The nitty gritty of Moon soil
Scientists develop a new way to study lunar chemistry.
By Nick Carne
US scientists have started making a very detailed analysis of Moon rocks and soil – atom by atom.
They say that not only allows them to find things that might otherwise be missed, it also ensures they make good use of a limited supply of precious raw material that hasn’t been replenished since Apollo 17 returned to Earth in 1972.
Jennika Greer and colleagues from Field Museum and the University of Chicago applied a technique called atom probe tomography (APT), which is normally used by materials scientists working to improve industrial processes.
"It's the first time a lunar sample has been studied like this,” says Greer. “We're using a technique many geologists haven't even heard of.”
In a paper in the journal Meteoritics & Planetary Science, the researchers describe Greer's analysis of single grain of soil, about as wide as a human hair, in which she identified products of space weathering, pure iron, water and helium that formed through the interactions of the lunar soil with the space environment.
To do that, she first used a focused beam of charged atoms to carve a tiny, super-sharp tip into the grain’s surface. A sample then went into an atom probe at Northwestern University, where it was zapped with a laser, which knocked off atoms one by one.
Each struck a detector plate, but heavier elements, such as iron, took longer to reach the detector than lighter elements like hydrogen. By measuring the time between the laser firing and the atom striking the detector, the instrument was able to determine the type of atom at that position and its charge.
Finally, Greer reconstructed the data in three dimensions, using a colour-coded point for each atom and molecule to make a nanoscale 3D map of the Moon dust.
This allowed the researchers to see both the type of atoms and their exact location in a speck of lunar soil for the first time. And because Greer’s study used a nanosized tip, the grain of lunar dust is still available for future experiments.
The researchers say APT is particularly valuable for studying space weathering as it allows them to look for differences between weathered surfaces and unexposed Moon dirt in a way that no other method can.
By understanding the kinds of processes that make these differences happen, they can more accurately predict what's just under the surface of moons and asteroids that are too far away to bring to Earth.
Greer encourages other “cosmochemists” to try the new approach
"It's great for comprehensively characterising small volumes of precious samples," she says.
"We have these really exciting missions like Hayabusa2 and OSIRIS-REx returning to Earth soon – uncrewed 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."
She and her colleagues will certainly be doing more of the same. NASA has asked them to spend three years studying different types of lunar dust with APT to quantify its water content and other aspects of space weathering.