What if we could determine what material built our solar system, how old it is, and even the type of star it came from? Emilie Dunham, postdoctoral researcher in the Nuclear and Chemical Sciences Division at Lawrence Livermore National Laboratory (LLNL), is exploring the answers to these very questions, all while following a childhood passion.
Dunham, a 2023 Lawrence Fellow, became interested in space at a young age when her father would wake her for stargazing in the middle of the night to watch lunar eclipses. Her interest continued through middle and high school, inspiring her choice to study astronomy at Case Western Reserve University. In her junior year, she took a class on meteorites, which piqued her interest and charted a new path for her future: a graduate education focused on meteorites.
During her Ph.D. research at Arizona State University, Dunham was thrust right into the world of meteorites as she began studying some of the first solids that formed in the solar system. At over 4.5 billion years old, these materials provide insight about what was happening when the Sun "turned on" and planets began forming.
While Dunham recalls a steep learning curve due to the broad knowledge base required to study meteorites, she adapted quickly and never looked back.
"Meteorites are at this intersection of astronomy, geology, chemistry and physics, so you have to learn a bit of everything, which I really enjoy," Dunham said.
After completing her Ph.D., Dunham ventured on a month-long meteorite hunting expedition in Antarctica as part of Case Western's Antarctic Search for Meteorites team. She returned to the U.S. to continue studying early solids as a 51 Pegasi b postdoctoral fellow at the University of California, Los Angeles.
The experiences she gained throughout her Ph.D. work and postdoctoral fellowship helped Dunham prepare for her current project at LLNL, where she is exploring a component in meteorites called "presolar grains."
Also referred to as stardust, presolar grains were made by stars before the formation of our sun and solar system. Near the end of their lives, some stars eject much of their material in intense pulses of hot gas, which eventually cools and allows dust grains to condense. The dust carries the elements and isotopes (atoms with unusual numbers of neutrons) that were forged in the star during its lifetime. Therefore, dust grains offer a glimpse into their parent stars and provide information about the material that made the solar system - and us.
Following their formation, presolar grains floated in space for a long time before eventually becoming part of the dust and gas that formed our solar system. Planets, comets and asteroids formed out of this dust and gas in the same way you make a snowman, starting with a small clump of dust grains (or snowflakes) that grows as it collects more material. While most presolar grains were destroyed during this process, a precious few are still found today in primitive meteorites - chunks of broken asteroids or planets that end up on Earth - and are key to understanding the types and ages of the stars that our solar system is recycled from.
To study these rare grains, Dunham first separates them from the rest of the meteorite. Conveniently, presolar grains are acid-resistant, so dissolving the meteorite material with acid leaves behind stardust. Less conveniently, the process is painstaking and requires months of careful work in a clean laboratory.
"People like to call it 'burning down the haystack to find the needle,'" Dunham said, "but the needles are still really hard to find."
Next, the isotopic fingerprint of the grain's parent star can be determined using mass spectrometry. Nanoscale secondary ion mass spectrometry, a rare instrumental capability that Lawrence Livermore possesses, allows for the measurement of lighter elements, including oxygen, silicon and carbon, without destroying the presolar grains. The grains are usually less than ten microns in size, or around 10 times smaller than the width of a human hair.
This step tells Dunham what type of grain she's working with, providing insights into what type of star it came from, such as a supernova - which is the result of a cataclysmic explosion at the end of a massive star's life - or an asymptotic giant branch star, which started with a mass closer to that of our Sun's and gradually shed its outer layers over time.
Resonant ionization mass spectrometry, practiced only at LLNL and a few other laboratories around the world, can then be used to measure the heavier elements in the sample - such as zirconium, titanium and molybdenum - offering a glimpse into how atoms were created inside the parent stars.
Isotopic signatures can also tell Dunham the ages of presolar grains. While they float in space, and before they coalesce into solar system bodies, the grains are bombarded by high-energy particles, which create specific noble gas isotopes within the grains. Higher levels of these isotopes indicate that the presolar grains floated in space for a longer time.
"Only a small number of presolar grains have been measured for their age, and a few of those measured formed more than a billion years before the solar system, making them least 5.5 billion years old," Dunham said. "These are literally the oldest objects that humans have ever held in their hands."
Dunham is part of a tiny group of researchers pursuing answers about stardust. "There are only about 30 people in the world studying these types of grains, which makes it a really tight-knit community." She says the support and assistance offered by this small community, as well as from her colleagues here at LLNL, has been crucial as she navigates such difficult research.
To bring awareness to this field, Dunham finds it extremely rewarding to serve as a mentor and plans to host an LLNL summer student this year. She is also a pen pal with a student in California through the Letters to a Pre-Scientist program, where she offers insights about her experience as a scientist.
Through her research and outreach activities, Dunham is making strides toward understanding how our solar system formed and showing the next generation of scientists that they, too, can shoot for the stars.
-Lilly Ackerman
