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Single moon dust grain collected during Apollo 17 'preserves millions of years of history'

Posted February 7, 2020 2:29 a.m. EST

— Although the moon is conveniently close for observations, astronauts haven't stepped on the lunar surface since 1972.

Fortunately, this will change in the future when NASA's Artemis program lands the first woman and next man on the moon by 2024.

In the meantime, that makes lunar samples returned by Apollo astronauts a rare and precious commodity. For the first time, researchers used a technique to look at individual atoms in a single grain of moon dust from an Apollo 17 moon sample.

Considering that astronauts Gene Cernan and Harrison Schmitt returned 245 pounds of lunar rocks and soil to Earth after their 1972 mission, studying one dust grain at a time would definitely help conserve samples.

The technique, known as atom probe tomography, provided a new way to study the moon's surface. It's typically used in materials science with nanowire production because it can analyze materials on a tiny scale.

NASA granted access to Apollo 17 lunar soil samples to researchers from Chicago's Field Museum, Northwestern University and Purdue University. Previously, those at the Field Museum and Northwestern had used atom probe tomography on tiny samples, like nanoscale-sized grains from an iron meteorite, said Jennika Greer, study author and PhD student at the Field Museum and University of Chicago.

"We're analyzing rocks from space, atom by atom," Greer said. "It's the first time a lunar sample has been studied like this. We're using a technique many geologists haven't even heard of."

Elements like iron or water create small, complex structures in lunar soil. Studying the soil atom by atom can provide a detailed look at the moon's composition.

The technique involved using a beam of charged atoms to carve a sharp tip into the surface of the dust grain. The sharp tip was a few hundred atoms wide. For reference, one sheet of paper is hundreds of thousands of atoms in thickness.

"We can use the expression 'nano-carpentry,'" said Philipp Heck, the study's co-author, a curator at the Field Museum and an associate professor at the University of Chicago. "Like a carpenter shapes wood, we do it at the nanoscale to minerals."

The sample was placed inside the atom probe at Northwestern and hit with a laser, which displaced atoms individually. Each atom then struck a plate designed to detect them. This allowed the researchers to determine the type of atom and its charge.

In the single grain, they found evidence of water, iron, helium and space weathering on the lunar surface. Iron is a heavier element, so its atom took longer to strike the detector plate than other, lighter elements -- which made it identifiable.

"We can apply this technique to samples no one has studied," Heck said. "You're almost guaranteed to find something new or unexpected. This technique has such high sensitivity and resolution, you find things you wouldn't find otherwise and only use up a small bit of the sample."

Greer was able to use the data for a 3D reconstruction, color-coding the atoms to map the moon dust grain. This enabled scientists to see exact atoms and their locations in moon dust for the first time.

The moon has no atmosphere, and it's impacted by small meteorites, solar wind and cosmic rays streaming from the sun -- which is known as space weathering. So while the moon isn't geologically active, like Earth and its shifting tectonic plates, the surface soil of the moon does change. Underneath this top layer of soil, things look very different. Unexposed moon soil can tell us about the history of the moon.

Understanding resources in lunar soil could make them useful for astronauts on future missions to the moon. And in the grand scheme of things, atom probe tomography could also be used to study the samples returned from asteroids.

"It's great for comprehensively characterizing small volumes of precious samples," Greer said. "We have these really exciting missions like Hayabusa2 and OSIRIS-REx returning to Earth soon -- un-crewed spacecrafts collecting tiny pieces of asteroids. This is a technique that should definitely be applied to what they bring back, because it uses so little material but provides so much information."

Comparing what they learn about the samples through atom probe tomography with telescope observations of the moon or asteroids can allow scientists to understand what they're seeing when sampling isn't practical or possible.

"It's important to understand these materials in the lab so we understand what we're seeing when we look through a telescope," Greer said. "Because of something like this, we understand what the environment is like on the moon. It goes way beyond what astronauts are able to tell us as they walk on the moon. This little grain preserves millions of years of history."

And the sample grain of dust used in the study can be studied by others in the future.

"Fifty years ago, no one anticipated that someone would ever analyze a sample with this technique, and only using a tiny bit of one grain," Heck said. "Thousands of such grains could be on the glove of an astronaut, and it would be sufficient material for a big study."

Given that future missions are planned for returning to the moon, the researchers hope that new, diverse samples will be returned.

Only examining space weathering from one place on the moon is "like only analyzing weathering on Earth in one mountain range," Greer said it's like only analyzing weathering on Earth in one mountain range," Greer said.

"We need to go to other places and objects to understand space weathering in the same way we need to check out different places on Earth, like the sand in deserts and outcrops in mountain ranges."

Greer is eager for samples from the far side of the moon, which is the side that continually faces away from Earth. It's a challenge, observationally, and the material that can be collected there might prove to be very different from the sites we've sampled on the Earth-facing side of the moon, she said.

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