The identification of airbursts in the past: insights from the BIT-58 layer

Airbursts are estimated to be the most frequent type of destructive impact events. Yet, confirmation of these events is elusive, resulting in a major gap in the impact record of Earth. The recent discovery of igneous chondritic spherules produced during a new type of touchdown airburst 430 thousand years (kyr) ago over Antarctica, in which a projectile vapor jet interacts with the Antarctic ice sheet, provided the first trace of such an impact in the geological record. In terms of petrology and geochemistry, particles constituting the BIT-58 dust horizon, which was found in surface ice at near Allan Hills in Antarctica, are almost identical to those produced 430 kyr ago. We demonstrate here that BIT-58 particles were indeed formed during a touchdown event between 2.3 and 2.7 million years (Myr) ago. This represents the oldest record of an airburst on Earth identified to date. Slight geochemical differences with 430 kyr old airburst spherules provide additional constraints on spherule condensation in large airburst plumes. Finding confirmation of airbursts in the paleorecord can provide insight into the frequency of the most hazardous impacts and, thus, has implications for planetary defence.