Transcript Lecture 7: The faint young Sun problem_Part 2_CH4 feedbacks

The Evolution of Earth’s
Atmosphere and Surface
James F. Kasting
Department of Geosciences
Penn State University
Brief review of theories of early
atmospheric composition
1.
•
•
•
Oparin (1924, 1938),
Haldane (1929)
Atmosphere was a strongly
reduced mixture of CH4, NH3,
H2, and H2O
Good from an origin of life
standpoint
Belief in this model was
greatly strengthened by the
success of the Miller-Urey
experiments (early 1950’s):
spark discharge in such a
mixture created many
organic compounds,
including amino acids
Image from Wikkipedia
Problems with the old model
• The giant planets are not good analogs for the early
terrestrial atmosphere, as Urey had argued
• CH4 and (especially) NH3 can be photolyzed and
converted into other gases
• Modern (surface) volcanic gases are relatively oxidized,
as nicely documented in books and papers by Dick
Holland. The major constituents, in decreasing order of
abundance, are H2O, CO2, CO, H2, and sometimes SO2
and H2S. Little or no CH4 is released from surface
volcanoes.
– And there is now good evidence that the redox state of the upper
mantle has not changed with time
Brief review of theories of early
atmospheric composition
2.
•
•
•
Rubey (1955), Walker
(1977), Kasting (1993)
The atmosphere was a
weakly reduced mixture of
N2, CO2, and smaller
amounts of H2 and CO
The H2 concentration was
determined by the balance
between outgassing from
volcanoes and escape to
space
Hydrogen was assumed to
escape at the diffusionlimited rate
J. F. Kasting, Science (1993)
Diffusion-limited escape
• Hydrogen escape can
be limited either at the
homopause (~100 km
altitude) or at the
exobase (~500 km
altitude)
• On Earth today, H
escape is limited by
diffusion through the
homopause
• This may not be true,
however, in hydrogenrich primitive
atmospheres
Homopause
Exobase
100 km
500 km
• If hydrogen escape was
limited by energy rather
than by diffusion, then the
escape rate could have
been 10-100 times slower
than the maximum rate
• This could have helped
the atmosphere remain
highly reduced for a long
time
• But, the problem is
complex because the
young Sun should have
been much more active,
leading to greater upper
atmosphere heating
Hydrogen escape rate (cm-2s-1)
Slow hydrodynamic escape?
Homopause hydrogen mixing ratio
F. Tian et al., Science (2005)
Solar EUV flux vs. time
 Studies of nearby young solartype stars give estimates for
the time history of shortwavelength solar emissions
(CoRoT press release drawing
of young solar-analog star)
• Young stars rotate faster, are more
magnetically active, and hence have
greatly enhanced EUV and XUV
I. Ribas et al., Ap. J. (2005)
emissions 
Estimated exospheric temperatures
on the early Earth
• Helmut Lammer’s group has
worked on this problem. They
calculate high exospheric
temperatures on the early
Earth (Kulikov et al., Space Sci.
Rev., 2006)
– Solar XUV flux was 100 times
greater at 4.5 Ga, 10 times
greater at 3.8 Ga
– Based on observations of
young, solar-type stars
– Earth’s atmosphere would be
swept away by the solar wind
if not for the presence of a
magnetic field
– But, these atmospheres were
hydrostatic…
Bauer and Lammer, Planetary Aeronomy
(2004)
Earth’s present thermosphere at different
XUV fluxes: hydrodynamic model
• Work in progress with
Feng Tian
• Physics packages from
the NCAR TIEGCM
• Atmosphere is dynamic,
so as it expands into
space, it cools
adiabatically
• Still needs work to
accurately model H2-rich
atmospheres
--Enhanced solar EUV flux
--Present Earth atmosphere
composition
F. Tian et al., JGR Planets (2008)
 EUV
An aside: Thermal escape of heavy
gases (C and O) from early Mars
• High solar EUV flux from the young Sun, combined with low gravity, leads
to fast thermal escape of C and O
• If most CO2 is lost by photodissociation and escape, as opposed to
formation of carbonates, then the atmosphere can become oxidized
• Helps explain early Mars’ climate
--NO2 and O3 provide additional warming (by lowering the albedo)
• Could provide another “false positive” for life
Back to Earth…
• Impacts may also
have helped shape
the early environment
Artist’s rendition of the K-T impact
(http://commons.wikimedia.org/wiki/
File:KT-impact.gif)
The late heavy bombardment
• The Moon, Mars, and
Mercury are all covered
with impact craters
• These impacts must
have postdated the
oldest Moon rock, which
has an age of ~4.44 Ga
• Many of the Apollo
moon rocks have ages
of 3.8-3.9 Ga
1200 km
Mare Imbrium
Mare Orientale
• Located just over the
western limb (on the
lunar farside)
• Crater is 930 km in
diameter
• Requires a 100-km
diameter impactor
• The Earth must have
been getting hit with
objects at least this big
Photograph from NASA Lunar
Orbiter 4 (1967)
$64,000 question:
• Was there a “pulse” of bombardment at
~3.9 Ga, or was there a 700-m.y.
exponential tail to accretion?
Probable answer: Yes, i.e., both of these
are true
The Nice model
• A new theory of outer
Solar System evolution
can explain the
clustering of lunar age
dates near 3.9 Ga
• Current orbital periods:
Jupiter – 11.8 yrs
Saturn – 29.4 yrs
• In the Nice model,
Saturn starts closer in,
and its orbital period is
less than twice that of
Jupiter
http://images.spaceref.com/news/2006/
1196_web.l.jpg
Orbital evolution of the giant planets
Neptune
Uranus
Saturn
Jupiter
Tsiganis et al., Nature, 2005, Fig. 1
• Simulated evolution of outer planets interacting with a ‘hot’ disk
of planetesimals
• Uranus and Neptune flip positions as Jupiter and Saturn pass
through the 2:1 mean motion resonance!
Implications of the Nice model
• Asteroids would be continually kicked into new orbits
(some of them Earth-crossing) as Jupiter’s mean motion
resonances swept through the asteroid belt
– Helps to explain asteroid belt clearing
– This would give rise to more-or-less continuous bombardment in
the inner Solar System
• Comets would be strongly perturbed when Uranus and
Neptune make their move
– This may have created a pulse of impacts near 3.8-3.9 Ga
• Impacts involving both types of objects would have
influenced atmospheric composition (creating CO and
CH4) and may have frustrated the early origin of life
What was Earth’s surface like prior to 4.0 Ga?
– Atmosphere was probably weakly reduced,
assuming that the impact flux was not too large
– No evidence for atmospheric O2 until about 2.4 Ga
(despite continued claims to the contrary)
– Climate is a big question: Was it hot, or were
temperatures relatively cool?
– Information about early climates can, in principle
be derived from O isotopes in various types of
sedimentary rocks
O isotopes—the last 900 k.y.
• O isotopes are measured in carbonate minerals in deep
sea drill cores
• Low 18O values correspond to warm temperatures
after Bassinot et al. 1994
Marine carbonate 18O vs. time
Time 
Cold
Warm
Shields & Veizer, G3, 2002
• On longer time scales, the O isotope composition of carbonates exhibits
a steep trend
• Taken at face value, a decrease of 10‰ in 18O of carbonates
corresponds to a temperature increase of ~54oC, putting the Archean
Earth at roughly 70oC!
18O of modern and ancient cherts (SiO2)
(SMOW)
 Time
Cold
Warm
• Cherts, which are better preserved, tend to show the same
trend, i.e., they become increasingly depleted in 18O as they
get older
P. Knauth, Paleo3 219, 53 (2005)
Chert data:
• Mean surface temperature was 7015oC at 3.3
Ga
– Ref.: Knauth and Lowe, GSA Bull., 2003
Carbonate data:
• Surface temperatures remain significantly
elevated until as recently as the early Devonian
(~400 Ma)
• These arguments have been supported by
evidence from silicon isotopes (Robert &
Chaussidon, Nature, 2006) and from molecular
biology (Gaucher et al., Nature, 2008)
• I don’t believe this, however, because
there were glaciations at ~2.4 Ga and 2.9
Ga
Geologic time
First shelly fossils (Cambrian explosion)
Snowball Earth ice ages
Warm
Loss of methane greenhouse
Rise of atmospheric O2 (Ice age)
Ice age
Warm (?)
Origin of life
The faint young Sun problem
• Furthermore, the Sun was less bright back at that time,
making it even more difficult to make the early Earth hot
Kasting et al., Scientific American (1988)
CH4/CO2/C2H6 greenhouse with haze
Late Archean Earth?
Paleosols
Water freezes
• One can compensate for low solar luminosity by having higher
concentrations of greenhouse gases, specifically, CO2 and CH4
• Warming by CH4 is limited, however, by the formation of organic haze
• Hence, one needs very high CO2 in order to make the early Earth hot 
J. Haqq-Misra et al., Astrobiology (2008)
Surface temperature vs. pCO2 and fCH4
3.3 Ga
S/So = 0.77
• Getting surface temperatures of ~70oC at 3.3 Ga would
require of the order of 3 bars of CO2 (10,000 times present)
• pH of rainwater would drop from 5.7 to ~3.7
• Would this be consistent with evidence of paleoweathering?
Probably not…
Kasting and Howard, Phil. Trans. Roy. Soc. B (2006)
Possible explanations for the O
(and Si) isotope data
• The oxygen isotope
composition of seawater
may have changed with
time
– Kasting et al., EPSL, 2006
• The cherts may have
been formed by mixing of
warm, hydrothermal fluids
with cold seawater on the
ocean floor
– Hofmann et al.,
Precambrian Res., 2005;
van den Boorn et al.,
Geology, 2007
• Seawater isotopic composition is
thought to be controlled by waterrock interactions within the midocean
ridge hydrothermal circulation
systems (Muehlenbachs and Clayton,
1976)
Conclusions
• The Earth’s atmosphere prior to 4.0 Ga was probably
weakly reduced (mostly CO2 + N2). Some CH4, however,
was almost certainly present, especially after the origin
of life
– O2 was not present until about 2.4 Ga
• The early surface environment of the Earth, along with
climate and atmospheric composition, depends critically
on the impact history of the early Solar System
– The Nice model suggests that the heavy bombardment included
a pulse at 3.8-3.9 Ga, along with an extended tail to accretion
– Going back to the Moon with people may allow us to determine if
this is really true
• Despite the apparent congruity between isotopic and
biological data that support the hot early Earth
hypothesis, it seems unlikely that this was actually the
case