X-ray Fluorescence

Legacy Tech Rocks

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By Anthony Denicola

As the clouds rolled in and the rain threatened to end the day early, a two-woman team of NASA geologists stood on an outcrop of frozen lava, in the middle of the Kilauea lava fields, planning their next move.

“I want to blanket this whole area in XRF measurements and maybe we can contour map some of these weird alteration products,” said Kelsey Young, a precocious post-doc, as she intently surveyed the expanse of grey terrain in front of her.

“I think that would be fabulous,” responded Cynthia Evans as enthusiastically as only a geologist with more than 20 years of field experience could.

Meet the team

Kelsey Young

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Cynthia Evans

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Sometimes it’s easy to take the natural world for granted and forget that everything, down to the rocks under your feet, is a wonder. For geologists, the rocks are the best part, and for Kelsey Young and Cynthia Evans, this is no different. They comprise the X-Ray Fluorescence (XRF) unit of the RIS4E field team, which consists of four different instrument teams in total, working together to lay the ground- work for future missions to the Moon, Mars and other planetary bodies.

“Our mission, here in the field, has two objectives: Science operations and actual science,” said Young, XRF team leader and deputy lead of the entire RIS4E field team. “Science operations refer to a process of testing these various instruments in the field through simulated Extravehicular Activities (EVA’s) to assess their effectiveness and worth to missions on other planetary surfaces.”

Each RIS4E instrument team has science objectives and questions that they want to answer. The X-Ray Diffraction (XRD) team wants to answer mineralogical questions while the Light Detection and Ranging (LiDAR) team answer questions of topography. For the XRF team, this means questions about the chemical composition of rocks on the surface. All of this serves to provide scientists with the clearest possible picture of the surface they are studying.

“XRF provides a very broad brush stroke picture of a sample, but still provides information that couldn’t be known just by looking,” explained Evans. “Two rocks may look alike but could be, compositionally, very different.”

Looking like a futuristic ray gun, the hand-held XRF scanner takes measurements by firing two X-Ray beams, operating at different wavelengths, into the surface of the rock. Each element on the periodic table produces a unique set of X-Rays at different wavelengths that form a “fingerprint” for that specific element. The XRF’s beams, one looking for wavelengths of higher energies while the other looks for lower, scans for these wavelengths to provide real-time, compositional data of the sample and present that information on an easy- to- read, digital display.

“I don’t think it is going to change the way geology is done, but I think it will increase efficiency,” said Young of her hopes for XRF technology. “The field instruments can really help maximize our efficiency and ability to collect the right samples the first time, and really help inform our understanding of the geologic context of what we are looking at in real-time.”

The RIS4E project represents the maiden field-voyage for this hand-held XRF and as such, it comes with some limitations. As of now, XRF technology is limited in the sense that it cannot detect the 11 elements lighter than magnesium on the periodic table due to their low energy wavelengths. On the XRF’s digital display these 11 light elements, which include naturally abundant chemicals like sodium, carbon and nitrogen, are lumped together under the distinction, “LE” or “Light Elements” and often account for up to 40 – 60 percent of a sample. As with most new technology, improvements are needed.

“There will be calls to make the hand- held XRF better than what it is and capable of measuring lighter elements than magnesium,” said Evans. “The technology will become more sophisticated. One of the main things is we will be better able to make decisions in the field based on real time data.”

While the XRF looks like a ray gun, its functionality is not so simple as, point and shoot. The user must hold the XRF’s scanning window flush against the surface of the sample they intend to measure for no less than 60 seconds. It must be flush because the quality of data can be influenced by surface roughness and it needs 60 seconds to give the scanner time to integrate the two beams’ measurements and present the data on the screen.

The amount of time required to take a sample in the field is important, especially on the surface of a planet with no oxygen, and as Young discovered, it can come with risks.

“I was lying on my back, taking the measurement when I felt something crawling on my neck,” Young said of a particular scan that took place in the mouth of a cave-like lava formation. “I knew it was a spider or something, and I hate bugs, but if I moved the scanner or dropped it, the measurement would have been ruined.”

Young held on for all 60 seconds and got her measurement before squirming out of the cave mouth to do the age-old dance of bug swatting.

Fortunately for astronauts, they can probably count on not running into any spiders on the Moon. But there are other considerations that make time an important factor. Even if scientists plan an extended stay on the surface, every second in the field needs to be maximized.

“Anytime you are stopping to take a measurement, that is time you are not doing something else,” Young explained. “You want to make sure you have a good reason and that everything you do while on an EVA is useful.”

The lava fields of Kilauea are an ideal place for this kind of study because of their topographical and chemical similarities to the planetary surfaces that scientists want to explore. While they are a far cry from the Moon or Mars, they do give scientists like Young and Evans a window through which to study other worlds, in addition to our own.

“All these textures and forms of basaltic lava flows are just wild and I have never seen anything like it,” remarked Evans. “I have begun to understand a little bit of what is really going on in a lava flow. As a geologist, anytime you go into the field, you understand a little bit better how the planet works.”