Researchers have found meteoritic spherules on top of Walnumfjellet in the Sør Rondane Mountains, Antarctica.
The breakup of a giant asteroid in the main asteroid belt. Image credit: NASA / JPL-Caltech.
“Remnants of hypervelocity impacts on Earth’s surface are mainly preserved as impact craters, generally circular depressions resulting from asteroids large and/or dense enough to reach ground level without suffering substantial atmospheric disruption,” said corresponding author Dr. Matthias Van Ginneken from the School of Physical Sciences at the University of Kent and colleagues.
“Crater formation is accompanied by the production of a characteristic set of shock-metamorphic effects and formation of high-pressure mineral phases in target rocks, resolvable geochemical anomalies, and ejection of target/projectiles materials with high velocity.”
“Identifying hypervelocity impacts in the geological record is relatively straightforward if one or several of these features are identified,” they noted.
“However, impactors several tens up to 150 m in size are totally fragmented and vaporized during atmospheric entry, resulting in a low-altitude airburst, similarly to the Tunguska and Chelyabinsk events over Russia in 1908 and 2013, respectively.”
“Observation by direct eye witness accounts and indirect infrasound, seismic, video cameras, and numerical modeling of medium-sized airbursts have shown that these impacts represent a notable fraction of the extraterrestrial material accreted to Earth, with Tunguska-like events occurring once every 100 to 10,000 years, which is orders of magnitude more frequent than large crater-forming impacts.”
“However, evidence of these events is scarce in the geological record, principally due to difficulty in identifying and characterizing potential residues.”
“Finding evidence of these low-altitude meteoritic events thus remains critical to understanding the impact history of Earth and estimating hazardous effects of asteroid impacts.”
Scanning electron backscattered images of particles from the summit of Walnumfjellet, the Sør Rondane Mountains, Antarctica; these particles were produced during a single-asteroid impact 430,000 years ago. Scale bars – 100 μm. Image credit: Van Ginneken et al., doi: 10.1126/sciadv.abc1008.
Seventeen extraterrestrial condensation spherules found by the team on the summit of Walnumfjellet indicate an unusual touchdown event where a jet of melted and vaporized meteoritic material resulting from the atmospheric entry of an asteroid at least 100 m in size reached the surface at high velocity.
This type of explosion caused by a single-asteroid impact is described as intermediate, as it is larger than an airburst, but smaller than an impact cratering event.
The chondritic bulk major, trace element chemistry and high nickel content of the debris demonstrate the extraterrestrial nature of the recovered particles.
Their unique oxygen isotopic signatures indicate that their interacted with oxygen derived from the Antarctic ice sheet during their formation in the impact plume.
“To complete Earth’s asteroid impact record, we recommend that future studies should focus on the identification of similar events on different targets, such as rocky or shallow oceanic basements, as the Antarctic ice sheet only covers 9% of Earth’s land surface,” Dr. Van Ginneken said.
“Our research may also prove useful for the identification of these events in deep sea sediment cores and, if plume expansion reaches landmasses, the sedimentary record.”
“While touchdown events may not threaten human activity if occurring over Antarctica, if it was to take place above a densely populated area, it would result in millions of casualties and severe damages over distances of up to hundreds of kilometers.”
The team’s paper was published in the journal Science Advances.
_____
M. Van Ginneken et al. 2021. A large meteoritic event over Antarctica ca. 430 ka ago inferred from chondritic spherules from the Sør Rondane Mountains. Science Advances 7 (14): eabc1008; doi: 10.1126/sciadv.abc1008
