1. Formation of Methane in Comet Impacts and the Implications for Earth Meredith N. McCarthy December 7, 2004 Based on the paper Formation of Methane in Comet Impacts: Implications for Earth, Mars and Titan by Monika E. Kress and Christopher P. McKay

2. Outline Brief intro to the frost line and faint young sun quandaries Methane formation from comet impacts - Impact model - Impact chemistry - Transition metal catalyzation Results Implications

3. The frost line and faint sun problems for solar system models The frost line: - Volatile materials (water and organics) had to form beyond ~5 A.U. from sun - Comets and asteroids are the most likely seeding methods for the young Earth The faint young sun: - The early sun was ~25% less luminous than it is presently – the Earth would be completely frozen - The greenhouse effect is invoked to compensate for this effect – but there are serious problems with the abundance of CO 2.

4. Fixing the Greenhouse Effect The predicted abundance of CO 2 in the early atmosphere is not sufficient. CH 4 is hypothesized to be the gas that compensates for the additional GH effect. Mixing ratios as low as 10 -4 can provide the necessary heating. Most models use biological processes (microbes, thermal decomposition) to produce this CH 4. This research suggests that the fireball resulting from an impact provides suitable conditions for CH 4 to form, and historical impact rates will provide enough of these fireballs to reach the critical mixing ratio.

5. Simple Impact Model fireball explosion with evaporated impactor (typical comet abundances) all molecules completely dissociated fireball expands and cools from 5000K (spherical, isobaric, isothermal, no mixing with atmosphere) chemical equilibrium internally maintained until ~2000K

6. Impact Chemistry Impact Chemistry: - at 2000K, gas-phase reactions are quenched - Carbon gets locked into CO - Other reactions are kinetically inhibited Unlocking CO 2 and CH 4 - CO 2 forms photochemically from CO - Catalyzed CH 4 reactions lower the quenching temperature to ~500K

7. Catalyzing CH 4 Transition metals on dust grains provide locations for covalently bonded CO to dissociate and react - Iron and Nickel are good at this – already used in industry to form CH 4 from CO Impacts produce “iron-silicate smokes” which are highly reactive, but the rates are not parameterized well Thus this research uses well studied industry modeled reaction rates (CO, CO 2, H 2 O, H 2 and CH 4 reactions) - same temperatures and pressures as impact - same type of catalyst (Ni/MgAl 2 O 4 ) - complete reactions, not just CH 4 production

8. The Model and its parameters Cooling of fireball: Temp range: 5000K to cool (~500K) Cooling efficiency (ε) chosen for least efficient cooling before equilibrium Timescale 10 3 sec Larger mass – more CH 4 Higher atmospheric pressure does not increase CH 4 production More catalyst – more CH 4 to the equilibrium limit

9. Implications for Earth 1kg comet could produce about 5ppb of CH 4 Impact rates were large enough during the late heavy bombardment (~3.8 Gyr ago) to produce enough methane to keep water above freezing Only around 100-300 ppm needed to warm the Proterozoic Earth (~2.5 Gyr ago) (Pavlov et al, 2003) What happens in the time gap between these? Not sure, but even the most optimistic rates of comet impacts can’t maintain 100 ppm.

10. References Anders, E., Hayatsu, R., Studier, M.H., 1974, Astrophys. J. 192, L101-L105 Delsemme, A., 1988, Philos. Trans. R. Soc. London Ser. A 325, 509-523 Kress, M.E., McKay, C.P., 2004, Icarus 168, 475-483 Pavlov, A.A. et. al., 2001, J. Geophys. Res. 105, 11981-11990 Pavlov, A.A. et al., 2003, Geology 31, 87-90 Xu, J., Froment, G.F., 1989, Amer. Inst. Chem. Eng. J. 35, 88-96