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The Role of :Impacting Processes in the
Chemical Evolution of the
Atmosphere of Primordial Earth
LEV M. MUKHIN AND M.V. GERASIMOV
Institute of Space Research
ABSTRACT
The stability of the chemical composition of the planets' atmospheres
on any time scale is determined by the ratio of source and sink strengths for
various atmospheric constituents. Beginning with the Earth's formation and
continuing throughout its history, these ratios have undergone significant
alterations. Such changes are determined by various physical processes that
are tied to the evolution of the Earth as a planet. A complete theory of the
origin and evolution of the Earth's atmosphere is still far from complete
at this time. This is due, in particular, to a certain randomness in the
selection of a number of important physical parameters of the preplanetary
cloud. They include, for example, the time scale for the dissipation of the
gaseous nebula component; the planet accretion scale; and the chemical
composition and distribution by size of planetesimals. It will therefore be
useful to consider some physical constraints on the process by which the
Earth's atmosphere was formed.
Currently existing paleontological data offer clear evidence of the
presence of life on Earth 3.5 billion years ago (Schopf and Packer 1987~.
Furthermore, we can make the fundamental conclusion from the analysis
of carbon isotope ratios i3C/~2C that an almost contemporary biogeochem-
ical carbon cycle (Schidlowski 1988) existed on Earth 3.8 billion years ago.
Moreover, there is some reason to believe that this last dating could be
pushed further back to four billion years (Schidlowski, personal communi-
cation). There is no doubt that both the prebiological evolution processes
and the global biogeochemical carbon cycle can only occur in a sufficiently
203
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dense atmosphere and hydrosphere that have already formed or are form-
ing. Taking into account data from isotope systematics (Faure 1986), with
which we can estimate the Earth's age at 4.55 to 4.57 billion years old, we
conclude that the time scale for prebiological evolution and the emergence
of life on Earth was sufficiently brief: possibly less than 0.5 billion years.
This fact means, however, that the Earth's protoatmosphere must have
been formed prior to the appearance of the biogeochemical cycle, that is,
over a period of time significantly less than 0.5 billion years. Additional
indirect evidence of the early emergence of the Earth's atmosphere can be
found in analyzing the isotope relationships of the noble gases Ar and Xe
(Ozima and Kudo 1972; Ozima 1975; Kuroda and Crouch 1962; Kuroda
and Manuel 1962; Phinney et al. 1978~.
Therefore, existing and observed data provide evidence of the very
early formation of a dense atmosphere on Earth. We shall consider possible
scenarios for the formation of the Earth's early atmosphere and its initial
chemical composition.
EARTH'S INITIAL ATMOSPHERE
A model for Earth's early atmosphere, formed from solar composi-
tion gas captured gravitationally during the final stages of accretion, was
explored in Hayashi's studies (1981, 1985~. Hydrogen and helium are the
primary components of this atmosphere. According to the estimates of
several authors, the total mass of the initial atmosphere could have reached
1026-103i g (Hayashi et al. 1985; Cameron and Pine 1973; Lewis and Prinn
1989~. However, the inference by these models that the entrapment pro-
cess occurred isothermically may lead to significant error: they overlook
the heating up of the gas during accretion (Lewis and Prinn 1984~. We will
note that there are some additional difficulties in a model of an isothermal,
initial atmosphere. They stem from the diffusive concentration of heavy
gases in the initial atmosphere (Walker 1982~. The pressure of the initial
atmosphere for Earth is only 0.1 bar in the more realistic models of the
adiabatic, gravitational capture of gas from a protoplanetary nebula (Lewis
and Prinn 1984~.
Clearly, an initial atmosphere could only have formed if the processes
of gas dissipation from the protoplanetary nebula had not ceased by the time
Earth's accumulation had concluded. Gas dissipation from a protoplanetary
nebula is determined by a solar wind from a young T Tauri Sun and EUV
(Sekiya et al. 1980, Zahnle and WaLker 1982; Elmegreen 1978; Horedt
1978;~. Canuto et al. (1983) have estimated the time scale for dissipation
of the gaseous component of a protoplanetary nebula, using observed data
on T Tauri star luminosity. Their estimates show that this time scale is not
more than 107 years. It may only be several million years, beginning with
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205
the onset of the convective phase in the history the Sun's development. This
time scale is appreciably shorter than the approximately 108 years estimated
for Earth's accumulation (Safronov 1969; Wetherill 1980~. Hence, current
theories of stellar evolution, coupled with observed astronomical data, raise
the possibility that only a very weak initial atmosphere existed at the very
inception of planets' accumulation process. This is a very serious argument
against the formation of an initial atmosphere on Earth as proposed by
Hayashi.
Nevertheless, one should note that the accuracy of estimates of the time
scales for a significant portion of cosmogonic processes at the early stages of
protosolar nebula evolution is not reliable enough to rule out, with absolute
certainty, the possibility that an appreciable initial atmosphere existed
on Earth. According to this scenario, the initial atmosphere encounters
considerable additional difficulties stemming from its chemical composition.
Adopting, again, the adiabatic model of the gravitational capture of gas
from a protoplanetary nebula (Lewis and Prinn 1984), we can see that
with a 0.1 bar value of the pressure of the primordial atmosphere, the
nitrogen levels in it would be 105 times lower than present levels. At the
same time, neon levels would exceed current neon atmospheric levels by
approximately 40 times. WaLker's more detailed computations, where he
accounts for the radiative-convective structure of the primordial atmosphere
(1982), also show an inconsequential pressure for this kind of atmosphere
at the surface (approximately 0.2 bars). This means that arguments raised
by Lewis and Prinn (1984) against a primordial atmosphere remain valid
in this instance. Hence, various scenarios for a dense initial atmosphere
appear to be highly improbable for the above reasons. Therefore, the very
rapid formation of a dense Earth atmosphere was apparently a function of
different physical processes.
CONTINUOUS DEGASSING
One of the suggested mechanisms for the rapid formation of the
atmosphere is the intense, continuous degassing of Earth in the conven-
tional sense of this term, including magmatic differentiation and volcanism
(Walker 1977; Fanale 1971~. There is still no answer to the question
of whether continuous degassing of the Earth could have been intensive
enough to allow for the formation of the atmosphere and the hydrosphere
over a very short time span (< 108 years). Such a possibility exists where
there is a strong, overall heating of the planet during its accretion. However,
this entails the inclusion of a number of additional inferences regarding a
very brief accretion scale: < 5 x 105 years (Hanks and Anderson 1969~.
Walker's recent analysis of the process of continuous degassing (Walker,
this volume) shows that the present rate of degassing is dearly insufficient
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to be responsible for forming the atmosphere in a very brief time scale. At
the same time, Walker's proposed numerical estimate of the accumulation
rate of hot lava on primordial Earth, which could have provided for the
necessary intensity of degassing (1.3 x 10~9 g/yr), appears to be unjustifiably
high. Such intensive volcanism infers that ~ 1.3 x 1027 g of magma would
pour out over 108 years of accumulation on the surface, or a quarter of the
entire present mass of the Earth.
Under the currently adopted planetary accumulation models with an
approximate 108 year time scale, such a hypothesis would only be justified
in the case of a gigantic impact (Kipp and Melosh 1986; Wetherill 1986~.
There is no question that a considerable portion of the planet would have
melted as a result of a gigantic impact which would have released a huge
quantity of gases. This amount would have been sufficient to have formed
an atmosphere. Yet, there is no detailed, physical-chemical model of this
process at the present time. Furthermore, it cannot be considered that the
very fact of a giant impact in the Earth's history has been firmly established.
Clearly, if such a catastrophic event did actually take place in the earliest
history of our planet, its consequences would have been so great that they
would have been reflected in the geochemical "records." However, the
scenario explored by WaLker (this volume) of two reservoirs of volatiles,
one of which is virtually entirely degassed (the upper mantle), and the
other which has conserved its store of volatiles (lower mantle), is difficult
to reconcile with the idea of a gigantic impact. Duly, such a clear separation
of the two mantle reservoirs, whose presence has been determined with
sufficient certainty from observed data (AIlegre et al. 1987) is difficult to
expect in the case of a gigantic impact. An attempt should be made to
analyze the possible geochemical consequences of a gigantic impact and
reconcile them with existing data. For now, a gigantic impact remains
a reality only in computational models of the evolution of preplanetary
swarms of bodies. However, this episode is only a specific case of the
impact-induced degassing of matter. Yet impact-induced degassing was,
apparently, a determining physical process which led to the very rapid
formation of protoEarth's atmosphere.
IMPACT PROCESSES AND THE EARTH'S PROTOATMOSPHERE
The role of impact processes in forming Earth's protoatmosphere was
discussed more than 20 years ago in F-lorensly's study (1965~. During
the years that followed, this idea was developed in a number of studies
(Arrhenius et al. 1974; Benlow and Meadows 1977; Gerasimov and Mukhin
1979; Lang and Ahrens 1982; Gerasimov et al. 1985~. Impact-induced
degassing is rooted in the idea of Earth accumulation from large solids. This
idea does not require the inclusion of any serious additional proposals. It is
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this circumstance that brings us to the conclusion that impact degassing was
a real mechanism in the formation of Earth's atmosphere and hydrosphere.
Departing from the gigantic impact issue, we can see that the maximum
dimension of those bodies which regularly fell on the planet, and from which
Earth was formed, could have attained hundreds of kilometers (Safronov
1969~. These dimensions fit with estimates of the diameters of impacting
bodies which formed the largest craters on the terrestrial planets and their
satellites (O'Keef and Ahrens 1977~. The velocity of the collision of a
random body of a preplanetary swarm with the embryo of primordial Earth
is no less than the escape velocity. Therefore, the range of velocities of
planetesimal collision with a growing planet embryo varies from meters per
second at the initial stages of its expansion, to > 12 kilometers per second
at the final stages of accumulation. In reality, collision velocities could have
been significantly higher in the case of a collision between Earth and bodies
escaping from the asteroid belt, or with comets. In this case, when the mass
of an expanding Earth reached approximately 10% of its current mass, the
escape velocity became equal to roughly five kilometers per second; the
melting of silicate matter began during a collision between planetesimals
and an embryonic planet (Ahrens and O'Keef 1972~. Beginning with the
point where the mass of the embryo was equal to approximately 0.5 of the
mass of present-day Earth, impact processes were paralleled by the partial
vaporization of silicates (Ahrens and O'Keef 1972~. The more high-speed
planetesimals, reaching a speed of more than 16-20 kilometers per second,
were completely vaporized. Several works (Ahrens and O'Keef 1972; Gault
and Heitowit 1963; McQueen et al. 1973; Gerasimov 1979; O'Keef and
Ahrens 1977) analyze in detail the physics of how colliding matter is heated
as a shock wave passes through that matter and estimate the efficiency of
impact-induced degassing. They demonstrate that impact degassing is most
efficient when melting and partial vaporization of silicate matter begin. It is
worth noting that the release of the primary volatile components begins long
before the point when the planet's accumulation process reaches collision
velocities corresponding to the melting point of matter. Pioneering works
(Lang and Ahrens 1982, 1983; Katra et al. 1983) establish the beginning
of volatile loss at extremely low collision velocities of approximately one
kilometer per second. Using the Merchison meteorite as an example, the
authors established that about 90% of the initial amount of volatiles is
already lost at a collision velocity of 1.67 kilometers per second (1Uburcy
et al. 19~86~. This loss is due to breakdown of the meteorite's water-,
carbon-, and sulphur-containing minerals at impact. These experiments
are proof of the beginning of volatile release at a very early stage of the
Earth's accumulation. Its mass was less than 0.01 of its final mass. Water
and carbon dioxide are the main constituents involved in the processes of
shock-induced dehydration and decarbonatization of minerals.
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As the mass of the embryonic planet increases, escape velocity rises.
Consequently, there is also a rise in the velocity at which planetesimals
fall. There occurs a corresponding increase in the heating of matter of the
planetesimal and the surface of the planet in the central shock zone. The
nature of degassing processes undergoes qualitative changes. The shock
process becomes a considerably high-temperature one. Chemical reactions
occurring in the vaporized cloud become increasingly significant. High-
temperature chemical processes begin to play a predominant role during
the final stages of Earth's accumulation, instead of the relatively simple
processes involved in the breakdown of water-containing compounds and
carbonates. Colliding matter undergoes total meltdown in the central zone
of impact at collision velocities of five to eight kilometers per second:
volatile components are released from the melted matter and interact with
it. The chemical composition of the released gases must correspond to the
volcanic gases for magma of corresponding composition and temperature.
CO2, H2O, and SO2 will clearly be the primary components in such a gas
mixture. The gases CO, H2, H2S, CH4, and others may be competitors to
these components, depending on the extent to which planetesimal matter
is reduced (Holland 1964~. Where collision velocities exceed eight to nine
kilometers per second, vaporization at impact of a portion of planetesimal
matter becomes significant.
Vaporization of silicate matter is supported by thermodissociation of
planetesimal mineral constituents. As a result of this process, a considerable
quantity of molecular and monatomic oxygen is released to the cloud of
vaporized matter. Melt-vapor equilibrium determines the conditions for
vaporization. The characteristic vaporization temperature for silicates is
approximately 3000-5000 K Vapor pressure is approximately 1-100 bars
(Bobrovskiy et al. 1974~. In these conditions, thermodynamic equilibrium
in a gaseous phase is reached over a time scale tchem < 1O-5 seconds
(Gerasimov et al. 1985~. Therefore, when large-scale impact episodes
occur, where the characteristic time scale for the expansion of a cloud of
vaporized matter tcool is counted in seconds, thermodynamic equilibrium
(scoot ~ tchem) iS clearly reached at the initial stage of the expansion
of such a cloud. Consequently, gases are formed in the cloud from the
entire range of volatile elements present, regardless of what form they
displayed in the initial matter. In such conditions, H2O and CO2 will
be formed inside the dense, hot cloud if hydrogen, carbon, and oxygen
are present in it. This will occur regardless of the initial presence in
the planetesimals of carbonates and hydrated minerals. At the same time,
where carbonates and hydrated minerals are present in planetesimals, a
portion of the hydrogen and carbon in these minerals will be used to form
other hydrogen- and carbon-containing components. Less H2O and CO2
will be produced than with the simple thermal breakdown of minerals. As
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the vapor-gas cloud expands, its density and temperature drop, and tchem
increases. The expansion process commences when tcoo' ~ tchem. This is
the point where the reaction products undergo chemical hardening, since
the time scale for chemical reactions becomes greater than the characteristic
time scale for cloud expansion as the cloud expands further. Gases with a
chemical composition corresponding to the moment of hardening are mixed
with atmospheric gases, interacting with both these gases and bedrock from
the uppermost layers of the planet (regolith). If the atmosphere is dense,
mixing of gases in an expanding cloud with atmospheric gases may occur
earlier, when overall pressure of the expanding cloud pv2/2 + P (p =
density, v = mass velocity, P = pressure in the vapor-gas cloud) becomes
approximately the pressure in the atmosphere. The portion of the vaporized
silicate matter c', which is condensed in the cloud by the time constituents
harden (occurring at a given temperature T*), is defined by the simple ratio
(Anisimov et al. 1970~:
c~~ 1—T*/To'
where To denotes temperature in the vaporized cloud as it begins to expand.
Estimates demonstrate that for impact of a planetesimal with a dimension
of approximately 100 kilometers, the value is T* ~ 2000 K Therefore, for
a vaporization temperature of To ~ 5000 K, the value is ~ ~ 60%. This
means that approximately one half of molecular and monatomic oxygen,
comprising ~ 30% of the cloud's pressure, (Gerasimov e! al. 1985) will
be released into the atmosphere with each impact. Clearly, the impact-
vaporization mechanism is a powerful source of free oxygen in Earth's
earliest atmosphere.
The chemistry of a high-temperature, gas-vapor cloud is too complex
to be able to judge it solely in terms of the thermodynamical equilibrium
in its gaseous phase. Condensation of silicate particles and catalytic ac-
tivity occur during expansion and cooling of the vaporized cloud. These
processes can significantly alter the equilibrious gas chemical composition.
This circumstance imposes certain requirements on the search for both
theoretical and experimental approaches to the study of chemical processes
in a cloud of impact-vaporized matter.
Studies (Gerasimov et al. 1984; Gerasimov and Mukhin 1984; Gerasi-
mov e! al. 1987) used laser radiation to examine the chemical composition
of gases which form during high-temperature, pulse-induced vaporization
of the Earth's rockbed and meteorites. They demonstrated that molecular
oxygen is actually the most abundant constituent in a cloud of vaporized
matter (Gerasimov e! al. 1987), and that it determines the chemical pro-
cesses occurring in the cloud. Regardless of how reduced the initial matter
is, the primary volatile elements H. C, and S are released as oxides: H2O,
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CO, CO2 (CO/CO2 > 1), and SO2. In addition to the oxides, a certain
amount of reduced components and organic molecules is formed in the
cloud: H2, H2S, CS2, COS, HCN, saturated, unsaturated, and aromatic
hydrocarbons from CH4 to C6 and CH2CHO. Molecular nitrogen is re-
leased. The vaporization-induced gaseous mixtures for samples belonging
to both crust and mantle rock, as well as for conventional and carbonaceous
chondrite, are qualitatively homogenous. This is seen in the preponder-
ant formation of oxides, and in both the comparable (within one order
of magnitude) ratios of the gases CO/CO2 and the correlation between
the various hydrocarbons. The gas mixtures formed at high temperatures
are in nonequilibrium for normal conditions. Therefore, their chemical
composition will be easily transformed under the impact of various energy
sources.
One should note that the passage of a planetesimal through the atmo-
sphere exerts a substantial influence on the formation of the atmosphere's
chemical composition. Studies (Fegly et al. 1986; Prinn and Fegley 1987;
Fegley and Prinn 1989) have analyzed this issue in the greatest detail
in recent years. In the physics of the process, a body of large dimen-
sion (approximately 10 kilometers in diameter) passes through primordial
Earth's atmosphere at a speed of approximately 20 kilometers per second;
shock waves send a large amount of energy into the atmosphere. If we
put the planetesimal density at ~ 3 g/cm3 (which matches the chondrite
composition), and the angle of entry into the atmosphere at 45°, as first
approximation, we would have Z2 x 1027 ergs. This figure is 0.07% of
the energy of an asteroid. The energy passes directly into the atmosphere
as the body flies through, and even more energy (~ 3.2 x 1029 ergs) is
"pumped" into the atmosphere by a supersonic discharge of matter from
the impact crater (Fegley and Prinn 1989~. The shock wave front (formed in
the atmosphere during this process) compresses and heats the atmospheric
gas to several thousand degrees Kelvin. It is clear that various thermo-
chemical and plasmochemical reactions are occurring in this region. Due
to recombination processes, new compounds are formed as cooling occurs.
This substantially alters the initial chemical composition of the atmosphere.
We can estimate the chemical composition of the gas mixtures at high
temperatures, using the standard methods of thermodynamical equilibrium
(given the presupposition that the time scale for the breakdown of a given
molecule is less than the time required to cool the elementary gas exchange.
The most detailed computations of these processes were made in the studies
of Fegley e! al. (1986) and Fegley and Prinn (1989~. It seems obvious
that these findings must be critically dependent on the initial chemical
composition of an unperturbed atmosphere. The atomic ratio C/O is an
important parameter here: it determines the "oxidized" and "reduced"
state of the atmosphere. Fegley and Prinn (1989) demonstrated that if
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211
C/O > 1 (reduced atmosphere), precursors of biomolecules, such as HCN
and Hz CO, are formed as the asteroid passes through this atmosphere.
Nitrogen oxides appear in the oxidized atmosphere instead of prussic acid,
supporting the formation of nitric acid if rain falls. Fegley and Prinn
(1989) considered several possible chemical compositions for unperturbed
atmospheres, and they calculated the commensurate alterations in the
composition owing to the passage of large bodies through the atmosphere.
I would like to make the following comment regarding their work The
authors used computation methods employed for purely gaseous reactions.
Heterogeneous catalysis on mineral particles (present in the atmosphere
during the impact-induced discharge of matter) must play a significant role
in the actual natural process. Heterophase reactions must considerably af-
fect the evolution of the atmosphere's chemical composition during impact
reprocessing. However, it is quite difficult to account for these reactions at
this time. An account of the fluxes of such important components as prussic
acid and formaldehyde is an unquestionable achievement in the work of
Fegley and Prinn. Their efforts made it possible to estimate the stationary
concentrations of these components in a modeled early atmosphere. We
shall note that the numerical values of the strength of the source of cyanic
hydrogen formation in atmospheric reprocessing and in experiments on
laser modeling of the processes of shock-induced vaporization are compa-
rable (Mukhin et al. 1989~. Hydrocarbon and aldehyde output in the latter
case is significantly higher.
THE EVOLUTION OF AN IMPACT-GENERATED ATMOSPHERE
Gas fluxes from the atmosphere present one of the most difficult
issues related to the early evolution of the Earth's atmosphere. There
are no accurate estimates at this time of the reverse fluxes of released
gases to the surface rock of the young planet. It is therefore impossible
to reliably estimate the stationary concentrations of these gases in the
protoatmosphere.
The strength of a shock source of atmospheric gases is so great that the
atmospheric accumulation of gases released during the fall of planetesimals
at the early stage of accretion would create a massive atmosphere from
water vapors. A runaway greenhouse effect would develop. Such a scenario
was developed in several studies (Abe and Matsui 1985; Matsui and Abe
1986; Zahnle et al. 1988~. The massive atmosphere "locks in" heat released
by the impact. This in turn triggers the heating of the atmosphere and
melting of the upper layers of the planet's silicate envelope. Zahnle et al.
(1988) have estimated that the massive water atmosphere should condense
and form an ocean as planetesimals cease falling. The problem however,
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213
absorption by condensed particles of oxygen released during vaporization is
an important experimental finding (Gerasimov 1987~. This is evidence that,
despite the impact-induced release of large amounts of oxygen into the
atmosphere, molecular oxygen was absent in the primordial atmosphere:
it was almost instantaneously absorbed by the regolith. It is possible
that the same fate (albeit with greater time scales) is also characteristic
for other chemically active gases. In the future, absorbed gases will be
repeatedly released into the atmosphere owing to shock-induced processing
of planet surface matter by falling planetesimals. Chemically inert gases,
such as the noble gases and molecular nitrogen, may have accumulated
in the protoatmosphere. Therefore, the degassing fate of noble gases and
chemically active gases may have been entirely different (Gerasimov et al.
1985~.
If we extrapolate the conditions on earliest Earth to the following
period of "continuous" degassing, with a weaker source of atmospheric
gases, atmospheric density would have been extremely small. This is due to
the consecration of high flux rates. The appearance of a volatile-enriched
(particularly oxygen) protocrust is one of the most important conditions
for atmospheric stabilization. The appearance of an acidified protocrust
ensures low flux rates for the majority of atmospheric gases and the accu-
mulation of an ocean. In turn, the formation of an ocean also governs the
composition and density of the atmosphere. The appearance of a protocrust
is currently being considered in models of magmatic differentiation (Taylor
and McLennan 1985~. The earlier appearance of a protocrust in this model
runs up against the same difficulties as the catastrophic formation of an
atmosphere in the continuous degassing model. One possible avenue for
the appearance of a protocrust is shock-induced differentiation (Mukhin
et al. 1979, Gerasimov e! al. 1985), as a result of which the atmosphere
and the protocrust are formed within the same process of accumulation.
However, the question of shock differentiation has yet to be theoretically
analyzed in-depth and requires additional investigation.
The shock-degassing source was operative virtually throughout the
planets' entire accumulation process. The formation of a sufficiently dense
atmosphere at the earlier stages of accumulation of the Earth and the
terrestrial planets is problematic owing to the possible rapid flux of atmo-
spheric components to the regolith. Other factors also had an effect, such
as shock-induced "cratering" of the atmosphere (Walker 1986), intensive
T lburi-like solar wind, EUV, and thermal volatilization. The probability
of the formation of a dense atmosphere increases with the accumulation
of the planet's mass, since by the time accretion is completed there is not
likely to be any action of intensive solar wind and EUV. The loss of gases
from a planet with a large gravitating mass is also difficult. Uncertainties in
estimating the density of an impact-generated atmosphere at the final stage
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of accretion are largely related to uncertainties in estimating the rates at
which gases flow into the regolith.
Therefore, impact-induced degassing, despite the possible parallel in-
put of an accretive source and a continuous degassing source was clearly
the most probable and important source of atmospheric gases during the
earliest epoch of the Earth's evolution: the period of its accumulation.
Achievements made in recent years in investigating the impact source allow
us to assess its strength and the chemical composition of the gases that were
released. Nevertheless, the question of the evolution of the composition
and density of the impact-generated atmosphere continues to remain an
open one, primarily due to ambiguities in estimating gas flux rates from the
atmosphere during accumulation. Therefore, it is our view that fluxes are
one of the most important issues pertaining to the origin of Earth's earliest
atmosphere.
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Allegre, CJ., T. Staudacher, and P. Sarda. 1987. Rare gas systematics: formation of the
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Fegley, B., Jr., R.G. Prinn, H. Hartman, and G.H. Watkins. 1986. Chemical e~ects of large
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Gault, D.E., and E.D. Heitowit. 1963. The partition of energy for hypervelocity impact
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Representative terms from entire chapter:
initial atmosphere
