When a comet explodes: Cosmic airbursts are more common than we thought

When most people think of space impacts, they imagine massive craters – but an airburst can deliver the same level of destruction without leaving a visible scar. In these rare events, a comet or asteroid explodes in the atmosphere, sending searing heat and shockwaves across the land.

Recent studies from UC Santa Barbara are shedding new light on how destructive airburst events can be, and why they might be far more common than we’ve realized.

“Touchdown events can cause extreme damage through very high temperatures and pressures. 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,” noted Professor James Kennett.

Evidence of ancient airbursts

In one study, the research team uncovered traces of a 12,800-year-old cosmic explosion preserved in deep-ocean sediments from Baffin Bay, off Greenland’s western coast.

“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,” said Professor Kennett.

According to the Younger Dryas Impact Hypothesis, a fragmented comet exploded above Earth, triggering a sudden global cooling period, the extinction of large animals, and major cultural shifts in humans.

Evidence of the event is often found in a carbon-rich “black mat” layer containing platinum, iridium, metallic melt spherules, shocked quartz, and minerals fused into meltglass. “They’re preserved in marine sediments as deep as about 2,000 meters,” noted Professor Kennett.

While the material doesn’t reveal the exact shock strength, it does show the scale and reach of the blast. “The material was thrown up into the atmosphere, and was globally transported and deposited in a broadly distributed layer that we earlier have described.”

A lake that might be a crater

For decades, scientists have debated whether any crater exists from the Younger Dryas boundary period. Finding one is tricky – touchdown airbursts don’t always leave obvious marks on the landscape.

“Previously, there has been no evidence for the Younger Dryas boundary (YDB) event of any crater or possible crater,” said Professor Kennett. “So these events are more difficult to detect, especially when they are older than a few thousand years and after being buried, leave little or no superficial evidence.”

But in Louisiana, a shallow seasonal lake near Perkins could change that. First speculated in 1938 to be an impact site due to its shape and raised rim, the depression was later found to contain spherules, meltglass, and shocked quartz – all consistent with cosmic impacts.

Radiocarbon dating links the material to the Younger Dryas event. Still, the scientists caution that more research is needed to confirm it as a crater.

Airbursts may leave unique signatures

Shocked quartz is one of the clearest markers of high-energy impacts. Traditionally, scientists have used it to confirm crater-forming events. But the new research expands that definition to include a wider variety of fracture patterns – a clue that airbursts may leave their own unique signatures.

One study focused on Tunguska, Siberia, where in 1908 a fireball flattened more than 2,000 square kilometers of forest. “The interesting thing about Tunguska is that it is the only recorded historical touchdown event,” Kennett said.

In addition to historical accounts and photographs of devastation, the team found shocked quartz grains, meltglass-filled fractures, impact spherules, and melted metal and carbon. This evidence suggests that the blast’s energy may have created small ground depressions now visible as swamps and lakes.

Another study examined Tall el-Hammam in the southern Jordan Valley, where a Bronze Age city is thought to have been destroyed by a similar event around 3,600 years ago.

Here too, the team identified spherules, meltglass, rare minerals, and a range of shocked quartz fracture patterns – some classic parallel cracks, others web-like or curved, hinting at varied blast pressures and directions.

Why airburst events deserve more attention

Taken together, these studies suggest that touchdown airbursts happen more often than the massive asteroid strikes that leave craters. And while they may be harder to detect, they can cause far more widespread destruction.

“They’re far more common, but also possess much more destructive potential than the more localized, classic crater-forming asteroidal impacts,” said Professor Kennett.

“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.”

The full study was published in the journal PLOS One.

Image Credit: UC Santa Barbara

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