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Astronomie - Solar system detectives search for clues in crumbs left over from early solar system

30.01.2023

NASA awarded nearly $3 million to the University of Arizona Kuiper Materials Imaging and Characterization Facility to support OSIRIS-REx sample science and much more.

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Students use the transmission electron microscope, or TEM, to study a meteorite sample at the Kuiper Materials Science Facility. The TEM, encased in the large enclosure on the left, operates at the smallest scales, allowing scientists to see individual atoms. Maria Schuchardt/LPL

A magnifying glass just won't cut it for the high-tech "detectives" at the University of Arizona Kuiper Materials Imaging and Characterization Facility. The scientists, who can be found in the basement of the university's Kuiper Space Sciences Building, are working to decode the stories archived in rocks and dust left over from the earliest days of the solar system.

Part of the UArizona Lunar and Planetary Laboratory, or LPL, the facility has been a resource for public and private science programs, both on and off campus, since 2016. Now, thanks to a four-year, nearly $3 million grant from NASA to support facility operations, scientists will be able to dig deeper into scientific questions than ever before.

"The history of the solar system is encoded in asteroids – the planetary crumbs left over from its birth over 4.5 billion years ago," said facility director Thomas Zega, a professor in the UArizona Department of Planetary Sciences. "The university and NASA are both investing a lot of money and resources in bringing back a sample from Bennu, a carbonaceous asteroid, and this is the first asteroid sample return mission in NASA's history, so it's important that we're properly equipped as a science team to analyze the sample when it comes back."

Facility co-investigators include assistant professors of planetary sciences Jessica Barnes and Pierre Haenecour, as well as Regents Professor of Planetary Sciences Dante Lauretta, principal investigator of NASA's OSIRIS-REx mission, which will return a sample from asteroid Bennu to Earth later this year.  

In addition to asteroid samples, scientists use the facility to analyze meteorites and debris from asteroids and other planetary bodies that fall to Earth. The facility has cutting-edge instrumentation and is open to users on campus as well as from other universities or the private sector. The new grant will allow researchers who already receive funding through NASA to use the fee-based facility at a reduced rate.

Other NASA programs that use the facility include the Interdisciplinary Consortia for Astrobiology Research, Laboratory Analysis of Returned Samples and Emerging Worlds. The facility will also serve research efforts on planetary materials returned by other space agencies' sample return missions, such as Japan's Hayabusa 2, which is OSIRIS-REx's "sister mission."    

"There's even more sample science to look forward to in the future," Zega said.

For example, NASA's Artemis missions will return lunar samples. And UArizona researchers are pursuing funding for the Comet Astrobiology Exploration Sample Return, or CAESAR mission, which would return a sample from a comet.

"The U.S. has been a world leader in sample science, and we want to maintain that, especially here at the University of Arizona," said University of Arizona President Robert C. Robbins. "Extracting the maximum amount of scientific information from modest samples is no easy feat and requires high-tech instrumentation like we have on our campus. I am honored by NASA's continued faith in our expertise, and I look forward to what we will learn."

Scale is everything

The university-led OSIRIS-REx mission was designed to return 60 grams – a little over 2 ounces – of surface material from asteroid Bennu. The mission team estimates that it's collected quite a bit more than that, and the mission's science team members, who are spread all over the world, will be allotted 25% of the total mass collected. A fraction of the sample will be released to investigators who are not part of the OSIRIS-Rex science team, and the remainder will be curated for future generations of researchers.

"We want to be sure we can look at the samples at multiple scales, from something you can see in the palm of your hand, all the way down to the atomic level," Zega said. "To do this, we need extremely sophisticated instrumentation."

Artist's illustration of the OSIRIS-REx spacecraft kicking up dust and rocks during its touchdown on asteroid Bennu.
Artist's illustration of the OSIRIS-REx spacecraft kicking up dust and rocks during its touchdown on asteroid Bennu. Scheduled to arrive on Earth in September, portions of the sample will be analyzed at the UArizona Kuiper Materials Imaging and Characterization Facility.NASA’s Goddard Space Flight Center/CI Lab/Jonathan North

The Kuiper Materials Imaging and Characterization Facility includes a focused-ion-beam scanning electron microscope, transmission electron microscope, an electron microprobe laboratory and scanning electron microscopes. A NanoSIMS instrument for measuring chemical elements in a sample is scheduled to arrive in June.

"There are different types of analysis we have to do on samples, and most chemists who study planetary materials specialize in one or several measurement techniques," Zega said. "We all have different specialties, and together our expertise complements one another and rounds out the analytical portfolio that we wanted to build at the university."

The tools: From microscopes to atomic probes

The first in a line of sample probing tools is the light microscope, familiar to many and used for centuries. It helps scientists visualize samples several hundred nanometers to micrometers in size, about the scale of bacteria and cells.

"Visible light microscopes are not able to 'sniff out' the chemical makeup of a sample, but they provide us with images, which might reveal textures and some information on its microstructure," Zega said.

"It also might reveal areas in your sample that you may want to target further," he said. "It might give you a sense of spatial relationship, which might tell you a little bit of the story to start piecing together some history of the sample. But it's not until more sophisticated methods that you start getting more of the picture."

The scanning electron microscope, or SEM, and electron microprobe are used for analyzing samples at a slightly smaller scale. An electron microprobe, also known as an electron probe microanalyzer, is similar to a scanning electron microscope, but offers the added capability of revealing clues about the sample's chemical composition.

A fragment of the Allende meteorite next to a microscopic image showing a lattice of individual atoms in a sample taken from it.
The image on the left shows a fragment of the Allende meteorite, the largest of its kind that has been found on Earth. Using the sophisticated instrument suite at the Kuiper imaging facility, researchers in Zega's team were able to probe the crystal structure of an Allende sample down to individual atoms (image on the right).H. Raab/Wikimedia Commons, Tom Zega

"The microprobe allows us to image and map out the chemical heterogeneity in a sample in two dimensions at the micrometer scale, less than half the length of an average-sized bacterial cell," Zega said. "The SEM can do the same thing, although not quite the same level of precision. Both can image and give us compositional information at the microscale, and both are critical in analysis of the sample from Bennu, for example, because that level of information will tell us where in the sample we might want to probe further using NanoSIMS or TEM."

The NanoSIMS instrument measures the chemical elements in a sample, which is important for understanding the origins of the material. Unlike the SEM or microprobe, the NanoSIMS can reveal the isotopic composition of a sample. Isotopes are different varieties of chemical elements.

"The isotopic composition of a planetary material can tell us something about its origins and history that the elemental information alone may not," Zega said. "The NanoSIMS also lets us measure trace elements, which are present in extremely small amounts, at the scale of tens of a nanometer."

The transmission electron microscope operates at the smallest scales, allowing scientists in Zega's lab to see individual atoms.

In 2021, Zega's team used the tool, combined with quantum mechanics, chemical thermodynamics and astrophysical modeling, to reconstruct the origin journey of a dust grain through the nascent solar system.

"Because humans were not around 4.6 billion years ago to witness all of this chemistry happening, we have to examine the leftovers and reverse engineer their origins," Zega said. "That is what these sophisticated analytical tools enable us to do."

Decades in the making

"Our meteoric record is incomplete," Zega said. "Those of us who study meteorites are at the mercy of what falls from the sky; we don't know exactly where they come from, so we try and piece it together."

In the early 2010s, Mike Drake, who served as OSIRIS-REx principal investigator until his passing in 2011, and Lauretta, the mission's current principal investigator, realized that the university needed to build up capabilities in sample science if it was going to take on the mission, according to Zega.

"These guys were visionaries; they knew that we needed a sample return mission, and that was a major catalyst for building out the facility," Zega said. "Since then, we have made an effort to hire the right faculty to lead the lab. This is the culmination of 20 years of that effort."

Quelle: UArizona News

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