we set out to do”.

Based on the data the authors were able to tease from their samples, they concluded that the particle formed in a region of the protoplanetary disk not far from where Earth is now then made a journey closer to the sun, where it was progressively hotter, only to later reverse course and wash up in cooler parts farther from the young sun. Eventually, it was incorporated into an asteroid, which later broke apart into pieces. Some of those pieces were captured by Earth’s gravity and fell as meteorites.

This study’s samples were taken from inside of a meteorite and are considered primitive, or unaffected by environmental influences. Such primitive material is believed to not have undergone any significant changes since it first formed more than 4.5 billion years ago, which is rare.

Whether similar objects occur in asteroid Bennu, samples of which will be returned to Earth by the UArizona-led OSIRIS-REx mission in 2023, remains to be seen, for now, scientists rely on samples that fall to Earth via meteorites. Venkat Manga, co-author of the paper said “This material is our only record of what happened 4.567 billion years ago in the solar nebula”, they also said, “Being able to look at the microstructure of our sample at different scales, down to the length of individual atoms, is like opening a book.”

Zega said also, “Perhaps at some point we can peer into evolving disks, and then we can really compare our data between disciplines and begin answering some of those really big questions”, he also said “Are these dust particles forming where we think they did in our own solar system? Are they common to all stellar systems? Should we expect the pattern we see in our solar system – rocky planets close to the central star and gas giants farther out in all systems?”.

Finally, he said “It’s a really interesting time to be a scientist when these fields are evolving so rapidly,” he added. “And it’s awesome to be at an institution where researchers can form transdisciplinary collaborations among leading astronomy, planetary, and materials science departments at the same university.”