THEORETICAL CONSTRAINTS ON EARLY EARTH’S ENVIRONMENT
Eiichi Tajika
Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
tajika@eps.s.u-tokyo.ac.jp(Received August 25, 2008; Accepted September 10, 2008)(Abstract)
The only geological evidence for the early Earth iszircon which suggests existence of continental crustand oceans at 4.404 billion years ago. Based on therecord of the lunar impact craters, it has been
considered that heavy bombardment of small solar
system bodies to the Earth-Moon system occurred
during the first several hundred million years. There ishowever another possibility that an intense, but
short-term impact event (called the cataclysm) might
have occurred at around 3.9 billion years ago, which
could have been caused by rapid migration of the giant
planets. The atmospheric composition during the early
Earth was reducing, irrespective of source of volatiles.
This may have been favorable for the prebiotic
synthesis of organic materials and so the origin of life.
The early Earth might have been globally glaciatedbecause the heavy bombardment produced a large
amount of impact ejecta which consumed theatmospheric CO2 effectively through chemical
weathering followed by carbonate precipitation.
However, the effects of mantle overturn and large-scale
resurfacing on the early Earth’s environment remains
unknown.
(Keywords)
early Earth, Hadean, heavy bombardment, impact,
carbon cycle, snowball Earth
1. Introduction
Environment of the early Earth, from 4.55 to 4.0 Ga(billion years ago) has been largely unknown, because
there is almost no geological evidence for this timeinterval. This period is called "Hadean".
For the Hadean environment, we usually assume an
extremely hot climate, very intense volcanism andhydrothermal activity, heavy bombardment of
extraterrestrial objects, large tides because of the Moon
orbit much closer to the Earth, and a reducing
composition of the atmosphere and ocean [e.g., 1-3].
While some of these factors may be reasonable, others
would be still uncertain.
It has been known that there are zircon grains of the
Hadean age [4]. Recent studies of oxygen isotopic
composition of the zircons provide information on theenvironmental conditions for the Hadean Earth:
existence of continental crust and oceans since theearliest history of the Earth [5-8].
On the other hand, there is reduced carbon with low
carbon isotope ratio in the sedimentary rocks of theearliest Archean age, suggesting that photosynthetic
activity could have already operated by 3.8 Ga [9-15].
It is therefore probable that life emerged during theearliest Archean or even during the Hadean.
Considering the only geological evidence from the
Hadean Earth is zircon grains, theoretical constraints on

Viva Origino 36 (2008) 55 - 60
ⓒ 2008 by SSOEL Japan

the Hadean Earth is required for further understandingof the Earth's earliest environment. Because
environmental condition of the Hadean Earth should be
important to consider the origins of life on Earth, it isuseful to summarize the present understanding of theearly Earth.
In this paper, the recent studies on the early Earth's
environment will be reviewed, and the possible
conditions for the Hadean Earth will be discussed.
2. Geological evidence for the early Earth
The oldest known material on Earth is a mineralcalled zircon. Zircon (ZrSiO4) is a common tracemineral that is highly resistant to erosion, weathering,and metamorphism. Detrital zircons of the Hadean age
have been found within 3-Gyr-old quartzitic rocks in
Mt. Narryer and Jack Hills in the Narryer Gneiss
Terrain, Yilgarn Craton, Western Australia [4]. Amongthe zircon grains, the oldest age obtained so far with
U-Pb dating is 4404±8 Ma (million years ago) (Fig. 1)[5].
The zircons are zoned with respect to rare earth
elements and oxygen isotopic composition. In
particular, oxygen isotopic composition (δ18O value) of
the zircon shows values ranging from 7.4 to 5.0 ‰ [5].
Heavy oxygen isotopic composition is produced by
low-temperature interactions between rocks and liquidwater, suggesting that the rocks which melted to form
the magma included components that had been at thesurface in the presence of liquid water. In other words,
the zircon formed from an evolving magmatic source,
probably a granitic melt, which contained a significant
component of re-worked continental crust formed in thepresence of oceans.
The zircon therefore suggests that there were
continental crust and oceans on the Earth before 4404
Ma [5].
The zircon grains with the age of 3910 to 4280 Ma
also have large δ18O values ranging from 5.4±0.6 to
15.0±0.4 ‰, suggesting the interactions with
continental crust and oceans [6]. These zircons also
show hafnium isotope ratio with large positive andnegative deviations from those of the bulk Earth [7, 8],suggesting the development of a Lutetium-Hafnium
reservoir and widespread depletion of the upper mantle.
This also implies the formation of continental crust by
4.4 to 4.5 Ga and its rapid recycling into the mantle.
Because the Earth is thought to have formed at 4.55
Ga, it appears that continental crust and oceans formedat the very beginning of the Earth's history.
The Acasta Gneiss in the Slave craton in
Northwestern Territories, Canada, is known as theoldest rock on the Earth, which is dated 4031±3 Ma(Fig. 1) [16-18]. It is suggested that tonalitic magmas
was produced by partial melting of pre-existing crust,
implying existence of continental crust and liquid water- 55 -

Viva Origino 36 (2008) 55 - 60
before 4.0 Ga.
The oldest sedimentary rock on Earth, about 3.8 to
3.7 Ga, is contained in the Isua greenstone belt insouthwestern Greenland (Fig. 1). In the Isuasupracrustal rock, the presence of 13C-depleted reduced
carbon (the δ13C values of -5.9 to -24.9 ‰) has beenreported [9-13]. Because the difference in δ13C values
between inorganic and organic carbon (magnitude ofcarbon isotopic fractionation) indicates carbon fixation
by autotrophs employing the Calvin cycle, Schidlowski[12] proposed biological (photosynthetic) origins ofthese reduced carbon which was later in reequilibration
with carbonate, although this interpretation has been
controversial. Mojzisis et al. [13] showed that there arecarbonaceous inclusions with low δ13C values of -30 to-37 ‰ in apatites from banded iron formation in Isuaand the sedimentary rocks in Akilia Island. There are
not any known abiotic processes which can explain thedata. The reduced carbon may have been therefore
biogenic, although the age of the Isua samples might bemuch younger [14]. On the other hand, Rosing [15]found the reduced carbon of 2 to 5 micrometer graphite
globules whose δ13C values are about -19 ‰ inturbiditic and pelagic sedimentary rocks withwell-preserved sedimentary structures. Based on thedata and the mode of occurrence, he insists that the
reduced carbon represents biogenic detritus, perhaps
derived from planktonic organisms [15].
Structures interpreted as the oldest (3465 Ma)
cyanobacterial microfossils have been found in Apexcherts of the Warrawoona Group in Western Australia[19, 20]. If it were the case, oxygen-producing
photosynthesis began at least from 3465 Ma. It is
however revealed that the microfossil-like structures
should be secondary artifacts formed from amorphous
graphite within multiple generations of metalliferoushydrothermal vein chert and volcanic glass [21].
Because the micron-sized filament-like structures andalso the stromatolite-like structures can be formed
inorganically [22, 23], it is difficult to identify
biological activities in Precambrian only from themorphologies.

On the other hand, there is convincing evidence forthe existence of an ecosystem including photosynthetic
microbes at 3416 Ma in the Buck Reef Chert in South
Africa [24]. The isotopic composition of carbonaceous
matter is -35 to -20
which is consistent with
fixation by autotrophs. The presence of siderite, lack ofprimary ferric oxides, and the restriction of microbial
mats to shallow water which may reflect confinement
to the euphotic zone (<150 m) indicate that anoxygenic
photosynthetic microbes were active at that time [24].
Together with the possibility of biological origin of
reduced carbon found in the Isua supracrustal rock, it ispossible for the origins of life to have been before 3.8
Ga, or even during the Hadean. All the geologicalevidence for the Hadean is, however, the zircon grains.
We therefore need theoretical constraints on the Hadean
Earth for further discussion of the earliest environmentof the Earth.
3. Theoretical constraints on the early Earth
3.1. Late heavy bombardment
Based on the impact cratering record on the Moon,
heavy bombardment of extraterrestrial bodies to the
Earth-Moon system for the first several hundred million
years has been suggested. Assuming an exponential
decrease of impact flux with time, the frequency of
impact on the early Earth would have been 103 to 109
times the present flux (Fig. 2) [25]. During the Heavy
Bombardment period, there were several impact eventswith an impact energy corresponding to the total
evaporation of the ocean [26, 27]. This might havedamaged the earliest biosphere on the Earth, if itexisted, and resulted in multiple origins and extinctionsof life [26, 27].
It is however known that there is no impact melts
older than about 4.0 Ga in the lunar highland samples[28]. Recent study of lunar meteorites also shows thelack of impact melt older than 3.92 Ga [29]. These facts
might suggest that a very intense impact event occurredat around 3.9 Ga (Fig.2) [30]. This episode is called a"terminal lunar cataclysm" or "late heavy
bombardment".

Figure 2. Impact flux to the Earth. Solid curve represents
the flux with exponential decay, and dotted curve
represents the late heavy bombardment (i.e., the cataclysm).
Ages of the major impact basin on the Moon are indicated(modified from Koeberl (2006) [30]).

Figure 1. The earliest history of the Earth.

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Viva Origino 36 (2008) 55 - 60
The cataclysm hypothesis might explain why there
is no terrestrial material other than zircon grains fromthe Hadean Earth and why the oldest sedimentary rockleft on the Earth is 3.85-Gyr-old Isua supracrastal rock.
A mechanism for the cataclysm about 650 million
years after the formation of the Earth and Moon may be
explained by gravitational perturbation induced fromthe rapid migration of the giant planets at that period[31-33]. The planetesimal disk outside the orbits of the
giant planets and the asteroid belt were destabilized,
which resulted in a massive delivery of planetesimals to
the inner Solar System [31].
It is however noted that the extremely intense fluxof impact in the first hundred million years could havedestroyed crustal rocks formed before 4.0 Ga and reset
K-Ar ages of the lunar rock samples by shock effects[25]. In that case, an apparent peak in the lunar rock age
distribution is expected even when the impact flux
decreased monotonically.
In either case, impacts must have significantly
affected the surface environment of the Earth, and so
the origin and early evolution of life. It is therefore
really important to reconstruct the impact history of the
Earth-Moon system during the first several hundred
million years.

characterized by chaotic plate motion, frequent mantle
overturn and large-scale resurfacing, hence very
unstable surface condition. This is consistent with
virtually no geological evidence left in the first several
hundred million years. Such a condition may not beappropriate for early evolution of life, if life emerged
during the Hadean.
3.3. Composition of early atmosphere
A growing Earth was surrounded by protoplanetary
disk gas (so-called the "solar nebula", composed mainlyof H2 and He) in which planetesimals accreted witheach other [e.g., 36, 37]. The proto-Earth could havecaptured the solar nebula gas as a primordial
atmosphere, because the gravity was strong enough tocapture the surrounding gas when the mass of the
proto-Earth was larger than 1/100 of the present Earth's
mass (about the mass of the Moon) [34]. As aconsequence, a primordial atmosphere of the Earth wasvery reducing.
Impact degassing of volatiles from planetesimals
due to hypervelocity impact may have also contributed
primordial atmosphere during the main accretionary
phase. It is probable that the atmosphere derived from
impact degassing contained abundant H2O, hence thesurface of the proto-Earth may have been under the
runaway greenhouse condition [38-40].
The runaway greenhouse condition is achieved only
when the sum of the energy input to the planetary
surface due to net solar incident flux, planetesimal
impacts, and geothermal heat flux is larger than 310
W/m2 [41-43]. Under the runaway greenhousecondition, the surface of the planet is covered withmagma ocean because there is an excess energy input
larger than the upper limit of outgoing radiation fromthe steam atmosphere, resulting in a runaway increasein the surface temperature. It is suggested that metallic
iron in the magma ocean was segregated to form thecore. At that time, chemical reactions of volatileelements with metallic iron resulted in formation of
reducing atmosphere. In the presence of metallic ironand silicates in the magma ocean, oxygen fugacity iscontrolled by the iron-wüstite buffer [44, 45]. In suchan atmosphere, H2/H2O and CO/CO2 may be >1 and ~5,
respectively [34].
This primordial atmosphere might have been,
however, largely removed. One possible process ishydrodynamic escape due to heating of hydrogen-richatmosphere by strong X-ray and extreme ultraviolet(XUV) radiation thought to have been emitted from theyoung Sun [e.g., 46, 47]. Another possible process is
atmospheric loss due to giant impacts of Mars-sized
proto-planets during the late accretionary stage [48]. It
appears that the planetary atmosphere could haveescaped by giant impact when the planet was covered
with an ocean, rather than covered with a steam
atmosphere [48]. It is therefore possible that asignificant fraction of the pre-existing atmosphere of
the Earth might have been lost owing to the giant
impacts at the last stage of accretion.
After the moon-forming giant impact, theatmosphere of the Earth was composed mainly ofsilicate vapor at first, and then, evolved to water vapor[49]. Hence the steam atmosphere appeared again.
Cooling of the steam atmosphere resulted in the

3.2. Mantle convection
Magma oceans were probably formed by giant
impacts in the last stage of the Earth’s formation. Water
vaporized to form a steam atmosphere which, in turn,affects surface temperature through absorption of
infrared radiated from the surface. As the surface
temperature decreases with decreasing heat flux fromthe interior of the Earth, magma ocean evolved from asuper-liquidus (completely molten) state to a partial
melting (partially molten) state. Proto-crust formed
when the steam atmosphere collapsed to form
proto-ocean.
There was still an internal magma ocean beneath theproto-crust during the earliest history of the Earth. Heattransport was controlled by melt-solid separation rather
than thermal convection, and chemical differentiationof the mantle proceeded [34]. This stage would havecontinued several hundred million years until thegeothermal heat flow from the interior of the Earthdecreased to 1 to 0.1 W/m2 [34]. After this stage,solid-state convection has been working in the mantle
until today. It is therefore suggested that there was aninternal magma ocean, and so extensive volcanism andhydrothermal activities are expected to have occurred
during the early Earth.
Heat production due to decay of radioactive
elements, such as 235U, 238U, 232Th, and 40K, in the
mantle was about 4 times higher than that of today(about 30 pW/kg, i.e., mantle heating rate of 1000
K/Gyr), hence vigor of mantle convection should havebeen far strengthened compared to the present one.
According to a study of numerical simulation of the
mantle convection under such a condition, there areseveral characteristic features of the mantle convection[35]. Strong internal heating of lower mantle induces
mantle overturn which causes vigorous magmatic
activity. As a consequence, large-scale resurfacing(formation of magma pond) occurs repeatedly [35].
It is suggested that the Hadean Earth may have been-57-

Viva Origino 36 (2008) 55 - 60
formation of the ocean and atmosphere on the Earth.
The chemical composition of the atmosphere at thattime depends on an existence of metallic iron in the
pre-existing magma ocean and on the quenchingtemperature of the chemical reactions. When thequenching temperature of the reactions is lower than800 K, abundance of CH4 increases significantly [52].
There is however still the source of volatiles to the
Earth's surface. It is well known that the upper mantleof the Earth today contains highly siderophile elements
much more than that predicted from the equilibriumwith metallic iron [50]. It is therefore considered that
accretion of oxidizing materials, such as CI chondrites
or icy planetesimals (comets), should have occurred
after the end of core formation (late veneer hypothesis).
It is generally believed that the late veneer might havesupplied H2O and CO2 to the Earth's surface for several
hundred million years, and created oxidizing
atmosphere [51].
However, even the CI chondrite, the most oxidizing
materials in the solar nebula, should form a reducing
atmosphere with abundant H2 and CO [52]. The CI
chondrite contains organic matter (reduced form ofcarbon) of 3.5 wt% which should be oxidized by 32.6
wt% of ferric iron. The typical content of iron in CI
chondrite is, however, at most 20 wt%, resulting in a
deficiency in oxygen to oxidize all the reduced carbon.
As a consequence, the atmosphere formed by lateveneer of CI chondrite should have been also reducing[52].
It is therefore suggested that the atmosphere in theearly Earth was almost certainly reducing.
The atmosphere has been oxidized owing to theescape of H2 to space. The reducing gases such as CO,
CH4, and NH3 are known to be unstable when there iswater vapor. They should be oxidized by OH radicals
formed through photochemical reactions of water vapor,resulting in the formation of H2 which would escape tospace [53, 54].
The mechanism for H2 escape may behydrodynamic escape due to large XUV flux from theyoung Sun. The timescale of H2 escape determines alife time of reducing atmosphere on Earth, although theprecise timescale have not been established yet.
According to the recent study [55], the timescale of H2
escape might be longer than that previously thought.
When we assume the hydrogen escape flux of about
6x1015 molecules m-2 s-1 [55] for the solar XUV flux ofabout 10 times the present value at 3.8 Ga [56], thehydrogen equivalent to the terrestrial ocean escapes tospace in 500 Myr [52].
It is therefore concluded that there has been
reducing atmosphere for the first several hundred
million years during the Earth's history. This might
have been favorable for the prebiotic synthesis oforganic materials and so the origin of life on Earth.
4. Carbon cycle and Hadean snowball Earth
The Moon-forming impact created a silicate vapor
atmosphere which lasted probably for 1000 years [49](Fig. 3). And then, a steam atmosphere was left on thesurface of the Earth. Because of very high geothermalheat flow from the interior, the runaway greenhousecondition is maintained, resulting in the surface
temperature of 1500-2000 K and the surface of the

Earth being covered with magma ocean [49]. Becauseof a negative feedback mechanism for the surface
temperature due to solubility equilibrium of water
vapor with magma ocean [38-40], this stage would
have lasted for about 2 million years [49].
As the mantle froze, the steam atmosphere became
unstable and condensed to form a warm (500 K) water
ocean [34, 49]. The cooling time of 100 bar atmosphereis about 700 years, and the duration of heavy rain is 150
to 250 years with average net precipitation of 4000 to7000 mm/year [34].
The atmospheric composition of this stage might
have been reducing, but the dominant greenhouse gas
may have been CO2. If there were 100 bar of CO2,
warm ocean stage would have lasted for 10 to 100 Myr,
depending on the timescale of subduction of seafloor
carbonates into the mantle.
The climate of the Hadean Earth was, then,controlled by carbon cycle which might have been quite
different from that at present. The supply of CO2 via
volcanism to the atmosphere was probably much more
efficient than today. If the continental crust was very
little, chemical weathering of seafloor basalt followedby carbonate precipitation could have been thedominant CO2 sink. It might have occurred mainly at
mid-ocean ridge system.
There may have been, however, another dominant
sink of CO2 during the Hadean: chemical weathering ofabundant ultramafic volcanics and impact ejecta,
followed by carbonate precipitation [49, 57-59]. The
Heavy Bombardment (described in the section 3.1)
should have produced large amount of impact ejecta
which included very fine particles and were effectively
weathered to consume CO2, resulting in global
glaciations [49,57-59].
In the snowball Earth condition, the surface of the
Earth is completely covered with ice. However, becauseof very high geothermal heat flow during the early
Earth, the thickness of ice shell over the ocean was very
thin, probably 10 to 100 m thick. Such a thin ice shell
may have been easily destroyed by large impact events,
which might have triggered a brief impact summer[49].

Figure 3. Schematic diagram of the early evolution of
atmosphere and climate of the Earth. Solid curve is surface
temperature, dashed curve is amount of H2O in theatmosphere, and dotted curve is amount of CO2 in theatmosphere (modified from Sleep (2006) [49]).

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Viva Origino 36 (2008) 55 - 60
The possibility of the early ice-covered Earth may
be tested by observation of terrestrial planets aroundyoung stars in the future. Such a “snowball planet” mayexist as a typical type of water-rich terrestrial planets inthe extrasolar planetary systems [60], because globally
ice-covered is one of the stable climate states of thewater-rich terrestrial planets.
It is however noted that a net result due tocompetition between the effects of the Heavy
Bombardment and the intense volcanism due tofrequent mantle overturn has not been studied so far.
Very high heat flux localized due to intense volcanism
and resurfacing, and/or efficient recycling of subducted
CO2 to the surface might have prevented the early Earthfrom complete freezing. Further studies are therefore
required to understand the climate of the Hadean Earth.

10.

11.

12.
13.
14.
15.
16.

5. Conclusions
The early Earth’s environment has been largely
unknown because there is no geological evidence otherthan zircon grains in this period. However, oxygenisotopic composition of the zircons suggests existenceof continental crust and ocean before 4.404 Ga. The
atmospheric composition during the early Earth musthave been reducing even when the atmosphere formed
by degassing from CI chondrite (the most oxidizing
material in the solar nebula). During the early Earth, theHeavy Bombardment of extraterrestrial objects
occurred, which might have resulted in global
glaciations. However, the effects of intense volcanism
and large-scale resurfacing on the early Earth’s
environment remain uncertain.

17.
18.
19.
20.
21.

22.

Acknowledgments
I would like to thank Dr. Akihiko Yamagishi for thereviews and helpful comments. This research waspartially supported by a Grant-in-Aid for Scientific
Research (No. 18340128) of the Japan Society for the
Promotion of Science.

23.
24.
25.

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