Roughly 430,000 years ago, an incandescent ball of hot gas came barreling out of the sky and slammed into Antarctica — and now, scientists have found tiny bits of debris formed by that impact.
The team scooped up the mineral particles from Walnumfjellet in the Sør Rondane Mountains of Queen Maud Land, Antarctica, which is located south of Africa on the eastern side of the continent. Antarctica offers the perfect environment to scout for meteorite remnants, due to its dry, frigid climate and minimal human presence, first author Matthias van Ginneken, a geoscientist who specializes in the study of micrometeorites, or extremely tiny meteorites the size of dust particles, told Live Science.
"It was my first Antarctic expedition … and we found this very ideal sampling area on top of a Sør Rondane mountain," said Van Ginneken, who now conducts research at the University of Kent in the United Kingdom, but during the study, held positions with the Free University of Brussels, Vrije Universiteit Brussel and Royal Belgian Institute of Natural Sciences. After gathering sediment from the summit, Van Ginneken scanned the samples with an electron microscope.
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"To my great surprise, I found these very weird looking particles that did not look like terrestrial particles ... but they didn't look like micrometeorites either," he said. Unlike micrometeorites, which resemble fine dust, about half of the samples looked like several teeny stones fused together. Some carried tiny flecks of material on their surfaces, while others bore distinct, almost snowflake-like markings, he said.
The chemical composition of the particles suggested that they formed hundreds of thousands of years ago during an airburst in the lower atmosphere, which occurs when a meteorite becomes vaporized before hitting the ground, according to the new study, published online March 31 in the journal Science Advances.
"If more of these unique touchdowns can be identified and then even older particles are investigated, maybe we can use them to understand the characteristics of early Earth's atmosphere," Maitrayee Bose, an isotope cosmochemist at Arizona State University (ASU) in Tempe, who was not involved in the study, told Live Science in an email.
Understanding the nature of these impacts could also help us prepare if such a meteor came zooming toward Earth again, but this time aimed at a bustling city instead of the Antarctic wilderness, Van Ginneken said.
Reconstructing the impact
Upon first discovering the unusual particles, "I said, 'Bingo! This is fantastic, fantastic stuff,'" Van Ginneken said. But the discovery was just the start of the story — to learn how these particles came to be, the team conducted thorough chemical analyses, searched the literature for reports of similar particles and created numerical models to visualize the original asteroid that created them.
"The paper does detailed analysis at each step ... and does an excellent job of convincing me that such an event may have occurred in Earth's recent past," Bose told Live Science.
The particles themselves measured about 0.004 to 0.01 inches across (100-300 micrometers) and mostly contained the minerals olivine and iron spinel, which formed the snowflake-like patterns on some of the particles. These minerals were fused together by a small amount of glass. This composition closely matched a class of meteorites known as CI chondrites, confirming that the particles contained material from an asteroid, Van Ginneken said.
The high quantity of nickel in the particles also pointed to an extraterrestrial origin, because nickel is not very abundant in the Earth's terrestrial crust, he added.
Knowing that these particles contain material from space, the authors then wanted to figure out where and how they formed once their parent meteoroid entered Earth's atmosphere. The oxygen isotopes in the particles — meaning forms of oxygen with different numbers of neutrons — revealed how much oxygen was present during the particles' formation, Van Ginneken said.
Compared with typical chondrite material, the samples were very rich in oxygen, overall, suggesting they formed in the atmosphere, but relatively close to the ground. That said, the particles contained very few heavy oxygen isotopes, and specifically lacked an isotope called oxygen-18, the team found. This mimics the chemical composition of Antarctic ice, which contains little oxygen-18; based on this, the team concluded that the particles interacted and mixed with the ice during their formation.
Next, to estimate when these particles formed, the team went hunting for reports of similar meteorite touchdowns. It turned out that similar particles had been captured in ice cores drawn from other regions of Antarctica, including two summits known as the EPICA Dome C and Dome Fuji. Studies suggest that these meteorites fell to Earth 430,000 and 480,000 years ago, respectively, and by comparing the newfound particles to these other ones, the authors estimated that the Walnumfjellet particles formed 430,000 years ago.
"The mineralogic and textural evidence used in the paper shows similarities between particles from the different regions in Antarctica," but despite these overlaps, the absolute age of the Walnumfjellet particles remains unknown, Bose said. Future analyses will be needed to nail down their precise age, more conclusively, she said.
Considering the size, shape and density of the particles, the team was also able to produce a "very rough calculation" as to the size of their parent asteroid, Van Ginneken said. The particles' fused appearance hints that the cloud of hot gas in which they formed was very large and very dense, which allowed the minerals to collide and melt into one another on their way to Earth. This hints that the original asteroid was likely between 328 feet and 492 feet (100 and 150 meters) in diameter.
Based on their numerical models, "it turns out that such an asteroid will not reach the ground … basically it would be vaporized into a cloud of superheated meteoritic gas," Van Ginneken said. The cloud of gas would then continue descending toward the ground at a similar rate to the original asteroid — "we are talking kilometers per second," he said.
"This very dense, incandescent plume that would reach the surface, this is extremely destructive. This could destroy a large city in a matter of seconds, and do severe damage over hundreds of kilometers," Van Ginneken said.
Airburst events occur much more frequently than asteroid impacts that create large craters in the crust, he added. For example, an airburst event took place in Chelyabinsk, Russia in 2013, and scientists also suspect that the massive explosion that leveled forests near Tunguska, Russia in 1908 was an airburst, the authors wrote in the Science Advances report.
Tunguska-like events are estimated to occur "once every 100 to 10,000 years, which is orders of magnitude more frequent than large crater-forming impacts," the authors wrote. Studying the newfound Walnumfjellet particles could help scientists better understand how often these impacts occur and how severely they damage the earth below, Van Ginneken said.
The study suggests "that we should worry more about smaller asteroids, between a few tens of meters and 200 meters [32-656 feet in diameter], than much larger asteroids resulting in impact-cratering events," because the smaller asteroids touch down on our planet more often, he said. Should such an asteroid start hurtling toward a small country, a mass evacuation would likely be required to spare people from the fiery plume, he said.
Originally published on Live Science.