In 1972, when NASA’s Apollo 17 astronaut crew returned home from the space agency’s final crewed mission to the Moon, they brought with them 250 pounds of rocks and soil from the lunar surface.
For nearly 50 years, scientists have had to balance preservation of this scarce resource with a natural curiosity to study the material in hopes of uncovering new insights about the Moon and our solar system.
When Philipp Heck, Robert A. Pritzker Curator for Meteoritics and Polar Studies and head of the Robert A. Pritzker Center at the Field Museum, and Jennika Greer, a resident graduate student at the Field Museum, gained access to a sample of lunar soil in 2019, they turned to Northwestern Engineering materials scientists David Seidman and Dieter Isheim to help them uncover its secrets.
To do so, the researchers needed only a single grain of the sample.
Using the LEAP 5000XS atom probe tomograph, the flagship instrument at the Northwestern University Center for Atom-Probe Tomography (NUCAPT), Seidman and Isheim visualized the lunar soil’s atomic structure and determined the precise location and chemical identities of individual atoms within the grain. Normally used to study metals, semiconductors, and ceramics, the atom probe unleashed a pulsing ultraviolet laser onto the tiny lunar sample, releasing its atoms and providing data to record spatial locations in three dimensions.
“With the atom probe tomograph, we conduct chemical analyses by counting individual atoms. Our typical datasets may include tens of millions of atoms, but that’s still a microscopic volume,” says Seidman, Walter P. Murphy Professor of Materials Science and Engineering and director of NUCAPT. “We don’t need much material to gain an understanding of the chemical composition of a sample, which is useful when the primary source, like lunar soil, is extremely finite.”
In analyzing the speck of lunar dust under the atom probe, the researchers found evidence of space weathering, a phenomenon where exposure to solar and cosmic radiation induces chemical changes to the outer layer of the material. The ionized hydrogen atoms (protons) and trace amounts of helium detected, they confirmed, came from our sun. The analysis also found the presence of iron and water molecules that formed in the dust grains due to exposure to solar wind.
By pinpointing the differences between the chemical composition of lunar soil that has been exposed to space weathering and soil that has not, scientists may better understand the composition of other materials in our galaxy.
“This technology to study lunar soil wasn’t available in 1972. We can answer so many more questions today with the same samples that were collected by the Apollo 17 crew,” Heck says. “This work has not only reinforced the value of mission-return samples, but also how great a resource the atom probe has been for extraterrestrial materials science research at the Field Museum and our ongoing work with Northwestern.”
