The Apollo astronauts didn't know what they'd find when they explored the surface of the moon, but they certainly didn't expect to see drifts of tiny, bright orange glass beads glistening among the otherwise monochrome piles of rocks and dust.
The beads, each less than 1 mm across, formed some 3.3 to 3.6 billion years ago during volcanic eruptions on the surface of the then-young satellite. "They're some of the most amazing extraterrestrial samples we have," said Ryan Ogliore, an associate professor of physics in Arts & Sciences at Washington University in St. Louis, home to a large repository of lunar samples that were returned to Earth. "The beads are tiny, pristine capsules of the lunar interior."
Using a variety of microscopic analysis techniques not available when the Apollo astronauts first returned samples from the moon, Ogliore and a team of researchers have been able to take a close look at the microscopic mineral deposits on the outside of lunar beads. The unprecedented view of the ancient lunar artifacts was published in Icarus. The investigation was led by Thomas Williams, Stephen Parman and Alberto Saal from Brown University.
The study relied, in part, on the NanoSIMS 50, an instrument at WashU that uses a high-energy ion beam to break apart small samples of material for analysis. WashU researchers have used the device for decades to study interplanetary dust particles, presolar grains in meteorites, and other small bits of debris from our solar system.
The study combined a variety of techniques -- atom probe tomography, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy -- at other institutions to get a closer look at the surface of the beads. "We've had these samples for 50 years, but we now have the technology to fully understand them," Ogliore said. "Many of these instruments would have been unimaginable when the beads were first collected."
As Ogliore explained, each glass bead tells its own story of the moon's past. The beads -- some shiny orange, some glossy black -- formed when lunar volcanoes shot material from the interior to the surface, where each drop of lava solidified instantly in the cold vacuum that surrounds the moon. "The very existence of these beads tells us the moon had explosive eruptions, something like the fire fountains you can see in Hawaii today," he said. Because of their origins, the beads have a color, shape and chemical composition unlike anything found on Earth.
Tiny minerals on the surface of the beads could react with oxygen and other components of Earth's atmosphere. To avoid this possibility, the researchers extracted beads from deep within samples and kept them protected from air exposure through every step of the analysis. "Even with the advanced techniques we used, these were very difficult measurements to make," Ogliore said.
The minerals (including zinc sulfides) and isotopic composition of the bead surfaces serve as probes into the different pressure, temperature and chemical environment of lunar eruptions 3.5 billion years ago. Analyses of orange and black lunar beads have shown that the style of volcanic eruptions changed over time. "It's like reading the journal of an ancient lunar volcanologist," Ogliore said.