Scattered throughout the Solar System, asteroids are much more than clumps of rocky debris. To a trained eye, they are akin to archaeological artifacts. Obtaining and analyzing even a few milligrams of asteroid material provides scientists a glimpse into the series of events that transformed the Solar System from a disk of gas and dust into the planets and moons observed today. The scientists who conduct this analysis feature cosmochemists, who use a host of laboratory methods, chemical and isotopic data, and statistics to piece together billions of years of Solar System evolution.
Lawrence Livermore National Laboratory is one of a select group of institutions that houses, handles, and analyzes these relics of the Solar System’s distant past. Insights from Livermore’s research are advancing our understanding of the Solar System’s transformation. “Scientists can obtain incredible telescope images of objects in the Solar System and beyond, but until we were able to bring a piece of one into the Laboratory, the information we could glean about their composition and formation was limited,” says staff scientist Greg Brennecka. “With the ability to analyze physical samples directly from space, we can better address questions about the history of the Solar System, the formation of Earth, and even the origin of life—questions that people have been asking for millennia.”
Paradigm Shift
Studying extraterrestrial material such as asteroids has long been challenging because samples were limited to the chance occurrence of meteorite impacts. Moreover, once a meteorite reaches the ground, its physical and chemical characteristics are different from when it was an asteroid in space. When a meteorite descends to Earth’s surface, it passes through the atmosphere at tens of thousands of kilometers per hour, faster than any returning space capsule, and its exterior is baked to thousands of degrees. Only a fraction of the original mass survives to hit the ground, at which point water and biological materials from Earth’s surface will farther alter the meteorite.
Space exploration has changed the paradigm for studying extraterrestrial fragments. Whether through historic crewed lunar missions—as NASA conducted during the Apollo Program—or now by sending uncrewed space missions, researchers have greater access to extraterrestrial materials, including asteroids, than ever before. In recent asteroid–return missions, uncrewed spacecraft outfitted with sophisticated flight controls, instruments, and robotics intercepted small asteroids, collected material samples, and returned them to Earth for study. Retrieving asteroid samples from space is beneficial because it circumvents the contamination and thermal damage that meteorites face when they impact Earth. “These asteroid materials have been preserved in outer space for over 4.5 billion years, providing a time capsule for researchers to look into the history of the Solar System,” says Brennecka. The curation team from the Astromaterials Research and Exploration Science Division at NASA’s Johnson Space Center in Houston, Texas, went through a careful process to maintain the pristine nature of the samples for distribution to researchers.
Scientists can use physical samples to more accurately determine three vital qualities of extraterrestrial materials: the sources of formation, processes of change, and rate of change over time. Livermore has in-house access to portions of the Ryugu asteroid, obtained by the Japan Aerospace Exploration Agency’s Hayabusa2 mission, and the Bennu asteroid, obtained by NASA’s OSIRIS-REx mission. Out of more than a million known asteroids in the Solar System, researchers targeted Ryugu and Bennu because the two are relatively accessible from Earth and just massive enough to intercept in space to obtain material samples. Even more important, analyzing the asteroids’ spectra—the wavelengths of light reflected off the asteroids’ surfaces which function as chemical fingerprints—indicated that they are carbon-rich bodies whose composition and mineral structures have changed little since the Solar System’s formation. Thus, they could harbor information necessary to improve understanding of the Solar System’s earliest stages. “More than 75,000 meteorite samples exist in collections around the world, but only six of them resemble the elemental makeup of the Sun as Bennu and Ryugu do. Just as scientists are still analyzing lunar rocks from the Apollo missions, globally, we will be able to conduct great scientific research on these asteroid materials for decades to come,” says staff scientist Quinn Shollenberger.
Analyzing Chemical Fingerprints
Samples from Bennu and Ryugu are not the first materials from outer space Livermore researchers have studied. The Laboratory’s analysis of extraterrestrial samples began shortly after NASA’s 1969 Apollo 11 mission returned pieces of the Moon, enabling researchers to begin unpacking the history of the Earth–Moon system. These efforts required mass spectrometry, an analytical process for quantifying the chemical elements, isotopes, and molecular structures present in a material. At the time, Livermore possessed the nation’s state-of-the-art mass spectrometry capabilities, and nuclear security needs gave rise to radiochemistry facilities, concentrated analytical expertise, and established process controls. Livermore scientists analyzed materials from nuclear tests for abundance ratios of isotopes, which reflected weapons performance and informed design changes. This foundation led the Laboratory to become a premier site for collecting data on nuclear half-lives and reaction cross-sections that cosmochemists use to understand the Solar System.
Just as isotope analysis characterizes nuclear test materials, similar scientific methods help cosmochemists unravel the Solar System’s history. Early in the process, scientists use mass spectrometry to measure how much of the elements on the periodic table are present in the sample. They then compare these measurements to cataloged meteorites to identify the general type of primordial from which it originated. Planetary bodies that coalesced close to the Sun rarely contain much water (which would have vaporized from intense solar radiation), while bodies formed further from the Sun typically contain more ice, volatile compounds (those that readily evaporate), and complex organic compounds due to the region’s thermal stability. Based on compositional data, researchers suspected that even though the OSIRIS-REx spacecraft intercepted Bennu near Earth, the asteroid is a remnant of a body that formed in the outer Solar System because the sample contained water, ammonia, and other volatile compounds.
Livermore uses isotope ratios to further narrow down a sample’s origins. By comparing the abundance of specific isotopes in an asteroid sample to those in a reference material (usually a terrestrial sample or a meteorite), scientists uncover additional information to estimate where and when the asteroid formed. “Isotope ratios tell us about the mixing and separation of mass in the early Solar System. Slight isotopic differences exist on the order of hundredths of a percent based on where the mass coalesced within the protoplanetary disk,” says staff scientist Jan Render, who is an expert in high-precision isotopic measurements. For example, asteroids formed further from the Sun exhibit higher amounts of the isotope zirconium-96, while asteroids formed closer to the Sun are more abundant in ruthenium-100. In addition to elemental composition and isotope ratios, researchers estimate a sample’s age through the degree of radiogenic decay observed in specific radioactive isotopes and through neutron capture signatures, which reflect the amount of cosmic radiation that struck the asteroid throughout its life.
As with many other mass spectrometry methods, acquiring high-precision isotope readings requires destructive analysis. Researchers pulverize the sample and dissolve it in concentrated mineral acids to break apart chemical bonds, purify the elements, and make the isotopic measurements. Since the work is destructive, handling asteroid samples is high stakes, and only researchers with the experience and the right techniques to address outstanding scientific questions can work on these precious samples. Furthermore, to ensure the asteroid samples’ compositions are preserved, they cannot be exposed to contamination. “Science agencies place substantial trust in the teams they allow to analyze these samples. After all, we are handling a 4.5-billion-year-old sample captured from a significant space mission,” says Brennecka. Rigorous preservation methods have paid off. Analyzing Bennu samples, other research teams focusing on organic research detected all five nucleotide bases found in DNA and RNA as well as 14 of the 20 amino acids associated with terrestrial biology, suggesting that material from meteorites hitting early Earth could have provided the organic ingredients that formed life. “I am constantly stunned at how we can obtain so much information from such a tiny, ancient piece of the Solar System,” says Shollenberger.
Evolution of a Discipline
Based on their elemental and isotopic compositions, meteorites have previously been sorted into two groups: the noncarbonaceous reservoir, which are thought to have formed in the inner Solar System, and the carbonaceous reservoir, forming beyond Jupiter’s orbit in the outer Solar System. However, some meteorites within the carbonaceous reservoir exhibit certain discrepancies or atypical characteristics, raising the possibility that other early formation reservoirs existed. During research on the asteroid Ryugu, Livermore scientists collected measurements for six separate isotope systems, including elements such as titanium, iron, and chromium. When compiling their findings, they faced the challenge of conveying whether Ryugu fell into the conventional reservoir distinctions. “We wanted to compare measurements from Ryugu to those from the many meteorites in our collections, but the standard way of presenting these statistics was not possible. We couldn’t make a six-dimensional plot to visualize all the measurements together,” says Render.
The team worked with Nipun Gunawardena, a researcher in Livermore’s Atmospheric, Earth, and Energy Division, through the Data Science Institute Consulting Service, a Laboratory resource that internally connects research groups with short-term consultants specializing in data science and statistical analysis. Noting the data set’s dimensionality, Gunawardena suggested principal component analysis (PCA) to reduce the data into a two-dimensional visualization. PCA is a technique that identifies principal components (uncorrelated directions of maximum variance) of a data set and enables researchers to view the data set from a carefully chosen perspective. Often implemented in scientific programming languages, PCA processes the data by breaking it down into its principal components with the first two being used to create a visual representation. Gunawardena explains, “A simple example of PCA is to think of dog breeds, such as the differences between Chihuahuas and Great Danes. Their size and weight dimensions vary significantly, but their number of legs does not. While all measured dimensions of a dog breed contribute to variance, weight and size are what contribute most to observed variance.” In a reduced-dimension scatterplot of dog breed similarity, the two breeds would therefore be spaced far apart because the dimensions of size and weight contribute heavily to the identified principal components.
While PCA has several caveats that are essential to avoid misinterpretation (such as how linear and structured the data is, how much variance can be captured by a reduced set of principal components, or the impact of changing the number of samples in the data set, among others), the method is a simple and well-established dimensionality reduction method. PCA is well suited to small sample sizes and is easily interpretable, making it a useful approach for Livermore’s analysis of asteroid samples.
Livermore cosmochemists compared six dimensions of isotope ratios measured in Ryugu to the existing measurements of meteorites, finding that Ryugu appeared distinct in the carbonaceous class of asteroids. “From a statistical perspective, we see evidence for three reservoirs,” says Gunawardena. Therefore, at least three isotopically distinct regions exist that surrounded the early Sun in which asteroids formed. “The newly found third reservoir is most representative of what the Sun is made from, which contains 99.9 percent of the mass in the Solar System. We now have a better reading of the bulk composition of the Solar System,” says Shollenberger, adding, “We’re excited to have strong evidence for a third reservoir only a few years after it was theorized.”
More than 50 years after the return of the Apollo lunar samples, the cosmochemistry field continues to grow, especially at Lawrence Livermore. “Access to these amazing samples is what brought many scientists and new research capabilities to Livermore in the first place. The Moon landing kicked off a worldwide craze for outer space exploration, and the opportunity to interact with extraterrestrial materials, such as those from Bennu and Ryugu, is a recruiting tool for the field and Lawrence Livermore today,” says Brennecka. As the Laboratory’s nuclear forensics and cosmochemistry work evolves, expanding scientific outreach to other disciplines, such as data science, could help unlock insights that may have otherwise remained hidden.
—Elliot Jaffe
For further information contact Greg Brennecka (925) 423-8502 (brennecka2 [at] llnl.gov (brennecka2[at]llnl[dot]gov)).
