Theme 4Maximizing Science From Returned Samples
Our History in Rocks
By Ali Sundermier
Four and a half billion years ago, a star was born. A cloud of gas and dust floating aimlessly in space suddenly collapsed, pulled in by gravity, spinning faster and faster, like a pirouetting dancer. As the cloud grew hotter and denser in the center, becoming a star, the surrounding material flattened like a disk, and particles that weren’t dragged into the center coalesced into clumps that got bigger and bigger until they formed an array of planets and airless bodies that orbited endlessly around that central star. A solar system.
In this solar system was a planet on which life evolved. Non-living matter became organisms that became fish that sprouted limbs and became bipedal creatures that would eventually name this planet Earth and develop telescopes and synchrotrons and theories about the mystery of life and the formation of the universe.
“The fundamental question is: Where did we come from?” said Timothy Glotch, the principal investigator of the project. “Some people answer that question philosophically. We can actually try to answer it physically.”
What were the conditions of the early solar system? How did life form? To investigate these questions, scientists working on Theme 4 of the RIS4E project are using X-rays to probe extraterrestrial material, such as interplanetary dust and meteorites, that might contain answers, or at least clues, about the formation of the world.
Currently, scientists use a method that requires large amounts of samples to analyze the composition of glass and minerals. But scientists have long wanted a technique that could analyze these samples at micro-scales.
“The problem is that we don’t have very many extraterrestrial samples,” said Darby Dyar, the lead investigator for Theme 4.
So in 1990, Dyar and her colleagues began working on a new approach, using immense particle accelerators called synchrotron light sources. Now, 25 years later, Dyar and her team are finally perfecting their synchrotron methods to measure and analyze microscopic samples of meteorites and interplanetary dust.
In the light sources being used for these experiments, an electron gun generates electron beams and feeds them into a linear accelerator, or a linac. Electromagnets and microwave radio-frequency fields are used to accelerate the electrons, which must travel in a vacuum to ensure they don’t encounter resistance.
Next, the electrons enter a booster ring, where they are accelerated to approximately 99.9 percent the speed of light, and then they are injected into a circular ring called a storage ring.
In the storage ring, the electrons are steered by an assortment of magnets. As the electrons go around turns in the storage ring, they decelerate slightly, losing energy. This energy loss is key to the whole experimental process.
The lost energy can be converted into different forms of electromagnetic radiation. For the purpose of this research, it takes the form of X-rays that are directed down beamlines running in straight lines tangential to the storage ring. At the end of the beamline, the X-rays crash into samples of whatever material is the subject of the experiment.
Scientists use the data that result from these crashes for, among other things, spectroscopy, which analyzes the chemical composition of materials by exciting the electrons in an atom. In micro X-ray absorption spectroscopy, the type of spectroscopy being done for Theme 4, scientists use light sources to focus extremely high energy beams of photons onto samples and look closely at the region of the electromagnetic spectrum where an iron peak appears.
The shape and energy of this peak allows them to determine how much iron metal, ferric iron, and ferrous iron are present. This is termed the redox ratio because it indicates whether iron atoms have been reduced—gaining electrons—or oxidized—losing electrons. Sophisticated computer algorithms are needed to extract this information from the spectra, using standards for which iron redox ratios have already been determined by alternate methods.
This redox ratio is important to understanding how rocks form, and reflects the amount of oxygen available to minerals when they are crystallizing, which influences the crystallization sequence of magmas, as well as the composition of the resulting minerals. The redox ratio also indicates the additional effects of subsequent interactions with the near surface environment—which may include melting, loss or gain of water and interactions with other rocks. By determining these factors, scientists can understand the emergence of oxygen and the evolution of the interiors of planetary bodies such as Earth, the Moon, and other bodies such as asteroids.
“We look at really old meteorites, the oldest pieces of old meteorites, and we look at the oxidation states in their different elements,” Glotch said, “and that tells us how much oxygen was around really early in the solar system.”
Juergen Thieme, the group leader for sub-micron resolution X-ray Spectroscopy (SRX) at the National Synchrotron Light Source II, or NSLS-II, at Brookhaven National Laboratory and co-investigator for Theme 4, explained that the interplanetary dust particles or remnants of meteorites that scientists are studying are typically less than a millimeter across.
“If you want to study them,” he said, “you need a microscope. And you need a microscope that is capable of doing chemistry studies as well so you can look at the constituents of these particles and try to identify what kind of metals are in there and how these metals are bound.”
Who Is Juergen Thieme?
The NSLS-II is the perfect instrument for this because of its small beam size. Scientists can currently focus the beam down to 15 nanometers, about six times the thickness of a single strand of human DNA.
Thieme explained that beyond studying the composition of meteorites, scientists can also use the NSLS-II to see the impact of space weathering on materials. This information will help engineers design the equipment that future astronauts will take into space, making it sturdy enough to stay in one piece as it travels.
Another use for the NSLS-II—and the goal of Theme 3 of the RIS4E project—is to study the interaction of space-dust particles with animal and human tissue.
Because the NSLS-II is still under construction, the beamline where this research will be done won’t be ready until this fall. In the meantime, scientists are using the Advanced Photon Source, a light source at Argonne National Laboratory in Chicago. Once the NSLS-II beamline is fully operational, the research for Theme 4 will be done at both Brookhaven and Argonne and at the Naval Research Laboratory in Washington, D.C., where scientists are using an electron microscope.
“We’ve finally gotten to the point where we have techniques,” Dyar said. “We’ve figured out how to make these measurements at very small scales, which is really exciting because it means that we can analyze little tiny spots in meteorites and little tiny pieces of interplanetary dust.”
But Dyar explained that there’s a catch: in order to analyze particular minerals, they need to have a calibration. In other words, they need to have data from a ton of other samples to use as standards, or models, as a point of comparison. To do this, Dyar and her colleagues need to find as many minerals as they can with variable redox ratios representing as many increments as possible along the range between zero and 100.
“We can compare an unknown to these standards and we can figure out which of the standards is most like the unknown,” Dyar said. “Then we know what the redox ratio of the unknown is.”
Glotch said that a large part of Theme 4 is figuring out the calibration to determine the redox ratio of certain types of geological material. And these calibrations can be time-consuming. Glotch said he anticipates that this work will take years to complete.
Nonetheless, he said, this research will be especially important for future space missions.
“We want to show that we can do great science with really small amounts of sample,” Glotch said, “so that when the time comes and these future sample return missions bring samples back we’ll be in really prime position to get those samples and do really good work on them.”
While some people like to spend their days pondering free will and the futility of existence, scientists like the ones working on Theme 4 of RIS4E are probing extraterrestrial rocks to figure out where our solar system came from and how our planet formed. These meteorites and interplanetary dust particles are like perfectly preserved time capsules, suspended in space or hurtling through our atmosphere.
“If we want to learn something about the origin of the solar system and the original soup from which it formed,” Thieme said, “these particles can tell us a lot.”
The Four Themes of RIS4E: Pathways to Space
Airless weather and extreme temperature fluctuations in space can alter samples in ways we don’t experience on Earth. To best interpret what the remote sensing data means, scientists must first do a lot of in-depth laboratory analysis on meteorite samples and lunar simulants to understand what data results from weathering. This laboratory data will go into libraries where it will be used to better analyze future remote sensing data.
RIS4E and the broader NASA initiative supporting it are about future exploration of planetary bodies. Theme 2—“Maximizing Exploration Opportunities”—is like a simulated advance team: Researchers going to places on Earth that are similar to the surfaces of the Moon and Mars to test and develop equipment that will help future astronauts know what to do when they get there. The expedition to Hawaii’s Mount Kilauea was Theme 2 in action.
It was the late 1960s, and the world was awed by what the United States was accomplishing in space. Landing men on the moon seemed the stuff of science fiction in 1969 but by the time the Apollo program ended three years later 12 American astronauts had firmly planted their space boots on the Moon’s surface, boldly stomping where no human had stomped before.
And kicking up a lot of lunar dust in their wake.
What were the conditions of the early solar system? How did life form? To investigate this, scientists working on Theme 4 of the RIS4E project are using X-rays to probe extraterrestrial material, like interplanetary dust and meteorites, which might contain answers, or at least clues, about the formation of the world as we know it.