In 1908, when an object entered the Earth’s atmosphere above the Podkamennaya Tunguska River, it flattened 80 million trees over nearly 2,200 square kilometers, and sent atmospheric shock waves reverberating around the world. Fortunately, this event was in a remote region and very few people were believed to be killed.
But research published in Nature’s Scientific Reports in 2022 by Tankersly et al. suggested that a similar, but even more powerful comet airburst in the Ohio River Valley may have been the death knell for the Hopewell civilization, some 1,600-1,700 years ago just outside modern day Cincinnati. However, other scientists rejected the arguments.
The initial paper points to several lines of evidence for an impact event in the region. The first is that archaeological sites in the area contain a surprising number of meteorites and objects made from meteoritic iron.
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Other archaeologists have suggested that these objects were prized in the culture and that the members of this civilization would trade for them using a continent-wide network of exchange. Indeed, the Hopewell culture is most famous for its burial practices, which involved interring the dead in massive earthwork mounds and burying them with prized possessions from across the continent. Examples include obsidian traced to the Rockies, shells from Florida, and other goods from distant lands. Specifically, previous authors suggested the meteoric iron may be from the Brenham meteorite in Kansas. However, the authors of this paper reject this as the chemical composition is notably different.
The next clue the authors point to is a widespread presence of minute iron and silicon-rich sand grain-sized particles in the regions surveyed. Particles of similar size and composition are found in other regions known to have experienced airburst or impact events, such as the KT boundary. Interestingly these meteoritic particles always existed in a relatively thin layer at all sites explored. The same layer of soil also tends to have more iridium – a relatively rare element on Earth, but frequently found in asteroids.
Perhaps even more interesting, the archaeological sites also contained a distinct layer of ash. The fire that caused the ash layer was sufficiently hot that it was able to convert the limestone to lime (calcium oxide). The authors describe this as “widespread synchronous fires resulting from a catastrophic cosmic airburst event.”
Following this, the authors propose that their civilization did continue on, and it was in this post-impact era that those who remained gathered the meteoric fragments and used them to make the well-studied items. But, the authors suggest that the impact event was not forgotten, and point to a comet-shaped earthwork as a testament to the event.
Comet-shaped Milford earthwork based on E. G. Squier and E. H. Davis’ 1848 Ancient Monuments of the Mississippi Valley Comprising the Results of Extensive Original Surveys and Explorations (Smithsonian Institution, Washington D.C.).
Lastly, they suggest that this potential impact was an unfortunate result of an increased period of risk in which there was an increased number of comets. To support this, they point to Chinese astronomers who documented 69 “near-earth” comets in the period between 220 and 589 CE, evidently including Halley’s comet, which they state came within 0.09 AU of earth (~35 times the distance of the moon).
The first response to these claims came from Neuhauser & Neuhauser and attacked several of the lines of evidence of the original authors.
Firstly, the Neuhausers note that the chemical analysis presented by the original authors is not consistent with comets in the first place. Rather, if anything, the material found was more akin to what should be expected from an asteroid.
Next, they criticized the claim that there was any increased period of risk, pointing out that authors of the original paper overstated and may have fundamentally misinterpreted the Chinese source for the comets. Specifically, the Chinese source gives no support for the claim that the comets observed were “near-earth”. Even the claim that Halley’s comet came so close is dubious; an even closer approach in 837 CE makes understanding of prior approaches uncertain, as their predictions would be highly dependent on just how close the comet came that year.
Worse, it’s not entirely established that the 69 comets the authors claimed were truly comets. Ancient astronomers had little understanding of the various types of transient objects, and often used very similar language to describe them, usually referring to any temporary astronomical phenomenon as a “guest star.” Thus, the underlying assumption that this period saw abnormal comet activity is unsubstantiated.
The Neuhausers then take aim at the claim that a comet shaped earthwork is evidence of anything. They note that the original authors failed to discuss the structure in the context of the surrounding structures. Neglecting to consider the wider archaeological context could lead to misinterpretation due to selection bias and pareidolia.
Lastly, they question whether or not there truly was a comet during the purported period that could have been the inspiration and source of a potential airburst. The Neuhausers turn again to the Chinese records, which they suggest are quite complete. They note that Chinese astronomers were quite observant of the sky for their astrological practices, and the period in which the supposed airburst took place was during a turbulent time in which court astrologers would have been especially observant. Yet, no such extraordinarily bright comet appears in their records, nor the records of any other astronomically inclined civilization.
Tankersly et al. responded to the criticism, conceding that the chemical composition was not indicative of a comet, but maintain that an airburst event occurred as the result of an asteroid progenitor.
However, the authors still lean into the comet shaped earthwork as significant, even though they no longer claim a comet or a cometary fragment as the cause of an airburst.
They also ask why, if meteor fragments were imported, does the iron/silicate rich fragments permeate the area?
A second response came last month from Nolan et al. with even harsher criticism, in which they claim that Tankersly et al. “misrepresent primary sources, conflate discrete archaeological contexts, improperly use chronological analyses, insufficiently describe methods, and inaccurately characterize the source of supposed extraterrestrial materials.”
First, Nolan et al. point to the claim that charred surfaces must be due to a cataclysmic astronomical event. They note that the charred areas Tankersly et al. point to were all found within ceremonial mounds in which historians expect there to have been large, ceremonial fires to have been prepared prior to the burials. Thus, the charred layers reported were not a single event, but several individual ones.
Next, the authors note that the claim that the Hopewell civilization declined sharply following the supposed event is false. Rather, the civilization continued but with a “gradual sociopolitical and economic reorganization” that led to the cessation of large earthwork construction.
The authors then demonstrate that Tankersly et al. have misrepresented their own data collection, suggesting that the site of a charred surface also had the higher traces of iridium. Yet Tankersley’s records from the excavations show that these were two separate excavations done at the Jennison-Guard site, but that the former came from the cultural hub of the site, whereas the iridium-enhanced area came from a second excavation 10m away where no artifacts were found and which also lies in a floodplain.
Likewise, Nolan et al. point out that Tankersley et al. conflated distinct archaeological contexts at another site, in which they assume that the age of one of their excavations would be identical to the age of an area nearly a kilometer away explored by other archaeologists without basis.
Nolan et al. then take aim at the claim that the spherules found were extraterrestrial in origin at all, regardless of whether they came from a comet or asteroid. They note that, while the spherules are high in iron and silicon, they are deficient in magnesium and nickel, which are typically found in asteroids. Thus, they conclude, that the spherules are a result of local soil chemistry and not of extraterrestrial origin.
This criticism by Nolan et al. was sufficient that the editors of the paper lost confidence in the original analysis and have retracted the paper.