Finding Water on the Moon
An advanced type of mass spectrometry has detected water in lunar particles.
Last week’s announcement of the discovery of evidence of water in lunar volcanic glass beads brought back from the moon during the Apollo missions in 1971–a finding that is causing scientists to rethink the conventional theory of the moon’s formation–was made possible by recent advances in an analytical technique called nano secondary ion mass spectroscopy or NanoSIMS.
NanoSIMS, which was developed by the French company CAMECA, is a variation of an established technique called secondary ion mass spectroscopy (SIMS), but it has a higher spatial resolution and can measure a handful of elements simultaneously, says Frank Stadermann, a senior research scientist at the Laboratory for Space Sciences at Washington University, in St. Louis.
With SIMS, a high-energy ion beam is focused on the surface of a sample. The impact of the beam causes atoms to be ejected from the surface; some of the atoms get ionized and then pass through a mass spectrometer that determines the composition of the sample material. The NanoSIMS instrument has an ion beam that can focus down to a diameter of less than one micrometer, whereas previous SIMS technology could only focus down to around 20 micrometers.
While researchers have been using NanoSIMS to study the isotopic composition of small particles, such as planetary dust and volcanic glasses, this is the first time that the technique has been able to detect traces of hydrogen. “The key part was, we developed a method for measuring very low water content on other SIMS instruments and applied the same methodology on the NanoSIMS,” says Erik Hauri, a staff scientist with the Carnegie Institution for Science, in Washington, DC, and one of several authors of the study last week describing evidence of water in lunar samples. “This allowed us to take measurements on a very small spatial scale. The limit for detecting water was about 50 parts per million at best. We developed a way to detect as little as five parts per million of water, and surprisingly, we found up to 46 parts per million in these tiny glass beads.”
The study was published on July 10 in the journal Nature and conducted by scientists from Brown University, Carnegie Institution for Science, and Case Western Reserve University.
To enable the technology to detect such a small amount of hydrogen, Hauri first improved the vacuum of the instrument. Any free gas gets deposited on the sample surface, contaminating it. Prior generations of the instrument have, on average, detection limits around 100 parts per million, but with a better vacuum the scientists were able to achieve a detection limit of five parts per million.
Hauri’s technique also included using a different high-energy primary ion beam. Most SIMS instruments use a beam of oxygen ions and collect positively charged ions. Instead, Hauri used a cesium beam to measure negatively charged hydroxyl ions that are ejected from the sample. The cesium ion beam is a much more sensitive method of analyzing water, says Hauri.
NanoSIMS has also allowed scientists to, for the first time, simultaneously measure several elements in the samples instead of just one, says Claude Lechene, a professor at Harvard Medical School and the director of the National Resource for Imaging Mass Spectrometry, in Cambridge, MA. In previous technology, if scientists wanted to analyze more than one element, they would have had to adjust the magnetic field. “The new system can measure five or seven masses concurrently,” says Stadermann, making the analysis much more efficient.
Stadermann says that the new work is an excellent use of the NanoSIMS technology. “This is one of the many steps of the broader uses of NanoSIMS,” says Stadermann. “I fully anticipate that there will be many more discoveries being made using the technology. You have capabilities that you did not have with instruments before.”
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June 11-12, 2019