New studies suggest that explosions of comets or asteroids above Earth's surface, known as touchdown airbursts, may have occurred more often and caused widespread destruction than previously recognized. Led by UC Santa Barbara's James Kennett, researchers have identified markers of these events in diverse locations, from ocean sediments to ancient ruins. These findings highlight the potential climate and societal impacts of such invisible cosmic threats.
Cosmic touchdown airbursts, where incoming objects like comets detonate above the ground, release intense heat and shockwaves without forming lasting craters, making them difficult to detect. UC Santa Barbara Earth Science Emeritus Professor James Kennett and his team emphasize that these events warrant greater scientific focus due to their destructive potential. "Touchdown events can cause extreme damage through very high temperatures and pressures," Kennett said. "And yet they don't necessarily form a crater, or they form ephemeral surface disturbances, but they're not the classic major craters that come from direct impacts."
Four recent studies provide evidence from multiple sites. In Baffin Bay off Greenland's western coast, a PLOS One paper reports the first marine sediment markers linked to the Younger Dryas Impact Hypothesis. Around 12,800 years ago, comet fragments are thought to have exploded, triggering global cooling, megafaunal extinctions, and shifts in human societies. These sediments, preserved up to 2,000 meters deep, contain platinum, iridium, microspherules, shocked quartz, and a carbon-rich "black mat" layer from widespread fires. "Baffin Bay is very significant because it's the first time we've found evidence for the Younger Dryas cosmic impact event in the marine record," Kennett noted.
A possible crater from this period appears in a shallow lake near Perkins, Louisiana, first noted in 1938 for its circular shape. Sediment cores since 2006 reveal meltglass, spherules, and shocked quartz dated to the Younger Dryas, though further verification is needed.
Reanalysis of the 1908 Tunguska event in Siberia, the only recorded historical airburst, uncovered shocked quartz with meltglass-filled fractures, impact spheres, and melted metals. Witnesses described a bright fireball flattening forests, and the blast may have formed local swamps. Similarly, at Tall el-Hammam in the Levant, destroyed around 3,600 years ago, diverse shocked quartz patterns support an airburst cause, including spherules, carbon, and rare minerals.
Kennett's team argues airbursts are more common and far-reaching than crater-forming impacts. "They're far more common, but also possess much more destructive potential than the more localized, classic crater-forming asteroidal impacts," he said. These events challenge traditional views of cosmic threats, urging better detection methods.