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It's Not Actually Size That Determines How Deadly a Meteor Is

16 DECEMBER 2021

We have a lot to thank meteorites for. Had they not instigated several mass extinction events, including wiping out non-avian dinosaurs, we probably wouldn't be here today.

But some things still don't add up about the massive scale of decimation they can cause.

 

"For decades scientists have puzzled over why some meteorites cause mass extinctions, and others, even really big ones, don't," says University of Liverpool sedimentologist Chris Stevenson.

Such mass extinctions are usually attributed to impact winters – where huge volumes of exploded ground smothers out sunlight, starving plants and algae and plunging the planet into coldness.

This would suggest larger meteorites, with the capacity to propel greater dust blankets into the sky, would have a bigger effect on the global biosphere than smaller ones.

But this is not what's been observed in Earth's geological records.

"It's surprising when we put together the data,'' explains Stevenson. "Life carried on as normal during the 4th largest impact with a crater diameter of ~48 km (30 miles), whereas an impact half the size was associated with a mass extinction only 5 million years ago."

Impact winters usually last for only a few years, but the lighter upflung dust can persist up to 100,000 years.

So geochemist Matthew Pankhurst from Spain's Technological and Renewable Energy Institute and colleagues analyzed this ejected dust from 44 meteor impacts over 600 million years.

 

"Using this new method for assessing the mineral content of the meteorite ejecta blankets, we show that every time a meteorite, big or small, hits rocks rich in potassium feldspar it correlates with a mass extinction event," says Stevenson.

This has been consistent over the last 600 million years.

"Meteorite impacts that hit potassium feldspar-poor rocks only correspond to background extinction intensities," the team explain in their new paper.

Feldspars are aluminum-silicate rocks, crystallized from magma, making up around 60 percent of Earth's crust. Potassium feldspar is common in many soils, and unlike other substances smashed into our atmosphere during these meteor impacts – like acid rain causing hydrocarbons – it is a safe and un-reactive chemical. 

However, potassium feldspar is a powerful ice-nucleating aerosol – meaning it can massively alter cloud composition. 

So the team proposed that once the immediate effects of blasting Earth's ground into the atmosphere (impact winters) wane, the chemistry of what lingers in the air starts to come into play. If it's normal clay dust, the climate system will rebalance, but if it's potassium feldspar it continues to disrupt Earth's cloud dynamics in two key ways.

 

More ice-nucleating minerals in the air mean clouds will contain a higher proportion of ice crystals as opposed to dense water droplets usually found in lower, warmer regions of the sky, making these clouds more transparent. This reduces the reflective effect water droplet clouds usually have (their albedo), allowing more light through to heat up the planet.

The weakened albedo also suppresses cloud cooling feedback mechanisms, increasing climate sensitivity. This in turn makes the whole climate system more vulnerable to other disturbances such as increased emissions from volcanic eruptions.

Some of the world's largest volcanic events are not associated with mass extinctions, but others are. And these ones also happen to be linked to more potassium feldspar in our atmosphere.

"Many kill mechanisms only variably correlate with extinction events through geological time: they coincide with these rare periods of climate destabilization by atmospheric potassium feldspar," the researchers write.

It's incredible how powerful something that is not at all directly harmful to us can be when it's in the wrong place.

"This strongly suggests that the driver of severe extinction episodes is a critical change in atmospheric function," Pankhurst and colleagues write.

"Anthropogenic activities may represent similar climate forcing with the rapid input of aerosols into the atmosphere that influence cloud dynamics."

This research was published in the Journal of the Geological Society.