An asteroid splashdown in one of Earth's oceans could trigger a destructive chemical cycle that would wipe out half the ozone layer, according to a new study. The massive loss of protection against the sun's ultraviolet (UV) radiation would likely force humans into a vampire-style existence of staying indoors during daylight hours.

The worst scenario based on an asteroid 0.6 miles (1 kilometer) wide would re-create the hole in the ozone layer that appeared over Antarctica during the 1990s, except this would be worldwide. UV levels in the study's simulation soared beyond anything measured so far on Earth by the daily forecasts of overexposure to UV radiation, and remained that way for as long as two years.

"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. "I was looking at the asteroid hazard from climatic effects."

To do that, Pierazzo combined her expertise in crater-impact modeling with simulations developed by U.S. 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 asteroid (500 meters) at a specific location and specific time of year.

They had no idea what to expect.

Breaking bad

The models showed how ozone destruction would result from an asteroid strike launching seawater vapor hundreds of miles up into the highest parts of the atmosphere. Chemical elements such as chloride and bromide that separated from the water vapor 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 vapor way up there — we're talking hundreds of kilometers," Pierazzo told LiveScience in a phone interview. "It really goes to the highest extent of the atmosphere."

Model results showed a 0.3-mile asteroid that hit at a latitude 30 degrees north 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.

Location of the asteroid strike matters because of atmospheric circulation patterns, Pierazzo explained. 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. (In the upper atmosphere, ozone forms when oxygen molecules are broken apart by the sun's UV light.)

A strike by the 0.3-mile asteroid saw a jump in ultraviolet radiation as measured by the ultraviolet index (UVI) to values above 20 in the northern subtropics for several months. Normally, a UVI of 10 or more can burn people with fair skin with just a few minutes of sun exposure, and some of the highest recorded UVI values on Earth (around the equator) have reached just 18. On certain days, a UVI of 20 was recorded at a high-altitude desert in Puna de Atacama, Argentina.

A strike by the larger of the two model asteroids boosted UVI values above 20 within a 50-degree latitude band north and south of the equator for about two years. Some areas within the band saw UVI spikes as high as 56. That band's northern end would include cities such as Seattle and Paris, while the southern end would extend into cities within countries such as New Zealand, Chile and Argentina.

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

The future threat

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. Those odds come from the fact that over 70 percent of the Earth's surface is covered by water, with about two-thirds covered by oceans more than a mile deep.

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

More cause for worry may come from smaller NEOs less than 1 kilometer wide. NASA has found just 5 percent of the estimated count for such NEOs, which leaves tens of thousands of unknowns.

"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, whose research was detailed online Oct. 2 in the journal Earth and Planetary Science Letters.

But finding NEO threats to Earth remains NASA's job. Pierazzo's next step with her colleagues will involve modeling the atmospheric impact of an asteroid strike on land. That may prove an even more complicated scenario, because of the combination of dust blocking out incoming sunlight and other possible chemical effects on the ozone.

Researchers have often suggested that a land asteroid impact would create a nuclear winter effect similar to what might follow a nuclear war. But researchers should hope that's where the comparison between space rocks and nukes ends — a past simulation showed that even a regional nuclear war could create a massive ozone layer hole across the world.

This article was reprinted with permission from LiveScience.

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