Asteroid strike in oceans can destroy ozone layer: study

In the worst scenario, the impact of a one-km-wide asteroid would re-create the ozone hole, which appeared over Antarctica during the 1990s. It could cause massive loss of protection against the sun's ultraviolet (UV) radiation and force humans stay indoors during the daytime, according to the researchers.

"An asteroid impact in the ocean is always dismissed as being a danger for coastal sites, but not much else has been discussed about it," said Elisabetta Pierazzo, a senior scientist at the Planetary Science Institute in Tucson, Ariz.

To evaluate the asteroid hazard from climatic effects, Pierazzo combined her expertise in crater-impact modelling with simulations developed by US and German atmospheric scientists that show the interactive chemistry of the atmosphere.

They tested scenarios with a 0.6-mile asteroid and a 0.3-mile (500 meters) asteroid at a specific location and specific time of year.The models showed how ozone destruction would result from an asteroid strike launching seawater vapour hundreds of miles up into the highest parts of the atmosphere.

Chemical elements such as chloride and bromide that separated from the water vapour could then wreak havoc by destroying the ozone layer that protects life on Earth from the worst of mutation-causing UV rays.

"The thing with the asteroid is that it ejects the water vapour way up there -- we're talking hundreds of kilometres," Pierazzo told LiveScience. "It really goes to the highest extent of the atmosphere."

The results, detailed in the journal Earth and Planetary Science Letters, showed a 0.3-mile asteroid that hit in the Pacific Ocean in January would lead to a local impact on the ozone layer -- though "local" still meant an ozone hole that spread across the entire Northern Hemisphere.

By contrast, the 0.6-mile asteroid strike led to a worldwide drop in UV protection -- at which point the "hole" ceases to be a hole, the researchers said.

According to Pierazzo, location of the asteroid strike matters because of atmospheric circulation patterns. Time of year in each hemisphere also matters, because the strength of the ozone layer changes by season based on the amount of sunlight reaching the atmosphere, she explained.

A strike by the 0.3-mile asteroid saw a jump in UV radiation as measured by the ultraviolet index (UVI) to values above 20 in the northern subtropics for several months. A UVI of 10 or more can burn people with fair skin with just a few minutes of sun exposure.

Long-term effects of such high UV radiation would include skin-reddening, changes in plant growth and genetic mutations for humans and other organisms.

Such scenarios represent the likelier outcomes of an asteroid impact on Earth. An asteroid has about twice the chance of striking water rather than hitting land as 70 per cent of the planet's surface is covered by water.

Asteroid hunters have found about 903 of an estimated 1,050 near-Earth objects (NEOs) with diameters of 1 kilometre or greater as of Oct 1. That still leaves well over 100 objects in the 1-2 kilometre size undiscovered.

"As you go down in asteroid size, there's a lot more objects out there that have not been identified that could be a threat," said Pierazzo.