Ancient Stardust Were Seeds for the Earliest Solids in the Solar System

How did the first solids that made up planets, moons, and asteroids form out of the "hot soup" that characterized our solar system's earliest years? New research from Caltech analyzing pieces of a meteorite has found that, in the same way that snowflakes crystallize around grains of dust, ancient stardust grains leftover from long-gone suns may have been the seeds of our solar system's earliest solids.

The research was conducted in the laboratory of Francois Tissot, professor of geochemistry and Heritage Medical Research Institute Investigator, and appears in the journal Science Advances on July 29.

In early 1969, just months before Apollo astronauts would return the first rock samples from the Moon, the Allende meteorite blazed through Earth's atmosphere and broke apart, scattering more than two tons of fragments over Mexico. Meteorites are like fossils from the early solar system, preserving a record of what our astronomical neighborhood looked like at the time of its formation 4.5 billion years ago. Allende is, to date, the largest primitive meteorite to be found on Earth and its many fragments contain a wealth of information that has helped scientists—such as Caltech researchers Gerald Wasserburg and Dimitri Papanastassiou (BS '65, PhD '70)—understand the formation and earliest history of the solar system.

In the 1980s, scientists at the University of Chicago studying similarly primitive meteorites discovered tiny nano-diamond grains that had chemical compositions totally different from anything in our solar system. These minerals, scientists concluded, were the remnants of an ancient star that existed and died before our own Sun was born.

"In general, the earth and meteorite samples are quite similar, with only very tiny chemical differences," says Ren Marquez (PhD '24), a former graduate student in Tissot's laboratory and first author on the study. "This led researchers to initially theorize that the early solar system was a giant homogenous soup of gases with everything formed out of the same starting materials. But detailed studies of these anomalous grains in primitive meteorites revealed signatures that are so wildly different that the only way to explain them is that they came from a different generation of stars that preceded our Sun. This was the first dramatic evidence that showed that the solar system may not be as homogeneous as we thought."

Since this discovery in the 1980s, scientists have uncovered a variety of other types of pre-solar grains. All these grains were found in one place: the carbon-rich matrix within primitive meteorites, regions which formed under cooler conditions. No pre-solar grains had ever been found in components formed in the solar system's hotter regions—until now.

After several years of developing new techniques with unprecedented precision, Marquez analyzed tiny fractions of the Allende meteorite that formed early in the solar system's history, when it was still quite hot. These components, called calcium–aluminum-rich inclusions or CAIs, were the very first solids to condense out of the hot environment of the early solar system. In a previous paper, Marquez and collaborators discovered signatures hinting that CAIs also contained pre-solar stardust.

The new study confirmed these earlier findings. Significantly, the work also shows that these pre-solar grains potentially shaped the makeup of the solar system material found alongside them. The researchers theorize that these fragments from earlier stars somehow survived intact through our own system's hot early years and acted as the nucleation points for the solar system's first formed solids. The grains, while not abundant, seem to have been a crucial structural substrate around which the rest of the CAI coalesced.

Many kinds of substances are known to precipitate through an initial seed, from proteins to magma to snowflakes.

"Nucleation is a very difficult process if there is no surface upon which to grow," Tissot explains. "Without the pre-solar dust grains disrupting an otherwise homogenous mix of gases, minerals should take a long time to condense as the solar system cooled. Pre-solar grains acting as seeds for this early condensation solves an otherwise unaddressed problem in cosmochemistry."

In future work, the researchers aim to determine the exact chemical compositions of the pre-solar grains.

Tissot emphasizes that fundamental science, such as this study, can also lead to unexpected benefits to society. The new techniques developed by Marquez are not only useful for studying fragments of ancient stardust but can also be applied to biomedical research for studying tiny samples of blood and tissue. Indeed, Tissot's lab is currently engaged in a project that is applying similar techniques to improve detection of osteoporosis.

"There can be a tension between funding fundamental research versus applied research with immediate obvious benefit, but we rarely know where the next most important technology comes from," Tissot says. "As my grandfather used to say, ‘We didn't discover electricity by studying the candle.'"

The paper is titled "Stardust as nucleation seeds for the earliest solids in the Solar System." In addition to Marquez and Tissot, Bruce Charlier of Victoria University of Wellington in New Zealand is a co-author. Funding was provided by NASA, a Packard Fellowship, Caltech, and the Royal Society Te Aparangi in New Zealand.