ASSESSING THE RISK OF SMALL ASTEROID COLLISIONS AND COMETARY AIR BURSTS TO SOCIETY FROM THE RECENT GEOLOGIC RECORD

The 2013 Chelyabinsk fireball and the 1908 Tunguska air burst above Siberia are well-known modern examples of threats from small asteroids and comets. But how common are such events in the recent history of our planet, and do we have cause for concern that the energy from such collisions could cause more acute or widespread damage from either the shockwave or radiant heating?

One important lesson was learned from the 2007 Carancas event when a small H5/6 ordinary chondrite approximately 2 meters in diameter slammed into the Peruvian Altiplano excavating a crater about 15 meters across (Tancredi et al., Meteoritics and Planetary Science, 2009). Despite infrasound data demonstrating that the bolide had fragmented in the atmosphere, the resulting debris, likely shepherded together by the bow wave from a large leading piece, struck the ground as a single mass traveling at more than 5 km/s. Had this impact occurred in a populated city, the results could have been tragic.

Another lesson comes from scattered occurrences of unique glasses that formed around the globe during the Pleistocene. Schultz et al. (Geology, 2022) recently reported evidence of a series of low-altitude air bursts over the Chilean Atacama Desert near the town of Pica approximately 12,000 years ago. The thermal energy and heated air from this event melted surficial sediments in discrete patches spread over almost 70 kilometers. Analysis of sand-sized mineral and rock fragments trapped in these Pica glasses revealed that the bolide most likely originated in the outer solar system, most probably a comet.

The Dakhleh glasses from Egypt and the Edeowie glasses from South Australia also have been suggested to have formed by similar air bursts, c. 150 ka and 700 ka, respectively (Osinksi et al., Meteoritics and Planetary Science, 2008; Haines et al., Geology, 2001). Our observations of mineral and rock fragments in these glasses reveal that they are very similar to those discovered in the Pica samples and implicate a similar air-burst origin from a body (or bodies) that originated as a comet. Because airburst-generated surface glasses are easily destroyed, further characterization of the objects responsible for these events and recognition of similar glasses around the Earth are important for accessing our risk.