December 1, 2010 | A team of scientists is tackling a scenario that is the stuff of Hollywood thrillers: What happens if a medium-sized asteroid strikes Earth? In particular, what if it crashes into the ocean?

The question is not fanciful. Well over 100 asteroids about 1–2 kilometers in diameter are thought to be orbiting in paths that could bring them close to Earth, and many smaller asteroids are looming undiscovered in our planet’s neighborhood. Although the odds of one of these objects striking Earth in the near future are very low, the atmospheric consequences of such an event have never been explored in detail until now.

Four NCAR scientists, working with colleagues from Tucson’s Planetary Science Institute and Germany’s Max Planck Institute, ran computer simulations using a three-dimensional shock physics code (SOVA) and an NCAR-based global model (WACCM) that extends from Earth’s surface up into the thermosphere.

The results, published in Earth and Planetary Science Letters in October, show that a 1-km asteroid striking the ocean would send vast amounts of water vapor and sea salt into the atmosphere, penetrating well beyond the stratosphere, to altitudes above 100 km. In the stratosphere itself, seawater chemicals such as chlorine and bromine would strip away significant amounts of Earth’s protective ozone layer. This would lead to a huge spike in ultraviolet radiation reaching Earth’s surface, with levels higher than any observed today. A smaller asteroid, measuring 500 meters across, would deplete global ozone to levels similar to the record ozone holes over Antarctica in the mid-1990s.

The effects of life on Earth would include increased rates of sunburn, skin cancer, and cataracts, as well as difficulty growing certain crops.

As noted in the study, past research suggests that an asteroid 500 meters wide or less strikes Earth about once every 200,000 years on average, with larger asteroid strikes occurring about once every 800,000 years. The study only analyzed asteroid strikes on the ocean; such a scenario is twice as likely as a land strike given that oceans cover about 70% of Earth’s surface.

E. Pierazzo, R.R. Garcia, D.E. Kinnison, D.R. Marsh, J. Lee-Taylor, and P.J. Crutzen, “Ozone perturbation from medium-size asteroid impacts in the ocean,” Earth and Planetary Science Letters, doi:10.1016/j.epsl.2010.08.036

This animation shows the impact of an asteroid 500 meters in diameter striking the ocean. Shaded colors represent changes in materials density, with lighter shades corresponding to lower densities. Blue represents the atmosphere, gray the seawater, and green the remnants of the asteroid. The lowermost horizontal black line corresponds to the 15 km height, designating the tropopause (the boundary separating the troposphere and stratosphere) at the latitude of impact. The uppermost horizontal black line corresponds to 140 km, which is approximately the top level represented by WACCM. (Image courtesy Elisabetta Pierazzo, Planetary Science Institute.)

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