Frequent ozone depletion resulting from impacts of asteroids and comets


The impact with the Earth’s oceans of a relatively small meteoroid, ≈260-300 m diameter, having a mean density of 2,500 kg/m3 and mean velocity of 17.8 km/s, would vaporize and loft sufficient seawater to increase the global stratospheric content of chlorine by 3 ppbv and possibly cause an Antarctic “ozone hole.” Meteoroids of this size or comets having equivalent kinetic energies are estimated to strike the Earth’s oceans about once every 29,000-40,000 years. A large, globally distributed ozone depletion is predicted to occur about two to four times less frequently, resulting from the a combination of increased chlorine content (10 ppbv chlorine every 75,000-110,000 yr), increased NOy content (10 ppbv every 75,000-186,000 yr), and increased water vapor content (4 ppmv every 66,000- 95,000 yr). The meteoroid diameter required to cause a large globally distributed ozone depletion is in the range 390-660 m for an average density and impact velocity. These hypothesized ozone depletions are predicted to occur with sufficient frequency that it may be possible to detect past asteroid impacts by examining the UV damage to DNA of pollen grains retrieved from ice cores such as the Antarctic Dome C core dating to 740,000 B.P. Model calculations using the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIMEGCM) to simulate coupled dynamics and chemistry in the altitude range 30-500 km confirm that oxides of nitrogen and water vapor lofted to altitudes above 70 km are rapidly dispersed horizontally. These simulations show that a large fraction of the injected NO and H2O survives the photochemistry of the upper mesosphere and thermosphere and is transported to the stratosphere, with maximum ozone depletions occurring about three months following the impact.

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