When a meteor streaks across the sky, it's not just beautiful. It's nature's way of delivering a time capsule to Earth. Contained within are hints about the very beginning of the solar system and how planets, including our own, formed.
Lawrence Livermore National Laboratory (LLNL) scientist Thomas Kruijer and collaborators describe how meteorites tell the story of the early solar system in a recent paper in Space Science Reviews that will also become a chapter in an upcoming textbook.
"The holy grail, the ultimate goal, is to understand how you form habitable planets like the Earth," said Kruijer. "How do you form the planet that we stand on and walk around on every day, and how do you form a planet that eventually is able to harbor life on it? That is still vigorously debated."
But researchers have some well-founded ideas. When the solar system began to form, gas and dust from a giant molecular cloud was pulled into a flattened disk of material around the sun. That protoplanetary disk was very hot at first but eventually started to cool and coalesce. Gravity caused dust clumped together into larger and larger pieces. Eventually this formed small bodies, 1-100 miles across, called planetesimals.
"These planetesimals are thought to be the building blocks of larger planets, so they're actually very important. To understand how the Earth formed, you also need to know how planetesimals formed," said Kruijer. "I think we are somewhat like detectives or historians, in a way. We are trying to develop a sequence of these events."
The main clue for this detective work comes in the form of meteorites. Many of these space rocks come from the asteroid belt, a graveyard for some of the first bodies that formed in the solar system. By analyzing meteorites in the laboratory - a field known as cosmochemistry - scientists can determine the age and compositions of samples that are over 4.5 billion years old.
"It's maybe a sample the size of your fingernail, and that rock that you hold is the oldest thing on Earth," said Kruijer. "For all these years it has stayed the same, and it has recorded that information about the time that it formed."
The authors describe the various types of meteorites and what information each provides. Undifferentiated meteorites come from planetesimals that mixed together and formed without melting. They harbor calcium aluminum-rich inclusions that were possibly the first materials to condense out of the protoplanetary disk. Also inside are chondrules, small spheres that can be easily dated and demonstrate when a body formed.
In contrast, differentiated meteorites underwent heating and melted. Heavier materials like iron sank inward to form a planetesimal core while light materials rose to the mantle.
"That's quite special because the earth is also thought to have an iron core, but it's so deep under our feet that we can never access it," said Kruijer. "By studying iron meteorites, you can also study the cores of planetary bodies."'
LLNL is home to many analytical tools that are used to analyze meteorite samples. The Laboratory specializes in measuring isotopes, ages and chemical compositions of very small samples very precisely and accurately. For example, when NASA's OSIRIS-REx mission became the first U.S. mission to collect a sample of material from an asteroid and return it to Earth, scientists at LLNL performed some of the analysis.
The team aims to do the same for samples from the upcoming Artemis missions to the moon. In preparation, they are analyzing historical lunar samples that were returned from the Apollo missions.
"We are currently ramping up our scientific capabilities in anticipation of Artemis," said Kruijer. "This is a major thrust area for cosmochemistry at LLNL. We want to sustain and strengthen our ability to study lunar samples."
These analytical techniques are mission-critical for the Laboratory. State-of-the-art capabilities in cosmochemistry feed into nuclear forensics, an area of research at LLNL that seeks to extract information about the provenance and history of nuclear materials.
Eventually, Kruijer wants to see results from meteorite sample analysis feed into large-scale astrophysical models of the protoplanetary disk. He hopes the paper's compilation of meteorite research will serve as a valuable resource for early-career scientists and experts in adjacent fields.
"You can ask AI to summarize the latest developments in this field. That gives you some idea, but there's a lot of nuance and precise language that's used in scientific papers," he said. "I think having a well-curated review paper that is written by experts who understand all the nuances, that's still really important."
