Uncovering Earth’s Secrets One X-ray at a Time

With over 148 million specimens and artifacts to explore, questions are part of daily life at the National Museum of Natural History. How did dentists fill cavities in the 1800s? How do biting insects snap their jaws? What tools did ancient Mesoamericans use to create their exquisite stone figurines? 

These are all questions that have intrigued NMNH researchers in recent decades. Thanks to cutting-edge technology in the museum’s analytical laboratories, we now have answers.   

In order to address natural science’s biggest mysteries, you have to start small. Very small. The Smithsonian’s Department of Mineral Sciences is home to three analytical microscopes that use electrons to reveal the chemical compositions, textures, and microscopic features hidden within the museum's research specimens. 

Each of these state-of-the-art instruments occupies an entire laboratory room. And they are a far cry from the typical optical microscope. The machines direct 15,000-volt beams of electrons onto museum samples, creating a series of interactions that can be used to image and analyze particles at a sub-micrometer scale.  

When an electron beam hits a sample, it forces the atoms of each element to release energy in the form of X-rays. These wavelengths are picked up by energy detectors on the microscopes, allowing researchers to build elemental maps across the different sections and layers of an object.

“We look at samples in very fine detail, and we are always discovering something new,” said Tim Rose, the manager of the museum’s analytical laboratories. Until electron analysis was revolutionized in the 1970s, this type of technology was completely unavailable to science, forcing researchers to draw conclusions without key data points. In the last 20 years alone, Rose and analytical lab technicians Tim Gooding and Rob Wardell have analyzed hundreds of museum artifacts and thousands of mineral specimens.

Although initially trained as a geologist, Rose now considers himself a materials scientist. While the analytical microscopes officially reside in the Department of Mineral Sciences, Rose lends his expertise to the museum’s other departments. He regularly works with samples provided by archaeologists and paleobiologists. 

Mineral, rock, and meteorite specimens are analyzed using the electron microprobe, the most precise and accurate of the three electron instruments. The probe can only analyze samples that are flat, smooth and highly polished, a requirement that is fulfilled by the museum’s sample preparation lab.

The preparation lab is brimming with diamond-plated saws, polishing wheels and tiny abrasive wires. These tools are built to handle everything from colossal meteorites to tiny crystals. “Most rock samples are embedded in epoxy to stabilize them, and then a flat surface is ground and polished into them,” said Gooding. These smooth surfaces will allow the microprobe to capture X-rays coming from the samples with extremely high accuracy. 

In front of the electron microprobe and surrounded by a wall of colorful computer screens, NMNH research geologist Mike Ackerson and mineral science fellow Wriju Chowdhury stare intently at a complex set of graphs and charts. “Right now, we are looking at the oldest material on the planet,” said Chowdhury. “Just another day in the office.”

The researchers are analyzing 3.6-billion-year-old zircon minerals from Western Australia.  The electron microprobe will reveal the chemistry of the samples, providing key information about the conditions where these zircons formed.  

“We rely on high precision instruments at the Smithsonian to help us reveal some of the big questions about the early planet,” said Chowdhury. Understanding zircon chemistry will help the researchers reconstruct Earth’s early environments. This will support the museum’s “Our Unique Planet” research initiative that seeks to answer fundamental questions about the origins of life, the ocean and the continents on Earth.  

Tim Rose has also utilized the electron microprobe over the last 30 years to study explosive deposits from Hawaii’s Kilauea Volcano. Characteristic data revealed that the volcano is far more energetic than previously thought, with centuries-long cycles of eruptions capable of sending material higher than the peak of Mount Everest.

“Right now, we are looking at the oldest material on the planet. Just another day in the office.” — Wriju Chowdhury, NMNH Mineral Science Fellow

The mineral science team can cut and grind down their specimens to create the ideal flat, smooth and highly polished samples.  But what happens when researchers from other departments bring over unwieldy stone figurines and delicate fossils? “In a natural history museum setting, it’s essential to be able to analyze objects in a way that is non-destructive without jeopardizing their research value,” Rose said.

This is where the museum’s analytical scanning electron microscopes (ASEM) come into play. Although ASEM instruments produce results that are slightly less accurate than the electron microprobe, they can also accommodate specimens in their natural states.

When Rose and NMNH anthropologist Jane Walsh traveled to Mexico City to see Teotihuacan’s famed stone masks, they knew they could not bring any samples of the rare artifacts back to the Smithsonian.  Instead, they created silicone molds of the masks, allowing them to preserve and study the marks and imperfections that had accumulated on the stone.

After placing the molds in the museum’s ASEM, Rose was surprised to find a series of strangely coiled microorganisms clinging to the surface of the textured silicone. These were traces of diatomaceous earth, a white powdery substance made of fossilized algae that had been used to polish the stone faces over a thousand years ago. Rose and Walsh found diatoms on seven of the masks, providing insights into the lives, culture, and ritual techniques used by the ancient Mesoamericans.

“The first thing that many archaeologists do when they find something in the ground is brush it off,” said Rose. “But much of my research is evidence that they shouldn't clean artifacts right away, because there can be a wealth of information stored in the dirt and earth that was surrounding the objects.” 

Although several unusual objects have been maneuvered into the ASEM over the years, a collection of American Civil War teeth might be the strangest.  For a research study on the history of dental work, NMNH anthropologist Doug Owsley asked Rose to analyze fillings in the teeth of Confederate soldiers who had perished after their ship, the H. L. Hunley, sank in 1863. Rose found that the soldiers had multiple types of dental fillings, supporting the theory that trained dentists were starting to take over from barbers around the time of the Civil War. 

With an assortment of powerful analytical tools at his fingertips, Rose has also been able to answer a few burning questions related to his own personal interests. As a saxophonist, Rose had long wondered about the bright orange pigments that colored some hard rubber saxophone mouthpieces in the early 20th century. Shockingly, Rose found that the pigments were made of mercury and sulfur from the mineral cinnabar.  While that may sound like a serious health hazard, in combination the two elements actually create a stable compound that would have been safe for instrumentalists. 

The museum’s third analytical microscope is also its newest. It was custom-built to analyze the Bennu asteroid samples that were retrieved from NASA’s OSIRIS-REx mission in September. “Everything on Earth is represented in the Bennu samples,” Rose said. “This instrument is helping us to analyze the tiniest spots preserved in the asteroid and build a story of our early solar system and the creation of the Earth itself.”

According to Rose, designing a custom ASEM instrument is similar to buying a car.  You can get the base model, the deluxe model and anything in between. While most ASEMs have one single detector measuring the X-rays released from samples as they are hit by the electron beam, this new microscope has two. The second detector prevents a shadow from being cast on rough and imperfect specimens, providing better coverage and clearer scans than ever before. 

According to Rose, the instrument will add new capabilities for the characterization of natural history objects from across the museum. “My role is to help researchers apply this crucial technology to their specific areas of research,” Rose said. “I consider myself lucky to have contributed to so many scientific and cultural discoveries, and I know it is just the beginning for the analytical labs.” 

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Emma Saaty | READ MORE

Emma Saaty is a Science Communications Specialist at the Smithsonian National Museum of Natural History.  She brings science to the public by covering natural history research for Smithsonian Magazine's, Smithsonian Voices.  Additionally, she tracks media coverage and coordinates filming activities for the museum’s Office of Communications and Public Affairs.  Emma recently completed her Bachelor of Science in Biological Anthropology with a minor in Journalism at George Washington University where she conducted research on Virunga mountain gorillas with GW’s Life History and Skeletal Biology laboratory.  You can find her here.