Scientists apply the atom probe tomography (APT) known from material sciences to space geology
Back in 1972, Nasa sent their last team of astronauts to the Moon in the Apollo 17 mission. These astronauts brought some of the Moon back to Earth so scientists could continue to study lunar soil in their labs. Since we haven’t returned to the Moon in almost 50 years, every lunar sample is precious. We need to make them count for researchers now and in the future. In a new study in Meteoritics & Planetary Science, scientists found a new way to analyze the chemistry of the Moon’s soil using a single grain of dust. Their technique can help us learn more about conditions on the surface of the Moon and formation of precious resources like water and helium there.
„We’re analyzing rocks from space, atom by atom,“ says Jennika Greer, the paper’s first author and a PhD student at the Field Museum and University of Chicago. „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.“
The technique is called atom probe tomography (APT), and it’s normally used by materials scientists working to improve industrial processes like making steel and nanowires. Greer’s analysis only required one single grain of soil, about as wide as a human hair. In that tiny grain, 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 study the tiny grain, Greer used a focused beam of charged atoms to carve a tiny, super-sharp tip into its surface. This tip was only a few hundred atoms wide – for comparison, a sheet of paper is hundreds of thousands of atoms thick. „We can use the expression nanocarpentry,“ says Philipp Heck, a curator at the Field Museum, associate professor at the University of Chicago, and co-author of the paper. „Like a carpenter shapes wood, we do it at the nanoscale to minerals.“
Once the sample was inside the atom probe at Northwestern University, Greer zapped it with a laser to knock atoms off one by one. As the atoms flew off the sample, they struck a detector plate. Heavier elements, like iron, take 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 is able to determine the type of atom at that position and its charge. Finally, Greer reconstructed the data in three dimensions, using a color-coded point for each atom and molecule to make a nanoscale 3D map of the Moon dust.
It’s the first time scientists can see both the type of atoms and their exact location in a speck of lunar soil. While APT is a well-known technique in material science, nobody had ever tried using it for lunar samples before.
„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,“ Greer says.
Studying soil from the moon’s surface gives scientists insight into an important force within our Solar System: space weathering. Space is a harsh environment, with tiny meteorites, streams of particles coming off the Sun, and radiation in the form of solar and cosmic rays. On Earth the atmosphere is a protection against those influences.
Greer and Heck emphasize the need for missions where astronauts bring back physical samples because of the variety of terrains in outer space. „If you only analyze space weathering from the one place on the Moon, it’s like only analyzing weathering on Earth in one mountain range,“ Greer says. „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 on Earth.“
(25.02.2020, USA: 02.25.2020)