Robert A. Pritzker Center for Meteoritics and Polar Studies
Congratulations JENNIKA GREER on a SUCCESSFUL PHD DEFENse
Jennika Greer, UChicago resident graduate student at the Field Museum’s Robert A. Pritzker Center and Negaunee Integrative Research Center successfully defended her PhD. Her dissertation is titled “Atom Probe Tomography of Lunar Materials,” and her committee includes Philipp Heck (primary advisor), Nicolas Dauphas, Andy Davis, and Edwin Kite.
AGUAS ZARCAS METEORITE LIKELY RECORDED NEWLY DISCOVERED GEOLOGICAL PROCESS
Kate Golembiewski/FMNH: In 2019, NASA’s OSIRIS-REx spacecraft sent back images of a geological phenomenon no one had ever seen before: pebbles were flying off the surface of the asteroid Bennu. The asteroid appeared to be shooting off swarms of marble-sized rocks. Scientists had never seen this behavior from an asteroid before, and it’s a mystery exactly why it happens. But in a new paper in Nature Astronomy, researchers show the first evidence of this process in a meteorite.
”It’s fascinating to see something that was just discovered by a space mission on an asteroid millions of miles away from Earth, and find a record from the same geological process in the museum’s meteorite collection,” says Philipp Heck, the Robert A. Pritzker Curator of Meteoritics at Chicago’s Field Museum and the senior author of the Nature Astronomy study.
Meteorites are pieces of rock that fall to Earth from outer space; they can be made of pieces of moons and planets, but most often, they're broken-off bits of asteroids. The Aguas Zarcas meteorite is named after the Costa Rican town where it fell in 2019; it came to the Field Museum as a donation from Terry and Gail Boudreaux. Heck and his student, Xin Yang, were preparing the meteorite for another study when they noticed something strange.
“We were trying to isolate very tiny minerals from the meteorite by freezing it with liquid nitrogen and thawing it with warm water, to break it up,” says Yang, a graduate student at the Field Museum and the University of Chicago and the paper’s first author. “That works for most meteorites, but this one was kind of weird-- we found some compact fragments that wouldn’t break apart.”
Heck says that finding bits of meteorite that won’t disintegrate isn’t unheard of, but scientists usually just shrug and break out the mortar and pestle. “Xin had a very open mind, he said, ‘I’m not going to crush these pebbles to sand, this is interesting,’” says Heck. Instead, the researchers devised a plan to figure out what these pebbles were and why they were so resistant to breaking apart.
“We did CT scans to see how the pebbles compared to the other rocks making up the meteorite,” says Heck. “What was striking is that these components were all squished-- normally, they’d be spherical-- and they all had the same orientation. They were all deformed in the same direction, by one process.” Something had happened to the pebbles that didn’t happen to the rest of the rock around them.
“This was exciting, we were very curious about what it meant,” says Yang.
The scientists had a clue, though, from the 2019 OSIRIS-REx findings. From there, they put together a hypothesis, which they supported with physical models. The asteroid underwent a high-speed collision, and the area of impact got deformed. That deformed rock eventually broke apart due to the huge temperature differences the asteroid experiences when it rotates, since the side facing the sun is more than 300° F warmer than the side facing away. “This constant thermal cycling makes the rock brittle, and it breaks apart into gravel,” says Heck.
These pebbles are then ejected from the asteroid’s surface. “We don’t yet know what the process is that ejects the pebbles,” says Heck-- they might be dislodged by smaller impacts other space collisions, or they might just get released by the thermal stress the asteroid undergoes. But once the pebbles are disturbed, Heck says, “you don’t need much to eject something-- the escape velocity is very low.” A recent study of Bennu revealed that its surface is loosely bound and behaves like popcorn in a bucket.
The pebbles then entered a very slow orbit around the asteroid, and eventually, they fell back down to its surface further away where there was no deformation. Then, Heck and Yang say, the asteroid underwent another collision, the loose mixed pebbles on the surface got transformed into a solid rock. “It basically packed everything together, and this loose gravel became a cohesive rock,” says Heck. The same impact may have dislodged the new rock, sending it careening into space. Eventually, that chunk fell to Earth as the Aguas Zarcas meteorite, carrying evidence of the pebble mixing.
This could explain the pebbles present in Aguas Zarcas, making the meteorite the first physical evidence of the geological process observed by OSIRIS-REx on Bennu. “It provides a new way of explaining the way that minerals on the surfaces of asteroids get mixed,” says Yang.
That’s a big deal, Heck says, because for a long time, scientists assumed that the main way that the minerals on the surfaces of asteroids get rearranged is through big crashes, which don’t happen very often. “From OSIRIS-REx we know that these particle ejection events are much more frequent than these high-velocity impacts,” says Heck, “so they probably play a more important role in determining the makeup of asteroids and meteorites.”
Aguas Zarcas is the first meteorite to show signs of this behavior, but it’s probably not the only one. “We would expect this in other meteorites,” says Heck. “People just haven't looked for it yet.”
Heck joins OSIRIS-REx Team
Philipp Heck, Robert A. Pritzker Curator of Meteoritics and Polar Studies, was recently selected as a Participating Scientist Collaborator on NASA’s OSIRIS-REx mission. Philipp will collaborate with Yongsong Huang from Brown University to study samples from the asteroid Bennu that were collected as part of the mission and are scheduled to be brought to Earth in September 2023. (Read more on the mission on Eos.) The team will analyze the samples to determine how the asteroid and its organic compounds formed, and will compare them to other meteorite samples in the Field Museum’s collection. The proposal was made possible thanks to preliminary work supported by the Museum’s Science Innovation Award.
FOSSIL METEORITE COLLECTION ARRIVES AT FIELD MUSEUM
The Field Museum's Robert A. Pritzker Center received the world’s largest collection of fossil meteorites, which fell to Earth hundreds of millions of years before the time of the dinosaurs. Members of the media were invited on Monday to see the meteorites getting unpacked from giant shipping crates and added to the museum’s research collection.
In the 1980s, workers in a limestone quarry in Sweden noticed some of these fossil meteorites in slabs of limestone. Since then, more than 100 fossil meteorites have been identified, making them some of the rarest geological specimens in the world.
The Field Museum has received 115 fossil meteorites as a donation from the Boudreaux family. Terry and Gail Boudreaux were present to help unpack the meteorites, along with Field Museum scientists, interns, and volunteers. The meteorites are preserved in large limestone slabs from the Swedish quarry, sometimes alongside fossilized marine life.
“Fossil meteorites are really important because they can tell us about the evolution of the Solar System,” says Philipp Heck, “These fossil meteorites are remnants of a massive collision and provide insight into catastrophic events in our Solar System.”
LPSC 2022: Atom Probe and TEM of Lunar Soil
Resident UChicago graduate student Jennika Greer and collaborating Purdue graduate student Alexander Kling presented latest results from our NASA-funded project on lunar soil at LPSC 2022. We use TEM and atom probe tomography on the same lunar soil grains to study space weathering products, including water. Read our abstracts at "Nanoscale Analyses of Vesicles in Space-Weathered Lunar Soil Silicates and Ilmenite" and "Identification of Solar Wind-Sourced Water in the Space Weathered Rims of Lunar Soils". Direct evidence / That lunar space weathering / Forms water on Moon.
The Consulate General of Switzerland in Chicago started their inaugural Skyline Chat with a discussion on presolar grains with Philipp Heck, Robert A. Pritzker Curator of Meteoritics and Polar Studies. We thank Consul General Bruno Ryff, Joerg Oberschmied, Deputy Consul General & Senior Public Diplomacy Officer, and Roberta Neuhäusler, Executive Assistant & Project Manager, for inviting us and for their interest in presolar stardust!
MOON DUST Project FEATURED BY NORTHWESTERN
Our long-term collaboration with Northwestern University's Materials Science and Engineering Department was featured in the Northwestern Engineering Magazine. One of our current projects is in collaboratorion with Purdue University and Northwestern and looks at space-weathering products in lunar soil from the Apollo 17 mission and is funded by NASA. Learn More.
Asteroid Day 2021
June 30 was Asteroid Day, and members of the press were invited to meet with Field Museum postdoctoral scientist Dr. Maria C. Valdes (John Caldwell Meeker Postdoctoral Fellow) who described and classified a new brecciated eucrite NWA 13993. Maria performed X-ray microtomography, SEM/EDS and Raman spectroscopy on the rock. The specimen was collected in northwest Africa, purchased by Terry Boudreaux in 2020 and subsequently acquired by the Field Museum. Several Chicago media reported on this including the Chicago Sun Times, NBC Chicago, FOX32 Chicago.
ATOM PROBE on EXHIBIT AT THE FIELD MUSEUM
The Robert A. Pritzker Center's 2020 presolar stardust study published in PNAS made headlines again. The study was featured by National Geographic as the top story in "the top 10 awesome science discoveries you may have missed in 2020", by Yahoo! News as the top story in "The biggest and most important scientific breakthroughs of 2020", by Live Science among the "10 science records broken in 2020", by CNN among "Science's most fascinating and awe-inspiring discoveries in 2020", and by BBC Science Focus among the "20 moments in science to make you feel better about 2020". In our study we present cosmic-ray exposure ages of presolar stardust, the oldest dated solid samples available to science. We are honored by having been featured on these lists!
Hamburg (Mi) meteorite study published
The Hamburg meteorite that fell in January 16, 2018 was recovered quickly thanks to a map of the strewnfield that made possible by weather radar data. Because the meteorite landed on frozen lakes in winter it did not get exposed to liquid water, did not get weathered, and also did not have time to get contaminated much. This is what makes this meteorite special compared to others that were not recovered as quickly and compared to those that got rained on. Thanks to meteorite hunters and collectors this meteorite was quickly made available to science. Read our international consortium study featuring work from 29 scientists at 24 institutions that appeared on October 27, 2020 in Meteoritics & Planetary Science. The study is featured on the journal cover and was covered by news media including CNN, The Guardian, Gizmodo, Courthouse News Service.
Aguas Zarcas meteorite Featured In Science
We are excited that our work at the Field Museum's Robert A. Pritzker Center on Aguas Zarcas appeared in a feature article and video in Science Magazine. The main mass of Aguas Zarcas and additional material were donated to the Museum by the Boudreaux Family. The fragments are in cryogenic storage in liquid nitrogen and the main mass is kept in a nitrogen desiccator.
Educational and Outreach Videos
Our team is continuing to work hard on our various research projects. Besides we are preparing videos to reach museum visitors, students and interested members of the public while our Museum is temporarily closed due to the pandemic. The first video we share was prepared by our resident graduate student Jennika Greer on geological basics in Minecraft. The first episode starts with igneous rocks. Enjoy!
NEw insights into some of Earth's oldest Rocks
Jennika Greer, UChicago resident graduate student at the Robert A. Pritzker Center for Meteoritics and Polar Studies published a new paper on some of the oldest rocks on Earth. Earth is currently our only data point for life, and by understanding how life can arise on our planet, we can better understand how life can arise on others. However, our planet is incredibly geologically active, and constant resurfacing events have erased much of our planet’s early history. This is why analyzing the remnants of early crust that still survive is so important- and the Nuvvuagittuq supracrustal belt is one of the least studied of these. At 3.8 billion years old, and located in a relatively inaccessible part of northern Canada, this paper aims to better understand the geological context of these ancient rocks. This is done through chemical analysis of these rocks and uranium-lead dating of the zircons within them. Not only does this paper put these rocks into better geological context, it also provides compelling evidence of more ancient terrains in the area, previously undiscovered.
The field site is difficult for scientists to traverse (which explains why the area hadn't been explored before) but while collecting the rocks surrounding the Nuvvuagittuq supracrustal belt, scientists found other supracrustals floating like rafts in the younger granites. With further exploration of the area, which is simultaneously marshy and rocky, it is possible that scientists might find the oldest rock still preserved on our planet.
The paper is published in the scientific journal Lithos.
The meteoritical community lost a remarkable scientist, mentor, colleague and friend with the passing of Edward J. Olsen on January 30, 2020 at his home in Madison, Wisconsin. Ed is survived by his wife of 38 years, Lorain Olsen, his daughters Andrea Southwood and Ericka Olsen and his grandson Jacob Taggart. Read more.
Atom-probe tomography of apollo 17 soil
Back in 1972, NASA sent their last team of astronauts to the Moon in the Apollo 17 mission. These astronauts brought some of the Moon back to Earth so scientists could continue to study lunar soil in their labs. Since we haven’t returned to the Moon in almost 50 years, every lunar sample is precious. We need to make them count for researchers now and in the future. In a new study in Meteoritics & Planetary Science, scientists used Atom-Probe Tomography to analyze the chemistry of the Moon’s soil using a single grain of dust. Their technique can help us learn more about conditions on the surface of the Moon and formation of precious resources like water and helium there.
“We’re analyzing rocks from space, atom by atom,” says Jennika Greer, the paper’s first author and a PhD student at the Field Museum and University of Chicago.
Studying galactic star formation with presolar grains
By Kate Golembiewski — Stars have life cycles. They're born when bits of dust and gas floating through space find each other and collapse in on each other and heat up. They burn for millions to billions of years, and then they die. When they die, they pitch the particles that formed in their winds out into space, and those bits of stardust eventually form new stars, along with new planets and moons and meteorites. And in a meteorite that fell fifty years ago in Australia, scientists have now discovered stardust that formed 5 to 7 billion years ago-the oldest solid material ever found on Earth.
"This is one of the most exciting studies I've worked on," says Philipp Heck, a curator at the Field Museum, associate professor at the University of Chicago, and lead author of a paper describing the findings in PNAS. "These are the oldest solid materials ever found, and they tell us about how stars formed in our galaxy."
presolar grains workshop 2019 Chicago
The Robert A. Pritzker Center organized the 2019 Chicago Presolar Grains Workshop which brought together researchers from all of the US to share and discuss the latest research on presolar stardust in an informal setting. This workshop traditionally bridges cosmochemistry and astrophysics and includes scientists from both disciplines. The event included talks at the University of Chicago, tours of the meteorite collection at the Field Museum's Robert A. Pritzker Center and a banquet in the Marae Gallery of the Field Museum.
Main MASS of the Aguas Zarcas Meteorite donated
On October 7th, 2019 Terry and Evan Boudreaux visited the Pritzker Center to donate the main mass of the Aguas Zarcas meteorite that fell April 23, 2019 in Costa Rica. The Boudreaux family has supported the Pritzker Center over the last decade with many generous donations of scientifically important meteorites. Terry Boudreaux is one of the world's top meteorite collectors and loves to support scientific research with meteorites. Aguas Zarcas is an unusual carbonaceous chondrite, which at first resembles Murchison, but at a closer look reveals a more diverse collection of different lithologies that also include parts that were not aqueously altered. This makes it particular interesting to search for the earliest solar system condensates and presolar materials. The Field Museum is extremely grateful to the Boudreaux family for this scientifically highly valuable donation.
Dust from asteroid breakup may have caused Ordovician ice age
About 466 million years ago, long before the age of the dinosaurs, the Earth froze. The seas began to ice over at the Earth's poles, and the new range of temperatures around the planet set the stage for a boom of new species evolving. The cause of this ice age was a mystery, until now: a new study in Science Advances lead by Birger Schmitz, a Professor at Lund University and an international team of colleagues incl. Philipp Heck, Pritzker Associate Curator and University of Chicago Associate Professor (part time), argues that the ice age was caused by global cooling, triggered by extra dust in the atmosphere from a giant asteroid collision in outer space.
There's always a lot of dust from outer space floating down to Earth, little bits of asteroids and comets, but this dust is normally only a tiny fraction of the other dust in our atmosphere such as volcanic ash, dust from deserts and sea salt. But when a 93-mile-wide asteroid between Mars and Jupiter broke apart 466 million years ago, it created way more dust than usual. "Normally, Earth gains about 40,000 tons of extraterrestrial material every year," says Philipp Heck. "Imagine multiplying that by a factor of a thousand or ten thousand." To contextualize that, in a typical year, one thousand semi trucks' worth of interplanetary dust fall to Earth. In the couple million years following the collision, it'd be more like ten million semis a year.
"Our hypothesis is that the large amounts of extraterrestrial dust over a timeframe of at least two million years played an important role in changing the climate on Earth, contributing to cooling," says Heck.
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.
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
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.
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 – January 26, 2017
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
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.