Chemical Geology 169 Ž2000. 69–82
www.elsevier.comrlocaterchemgeo
Accretion of Moon and Earth and the emergence of life
G. Arrhenius a,) , A. Lepland a,b
a
Scripps Institution of Oceanography, UniÕersity of California, La Jolla, San Diego, CA 92093-0220, USA
b
Institute of Geology, Tallinn Technical UniÕersity, EE-0001 Tallinn, Estonia
Received 18 March 1999; accepted 9 June 2000
Abstract
The discrepancy between the impact records on the Earth and Moon in the time period, 4.0–3.5 Ga calls for a
re-evaluation of the cause and localization of the late lunar bombardment. As one possible explanation, we propose that the
time coverage in the ancient rock record is sufficiently fragmentary, so that the effects of giant, sterilizing impacts
throughout the inner solar system, caused by marauding asteroids, could have escaped detection in terrestrial and Martian
records.
Alternatively, the lunar impact record may reflect collisions of the receding Moon with a series of small, original
satellites of the Earth and their debris in the time period about 4.0–3.5 Ga. The effects on Earth of such encounters could
have been comparatively small. The location of these tellurian moonlets has been estimated to have been in the region
around 40 Earth radii. Calculations presented here, indicate that this is the region that the Moon would traverse at 4.0–3.5
Ga, when the heavy and declining lunar bombardment took place.
The ultimate time limit for the emergence of life on Earth is determined by the effects of planetary accretion — existing
models offer a variety of scenarios, ranging from low average surface temperature at slow accretion of the mantle, to
complete melting of the planet followed by protracted cooling. The choice of accretion model affects the habitability of the
planet by dictating the early evolution of the atmosphere and hydrosphere.
Further exploration of the sedimentary record on Earth and Mars, and of the chemical composition of impact-generated
ejecta on the Moon, may determine the choice between the different interpretations of the late lunar bombardment and cast
additional light on the time and conditions for the emergence of life. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Moon; Early Earth; Emergence of life; Lunar bombardment; Earth satellites; Archean sediments; Chemofossils
1. Introduction
The manifestations of a late heavy bombardment
of the Moon were first observed by Papanastassiou
and Wasserburg Ž1971. and Tera et al., Ž1974., its
significance was further discussed by Wasserburg et
)
Corresponding author.
E-mail addresses: [email protected] ŽG. Arrhenius.,
[email protected] ŽA. Lepland..
al., Ž1977.. It was later generally assumed that this
protracted event must have affected the Earth violently and rendered our planet uninhabitable well
past 3.8 Ga Ž1 Ga-10 9 yr., with major impacts
extending to 3.45 Ga ŽFig. 1.. Such a bombardment
is thought to have severely affected the possibilities
for life to have emerged on Earth in the earliest
Archean ŽStevenson, 1988; Sleep et al., 1989; Maher
and Stevenson, 1988; Chyba, 1993; Oberbeck and
Fogleman, 1989, 1990.. In contrast, the record from
0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 5 4 1 Ž 0 0 . 0 0 3 3 3 - 8
70
G. Arrhenius, A. Leplandr Chemical Geology 169 (2000) 69–82
Fig. 1. Radiometric ages of samples from the lunar highlands,
illustrating the culmination of the late lunar bombardment around
3.8–3.9 Ga and tailing off toward 3.5 Ga Žafter Dalrymple, 1991..
The dashed line shows the minimum age of the Isua metasediments and delineates the overlap of the decaying late lunar
bombardment with the early sedimentation record of Earth. Compare interpretations in Fig. 3.
the oldest known Isua and Akilia metasedimentary
rocks from southern West Greenland, extending back
in time beyond 3.75 Ga ŽFig. 2; Moorbath et al.,
1973; Rosing et al., 1996; Nutman et al., 1997. show
sequences of banded ironstones without any clearly
identifiable disturbances that could have formed at
asteroidal impacts. Such disturbances, in the form of
surge deposits, are exemplified by the Ordovician
Lockne impact structure in Sweden ŽSturkell, 1998.
and consist of coarse-grained rock fragments, transported and sorted by the strong tidal flow generated
by the impact. Crushing-, flow- and transport-effects
at impacts of the size indicated by the lunar maria,
and scaled up for Earth’s gravitation, would have
been correspondingly violent. They would have left
records that are likely to be distinguishable from the
layer structure of the banded iron formation, even
though the original sedimentary features bear an
overprint of metamorphism and tectonism. The Isua
and Akilia metasediments contain ubiquitous chemofossils with carbon isotopic composition suggestive
of biochemically advanced microbial life forms
ŽMojzsis et al., 1996.. Their presence indicates that
the planet was not sterilized by impacts during the
time periods of deposition of these sediments.
This apparent lack of counterparts on Earth, to the
late impact features on the Moon, together with new
data obtained from the studies of the Martian meteorites, require a re-evaluation of the events in
Earth–Moon space in this critical time interval for
the emergence of life. The 3.86 Ga age of the Akilia
formation determined by Nutman et al. Ž1997, 2000.,
has been questioned by Kamber and Moorbath Ž1998.
and Whitehouse et al. Ž1999., proposing an age of
3.65 Ga for this formation. Regardless of ultimate
consensus on the age difference between the Isua
and Akilia formations, this difference would, in the
present context, be insignificant relative to the spread
in time of the late impacts on the Moon, 4.0–3.45
Ga ŽFig. 1., essential for the comparison with the
record on Earth.
2. The picket fence model and the sedimentary
record
We are considering two possible explanations for
the apparent incongruence between the lunar and
terrestrial records. If the bombardment was caused
by invading translunar objects, it is likely to have
been episodic. The effects could then have been
overlooked in the discontinuous geologic record investigated from the early Archean. This concept is
embodied in the ‘‘picket fence’’ model ŽFig. 3.
proposed by Mojzsis and Zahnle ŽMojzsis et al.,
1998.. Life could, in each such major occult impact
event, have been extinguished, only to arise again in
the intervening quiescent time intervals of a few ten
or hundred million years. Or it could have found a
niche for survival in the deep ocean or crust, and
have spread again from there, between each pair of
major assaults ŽSleep et al., 1989..
Sequences of metasedimentary cherts and banded
iron formations in the Isua Supracrustal Belt ŽISB.,
G. Arrhenius, A. Leplandr Chemical Geology 169 (2000) 69–82
71
Fig. 2. Time scale of events related to accretion of Moon and Earth and the emergence of life. The 3.8 Ga age line indicates the culmination
of the late bombardment observed on the Moon.
referred to above, are exposed in outcrops and in a
series of several-hundred meter-long drill cores ŽAp-
Fig. 3. Three generalized interpretations of the late lunar bombardment data shown in Fig. 2. The concurrent record on Earth is
represented by the Isua sediments that have a minimum age of
3.75 Ga. Retrieved segments of these deposits do not bear evidence of contemporaneous bombardment of Earth at the scale
observed on the Moon. This suggests, either that the corresponding time–rock units on Earth are missing or have not been found
Žcf. Fig. 4., or that the late lunar bombardment was restricted to
lunar phase space. ŽZahnle and Mojzsis, unpubl., reproduced in
Mojzsis et al., 1998..
pel, 1991.. The metasedimentary units are commonly
intersected by intrusions of mafic rocks. Although
individual 10–100 m sections of coherent sequences,
represent comparatively long accumulation periods,
no certain evidence has so far been found for catastrophic surge deposits. Reliable stratigraphic
interpretation is, however, hindered by the tectonic
overprinting; the formation is isoclinally folded.
Therefore, it cannot be excluded that some of the
sequences represent repetitions of a single unit ŽRosing, 1998.. In the northeastern part of the ISB, the
sequence of finely-laminated banded iron formation,
contains strongly deformed 0.1–1 m thick layers,
sandwiched between undeformed strata. These deformed packages probably mark tectonic shear zones
that resulted from small-scale translations parallel to
the bedding. Some of the thicker deformed packages
ŽFig. 4., display grading of the component rock
fragments, opening the possibility that they could be
surge deposits formed by violent resuspension and
sorting during resettling. If meteorite impact is assumed as a cause of such implied surges, an enhancement should be evident of the platinum group
72
G. Arrhenius, A. Leplandr Chemical Geology 169 (2000) 69–82
Fig. 4. A rock sequence in the Isua banded iron formation which
could possibly indicate a disturbance of the primary sedimentary
structures by a seismic- or impact-related surge. The rock section,
seen in the photograph to be layered between sections of finely
laminated banded iron sediment, consists of rock fragments that
become successively finer from left Žbottom?. to right Žtop?..
Such grading may be interpreted as the result of violent suspension and sorting at settling of rock fragments, such as, after an
earthquake or an impact. However, most of the Isua sediment
sequences bear signs of tectonic deformation, therefore, indicating
that the described structure could also be the result of shear. The
possibility that this is an impact-related deposit can be tested by
analyses of the concentration of PGE that are characteristic components of undifferentiated extraterrestrial materials.
element ŽPGE. concentrations, in and above such
beds, but has not yet been observed.
3. Lunar bombardment restricted to the Moon?
As another possibility we suggest that the late
lunar bombardment was not a solar system-wide
process, but was largely limited to the lunar orbit.
This possibility may be evaluated against the background of several current theories for the origin of
the Moon, that can all be taken to provide more or
less compelling support for such a conjecture.
One currently popular scenario for the formation
of the Moon assumes that Earth, in a relatively
advanced state of accretion, including or followed by
core formation, would have collided with a hypothetical planet moving in an Earth-crossing orbit and
with a mass larger than that of Mars ŽHartmann and
Davis, 1975; Cameron and Ward, 1976; Wanke
et
¨
al., 1984; Cameron and Benz, 1991; Cameron, 1985,
1997; Canup and Esposito, 1996; Ida et al., 1997.
Ejecta from the impact would have been placed in
prograde equatorial orbit around Earth and coalesced
to form the Moon.
In view of the fact that large impacts occur relatively late on the Moon without leaving an observed
record on Earth, one may consider a scenario proposed by Canup and Esposito Ž1996.. They suggested that a massive proto-Moon coalesced from the
inner part of the ejecta ring and receded from Earth
due to tidal angular momentum transfer. The protoMoon later overtook and collided with a series of
originally formed smaller moonlets. This scenario
requires a lapse time of about 400 million years
between the initial emplacement of the Moon and the
late bombardment. The delay time, aside from the
orbital distance from Earth of such swept up moonlets, would depend on the rate of recession of the
proto-Moon from Earth, and thus, on tidal dissipation. Bodily tides are generally considered of minor
importance in the present solid Earth, compared to
the ocean, where dissipation is mainly restricted to
shallow seas and regions around submarine ridges
and seamounts ŽMunk and Wunsch, 1998.. In the
collisional theory for the origin of the Moon, considerable uncertainty is introduced for tidal dissipation
and rate of recession, since the hydrosphere must
have been totally evaporated, creating a thermal
blanket perpetuating this state for a long time. On the
other hand, extensive liquefaction of the outer layer
of the Earth, creating a magma ocean, would have
increased the dissipation factor.
Other theories for the formation of the proto-Moon
also offer possibilities for explaining late collision
events occurring in lunar phase space. With regard to
G. Arrhenius, A. Leplandr Chemical Geology 169 (2000) 69–82
formation and orbital evolution of the Moon, the
extensively analyzed capture theory ŽGerstenkorn,
1955, 1969; Lyttleton, 1967; Singer, 1968, 1972;
Alfven
´ and Arrhenius, 1963, 1969; Urey and MacDonald, 1971; Kaula, 1971. deserves further consideration.
The capture theory has, perhaps temporarily, become overshadowed by the collision theory. This
invokes, as supporting evidence, material similarities
between the Earth’s mantle and the Moon, and a
purported low probability of capture. Perhaps subliminally, it also draws on the computationally and
visually attractive drama of Worlds in Collision. At
closer scrutiny, the two former arguments against
capture, however, assume diminishing importance.
The low density of the Moon is comparable not only
to the silicate mantle–crust of the Earth but in
general, to circumsolar bodies beyond Earth, such as
Mars and the asteroids. The similarity of oxygen
isotope ratios of Moon and Earth, compared to Martian meteorites, has been invoked as another material
argument for the collision hypothesis. However, materials that isotopically are strikingly similar to those
of the Moon and Earth are represented by certain
types of meteorites ŽClayton and Mayeda, 1996. that
could have served as a source for the Moon and have
also been invoked as a likely source material for the
Earth ŽHerndon, 1996..
The collision cross-section of Earth is about six
times smaller than that of a presumed dissipative
Earth ring region, and the capture probability from a
corresponding encounter orbit is amplified by many
orders of magnitude by repeated passages. The improbability argument would seem to apply to the
collision hypothesis more seriously than to capture,
particularly when the efficient capture-dissipation
mechanism proposed by Kaula and Harris Ž1973. is
taken into account Žsee also, Wood, 1977.. This
mechanism involves collisions by the capturand with
particle swarms expected in near-Earth orbit, adding
significantly to energy loss by friction. Capture is
furthermore, generally considered as the most probable mechanism for emplacement of many other satellites with abnormal features ŽBurns, 1977., one of
the most striking examples being Neptune’s retrograde and massive moon, Triton ŽMcCord, 1966; cf.
Fig. 5.. Finally, capture of the Moon would take
place from a highly inclined orbit. The comprehen-
73
Fig. 5. Masses Žin grams. of systems of secondary bodies around
their magnetized primaries. The diagram shows the abnormally
large masses of the retrograde Neptunian satellite, Triton, which is
considered to be captured ŽMcCord, 1966., and of our present
Moon, which may likewise be captured from solar orbit. The
original, ‘‘normal’’ satellites, postulated to have been swept up by
the Moon and causing the major impacts in late lunar bombardment, would have had a total mass estimated by interpolation to
be of the order 10 21 –10 22 g, compared to 0.74=10 26 g for the
Moon. ŽFrom Alfven
´ and Arrhenius, 1972a..
sive study of the lunar orbital evolution by numerical
integration ŽGoldreich, 1963, 1966. characteristically
leads to a high inclination of the initial near-Earth
orbit of the Moon. A high initial inclination of the
proto-Moon can also be explained if it coalesced
from a debris ring around the Earth ŽWard and
Canup, 2000..
In the context of the present discussion of the late
lunar bombardment, the question of a capture vs.
collision origin of the Moon is, however, of secondary importance. The question of interest here is if
a time delay of several hundred million years can be
expected between emplacement of the Moon in
near-Earth orbit and its interaction with original,
much smaller satellites at a relatively large distance
from the Earth. Such a delay would mainly depend
on the distance from Earth of such moonlets and the
rate of recession of the proto-Moon. Initially ignoring the potentially delaying effect of Earth–Moon
resonances ŽAlfven
´ and Arrhenius, 1969. or Moon–
Venus resonances ŽKaula, 1971., the rate of increase
G. Arrhenius, A. Leplandr Chemical Geology 169 (2000) 69–82
74
in distance from Earth of the receding Moon is given
ŽMacDonald, 1964. by
dR
3k
s
dt
Q
Ž GM .
mr 5
1r2
Ž 1.
R 11r2
where R is the semimajor axis of the proto-lunar
orbit, k the Love number and Q the specific dissipation factor for Earth, G the gravitational constant, M
the mass, r the radius of Earth, and m the mass of
the proto-Moon.
The characteristic time for a given R is
ts
4p
R
s
d Rrd r
ž /
3
5r3
R 13r2
Q
3kG 1r2
M 7r6 m
Ž 2.
Integration of Eq. Ž2., coupled with modern measurements of the deceleration of the lunar longitude
and thus, the slowing of the Earth, give the startling
result of only 1.75 Ga for the evolution of the lunar
orbit from close vicinity to the Earth to its present
position, in clear contradiction of geological evidence ŽMacDonald, 1964; Kaula, 1971.. Lambeck
Ž1975. and Kaula and Harris Ž1973. have pointed to
the uncertainly in the value of Q over time as a
probable reason for this dilemma and the possibility
that the controlling oceanic topography may have
been different in the past, with substantially lower
dissipation Žlarger Q .. Munk Ž2000. has furthermore
pointed to the likelihood that in near-interaction between Moon and Earth, vorticity dissipation is likely
to have been saturated, introducing an effective delay
factor in the recession of the satellite, regardless of
its origin.
If the late lunar bombardment was due to collision
of the proto-Moon with original moonlets, these
must have been located at such a distance from Earth
that the Moon traversed between emplacement, about
4.4 Ga and bombardment between 4.0 and 3.5 Ga.
Small bodies could have formed outside of a major
lunar embryo in a collisionally generated accretion
disk ŽCanup and Esposito, 1996; Ida et al., 1997..
However, such moonlets close to Earth can not be
held responsible for late lunar bombardment since
the large exponent for R in Eq. Ž2. causes the
massive proto-Moon to traverse near-Earth space
comparatively rapidly, even if vorticity saturation is
taken into account.
The formation of regular satellite systems, such as
those of the outer planets, and by analogy, also of
the Earth, are considered in several theories. As an
example, the accretion theory developed by Safronov
Ž1954, 1960, 1972. based on orbital dynamics, predicts the formation of swarms of secondary bodies
around the planets ŽSafronov and Ruskol, 1977. but
does not specify their radial distribution. Considering
the seminal influence that Safronov’s general theory
has had on developments in the West, the neglect of
their application to satellite formation in the literature is notable.
More specific predictions about the radial distribution of planets and satellites are based on the
implied hydromagnetic control of circumstellar and
circumplanetary plasma as observed in the planetary
ionospheres and in the protoplanetary circumstellar
disk medium ŽAlfven
´ and Arrhenius, 1974, 1976;
Montmerle and Andre,
´ 1988; Hjalmarson and
Friberg, 1988; Bertout, 1988.. In the hydromagnetic
theory for secondary body formation, the emplacement process is thought to be controlled by the
critical velocity, Vc , of ionization for the infalling
neutral component of the interstellar cloud plasma
and by its interaction and partial co-rotation with the
magnetic field of the central body ŽDe et al., 1977..
As verified by effects observed in the space medium,
Vc s Ž2 eVionrMa .1r2 , where Vion is the ionization
potential of atoms with mass Ma and e the electron
charge. The critical velocity effect was discovered in
laboratory plasma experiments ŽDanielsson, 1970,
1973., showing selective ionization of neutral gas
moving through a magnetic field. Alfven
´ and Arrhenius Ž1974. visualized this effect as the physical
explanation of what Alfven
´ had previously pointed
out as a characteristic band structure of secondary
bodies around their magnetized primaries in the solar
system ŽFig. 6.. The statistical significance of the
proposed band structure was investigated by Arrhenius and Arrhenius Ž1988.. The physical basis of the
band structure model led De, in 1972 Žsee De, 1978.
to the prediction of rings inside the Roche limit of
Uranus; these rings were later discovered by Elliot et
al. Ž1977.; the relation of the prediction to the discovery is discussed by Brush Ž1996..
The observed parallelism in distribution of both
primary and secondary bodies in terms of gravitational potential, together with the proposed explanation ŽFig. 6., implies that the Earth would also have
been originally endowed with a satellite system of
G. Arrhenius, A. Leplandr Chemical Geology 169 (2000) 69–82
75
Fig. 6. Distribution of secondary bodies Žplanets and satellites. around their primaries ŽSun and the magnetized planets.. Orbital distances
Ž rorb . to the central bodies are normalized by relating them to the mass Ž Mc . of the central body as a measure of gravitational potential
energy, Eg , drawn as log Eg Žcmrg.. The individual systems are plotted in order of central body mass, normalized to the mass of the Sun
Ž M(.. The distribution of secondary bodies is perceived as a series of bands, corresponding to actual groups of critical velocities for
ionization of dusty gas falling into the circumsolar region from the interstellar source cloud. The ragged edges of the bands indicate
uncertainly in their extension; the physical reason for the slope of the bands is discussed in Alfven
´ and Arrhenius Ž1974, 1976.. In the
present context, the diagram serves the purpose of illustrating the possibility that Earth may originally have had two sets of small satellites,
swept up by the abnormally large Moon, during the evolution of its orbit. The location of the outer band corresponds to the approximate
Moon–Earth distance, 3.8–3.5 Ga. The Moon is slowly receding and is presently located at log Eg 17.2, just below the letter ‘‘R’’ in
‘‘EARTH’’. The Moon–Earth resonances, where recession of the Moon could have been retarded in its early history are discussed in Alfven
´
and Arrhenius Ž1969. Žfrom Alfven
´ and Arrhenius, 1972b..
possibly a dozen moonlets and inside the Roche
limit, perhaps also a ring. As is generally conceded,
our proto-Moon, regardless of its mode of formation,
by capture or collision, receded toward its current
76
G. Arrhenius, A. Leplandr Chemical Geology 169 (2000) 69–82
location at 60 Earth radii as a result of transfer of
angular momentum by tidal exchange with Earth. It
would, in this process, have slowly approached and
interacted with any normal satellites that may have
existed in its path, either by coalescence or ejection.
Creation of the maria by late impact with pre-existing Earth satellites was proposed by Kaula Ž1971..
The probability of ejection by interaction with such
moonlets, can perhaps best be judged by the fact that
our solar system contains a large number of planetary satellites that are considered to have been created by collisional accretion of smaller bodies.
One group of Earth’s original satellites would
have been located in the Martian band, between 14
and 34 Earth radii, and another in the transposition
of the Saturnian–Uranian band, between 2.9 and 6.3
Earth radii ŽFig. 6.. The latter, together with the
inside-Roche ring transposed from Ariel and Miranda, would have been swept up first, when the
precursor of our present Moon receded from nearEarth orbit and could have contributed to the frictional medium in the Kaula–Harris mechanism for
capture assistance. In the hydromagnetic model for
satellite formation, the putative moons that appear in
Earth’s gravitational equivalent to the Martian satellite band, would be the most likely candidates for
interaction with the proto-Moon when it, at the time
of the late lunar bombardment, had receded to 1r4–
1r2 of its present distance from Earth.
With sufficiently close similarity of orbital velocities at the time of the encounters, the collision
velocities could, under conditions outlined by Canup
and Esposito Žl.c.., have been close to the escape
velocity for the Moon, around 2.4 kmrs. Collision
velocities of the order of 2 kmrs would have substantial cosmetic effects on the surface of the Moon;
ejecta with speeds below the escape velocity would
have been recaptured and not be able to overcome
the energy barrier for escape and diversion to Earthimpacting orbits.
Debris from collisions exceeding escape velocity
may have remained in lunar intersecting orbits for an
extended time before ultimate capture by the Moon.
Resonance locking in the Earth–Moon system may
have contributed to such delay, so far, of undetermined duration ŽCanup and Esposito, l.c... Metastable
storage around the Lagrangian points, L4 and L5,
608 before and after the Moon in its orbit may also
have held ammunition for the late bombardment
until perturbations of the characteristic long orbital
excursions from the libration points brought some of
these bodies into collision with the Moon. Small
amounts of dust are observed trapped around the
Lagrangian points today ŽRoach, 1975; Winiarsky,
1989.. Observable dust particles are probably continually removed from the libration region by the
Poynting–Robertson effect and radiation pressure,
and replenished by accumulation of interplanetary
dust. A dynamic calculation is needed to evaluate the
actual residence time of larger and thus, practically
invisible solid bodies around the libration points.
Assuming the direct applicability of Eq. Ž2., an
estimate can be made from Eq. Ž3. of the radial
distance range from Earth in which the receding
proto-Moon would have encountered the outer band
of small regular Earth satellites in a time period
around 3.8 Ga, 600 million years after the initial
emplacement of the Moon in near-Earth orbit by
collision or capture, assumed to have taken place
around 4.4 Ga.
tb
ti
600
s
s
4400
Rb
ž /
60
6.5
Ž 3.
With t b being the orbital evolution time, about 600
Ma, up to the late lunar bombardment, the radial
distance, R b , of the Moon from Earth at the time of
bombardment is found to be 44 Earth radii. This
distance value is probably an upper limit, since the
recession of the Moon could have been retarded by
spin-orbit resonances with Earth ŽAlfven
´ and Arrhenius, 1972a. andror by resonance between Moon
and Venus ŽKaula, 1971..
The distribution of accreting material in the region around 40 Earth radii extrapolated by hydromagnetic considerations would thus, seem to support
the concept of the late lunar bombardment as a series
of collisions, possibly accompanied by ejections, as
the receding Moon crossed the orbits of an outer set
of original, proportionately small satellites of the
Earth. A potential impediment to such a hypothesis,
particularly in conjunction with capture, could result
from interference of the proto-Moon with the original small satellites already on its initial approach to
Earth during the early evolution of a capture orbit.
G. Arrhenius, A. Leplandr Chemical Geology 169 (2000) 69–82
The probability for such premature destruction of
these bodies would be decreased by the high inclination of a capture orbit. However, model calculations
of interactions at this stage would be desirable.
The Martian surface, if represented by the meteorite, ALH 84001, has not been melted since its
crystallization time, 4.5 Ga. ago. The surface layer of
all of the terrestrial planets would have been melted
if the projectiles causing the bombardment of the
Moon had come from an external source ŽStevenson,
1988.. Extended observations from Mars could
therefore contribute to the interpretation of the late
lunar bombardment as a parochial lunar event or
alternatively as an invasion, affecting the entire inner
solar system.
4. Evidence from terrestrial sediments
All current theories of lunar origin and collision
history, suffer from dynamic difficulties and remain
speculative. However, there are possibilities for verification of the circumstances around the late impacts.
One consists of detailed examination of long sequences of the oldest laminated sediments, searching
for and identifying depositional irregularities ŽSturkell, 1998., and meteorites, along with interplanetary
dust embedded in the sediments ŽSchmitz et al.,
1997.. Quantitative element and mineral analyses of
such sediments give information about their content
of extraterrestrial matter, and if the total rate of
sedimentation can be estimated, the rate of accretion
of the cosmic component can also be determined
ŽChyba, 1991.. Analyses by Ryder and Mojzsis
Ž1998. of the metasediments from Akilia Island, off
the coast of southern West Greenland, show no
indication of enhancement of PGE above the average
for crustal terrestrial rocks.
5. Evidence from late lunar impact ejecta
A second approach toward verification involves
analyses of such lunar samples that are chronologically identified with, and serving as the basis for
conclusions about the late bombardment on the
Moon. If the colliding objects were interplanetary
77
marauders, they would be expected to consist of
undifferentiated cometary- or meteorite-type material, conferring their characteristic excess of PGE to
the ejecta, with allowance for dilution with lunar
target material. If they were to be identified with
partly differentiated asteroids, some impact material
would be depleted in PGE, but other samples, enriched in metal, would have a correspondingly enhanced PGE content. If, however, such a signature is
found to be consistently absent in the samples representing the collision events, the most likely interpretation would be that the impacts were caused by
metal-depleted low density materials of the kind that
formed the Moon and possibly original Earth satellites. An integrating study ŽRyder and Mojzsis, 1998.
of the total surface accumulation of PGE over lunar
history, indicates that this amount is only 1.3% of
what would be expected if the source had been
meteoritic Žasteroidal. impactor material with a high
PGE content. Again, this suggests that the late lunar
bombardment was caused by projectiles with composition similar to the moon rather than asteroids.
The necessarily speculative views of the hardships
or even survival of life during the late lunar bombardment may now be replaced by searches in the
oldest preserved records on our planet. The manifestations of planetary impacts may probably be found
even in older samples from ancient terraces on Mars.
Life on Earth, instead of as is often quoted, originating 3.5–3.8 billion years ago, is found to have
already developed to a high degree of autotrophic
sophistication before these age limits.
6. Planetary accretion, the early surface state and
the emergence of life
The processes that formed our planet and modified it in its initial stages, must also have determined
the thermal state of the planetary surface and atmosphere. These developments predate the period of
late heavy bombardment of the Moon, discussed
above. The planetary accretion process and the initial
state of the Earth, have been subjects of intense
scientific inquiry in recent years, and it is generally
agreed that the heat balance would have been controlled mainly by the rate of growth of the planet by
78
G. Arrhenius, A. Leplandr Chemical Geology 169 (2000) 69–82
infall of extraterrestrial matter. If, as assumed in
many theories, all of the planetary source material
was emplaced around the Sun at one point in time
Ž‘‘instant nebula’’ theories., the accretion rate would
have increased exponentially with growing radius of
the planet, because of its concurrent increase in
gravitational attraction.
The result would be an accretional runaway catastrophe with an initial Earth, covered by a magma
ocean, extending perhaps to the depth of the upper
mantle and obviously detrimental for an early development of a hydrosphere and of life. High accretional temperatures would also lead to the early
removal of iron, which would sink toward the core,
heating the planet even further by the release of the
gravitational potential energy of the falling metal. In
the absence of such a development, metallic iron
would remain distributed in the outer layers of the
planet and would serve as a potential reductant and
hydrogen generator from water and hydroxyl ion as
discussed below.
With the high temperature of the planet, its
volatiles would have been evaporated into a dense
atmosphere which would form an effective blanket,
delaying radiative cooling for an extended time, as
on Venus. Protagonists of such scenarios therefore
propose that this heavy primitive atmosphere was
removed, e.g. hydrodynamically, with the aid of
hydrogen as a carrier gas, propagated by strongly
enhanced UV radiation from the early Sun ŽHunten,
1993.. Such a process may also explain the critical
features of noble gas abundances in the present
atmosphere ŽPepin, 1991.. Erosion of the atmosphere
by the impact of large bodies could also have effectively stripped off an early atmosphere but only
before Earth had accumulated a gravitational mass
comparable to Mars ŽMatsui and Abe, 1986; Ahrens
et al., 1989.. Consequently, these processes could
only have been effective in the early stages of solar
and planetary evolution. After formation of the
planet, stripping of the proto-atmosphere would,
however, also be likely to accompany a collision
event of the magnitude invoked in the currently
popular theory for the origin of the Moon and involving a colliding planet larger than Mars, as discussed above.
The stripping models also introduce the need for a
secondary atmosphere that could arise after radiative
cooling, following removal of the presumed dense,
thermally insulating proto-atmosphere. Such a secondary atmosphere would provide a medium for the
eventual emergence of life; its source and composition are consequently of importance in this context.
Different sources have been proposed for such a
post-cooling atmosphere, surviving in chemically
modified form today. One is an assumed ‘‘late veneer’’ of cometary material; another is the solidifying volatile-laden upper mantle, releasing gases
originally dissolved in equilibrium with a dense
proto-atmosphere. After arrival of a comparatively
high transparency of the atmosphere in the infrared
region, radiative cooling would rapidly allow a solid
crust to form, and internal heat could, as today, be
relatively efficiently transferred to the surface by
upwelling of molten rock in mid-ocean ridge systems.
In an intermediate accretion scenario, developed
by Wetherill Ž1994., calculations starting with already formed planetesimals, lead to a distribution of
orbits with a wide range of eccentricities and inclinations. In this case, planetesimals in near-Earth orbits
would have been captured at a high rate, leading to
an adolescent Earth at high surface temperature. The
remaining population of planetesimals of approximately lunar size and in highly eccentric and inclined orbits, would have been captured at a lower
rate, each with catastrophic local or regional effects
but because of their small size, incapable of necessarily melting the entire planet.
The heterogeneous accretion theory originally
suggested by Eucken Ž1944., and further developed
by Turekian and Clark Ž1969., specifically avoids the
catastrophic melting of the entire planet as a result of
core formation by gravitational segregation in the
already completed planet; a fateful consequence of
homogeneous accretion. The heterogeneous scheme
assumes that an early accretional episode involved
iron-rich material, spatially or temporally separated
in the space medium. Later episodes would then add
the silicate-rich space condensates making up the
main mass of the Earth, contained in its mantle and
similar in density to Mars, the Moon and asteroids.
Various processes have been proposed that could
have led to such separation Že.g. Orowan, 1969;
Danielsson, 1973; Canup and Esposito, 1996.. Considerations of the evolution of the Earth’s core based
G. Arrhenius, A. Leplandr Chemical Geology 169 (2000) 69–82
on isotopic measurements of Hf and W ŽHalliday et
al., 1996. would also be displaced from Earth to its
dispersed precursor medium if the fractionation took
place there. The actual occurrence of such fractionation in the inner solar system is suggested by the
large density differences between, on one hand, Earth
and Venus; on the other, Mars, Moon and the asteroids. Mercury represents material with particularly
high density, presumably richer in iron than the other
planets. In the case of the Moon, its proposed collisional removal from the mantle of a differentiated
Earth has been suggested as a reason for its low
density ŽWanke
et al., 1984., but this does not
¨
explain the large density differences between the
other bodies in the inner solar systems.
Alfven
´ and Arrhenius Ž1973, 1974. have emphasized control of the early accretional process by the
orbital dynamics of the charged solid particles in the
dusty plasma observed around young stars. They also
avoided the hypothesis of an ‘‘instant nebula’’ that
assumes all source material for the planets, at one
single time, to be present in the circumsolar region,
after falling in from the interstellar source cloud for
our solar system. As observed in young stellar objects surrounded by protoplanetary discs, such infall
of condensates from the molecular source cloud,
does not appear to be constant, but episodic ŽLada,
1988.. The variability of infrared luminosity in such
objects, thought to result from variations in material
supply, can now, for obvious reasons, only be
recorded on a human time scale. It is, however, also
reasonable to assume lower frequency events of enhanced infall of material into the solar nebula. Such
episodic emplacement events would have decisive
effects on the orbital dynamics of the accreting
system.
Elaborating on the hypothesis of heterogeneous
accretion, Alfven
´ and Arrhenius Žl.c.. assumed an
initial accretion of the present metallic core of the
Earth ŽFig. 7.. Later condensate emplacement
episodes would, with this gravitational center in
place, lead to the efficient capture of new material by
the Earth-embryo without an opportunity for this
material to form large separate planetisimals. After
runaway formation of the core, the mantle of the
Earth could, under such conditions, have been growing as a result of a large number of minor impacts,
each causing local melting and degassing, but with
79
Fig. 7. Accretional model for Earth, based on the assumption of
formation of the core Ž ; 0.5 R[ . from material initially emplaced
in Earth’s accretional feeding zone in the solar nebula. Accumulation of this mass would exponentially increase the gravitational
attraction of the planetary embryo and end with a catastrophic
runaway accretion phase. After temporary depletion of Earth’s
feeding zone, accretion of the core would be followed by accumulation of the mantle by gradual infall and sweep-up of material
from the interstellar source cloud. Observations of T-Tauri stars
support the notion of such episodic infall of matter onto their
circumstellar discs Žsee text.. This model provides rationales for a
separate accretional composition of core and mantle, and for a low
average temperature of the planetary surface during accumulation
of the mantle. It therefore, also permits gradual retention of liquid
water and emergence of life during the late stages of Earth
accretion. ŽFrom Arrhenius et al., 1974..
the average temperature of the Earth’s surface maintained low; a ‘‘hot spot accretion’’ of a type advocated by Schmidt Ž1944. and Vinogradov et al.
Ž1971.. In this extreme, the surface layer of the
growing Earth may never have been entirely molten
at any given time. A hydrosphere could begin to
develop as soon as the gravitational field permitted
the retention of liquid water at a size of the planetary
embryo somewhat larger than the core ŽFig. 7..
Heterogeneous accretion of the planets is also
favorable for models seeking to maximize the rate of
generation of methane and hydrogen from water and
carbon dioxide. This would be effectuated by the
oxidation of metallic iron forming a dispersed minor
component in the silicate mantle. The production of
hydrogen, methane and ammonia by this mechanism
would favor the formation of reduced organic com-
80
G. Arrhenius, A. Leplandr Chemical Geology 169 (2000) 69–82
pounds of biogenic importance. The destruction and
escape rate of reducing gases would also be minimized ŽMiller and Lyons, 1998.. The reduction process would, in such models, be localized in the
temporary hot spots generated by impacts, separated
in space and time.
In heterogeneous accretion scenarios with early
accretion of an iron-rich core, followed by comparatively slow growth of a silicate mantle at low average surface temperature, the planet could have
already become habitable during or soon after planetary formation, up to 4.5 billion years ago. This
would permit several hundred million years for the
emergence of life, which is first seen in already
enzymatically sophisticated form in rocks older than
3.75 billion years. Heterogeneous accretion theories,
thus, connect with biopoesis scenarios that for intuitive reasons, wish for the longest possible time
available for the emergence of life on Earth. It is
worth emphasizing, however, that there is no factual
or even speculative evidence, for the length of time
required for the origin of life ŽOrgel, 1998..
Acknowledgements
All of the considerations above, illustrate that
time is of essence. The contributions by Professor
Wasserburg and his collaborators to precision timing
and elucidation of processes on Earth and Moon and
in interplanetary space, have been of fundamental
importance for understanding events in the most
distant past. With this paper, we wish to express our
thanks for the inspiration and insights that he has
provided and doubtlessly, will continue to give.
The authors wish to thank Kevin Zahnle and
Stephen Mojzsis for extensive discussion of the range
of subjects at hand, and Peter Appel for supporting
and coordinating our field studies as part of the Isua
Multidisciplinary Research Project in southern West
Greenland and for discussion of the results. We are
also indebted to Stephen Moorbath, Minik Rosing,
Chris Fedo, Martin Whitehouse, Victor McGregor
and Alan Nutman for valuable views on tectonic,
sedimentary and chronological relationships in the
Isua complex. Constructive review comments were
received from Dr. Christopher McKay, Prof. Robert
Bostrom
¨ and two anonymous reviewers, and are
greatly appreciated. Generous research support was
provided by NASA’s Office of Space Science by
grants, NAGW-1031 and NAG5-4563, and by the
Marianne and Marcus Wallenberg Foundation.
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