COMETARY AIRBURSTS AND ATMOSPHERIC CHEMISTRY

1. COMETARY AIRBURSTS AND ATMOSPHERIC CHEMISTRY Adrian L. Melott (1), Brian C. Thomas (2), Gisela Dreschhoff (1), and Carey K. Johnson (3) 1Department of Physics and Astronomy, University of Kansas 2Department of Physics and Astronomy, Washburn University 3Department of Chemistry, University of Kansas

2. Did a comet impact North America 12,900 years ago, inducing megafaunal extinctions, ending the Clovis culture, and triggering massive reglaciation? Firestone et al 2007; PNAS 104, no 41 p 16016

3. About 12,900 years ago, a variety of species suddenly(?) disappeared from North America as the climate cooled, along with the “Clovis points” associated with a widespread human hunting culture. Why? The main ideas are climate change and overhunting by humans. Today I will discuss the results of our recent investigation of the comet impact idea.

4. A black organic carbon mat is found across North America, carbon-dated to 12.9 kya. Clovis points and megafaunal bones are found below it. Above it, neither is found. The terminal change is almost instantaneous (100 y or less), and coincides with sudden climate cooling.

5. The task: estimate atmospheric chemistry changes and compare with available data from ice cores • The kinetic energy of bolide impact can easily generate temperatures of order 105°K and deposit substantial energy • Such energies can dissociate nitrogen and oxygen molecules, resulting in synthesis of NOx • A major side effect is ozone depletion • Nitrate rainout can be recorded in ice cores • We can model the process and compare with data

6. A tale of two events Tunguska, Siberia ?Laurentide Ice Sheet? June 30, 1908 12,900 years ago? 5 × 107 kg5 × 1013 kg R = 15 m 1.5 km Both have been hypothesized as air-bursting comets; both are assumed to have come in at about 30 km/s relative speed, both are assumed to be half rock half “ice” (mostly water).

7. Model the energy input • In both cases the energy is assumed to be deposited in the atmosphere, from 10 to 50 km altitude, and quickly mixed over longitude. • We compute the nitrate output, again assuming mixing over the northern hemisphere, and get a deposition per unit area, to compare with ice core data.

8. Results and data—nitrate Tunguska: both simulation and ice core data from GISP2H (Greenland) show an integrated total of 1.5 mg/m2—excellent agreement, and coincidentally close to the Carrington solar flare in 1859. (Dreschhoff 2002—nitrate is black)

9. Extrapolate to YD Comet? • A million times the energy of Tunguska violates conditions of validity of the code. • In existing work, nitrate production scales linearly with energy, giving 1 kg/m2 for YD! • This is too much energy for valid scaling, but it suggests much more nitrate should be there. • Ice core data has poorer time resolution—at best about 5 years. A peak could be missed. • Do the best we can.

10. Summary of ice core data • In C, the solid line is the nitrate deposited mostly in the winter after Tunguska (solid line). • In A & B, we see with poorer time resolution a peak around the time of the YD event. • Dashed line is ammonium, attributed to forest fires by the YD comet authors. All 3 show an ammonium peak.

11. Comparison: Simulation & Data (1) Agreement for the Tunguska event with data is excellent, except that atmospheric ionization does not make ammonium ion, which was found in the ice core. The Tunguska forest fire (30 km across) was too small to account for this. (2) The ice core nitrate enhancement around the YD times was about 10-6 kg/m2, but a big peak could have been missed. The scalings predict a million times more but cannot be trusted. There is a comparable ammonium enhancement found in the ice core. (3) Comets contain ammonia; direct deposit from the comet could explain YD ammonium but again the Tunguska object was too small. (4) Can a consistent explanation be found assuming both are comets?

12. Alternative: the Haber Process • N2 + 3 H2↔ 2 NH3 • Developed in 1909; actually uses methane and water to make hydrogen, reacted with nitrogen to make ammonia (fertilizer & explosives). • Although it is exothermic, since 4 molecules become 2, it is favored by high pressures. • Cometary water ice, atmospheric nitrogen, high pressure (6000 atm). It can work. Tunguska needs water source: impacted a semi-frozen swamp.

13. Summary and conclusions ? • The ice core nitrates are easily produced by impact, though the models suggest YD should have produced more. We suggest higher time resolution studies in YD epoch ice cores. • All the events show an ammonium enhancement roughly comparable to the nitrate enhancement. Only an analog of the Haber process appears capable of such ammonia production. • The large NOx synthesis a YD comet would have caused would also cause large atmospheric ozone depletion, with a major Solar UVB disaster for at least a decade. • Cometary airbursts and atmospheric chemistry: Tunguska and a candidate Younger Dryas event. (Adrian L. Melott, Brian C. Thomas, Gisela Dreschhoff, and Carey K. Johnson) Geology, in press (April 2010).