Formation of Methane in Comet Impacts and the Implications for Earth

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
• 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
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
- The greenhouse effect is invoked to
compensate for this effect – but there are
serious problems with the abundance of CO2.
Fixing the Greenhouse Effect
•The predicted abundance of CO2 in the early
atmosphere is not sufficient.
•CH4 is hypothesized to be the gas that compensates for
the additional GH effect .
•Mixing ratios as low as 10-4 can provide the necessary
•Most models use biological processes (microbes,
thermal decomposition) to produce this CH4.
This research suggests that the fireball resulting from an
impact provides suitable conditions for CH4 to form, and
historical impact rates will provide enough of these
fireballs to reach the critical mixing ratio.
Simple Impact Model
• fireball explosion with
evaporated impactor
(typical comet
• all molecules
completely dissociated
• fireball expands and
cools from 5000K
(spherical, isobaric,
isothermal, no mixing
with atmosphere)
• chemical equilibrium
internally maintained
until ~2000K
Impact Chemistry
• Impact Chemistry:
- at 2000K, gas-phase
reactions are quenched
- Carbon gets locked into
- Other reactions are
kinetically inhibited
• Unlocking CO2 and CH4
- CO2 forms
photochemically from CO
- Catalyzed CH4
reactions lower the
quenching temperature
to ~500K
Catalyzing CH4
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 CH4 from CO
• Impacts produce “iron-silicate smokes” which are
highly reactive, but the rates are not parameterized
• Thus this research uses well studied industry
modeled reaction rates (CO, CO2 , H2O, H2 and CH4
- same temperatures and
pressures as impact
- same type of catalyst
- complete reactions, not
just CH4 production
CO  3H 2  CH 4  H 2 O,
CO  H 2 O  CO2  H 2 ,
CO2  4 H 2  CH 4  2 H 2 O
The Model and its parameters
• Cooling of fireball:
dT / dt  4 r 2 (Tamb
 Tfireball
) ( 1) / NR
• Temp range: 5000K to cool
• Cooling efficiency (ε)
chosen for least efficient
cooling before equilibrium
• Timescale 103 sec
• Larger mass – more CH4
• Higher atmospheric
pressure does not increase
CH4 production
• More catalyst – more CH4
to the equilibrium limit
Implications for Earth
• 1kg comet could produce
about 5ppb of CH4
• 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.
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