Robert A. Pritzker Center for Meteoritics and Polar Studies
Specimen of the Murchison meteorite in the collection of the Field Museum's Robert A. Pritzker Center for Meteoritics and Polar Studies.
This September we are celebrating the 50thanniversary of the fall of the Murchison meteorite, one of the most important meteorites to science. Since its fall near Murchison, Victoria in September 1969, the Murchison meteorite has been the source of numerous spectacular discoveries. Thanks to the large amount recovered, about 100 kg comprising of a large number of specimens, and its availability to the scientific community, the Murchison meteorite is one of the most studied meteorites of the type carbonaceous chondrite. The scientific community is grateful to the meteorite finders in Murchison to have made available the vast majority of the mass to science. The main fraction of Murchison was acquired by the Field Museum of Natural History in Chicago and since has been curated there another large fraction is at the Smithsonian Institution in Washington DC.
Some of the most important discoveries made by studying Murchison in the last 50 years includes the discovery of presolar stardust grains, solid samples of our parent stars more than 4.6 billion years old, which gave rise to presolar grains research, a new interdisciplinary subdiscipline within cosmochemistry and astrophysics. Other remarkable findings include the detection of a large variety of extraterrestrial organic matter incl. sugars, amino acids and urea, and the results obtained from studying refractory inclusions, which are among the first solids that formed in the solar system and are essentially time capsules from that time period. Murchison also served as an analog sample for the carbonaceous asteroid Bennu to test instruments of the OSIRIS-REx spacecraft. OSIRIS-Rex is scheduled to return to Earth with a sample of Bennu in 2023. The knowledge gained by studying Murchison significantly advanced our scientific understanding of the formation of our solar system 4.6 billion years ago.
The town of Murchison will hold an Anniversary Symposium on the anniversary weekend. Robert A. Pritzker Associate Curator Philipp R. Heck will speak there and will also give a public talk about the Murchison meteorite at the University of Melbourne. See this short video about Murchison.
The North Chile iron meteorite (FMNH ME 2937.1) from the Field Museum collection. This specimen is also informally named “Coya Norte,” one of its 16 unofficial names.
Best practices for meteorite names in publications
Philipp Heck (Robert A. Pritzker Associate Curator) is the lead author, with a large group of meteorite and astromaterial curators, of an article about best practices for the use of meteorite names in publications. The article appears in the early view section of the journal Meteoritics & Planetary Science. When meteorite specimens are loaned for research, recipients are not only expected to acknowledge the loaning institution, but also to refer to the loaned specimen in an unambiguous way to avoid confusion and enable reproducibility of the research. That means not only the meteorite name should be reported but also the specimen’s full catalog number (example at left). Knowing which specimen was studied can help resolve the rare cases of mislabeling, but is also very important when referring to meteorites with varied composition—for example, breccias can contain clasts of different meteorite types. In many cases, pieces of the same meteorites were recovered at different times and thus experienced varying degrees of alteration from terrestrial weathering (e.g., rain!). There are also cases in which specimens from the same meteorite have several different unofficial names because they were found by different people at different places at different times. The iron meteorite North Chile shown in the photo, for example, has accrued some 16 names! Many of the recommendations by Heck et al. may be transferrable to other collections. You can read the paper in Meteoritics & Planetary Science at https://onlinelibrary.wiley.com/doi/10.1111/maps.13291
Above: Electron microscopy image of mount with nanooxides (image width ~1mm). Below: Sharpened nanotip with nanooxide sample prepared for atom-probe tomography.
New NASA Grant to Robert A. Pritzker Center Team
Philipp Heck (Robert A. Pritzker Associate Curator for Meteoritics and Polar Studies) has received a grant from NASA’s Emerging Worlds program. Together with Resident Grad Student Jennika Greer (University of Chicago) and collaborators from Northwestern University and ETH Zurich, Switzerland, the research will focus on “Underexplored aspects of the history of our solar system’s presolar starting material.” The knowledge of the origin of the starting material of our Solar System is an issue of fundamental interest in planetary science. After the discovery of presolar grains in 1987 (by Edward Anders, Roy S. Lewis and their colleagues at the University of Chicago), it was possible to study solid samples of stars in the laboratory for the first time, providing a unique perspective on the origin and composition of the material from which the Solar System formed. However, most studies have focused on the larger size fraction of grains, which are rarer and therefore less representative. Philipp and colleagues will focus on presolar nanograins that are too small to study with conventional analytical techniques. Philipp’s group has pioneered the use of atom-probe tomography to study the composition of extraterrestrial samples with a goal of better understanding the origins of these understudied samples, and hence our origins, information not obtainable otherwise. The team will also significantly extend the known ages of presolar grains, information that is currently very limited. The team will apply a unique analytical method developed by Philipp and collaborators, and improved physics, to determine the presolar chronology of the Solar System’s starting material. The Murchison meteorite from the Field Museum’s collection will serve as the main source for presolar grains.
a New record of the early active sun
Our Sun's beginnings are a mystery. It burst into being 4.6 billion years ago, about 50 million years before the Earth formed. Since the Sun is older than the Earth, it's hard to find physical objects that were around in the Sun's earliest days--materials that bear chemical records of the early Sun. But in a new study in Nature Astronomy, ancient blue crystals trapped in meteorites reveal what the early Sun was like. And apparently, it had a pretty rowdy start.
"The Sun was very active in its early life--it had more eruptions and gave off a more intense stream of charged particles. I think of my son, he's three, he's very active too," says Philipp Heck, a curator at the Field Museum, professor at the University of Chicago, and co-author of the study. "Almost nothing in the Solar System is old enough to really confirm the early Sun's activity, but these minerals from meteorites in the Field Museum's collections are old enough. They're probably the first minerals that formed in the Solar System."
The minerals the team looked at are microscopic ice-blue crystals called hibonite, and their composition bears earmarks of chemical reactions that only would have occurred if the early Sun was spitting lots of energetic particles. "These crystals formed over 4.5 billion years ago and preserve a record of some of the first events that took place in our Solar System. And even though they are so small--many are less than 100 microns across--they were still able to retain these highly volatile nobles gases that were produced through irradiation from the young Sun such a long time ago," says lead author Levke Kööp, a post-doc from the University of Chicago and an affiliate of the Field Museum. In its early days, before the planets formed, the Solar System was made up of the Sun with a massive disk of gas and dust spiraling around it. The region by the sun was hot. Really hot-- more than 1,500 C, or 2,700 F. For comparison, Venus, the hottest planet in the Solar System, with surface temperatures high enough to melt lead, is a measly 872 F. As the disk cooled down, the earliest minerals began to form--blue hibonite crystals."
The larger mineral grains from ancient meteorites are only a few times the diameter of a human hair. When we look at a pile of these grains under a microscope, the hibonite grains stand out as little light blue crystals--they're quite beautiful," says Andy Davis, another co- author also affiliated with the Field Museum and the University of Chicago. These crystals contain elements like calcium and aluminum. When the crystals were newly formed, the young Sun continued to flare, shooting protons and other subatomic particles out into space. Some of these particles hit the blue hibonite crystals. When the protons struck the calcium and aluminum atoms in the crystals, the atoms split apart into smaller atoms--neon and helium. And the neon and helium remained trapped inside the crystals for billions of years. These crystals got incorporated into space rocks that eventually fell to Earth as meteorites for scientists like Heck, Kööp, and Davis to study. Researchers have looked at meteorites for evidence of an early active Sun before. But the findings could be explained by other mechanisms than direct particle irradiation of minerals by the early Sun. For the new study the team examined the crystals with a unique state-of-the-art mass spectrometer at ETH Zurich in Switzerland--a garage-sized machine that can determine objects' chemical make-up. Attached to the mass spectrometer, a laser melted a tiny grain of hibonite crystal from a meteorite, releasing the helium and neon trapped inside so they could be detected. "We got a surprisingly large signal, clearly showing the presence of helium and neon--it was amazing," says Kööp.The bits of helium and neon provide the first concrete evidence of the Sun's long-suspected early activity. "It'd be like if you only knew someone as a calm adult--you'd have reason to believe they were once an active child, but no proof. But if you could go up into their attic andfind their old broken toys and books with the pages torn out, it'd be evidence that the person was once a high-energy toddler," says Heck.Unlike other hints that the early Sun was more active than it is today, there's no other good explanation for the crystals' make-up. "It's always good to see a result that can be clearly interpreted," says Heck. "The simpler an explanation is, the more confidence we have in it." "In addition to finally finding clear evidence in meteorites that disk materials were directly irradiated, our new results indicate that the Solar System's oldest materials experienced a phase of irradiation that younger materials avoided. We think that this means that a major change occurred in the nascent Solar System after the hibonites had formed--perhaps the Sun's activity decreased, or maybe later-formed materials were unable to travel to the disk regions in which irradiation was possible," says Kööp.
Artist’s rendering of the space collision 466 million years ago that gave rise to many of the meteorites falling to Earth today. Illustration by Don Davis/Southwest Research Institute,
Today's rare meteorites were common 466 million years ago, study finds
Scientists reconstruct distribution of space rocks predating giant collision
Scientists reconstruct distribution of space rocks predating giant collision
By Kate Golembiewski (http://news.uchicago.edu/tag/source/kate-golembiewski) – January 26, 2017 Press Inquiries
About 466 million years ago, there was a giant collision in outer space. Something hit an asteroid and broke it apart, sending chunks of rock falling to Earth as meteorites. But what kinds of meteorites were making their way to Earth before that collision?
In a study published in Nature Astronomy, scientists tackled that question by creating the first reconstruction of the distribution of meteorite types before the collision. They discovered that most of the meteorites falling to Earth today are rare, while many meteorites that are rare today were common before the collision.
“We found that the meteorite flux—the variety of meteorites falling to Earth—was very, very different from what we see today,” said Philipp Heck, associate professor of geophysical sciences at the University of Chicago, the paper’s lead author. “Looking at the kinds of meteorites that have fallen to Earth in the last hundred million years doesn’t give you a full picture. It would be like looking outside on a snowy day and concluding that every day is snowy, even though it’s not snowy in the summer.”
Meteorites are pieces of rock that have fallen to Earth from outer space. They’re formed from the debris of collisions between bodies like asteroids, moons and even planets. There are many different types of meteorites, which reflect the different compositions of their parent bodies. By studying the different meteorites that have made their way to Earth, scientists can develop a better understanding of how the basic building blocks of the solar system formed and evolved.
“Before this study, we knew almost nothing about the meteorite flux to Earth in geological deep time,” said co-author Birger Schmitz, professor of nuclear physics at Lund University. “The conventional view is that the solar system has been very stable over the past 500 million years. So it is quite surprising that the meteorite flux at 467 million years ago was so different from (that of) the present.”
To learn what the meteorite flux was like before the big collision event, Heck and his colleagues analyzed meteorites that fell more than 466 million years ago. Such finds are rare, but the team was able to look at micrometeorites—tiny specks of space-rock less than 2 millimeters in diameter that fell to Earth. They are less rare. Heck’s Swedish and Russian colleagues retrieved samples of rock from an ancient seafloor exposed in a Russian river valley that contained micrometeorites. They then dissolved almost 600 pounds of the rocks in acid so that only microscopic chromite crystals remained.
Not having changed during hundreds of millions of years, the crystals revealed the nature of meteorites over time. Analysis of their chemical makeup showed that the meteorites and micrometeorites that fell earlier than 466 million years ago are different from the ones that have fallen since. A full 34 percent of the pre-collision meteorites belong to a meteorite type called primitive achondrites; today, only 0.45 percent of the meteorites that land on Earth are this type.
Other micrometeorites sampled turned out to be relics from Vesta—the brightest asteroid visible from Earth, which underwent its own collision event over a billion years ago.
Meteorite delivery from the asteroid belt to the Earth is a little like observing landslides started at different times on a mountainside, said co-author William Bottke, senior research scientist at the Southwest Research Institute. “Today, the rocks reaching the bottom of the mountain might be dominated by a few recent landslides. Going back in time, however, older landslides should be more important. The same is true for asteroid breakup events; some younger ones dominate the current meteorite flux, while in the past older ones dominated.”
“Knowing more about the different kinds of meteorites that have fallen over time gives us a better understanding of how the asteroid belt evolved and how different collisions happened,” said Heck, an associate curator of meteoritics and polar studies at the Field Museum of Natural History. “Ultimately, we want to study more windows in time, not just the area before and after this collision. That will deepen our knowledge of how different bodies in our solar system formed and interact with each other.”
—Adapted from a story first published by the Field Museum of Natural History.
Read the associated News & Views piece: DeMeo 2017, "Meteorites: A shift in shooting stars", Nature Astronomy, DOI: http://dx.doi.org/10.1038/s41550-017-0041
Funding: European Research Council and Tawani Foundation
3‐D APT reconstruction showing the kamacite–taenite interface in the iron meteorite Bristol. Isoconcentration surfaces for Ni and Fe show the distribution of the two major iron meteorite phases kamacite and taenite.
Atom-probe tomography of an iron meteorite
Former Field Museum postdoc Surya Rout together with Robert A. Pritzker Associate Curator Philipp Heck published an atom-probe tomography (APT) study in Meteoritics & Planetary Science on the Bristol iron meteorite together with collaborators at Northwestern University, Argonne National Laboratory and the University of Chicago. The study demonstrates that APT in conjunction with transmission electron microscopy (TEM) is a useful approach to study the major, minor, and trace elemental composition of nanoscale features within iron meteorites. This combined approached proved particularly fruitful for fast-cooled irons, as many of their features are on the nanoscale, and are well resolved with the near atomic spatial resolution of APT. The study measured composition of different phases in the specimen generated new knowledge about phase compositional changes during the fast cooling. The study also shows that the Bristol meteorite did not experience high shock pressures and temperatures due to impacts on its parent asteroid. The article can be read in full at Meteoritics & Planetary Science.
The team also present a new method using SEMGlu adhesive to speed up sample preparation for APT. Their method appeared in March 2018 issue of Microscopy Today.