X-ray Diffraction

Playing the Angles

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By Kristen O’Neill

Terra may not be the field leader of the RIS4E X-ray diffraction team, but out on the lava flows of Kilauea she was its star.

“I called her a diva because she was being a little temperamental,” Amy McAdam, the XRD team lead, said after their first day together in the field.

Terra, whose full name is In-Xitu Terra Portable XRD system, is the team’s portable X-ray diffraction instrument. X-ray diffraction is a method of finding what minerals make up a sample by shooting x-rays at it. For this fieldwork, the geologists wanted to know how fast that data could be found in the field, to help NASA create procedures for astronauts to potentially use these instruments on planetary bodies in space.

Meet the team

Jennifer Eigenbrode

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Amy McAdam

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The Terra comes complete with a bright orange case, weighs 35 pounds and has a habit of letting out a high-pitched whine while working. This means that while the other teams were usually grouped together close to the site being sampled, the XRD team was hiding behind rock and wind cover to avoid driving everyone else to mechanocide.

Jennifer Eigenbrode, the other human member of the XRD team, admits that Terra’s constant, pulsing hum can get to you after a while. But McAdam explained that the noise is a necessary evil: Without the right frequency, the sample won’t vibrate in just the right way to get the right data.

Despite this, everything seemed fine on the first field day. The sky was blue, the rocks were plentiful and Terra was running just fine, passing the test with flying colors. But when the team was called away for sampling, Terra was left in direct sunlight for about half an hour – in a desert where people were getting sunburn under layers of industrial-strength sunscreen. When the team returned, Terra had shut itself off, and no amount of patting or coaxing got it to turn back on.

“I was maybe coddling it a little,” McAdam said, laughing at how motherly she can be toward her “baby.” After multiple failed reboots, the team theorized that the detector inside the machine, which normally has to be cooled to about -31°F, had probably overheated. They had to cool down the machine, and fast.

Eigenbrode whipped off her scarf and draped it over Terra in an attempt to shield it from the sun. They covered the black top with their hands. They waited.

And waited.

And waited.

And finally Terra’s diva moment passed.

“You could just feel that the surface wasn’t as hot and it seemed to boot up okay,” McAdam said when Terra came back to life under the scarf. It just needed the right accessory for the hike. Which turned out to be a tinfoil dress to keep it cool for the rest of the day.

While the machine may be a little troublesome at times, it’s an amazing piece of tech. McAdam’s other XRD instruments in the lab are the size of a room. Terra is the size of a suitcase and gives very similar data.
“I just think it’s one of the coolest inventions,” McAdam said.

To get that data takes a little bit of work. First, a rock sample has to be ground up into a fine powder. This powder is put into the machine and vibrated, which makes all the little particles jump around. Then X-rays are shot through the sample, hitting the sides and edges of the particles randomly, causing the rays to diffract, or bounce off at random angles. If the particles weren’t moving, the X-rays would hit the same particles over and over again, and the data wouldn’t really represent their sample. Terra then takes the angle these X-rays bounce off a crystal, and how many X-rays made that same angle and spits out a graph. The graph shows up as a lot of peaks. A trained geologist can match up those peaks to peaks that pure mineral samples make to piece together the minerals in their rock.

The peaks get clearer over time as more X-rays hit the sample. In the field, they’re testing the machine for how quickly it can produce helpful information to determine if astronauts need to collect more samples for further analysis.

In her normal lab setting, McAdam would let a scan run for hours to find out exactly how much of each mineral is present. But for this trip, only the major mineralogy was needed. It was a hard adjustment for McAdam to go from perfect to “good enough.”

“I guess I’m sort of a data snob,” she said.

Eigenbrode adapted a little easier from her experience in an organics lab where any touching of the samples with bare skin is forbidden. For her, being able to touch everything and “get dirty” was liberating. “On this trip all we were looking at was minerals, we weren’t looking at organics,” Eigenbrode said. “So all the oils in the world from my hands – they didn’t matter.”

Which was good, because creating a sterile environment in the middle of a desert would have been tricky, as the team discovered when they found an interesting layer of ash deposit underneath a lava flow. And what do geologists do when they find something interesting that could contain organic, contaminable material? They climb into the bunny suits that they’ve conveniently brought with them to make sure the few samples they bring out are all clean and uncontaminated. The two looked more as if they were at a hazmat scene than in a rocky desert surrounded by a bunch of people in hiking gear. They started drilling into the side of a sandy pit before the field lead Jake Bleacher reminded them of their strict instructions to be non-invasive: No drilling. So the two abided by the rules, if somewhat reluctantly. They tucked away the drill and settled for scooping out their sample with a little shovel rather than risk being found out by park rangers.

Eigenbrode and McAdam have worked together before, both in the lab and the field as part of a NASA projected called Arctic Mars Analog Svalbard Expedition. In 2009 they worked on a research vessel that was looking at Mars analog sites, places with geology that mimics Mars, mainly in the volcanic rock called basalt that covers the planet’s surface. It was similar to this mission in Hawaii but with at least one notable difference – the vessel sailed around Svalbard, an archipelago of islands north of Norway.

Today, they are two of the many scientists on the SAM (Sample Analysis at Mars) instrument suite of the Mars Curiosity rover. SAM is made up of a lot of different instruments and one of them, like the Terra, uses X-ray diffraction to find the mineralogy of rocks on Mars. In this case, though, it’s trying to find evidence in the mineralogy of processes the rock underwent.

Many times these processes leave behind telltale minerals that could not only show whether the rock reacted with water, but even whether the water was salty or acidic. McAdam’s knowledge of these processes help her identify the environment that life would have to survive in. Eigenbrode uses her expertise in organic geochemistry to try to find the best conditions for life, and new ways to detect it.