New research suggests that Earth has experienced frequent and destructive cosmic “touchdown airbursts,” which are explosions caused by comets or asteroids detonating above the surface. Unlike traditional impacts that create craters, these airbursts release intense heat and pressure without leaving visible marks, making them difficult to detect. This discovery has significant implications for understanding the history of cosmic events and their effects on the planet.
According to a study led by Earth Science Emeritus Professor James Kennett from the University of California – Santa Barbara, these explosive encounters may occur more often than previously estimated, posing a greater risk than the well-known impacts linked to mass extinctions. “Touchdown events can cause extreme damage through very high temperatures and pressures,” Kennett noted. “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.”
The research, which includes four recently published studies, presents evidence of multiple airburst events throughout Earth’s history. These events are characterized by incoming objects, such as comets, detonating in the atmosphere and producing shockwaves and extreme heat that affect the surface.
New Findings on Younger Dryas
One of the notable studies, published in the journal PLOS One, documents the first discovery of impact markers related to the Younger Dryas Impact Hypothesis (YDIH) in marine sediments. This evidence was found in deep-sea cores collected from Baffin Bay, located off Greenland’s western coast. Kennett emphasized the significance of this finding, stating it marks the first time evidence for the Younger Dryas cosmic impact event has been identified in the marine record.
The Younger Dryas period, which occurred approximately 12,800 years ago, is believed to have been triggered by fragments of a comet that exploded in the atmosphere. This event is associated with a sudden global cooling episode, coinciding with the extinction of numerous large animals and significant changes in human populations. The explosions likely ignited widespread fires, resulting in a distinctive carbon-rich layer known as a “black mat,” found primarily across the Northern Hemisphere.
Researchers have identified various materials in the sediments, including rare elements linked to the original space object, glassy material created from melted Earth sediments, and shocked quartz exhibiting unique crack patterns. Kennett explained that these materials, while not directly measuring the explosion’s strength, indicate the event’s extensive impact on the climate.
Challenges in Identifying Cosmic Impacts
Cosmic impacts vary in frequency and magnitude, ranging from the continuous fall of fine extraterrestrial dust to massive collisions occurring every tens of millions of years. While large impacts typically leave craters that serve as clear evidence, touchdown airbursts often do not create permanent landscape alterations, complicating their detection.
Kennett pointed out that previous research has not identified a crater associated with the Younger Dryas Boundary (YDB) event. This poses challenges for scientists attempting to confirm such events. Notably, a shallow seasonal lake near Perkins, Louisiana, may represent the first known crater dating back to the Younger Dryas Boundary. Initial observations regarding the lake’s circular shape were made in 1938, but detailed sediment studies began only in 2006. The findings from sediment cores collected from the site revealed meltglass, spherules, and shocked quartz, with radiocarbon dating confirming their origin within the Younger Dryas period.
Researchers advocate for further investigation to test the hypothesis that this lake resulted from a cosmic impact, highlighting the need for continued exploration of ancient events.
Revisiting Historical Cosmic Events
Shocked quartz has long been regarded as evidence of extreme heat and pressure from cosmic impacts. Traditionally linked to large crater-forming events, recent studies indicate airbursts can generate a broader range of fracture patterns. Two additional papers published in the journal ScienceOpen analyzed samples from the site of the Tunguska explosion, which occurred in Siberia in 1908, and revisited findings related to Tall el-Hammam, an ancient city in the Levant believed to have been destroyed by a similar event about 3,600 years ago.
Kennett noted that the Tunguska event is the only recorded historical touchdown event. Witnesses reported a bright fireball, while photographs later documented vast areas of flattened forest. While previous studies focused on visible damage, researchers have now identified microscopic evidence of airburst-related materials at the Tunguska site. They discovered shocked quartz with clear planar fractures filled with meltglass, along with tiny impact-formed spheres and melted metal, suggesting the energy released by the explosion created small depressions that later formed swamps and lakes.
The team also reinforced the theory of an airburst over Tall el-Hammam during the Middle Bronze Age. Alongside previously documented spherules and carbon, they identified shocked quartz with various crack patterns, indicating complex blast directions similar to those observed at Tunguska.
Together, these studies suggest that cosmic impacts, particularly touchdown airbursts, may be more frequent and potentially more destructive than previously understood. “They’re far more common but also possess much more destructive potential than the more localized, classic crater-forming asteroidal impacts,” Kennett stated. “The destruction from touchdown events can be much more widespread, and yet they haven’t been very well studied, so these should be of interest to humanity.”
