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9 Early Earth
9.1 Introduction
To understand the origin and evolution of life, it is important to study the conditions on
early Earth and the emergence of life on our own planet. Earth provides the single
example where we can study the origin of life in great detail. The results help to identify
signatures of life also on extrasolar planets.
9.2 Origin of organic matter on Earth
Delivery of extraterrestrial organic matter to the early Earth
Carbon can be considered as an indicator of the amounts of abiotic organic matter.
Figure 1: Ratio of carbon to heavy
elements (all elements more massive
than H and He) for various Solar
System objects. The horizontal axis is
not to scale. Biotic organic matter
(life) is also indicated in the case of
Earth.
Inwards the asteroid belt the amount of organic matter declines abruptly. It appears that
during the time when life originated, the whole inner Solar System contained little
organic material, in contrast to the present (biotic) carbon abundance on Earth.
On the other hand, liquid water, which is another prerequisite for life, could be
maintained on planetary surfaces over the last 4.6 Gyr not beyond about 1.7 AU
(optimistic estimate).
This is a paradoxical situation as the two ingredients of life (liquid water and organic
matter) appear, in a very general sense, to occupy different areas of the solar system. A
solution to the paradox proposed by J. Oró in 1961: life could have been kick-started by
organic matter delivered to the early Earth by extraterrestrial objects (meteorites,
comets).
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A large amount of carbon and organic matter was brought to the Earth during the socalled period of late heavy bombardment, between 4 Gyr and 3.8 Gyr ago, as indicated
by Moon craters.
The estimated amount of organic material contributed by comets and meteorites to Earth
is 1019–1021 kg, with 91019 kg being the total carbon budget on Earth. Despite a large
uncertainty in the number of comets impacting on Earth it is clear that a significant
amount of organic matter was delivered to Earth by comets.
Table 1: The biological role and types of organic molecules (both monomers and polymers) found in life
and in meteorites.
The main difference between the organic constituents of life and meteorites is that
meteorites contain simple organic molecules (monomers) whereas life also contains
more complex polymerized versions of these molecules (polymers).
Synthesis of organic molecules on the early Earth
The surfaces of new formed planets and their atmospheres provide another opportunity
for the generation of organic molecules.
To combine relatively simple organic compounds into highly organized organic systems
of living organisms, energy is required
 to fuel synthesizing chemical reactions
 to sustain primitive life
Energy sources:
 total radiation from the Sun and UV light (the largest source)
 electric discharges (lightning)
 cosmic rays
 radioactivity
 volcanoes
 shock waves from meteorite impacts
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Miller and Urey’s experiment
1953: An attempt to recreate the type of chemical reactions that may have occurred on
the early Earth.
Figure 2: Miller and Urey’s apparatus
used in the abiotic synthesis of amino
acids. The lower flask containing boiling
water represents the primordial ocean.
Water vapor enters the upper flask,
representing the primordial atmosphere,
and mixes with methane, hydrogen, and
ammonia. Electrical discharges cause the
gases to combine into amino acids, which
then accumulate in a water-filled trap.
The experiment was allowed to run for one week. When the reaction products were
analyzed it became clear that a number of organic compounds needed for life, notably
amino acids, had been produced relatively simply and abiotically in a reducing
atmosphere. The ease at which these compounds could be produced suggested that they
should be abundant and widespread in the Universe.
This was also supported by the discovery of similar organic compounds in the
Murchison meteorite and Miller-Urey experiment (Table 2).
However, models of atmospheric evolution indicate that the early Earth would not have
had a methane- and ammonia-rich reducing atmosphere. It now seems that the more
stable molecules CO2, N2 and H2O dominated the Earth’s atmosphere (cf. Sect 8.5).
Under these less reducing conditions Miller-Urey synthesis is much more difficult. So it
appears that the environment of the early Earth might have been fit for sustaining life
but less suitable for the in situ production of life’s organic raw materials. This
emphasizes the role of the early bombardment and the importance of the cometary
contribution of organic matter.
The content of Earth’s atmosphere prior to the appearance of life is however not yet
finally settled. For example, it was suggested that H2 might have been lost to space
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more slowly than previously assumed. A sufficiently large H2 content would permit
synthesis of organic molecules even in a CO2 dominated atmosphere. This leaves still
some doubt on the relative contribution to organic matter by comets and synthesis on
Earth.
Table 2: Abundances of amino acids synthesized in the Miller-Urey experiment and those found in the
Murchison meteorite. The number of dots represents relative abundance. Those amino acids used by life
(i.e. in proteins) are indicated with stars.
9.3 Water on Earth
Water and oceans play a key role in the evolution of life. Indeed, life on Earth probably
evolved in water, and the oceans could have shielded organic molecules from the
massive UV radiation and protected living organisms from the heavy cometary and
meteoritic bombardment of our planet.
Origin of water
The water content of dust and planetesimals in the inner solar system was very low at
the time of terrestrial planet formation, because the high temperatures within the ice-line
strongly reduced the amount of ices. Therefore, the water was brought to Earth from
other regions in the solar system, from the asteroid belt or even further away from the
Sun. Possible candidates are dust and micrometerorites, asteroids, comets, or large
planetesimals.
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Knowing the timing of water delivery to the Earth may allow a choice to be made
among the possible scenarios of the ocean formation proposed until now: (1) an early
delivery of water during the accretion of the Earth, which implies that the oceanic water
inventory was available since the beginning, or (2) a continuous delivery of water
through eons, which implies expanding oceans. Currently, the most accepted hypothesis
is the former, but the extraterrestrial carrier of the water (chondiritic vs. cometary) and
the precise moment (during the planetary growth or at the end of accretion) are still
matters of debate.
There exist several findings that provide evidence of the existence of (deep) oceans
already on early Earth:
 4.4 Ga (Ga = billion years) old zircons contain information (based on oxygen
isotope ratios) on the presence of liquid water very early in the history of the Earth
(Peck et al. 2001, Valley et al. 2002).
 The minimum water depth needed to form the 3.2 Ga old Ironstone Pods in the
Barberton greenstone belt is 1000 m (de Ronde et al. 1997).
 Volcanic massive sulfides are common in Archaean terrains. Some of them are
analogous to sulfide deposits produced at present-day midocean ridges. To produce
such deposits, the ocean-floor pressure should be higher than the critical pressure,
which is equivalent to an ocean depth of about 3 km (Harrison 1999).
Micrometeorites (dust particles less than a millimeter in size) are therefore usually
excluded as candidates for the main water delivery on Earth. They would result in a
slow growth of the oceans, contradicting the evidence of early oceans.
The best constraints to choose between the potential candidates are isotope ratios
observed on Earth and on the different candidates. Further evidence is provided by
numerical modeling of the accretion history of the planets in the solar system, giving
information on the timing and the amount of accretion of bodies that originated from
different areas of the solar system.
Of particular importance is the ratio of deuterium and hydrogen (D/H ratio) in the water
molecule. The values of the D/H ratios in the terrestrial and extraterrestrial reservoirs
are reported in Fig. 3. In general, the D/H ratio in water increased towards the edge of
the proto-planetary nebula around the Sun, because the energy provided by UV
radiation and the cosmic rays in the interstellar medium shifted the equilibrium of the
exchange reaction H2O + HD  HDO + H2 towards a larger D/H ratio in water (Geiss
& Reeves 1981).
The water of Earth’s oceans has a D/H ratio of 155.710–6, while the whole Earth has a
D/H ratio of 149–15310–6. These values are close to those found in the carbonaceous
chondrites (128–18010–6, average 14910–6) and those measured in the Antarctic
micrometeorites (14010–6). The D/H ratio in the water of comets has been measured
only in comet Halley (316±34 10–6), in comet Hyakutake (290±100 10–6), and in
comet Hale-Bopp (320±120 10–6).
The obtained values of the D/H ratio in water on comets are 2–3 times larger than the
value found for modern seawater on Earth, suggesting that comets did not contribute
significantly to the delivery of water to Earth. A simple mass and isotopic balance
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shows that the amount of water delivered by comets on Earth is around 10% of the total
(Dauphas et al. 2000).
Figure 3: Frequency distribution of the
D/H ratios measured in carbonaceous
chondrites and Antarctic micrometeorites
compared to values for the proto-solar
nebula, Earth oceans and comets.
The three comets for which the D/H ratio has been determined belong to long-period
comets probably formed in the Uranus-Neptune region or the Kuiper belt. Comets from
that region might have arrived in large numbers relatively late during the accretion of
Earth simply because the time-scale of the Uranus and Neptune formation was larger
than the Jupiter/Saturn formation.
Comets from the Jupiter/Saturn region have also been suggested as candidates for water
delivery on Earth. Their D/H ratio was probably comparable to the ratio observed in the
asteroid belt and on today’s Earth, because Jupiter and Saturn are sufficiently close to
the inner solar system. However, the lifetime of comets from the Jupiter/Saturn region
was extremely short. Simulations suggest that they would have been dynamically
eliminated within 105 years afer the formation of Jupiter and Saturn, which was latest
about 10 Ma after the formation of the Sun. At that time only planetary embryos existed
in the terrestrial planet formation region. Any water brought to these embryos would
probably have been lost again during the numerous subsequent giant impacts leading to
the formation of the terrestrial planets.
Similarly, small asteroids appear to have been negligible agents for water delivery
(Morbidelli et al. 2000). They were more subject to interference by Jupiter and Saturn
than large planetesimals from the asteroid belt and would also have been lost too
quickly.
One possibility is that hydrated minerals, which have formed early in the outer asteroid
belt while the solar nebula was still present, could have migrated inward by gas drag.
These minerals could then be implemented into planetesimals at 1 AU out of which
Earth formed (Ciesla & Lauretta 2005).
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The currently most accepted hypothesis is that 90% or more of the water has been
delivered to Earth by a few, large planetesimals of chondritic composition that have
formed in the outer asteroid belt (Morbidelli et al. 2000). The remaining fraction would
have been delivered by comets. The timing of delivery is likely at the end of the
accretion of the Earth. It is plausible to assume that large planetesimals from the outer
asteroid belt consisted of about 10–20% of water similar to carbonaceous meteorites
from that region. It is possible that up to 10 times more water than necessary (water
content of today’s oceans: 1.41021 kg) was delivered in this way. Thus, a large fraction
can be lost during giant impacts and still retain the Earth’s current amount of water.
More and better numerical simulations of the accretion of terrestrial planets are however
necessary combined with detailed analysis of the chemical composition of Earth’s
mantel and of the isotope ratios of D/H but also of noble gases, to confirm this
hypothesis.
Evolution of oceans
The formation of the oceans on early Earth can be summarized in four steps represented
by Fig. 4.
Figure 4: Illustrations representing the main phases of the formation of the terrestrial oceans. Years
indicate the time before today. The impact shown in the first step represents the formation of the Moon.
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The first step is from the start of Earth’s accretion (4.56 Ga ago) to the segregation of
the core and the end of the interior outgassing, estimated at 4.45–4.50 Ga. During this
period water was delivered to Earth. Impacting asteroids and comets vaporized,
dispersing the trapped volatiles in the atmosphere. A small part of them has been
probably partitioned in the melt (the surface of the Earth was in a molten state) and
outgassed successively. All water of the present oceans was vaporized in the
atmosphere due to the high surface temperature.
A runaway greenhouse effect produced by the massive H2O and CO2 atmosphere
dominated the second phase of formation of the oceans. The greenhouse maintained the
surface of Earth close to the melting temperature as long as the heat flow from the
interior exceeded 150 W/m2.
For a surface heat flow lower than 150 W/m2, the runaway greenhouse could not be
maintained longer and the surface of the Earth rapidly cooled down and formed a crust
(3rd step). The oceans formed quickly due to the condensation of the atmospheric water
vapour. Abe (1993) suggested that the terrestrial oceans were produced in less than
1000 years due to heavy rain, with raining rates at 7000 mm/year, 10 times the present
raining rate at tropical latitudes. Assuming an atmospheric pressure at the surface of
several hundred bars, water starts to condense and precipitate at 600 K. Temperatures
lower than 600 K were probably available between 50 to 150 Ma after the formation of
Earth.
The newly formed oceans were too hot for the development of life. Only by about 4.3–
4.2 Ga (4th step), the oceans were cold enough to assure the survival of the first living
communities. However, it is likely that successive giant impacts have temporally
increased the temperature causing partial or complete evaporation of the oceans with
subsequent recondensation (Fig. 5). Oceans became stable at the end of the Hadean (3.9
Ga) when the massive meteoritic bombardment of the Earth ended.
Figure 5: Impact events on Earth (solid) and
the Moon (dark gray) as a function of time
(after Zahnle and Sleep 1997). On the right
hand side the amount of evaporated water is
indicated, with the dashed line giving the
mean depth of the oceans. The boxes marked
S.P.A., Im, Or, and Ir indicate impacts that
formed the lunar mares South Pole Aitken,
Mare Imbrium, Orientale, and Iridium. The
other symbols denote impacts that formed the
craters Tsiolkovsky, Hausen, Langrenus,
Copernicus, Tycho, Vredevort, Sudbury and
Chicxulub (responsible for the K/T boundary
event). Ovals mark the energies associated
with the formation of the Earth and the
Moon. Because the Earth has 81 times the
mass and 13 times the cross-section of the
Moon, it has a much higher chance (96%) of
sustaining an impact by comparison with the lunar surface (4%). In addition, the impact energy on Earth
is about a hundred times greater than for similar events on the Moon.
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Figure 6: Chronology of the first events that resulted in the formation of the oceans and the continents.
Chemical composition of the primitive oceans
If the volume of the oceans has not likely changed in the last 4 billion years, its
chemistry certainly has. The physical and chemical parameters, such as temperature,
pH, and the salinity have an impact on the development of life. The temperature will be
discussed in Sect. 8.5. Here we concentrate on pH and salinity. Results are based on
thermodynamical modeling and on the few traces that oceans left in the Precambrian
marine sediments.
The Archaean ocean was probably acid. This hypothesis is based on the fact that the
primitive ocean was in equilibrium with an atmosphere mainly composed of CO2. At
the present time, and probably already in the Archaean eon, the pH of the ocean is
controlled by its equilibrium with the carbonates (dissolution and precipitation of
CaCO3) and the CO2 fugacity in the atmosphere. Calculations in Fig.7 are based on an
initial partial pressure of CO2 of 10 bars. In the Hadean, the CO2 level could have been
much higher and the pH of the ocean directly controlled by the equilibrium of
carbonation of the oceanic crust rather than by precipitation of carbonates, requiring
more complicated models.
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Figure 7: Variations of the partial
pressure of CO2 in the atmosphere
and change of pH of the ocean, at
equilibrium. The different fields
represent the variations of the pH of
the oceans as obtained from different
thermodynamic calculations.
The salinity of the oceans derives from two distinct sources: weathering of the
continental crust and oceanic hydrothermalism. In the present day ocean, weathering is
the dominant source of salinity, but during the Hadean and the Archaean periods, it was
likely the oceanic hydrothermalism that was dominant. The volume of the continents
was lower than today (10–15% of the present-day volume) and the surface of the Earth
was composed by a larger amounts of small plates resulting in much larger total length
of midocean ridges and thus more hydrothermal activity. The Proterozoic era marks the
end of an ocean salinity dominated by hydrothermal sources and the beginning of a
salinity produced by weathering.
Measuring the isotopic variation of the Sr and Nd ratios in the Archaean and
Proterozoic marine sediments have permitted observation of the change in the salinity
sources of the oceans. And the concentration of the major cations and anions dissolved
in the Archaean fluids was determined from the analysis of the 3.2 Ga Ironstone Pods of
the Barberton greenstone belt in South Africa (Fig. 8).
The major contribution to the salinity of the oceans was likely, since the beginning,
halite (NaCl). The Na/Cl ratio in Archaean seawater was the same as in the present-day
ocean, namely 0.858), but the total amount of sodium and chlorine was a factor of 165%
higher in the Archaean (Fig. 8).
Figure 8: Variations of the concentration of the
main dissolved species in Archaean seawater and
in modern seawater. The main constituents, Na+
and Cl–, had a concentration of 165% of today’s
value.
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9.4 Earth’s continental crust
Life apparition, as well as organic molecule duplication, which is a prerequisite to life
development necessitated specific physical and chemical conditions and environment.
For instance, all proposed scenarios imply the presence of liquid water on the Earth
surface (ocean, lake, pond, etc), whereas others envisage alternation of aqueous and dry
periods (emerged continents). These conditions are classically realized in continental
domains or at the continent/ocean interface. Consequently, in order to address the
problem of the origin of life on our planet, it appears necessary, not only to know when
liquid water condensed on the surface of the Earth to form oceans, but also to determine
when the first continent formed and emerged.
On the other hand, the chemical composition of the ocean (i.e. pH) is directly correlated
to the mineralogical and chemical compositions of emerged continent. Indeed,
weathering and alteration of surface rocks render some chemical elements (i.e. alkali)
soluble, which, via rivers, are transported from continental crusts towards oceans, where
they can accumulate. From this point of view, detailed knowledge of the primitive
continental crust composition appears necessary to discuss the origin of life.
Most of the crust is formed by partial melting of deeper rocks (i.e. mantle peridotite).
There are two kinds of crusts: the oceanic crust and the continental crust.
Figure 9: Internal structure of the Earth. For sake of simplicity, iron is not identified in the different
mineralogical compositions mentioned on the figure (One can assume that 10% of Mg is replaced by Fe).
On the right is a simplified horizontally averaged temperature profile.
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Oceanic crust
Oceanic crust is formed by partial melting of the upper mantle during its adiabatic
upwelling beneath the 60000 km long midocean ridges. In the context of plate tectonics,
the oceanic crust returns and is recycled within the mantle at subduction zones. The
oldest known oceanic crust is about 180 Ma (“million years”) old.
Continental crust
The thickness of the continental crust varies between a few km at rifting zones to more
than 70km under mountains chains; its average thickness is about 35 km, which is
considerably thicker than the oceanic crust. It consists predominantly of granitic rock.
The continental crust is at least 10% less dense than the oceanic crust. Due to their
different density, their buoyancy is also contrasted such that their average altitude is
different resulting in a bimodal altitude distribution on Earth’s surface.
Continental crust is formed by more complex mechanisms of partial melting. Because it
is less dense than the oceanic crust it is more buoyant and only few continental crusts
can be recycled within the mantle. When plate motion pushes two continental
lithospheres one against another (India and Eurasia, for example), they collide; pile up
such that mountains form with deep continental roots due to isostatic equilibrium.
Figure 10: Growth and formation of continental crust. The hydrated, basaltic oceanic crust is partially
melted when it is subducted below another tectonic plate. Some material (e.g. islands that were created
over hot spots) can be accreted.
Age of oldest crust
At Acasta, in Northern Territories (Canada) smaller amounts (20 km2) of banded
Archaean rocks are exposed. These small outcrops constitute the oldest continental crust
so far discovered; it was generated at 4.030 ± 0.003 Ga (“billion years”) (Table 3).
Even larger outcrops (~3000 km2), though a bit younger, are exposed in Greenland.
They consist of magmatic rocks, now transformed into gneisses and called the Amitsôq
gneisses; they were dated at 3.822 ± 0.005 Ga. Associated volcanic and sedimentary
rocks from Isua yielded an age of 3.812 ± 0.014 Ga. Nearby, on Akilia island, a dyke
cutting a Banded Iron Formation (BIF) gave an age of 3.872 ± 0.010 Ga, thus
demonstrating that Akilia is still older than 3.872 Ga.
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Both Isua and Akilia formations contain the oldest known sedimentary rocks (BIF,
quartzites) that constitute a direct proof that liquid water existed on the surface of the
Earth at 3.87 Ga. In addition, these BIF contain organic matter with a 13C/14C isotopic
ratio depleted in 13C that could be interpreted as (oldest) biological signature, because
life preferably builds 12C into the organisms.
Table 3: Summary of the oldest crustal ages measured on terrestrial materials (from Gargaud et al. 2006).
Isua sediments:
3.8 Ga, oldest
evidence of life (?)
Acasta Gneiss:
4.0 Ga, oldest rocks
Jack-Hills zircons:
4.4 Ga, oldest mineral
Figure 11: Distribution of Archaean provinces. Exposed Archaean terrains are in dark blue, and areas
underlain by Archaean rocks are in light blue. The oldest known minerals, the oldest known rocks, and
the oldest sediments are highlighted.
Oldest minerals on Earth: Jack-Hills zircons
The oldest terrestrial material identified so far, are the zircons from Jack-Hills, found
about 800 km north of Perth, Australia. The oldest zircon has an age of 4.404±0.008 Ga
(Wilde et al. 2001). The Jack-Hills zircons tells us a lot about the conditions of Earth
just about 150 millions years after its formation.
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Zircon (ZrSiO4) is a common trace mineral in granitic rocks that preserves detailed
records of magma genesis. Once formed, zircon crystals are so durable that they can
persist even if their parent rock is exposed at the surface and destroyed by weathering
and erosion. Wind or water can then transport the surviving grains great distances
before they become incorporated into deposits of sand and gravel that may later solidify
into sedimentary rock. Indeed, the Jack Hills zircons—separated by perhaps thousands
of kilometers from their source—were found embedded in a fossilized gravel bar called
the Jack Hills conglomerate.
In addition to their longevity, they contain trace amounts of radioactive uranium, which
decays at a known rate to lead. Uranium atoms occasionally substitutes Zirconium
atoms as a trace impurity. Atoms of lead, on the other hand, are too large to comfortably
replace any of the elements in the lattice, so zircons start out virtually lead-free, which
makes zircons ideal minerals for radiometric dating.
Figure 12: Oldest zircon found so far. Arrows
point towards the oldest parts and a Quartz
inclusion. This zircon was found at Jack Hills,
Australia.
The zircons of Jack-Hills provide strong evidence that continents already existed as
early as 4.4 Ga ago. Rounded surfaces of some Jack Hills zircons show that wind and
possibly running water buffeted these crystals over long distances—possibly across a
large continental landmass—before they were finally laid to rest. Zircons found near
their place of origin retain their original sharp edges. The large number of ancient,
rounded Jack Hills zircons suggests their original source rocks were widespread.
Furthermore, the 18O/16O ratio of the Jack-Hills zircons provides evidence of liquid
water and relative low temperatures at the time of their formation (Fig. 13). These
zircons are enriched in 18O compared to the 18O/16O isotope ratio of Earth’s mantle. In
younger granites, such high 18O values indicate mixing of mantle magma with material
that was subjected to low-temperature alteration and then subducted and melted again. It
is significant that such alteration requires liquid water near the surface of Earth. Ocean
water has a much lower 18O/16O isotope ratio (it is the standard used to define 18O).
Contact of zircons with liquid water can lead to reactions that enrich the mineral with
18
O, or rather deplete the 16O content. The high 18O values of the Jack-Hills zircons
(larger than mantle values) are thus interpreted as (first) evidence of liquid water and
relatively low temperatures.
This does however not mean that liquid water was continuously present since that time.
Due to giant impacts parts of the water, possibly even all oceans might have evaporated
temporally (cf. Fig. 5).
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Figure 13: Comparison of 18O for the mantle
and the Jack-Hills zircons (from Valley 2006).
The conclusions obtained from the Jack-Hills zircons have lead to the concept of a cool
early Earth. Previously, it was often believed that Earth’s surface might have been
covered by a magma ocean during the whole Hadean eon. Arguments came from the
analysis of Moon’s craters that gave a relatively high impact rate around 3.9 Ga. For
earlier times the impact rate was extrapolated backward, assuming an exponential
reduction with time. Such frequent impacts would easily provide the energy to maintain
a magma ocean. The 4.4 Ga zircons indicate however a cool Earth and thus a much
lower impact rate during most of the Hadean. The formation of Moon’s creaters around
3.9 Ga are then attributed to a late heavy bombardment, possibly initiated by the
migration of Jupiter and Saturn that has caused resonances to move through the asteroid
belt with a subsequent increase of the impact rate in the inner solar system.
Figure
14: Estimates
of
meteorite impact rate during
Hadean and Archaean times. The
dashed line represents the
classical exponential decay of
impact rate whereas the solid line
represents the cool early Earth
with a late heavy bombardment
model.
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Evolution of oceanic and continental crust
The analysis of the layering in Archaean rocks demonstrates that plate tectonic operated
already since the early Archaean. The large amounts of internal heat produced during
Archaean times has necessarily been released, otherwise, accumulated heat should have
resulted in melting the external part of our planet, which is not shown by geological
record for this period. The total length of midocean ridges was significantly greater than
today to evacuate the great amount of heat (Fig. 15). Consequently, Archaean plates
were smaller than today and probably moved at a greater rate, such that the average age
of oceanic crust when it entered into subduction was < 20 Ma, whereas it is 60 Ma
today (the oldest oceanic crust existing today is 200 Ma old).
Horizontal tectonics allowed genesis of relief and mountain chains. However, due to
higher geothermal gradients, which lowered the crustal viscosity, Archaean relief should
have been lower than their modern equivalents.
The total area of continental crust during the Archaean was much smaller than today
(Fig. 15). It was however rapidly growing, although details of the continental growth
rate are not known. All models agree however that at least 75% of the continental crust
was generated and extracted from the mantle before 2.5 Ga. The Archaean-Proterozoic
boundary at 2.5 Ga marks the start of present-day geodynamic processes. Afterwards
continental growth was much slower, if at all present. Another example of geologic
difference starting at 2.5 G is a slight change of the mineral content of igneous rock
because of Earth’s cooling, which resulted in cooler magmas.
Figure 15: Schema comparing modern plate size (left) to its supposed Archaean equivalent (right). The
greater Archaean heat production resulted in a mosaic of plates smaller than today.
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9.5 Evolution of Earth’s atmosphere and climate
Formation of Earth’s atmosphere
During the accretion Earth might have possessed a (primary) atmosphere, probably a
mixture of H2, H2S, CO2, H2O and rare gases. This first atmosphere was probably lost
through erosion by the solar wind as well as by impact erosion, for instance, by the
massive impact with a Mars-sized planet that led to the formation of the Moon.
The first atmosphere was replaced by a slightly reducing to neutral atmosphere of CO2
and H2O (with minor amounts of other gases such as H2S, CH4, CO, N2). The second
atmosphere was produced by outgassing (by volcanoes), as well as by volatiles
imported via comets, meteorites and micrometeorites (by direct vaporization).
Ammonia (NH3) and methane (CH4), as used in the Miller-Urey experiment, were
probably not present in a significant amount. NH3 photolyzes rapidly in the absence of
UV screening by atmospheric O2 and O3. CH4 is not so strongly subject to this problem,
as it photolyzes only at wavelengths below ~145 nm, but photochemical models
indicate a lifetime of only about 10000 years in a low-O2 atmosphere. Furthermore,
today’s volcanoes produce mainly CO2, H2O and N2, which make it very probable that
the early atmosphere also consisted mainly of these molecules.
The pressure of the early atmosphere was very high. If we assume that all the water of
the present oceans (1.4 × 1021 kg H2O) was in the atmosphere (prior to the ocean
formation), the atmospheric partial pressure would be 270 bars. To this pressure, we
must add the partial pressure of CO2. The CO2 in the primordial atmosphere was
equivalent to the amount of carbon today preserved in carbonates, in continental
sediments, and in the biosphere. This amount would produce an atmospheric CO2
partial pressure of 40 bars. It has been proposed that the total inventory of CO2 in the
present mantle was concentrated in the primordial atmosphere. This amount is
equivalent to a partial pressure of 170 bars. The primordial atmosphere could have been
a dense mixture of 270 bars of H2O and 40–210 bars of CO2, an atmospheric
composition somewhat similar to that of Venus.
The faint young Sun problem
Earth is warmed by absorption of visible and near-IR radiation from the Sun and is
cooled by emission of thermal IR radiation. Earth has an atmosphere that warms the
surface by way of the greenhouse effect. Infrared radiation emitted by the planet’s
surface is absorbed and re-emitted by IR-active gases within the atmosphere. The extra
downward IR radiation helps to warm the surface. Thus, the global average surface
temperature Ts is ~288 K, about 33 K above the temperature it would have without this
greenhouse effect.
In Earth’s atmosphere today, the two most important greenhouse gases are CO2 and
H2O. H2O is responsible for approximately two thirds of this warming; CO2 accounts
for most of the remaining one third of the greenhouse effect. Lesser contributions, on
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the order of two to three degrees total, come from CH4, N2O, O3, and various
anthropogenic chlorofluorocarbons (CFCs).
The luminosity of the Sun was about 30% smaller when it formed compared to the
current luminosity, and it has increased more or less linearly in time. The incident
energy on Earth was correspondingly smaller. Assuming a constant CO2 concentration
and a fixed relative humidity (namely today’s values) the surface temperature would
have dropped below the freezing point of water prior to 2 Ga ago, which would almost
certainly have lead to a globally glaciated Earth. However, geologic evidence tells us
that liquid water and life were both present long before that time. This is called the faint
young Sun problem (Fig. 16). There exists actually evidence for global glaciations (see
text below) but only a few times during relatively brief periods. It is clear however that
the assumption of a constant composition of Earth’s atmosphere with time is not very
realistic.
Figure 16: Diagram illustrating the faint young Sun problem. The solid curve represents solar luminosity
relative to today. The dashed curves represent Earth’s effective radiating temperature (Te), and its mean
surface temperature (Ts), as calculated using a one-dimensional climate model. Fixed atmospheric CO2
and a fixed relative humidity profile were assumed in the calculation (from Kasting et al. 1988).
The solution to the faint young Sun problem probably lies in increased concentrations of
greenhouse gases at early times. Both CO2 and CH4 are plausible candidates. During the
Hadean CO2 (and H2O prior to the formation of oceans) were the main greenhouse
gases. Later in the Archaean it was probably both CO2 and CH4 that provided the
necessary greenhouse to prevent global glaciation as we will see below.
Climate feedbacks
The climate on Earth is strongly influenced by feedback mechanisms. Here we just
discuss briefly the most important ones.
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Water-vapor, which is near its condensation temperature (on Earth), acts as a nearly
instantaneous positive feedback. If the climate cools, the saturation vapor pressure
drops, and the atmospheric water-vapor concentration decreases proportionately. Less
water vapor results in a smaller greenhouse effect, which results in further cooling. Just
the opposite happens if the climate warms: Atmospheric H2O increases, thereby
increasing the greenhouse effect and amplifying the initial warming.
A second important climate feedback is the snow/ice-albedo feedback. An increase in
surface temperature causes a decrease in snow and ice cover, thereby decreasing the
planetary albedo, which causes an increase in surface temperature. This amplifies the
initial warming, so the feedback loop is again positive. Researchers believe that the
snow/ice-albedo feedback loop has played a major role in the advances and retreats of
the polar ice sheets throughout the past 2 million years.
The climate system must also contain negative feedbacks or it would be unstable. The
most basic negative feedback is the interaction between surface temperature (Ts) and the
outgoing IR flux (FIR). As Ts increases, FIR increases. However, Earth cools itself by
emitting IR radiation; thus, as FIR increases, Ts decreases. This creates a negative
feedback loop, the main reason that Earth’s climate is stable on short timescales. On
long timescales, the factors that affect climate (e.g., solar luminosity) can change; thus,
the IR feedback loop by itself no longer ensures stability.
Figure 17: Diagram illustrating the modern carbonate-silicate cycle, also referred to as the inorganic
carbon cycle. For illustrative purposes, we use the simplest silicate mineral, wollastonite (CaSiO3), to
represent all silicate rocks (from Kasting & Catling 2003).
On long time scales the carbonate-silicate cycle was most probably the most important
feedback that has stabilized Earth’s temperature. CO2 dissolves in rainwater to form
carbonic acid (H2CO3), which is a weak acid, but when it acts over long timescales, it is
strong enough to dissolve silicate rocks. The products of silicate weathering, including
calcium (Ca2+) and bicarbonate (HCO3–) ions and dissolved silica (SiO2), are
transported by streams and rivers to the ocean, where they precipitate as carbonates in
sedimentary rock. Due to plate tectonics the oceanic plates subduct below continental
plates at plate boundaries. When this happens, the overlying carbonate sediments are
carried down to depths where the temperatures and pressures are much greater. Under
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these conditions, carbonate minerals recombine with SiO2 (which by this time is the
mineral quartz) to reform silicate minerals, releasing CO2 in the process. This reaction is
termed carbonate metamorphism. The CO2 released from carbonate metamorphism
makes its way back to the surface and re-enters the atmosphere by way of volcanism,
thereby completing the carbonate-silicate cycle.
The carbonate-silicate cycle contains a negative feedback that stems from the
dependence of the silicate weathering rate on surface temperature, Ts. Weathering rates
increase both because of the direct effect of temperature on chemical reaction rates and
because evaporation (and, hence, precipitation) rates increase as Ts increases. As silicate
weathering is the loss process for atmospheric CO2, CO2 concentrations should tend to
fall as Ts rises and CO2 should increase as Ts falls. The response time of this feedback
loop is of the order of 60 million years. It is thus too slow to counteract human-induced
global warming, but fast enough to have a dominating effect on the billion-year
timescale of planetary evolution.
Evolution of Earth’s atmosphere
The originally very high CO2 content in the atmosphere has reduced during the Hadean
due to the carbonate-silicate cycle combined with a reduced bombardment by comets
and meteorites and a reduced volcanic activity (since Earth cooled down).
Geologic evidence indicates that the CO2 content in the Archaean was not large enough
to solve the faint young Sun problem. If the atmospheric concentrations of CO2 had
exceeded about eight times the present-day value of around 380 parts per million (ppm),
the mineral siderite (FeCO3) would have formed in the top layers of the soil as iron
reacted with CO2 in the oxygen-free air. But when the investigators studied samples of
ancient soils from between 2.8 billion and 2.2 billion years ago, they found no trace of
siderite. Its absence implied that the CO2 concentration must have been far less than
would have been needed to keep the planet’s surface from freezing.
It has therefore been suggested that methane was relatively abundant in the Archaean.
CH4 is destroyed both by photolysis and by reaction with the hydroxyl radical, OH. In
today’s atmosphere the lifetime of methane is about 10 years. In the Archaean its
lifetime was about 10000 years since oxygen and thus ozone (today the main OH
source) was negligible. Hence, in order for it to have been abundant in the early
atmosphere, it must have been resupplied by either biotic or abiotic sources. Abiotic
sources (mainly hydrothermal vents, although it is not clear whether part of this CH4
might actually be produced by methanogens living in hydrothermal vents) are 10 to 100
smaller than today’s biological CH4 flux.
The most reasonable and significant source of CH4 in the Archaean was thus life. The
methanogenic bacteria (or methanogens for short) can subsist by way of the reaction:
CO2 + 4 H2 → CH4 + 2 H2O. Due to the longer lifetime of CH4 compared to today it
was possible for methanogens to build up a substantial methane concentration in the
atmosphere. In addition methanogens would have even reduced the CO2 content further.
A humid greenhouse is the preferred climate for many methanogens; the warmer the
world became, the more methane they would have produced. This positive feedback
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loop would have strengthened the greenhouse effect, pushing surface temperatures even
higher. Concentrations of CO2 would have dropped (increased weathering and higher
consumption by methanogens) as those of methane continued to rise, until the two gases
existed in nearly equal amounts. Under such conditions, the behavior of methane would
have altered dramatically. Some of the methane would have linked together to form
complex hydrocarbons that then condensed into dustlike particles. A high-altitude haze
of these particles (similar to the conditions observed on Saturn’s moon Titan) would
have offset the intense greenhouse effect by absorbing the visible wavelengths of
incoming sunlight and reradiating them back to space, thus establishing a negative
feedback mechanism.
Figure 18: Evolution of Earth’s atmosphere (from Kasting 2004).
The appearance of oxygen producing bacteria (via oxygenic photosynthesis) has lead to
a strong increase in atmospheric oxygen 2.3 Ga ago (the O2 rise happened about 400
million years after the first oxygen producing bacteria; reason still under debate).
Geologic evidence for the O2 rise is well documented (oxidation of rock…). The O2 rise
has caused the establishment of an ozone layer that helped to protect life from UV
radiation (absorption mainly at 200–300 nm).
At the same time, around 2.3 Ga, clear evidence for a global glaciation exists on at least
three continents (North America, Africa and Australia). This correlation is unlikely to
be accidental.
Indeed, the rise of O2 would have eliminated most of the methane by reducing its
photochemical lifetime as well as by constraining the environments in which
methanogens could survive. The greenhouse effect would have been significantly
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reduced (due to the smaller methane concentration), causing the surface temperature to
drop below the freezing point of water (remember that the solar luminosity was only
about 80% of the present-day value). The lower temperatures would have reduced the
methane production by methanogens further. The result was a “snowball Earth” covered
with ice (the snow/ice-albedo feedback would have been happy to assist). Details are a
bit more complicated because geologic evidence actually indicates three separate
subsequent glaciations around 2.3 Ga.
The glaciation could only be stopped thanks to volcanoes. The CO2 stored in sediments
was released by volcanism back into the atmosphere over many million years until the
CO2 concentration became large enough to stop the glaciation. Note that the
atmospheric CO2 sink due to weathering was negligible during glaciation because of the
missing liquid water.
In the Late Proterozoic two additional “snowball Earths” may have occurred around 750
Ma and 600 Ma. The reason might be a further O2 rise combined with additional CH4
reduction.
Life thus had a significant influence on the evolution of Earth’s atmosphere. On the
other hand, the surface temperature could only be maintained at moderate, lifesupporting values thanks to the greenhouse effect and to negative climate feedbacks, in
particular the carbonate-silicate cycle. Plate tectonics and volcanism (only present
because Earth is not yet cooled down sufficiently) were thus at least in the case of Earth
a necessary prerequisite for the long term presence of life (otherwise the carbonatesilicate cycle would have stopped or global glaciations would not have been ended).
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