Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres
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96
Photochemistry
of
Planetary
Atmospheres
Figure
4.10
Correlation
of
26
Mg/
24
Mg
with
^Al/^Mg
in
Allende inclu-
sion.
After
Lee,
T.,
Papanastassiou,
D.
A.,
and
Wasserburg,
G. J.,
1977,
"Aluminum-26
in the
early solar system: fossil
or
fuel?"
Astrophys.
J.
Lett.
211, L107.
WA is a
coarse-grained
chondrule
containing
anorthite,
pyroxene,
spinel,
and
melilite.
in
exactly
the
condensation sequence predicted using
the
thermodynamic
equilibrium
chemistry described
in
section
4.5.
The
meteorites
are not all
primitive material. There
is
evidence
for
thermal
and
shock
processing
that
has
fundamentally
altered
the
composition
and
texture
of
some
of
the
meteoritic material. Theoretically,
we
would expect
a
planetesimal exceeding
the
size
of 10 km to
undergo
differentiation,
with
26
A1
as a
possible heat source.
The
net
result
of
this
process
is
shown
in figure
4.11.
On
breaking
up due to
collisions,
some
of the
differentiated material
is
released. This would account
for the
origin
of
the
highly processed material such
as
that
found
in the
stony-iron
and
iron meteorites
(see appendix
4.2).
The
existence
of
chondrules
in
meteorites provides another constraint
on the
phys-
ical conditions
in the
nebula. These
are
millimeter-sized igneous-textured spheroids
composed mostly
of
silicate minerals.
The
spheroid shape suggests that they were
once either
fully
or
partially molten droplets.
A
pulse
of
intense heating
is
required
for
their formation;
the
temperature must exceed
1800
K for not
more than
a few
minutes.
The
energetics
of
chondrule formation
are not
completely understood
and
pose
a
fundamental
challenge
to the
sources
of
disequilibrium energy discussed
in
section
4.5.
Current ideas include shock during
infall
of
material
to the
central plane
(see
figure
4.7),
melting during impact,
and
lightning.
Table
4.3
Extinct
nuclides
Radionuclide
Confirmed
26
Al
53
Mn
107p
d
129|
146
Sm
244
p,,
Strong
evidence
92
Nb
Hints
60p
e
135
Cs
182
Hf
Mean
life
(Myr)
1.1
5.5
9.4
23
149
120
231
1
3.3
13
Daughter
26
Mg
53
Cr
107
Ag
129
Xe
142
Nd
Fission
Xe
92
Zr
60
Ni
135
Ba
182
W
Abundance
26
A1/
27
A1
= 5 x
10-
5
53
Mn/
55
Mn
= 4 x
10~
5
107
Pd/
108
Pd
= 2 x
10~
5
129
I/
127
I
= 1 x
10-
4
i46
Sm/
i44
Sm
_
o
Q05
_
0
015
^Pu/^'U
=
0.004
-
0.007
92
Nb/
93
Nb
= (2 ± 1) x
10-
5
6°Fe/
56
Fe
<
2 x
10~
6
135
Cs/
133
Cs
<
2 x
10~
4
l82
Hf
= 1 - 10 x
10~
5
Comments
Shortest mean
life;
possible
heat source
Most recent discovery
Only
found
in
metal
First
discovered;
found
in
>75
meteorites
Uncertain
abundance
Also
ex-decays,
leaving
damage tracks
Based
on
single
Nb-rich
grain
Excess
60
Ni
observed; could
be
nucleosynthetic
effect
Deficit
in'
35
Ba
observed;
could
be
nucleosynthetic
Deficit
in
182
W
observed;
in
Hf-depleted
metal
Interesting
upper
limits
41
Ca
M7
Cm
0.19
22.5
41
K
235
V
41
Ca/
40
Ca
< 1 x
10~
8
™Cm/™U
<
0.004
Must
be
cogenetic
with
244
Pu
From
Swindle,
T. D.
(1993).
undifferentioted
asteroid
(ordinary
chondrilcs)
differentiated
asteroid
(achondriles.
stony
irons)
Figure
4.11
Schematic
cross
section
of
two
meteorite
parent
bodies:
undifferentiated
and
differentiated.
After
Wetherill,
G. W., and
Chapman,
C.
R.,
1988,
"Asteroids
and
Meteorites,"
in
Kerridge
and
Matthews
(1988;
cited
in
section
4.1),
pp.
35-67.
97
98
Photochemistry
of
Planetary Atmospheres
Table
4.4
Concentrations
and
molecular
characteristics
of
soluble
organic compounds
of
meteorites
Class
Ammo
acids
Aliphatic
hydrocarbons
Aromatic
hydrocarbons
Carboxylic
acids
Dicarboxylic
acids
Hydroxycarboxylic
acids
Purines
and
pyrimidines
Basic
N-heterocycles
Amines
Amides
Alcohols
Aldehydes
and
ketones
Total
Concentration
(ppm)
60
>35
15-28
>300
>30
15
1.3
7
8
55-70
11
27
>560
Compounds
identified
74
140
87
20
17
7
5
32
10
>2
8
9
411
Chain
length
2
C
2
-C
7
C,-C>
23
C6-C20
C
2
-C,
2
C
2
-C
9
C
2
-C
6
NA
NA
C,-C
4
NA
C,-C
4
C,-C
5
"From
Cronin,
J. R. et
al.
(1988).
NA
=
not
applicable.
Natural
remnant magnetization (NRM)
has
been observed
in
meteorites.
Estimates
of
paleointensities range from
100
/zT
in
Allende
to 18
/iT
for
Murchison.
The
observed
NRM is
consistent with
the
presence
of a
magnetic
field of
about
30
|*T
in
the
solar nebula that
is
inherited
from
the
interstellar cloud.
4.6.3
Organic
Material
Organic compounds
as
complex
as
amino
acids have been
found
in
meteorites.
Ta-
ble 4.4
lists important organic
species
identified
in
meteorites.
There
is no
doubt
as
to
their abiotic
and
extraterrestrial origin.
The
amino acids
in
meteorites have equal
amounts
of
levo
and
dextro chirality
and are
clearly distinguished
from
those
of bi-
ological origin with levo chirality only.
The
organic compounds
in CM
chondrites
include aliphatic
and
aromatic hydrocarbons, alcohols,
carbonyl
compounds, amines,
amides,
and
amino acids.
There
are
three possible origins
of
these organic compounds:
(a)
molecular cloud,
(b)
Urey-Miller
synthesis,
and (c)
Fischer-Tropsch process. Note
that
the first
source
is
pre-solar
nebula.
The
second
is in
situ
gas
phase chemistry driven
by
a
disequilibrium energy source.
The
third
is
equilibrium chemistry catalyzed
on the
surface
of
dust grains
in the
solar nebula.
These
sources
are not
mutually exclusive.
The
evidence that
at
least some
of the
organic material
in the
meteorites
is
from
the
molecular cloud
is its D/H
ratio. Some
of the
organic material
is
enriched
in D
by
factors
of
10-100
relative
to the
cosmic abundance. Such values
are
reminiscent
of
the
chemistry
of the
molecular clouds (see section
4.4.2).
There
is no
other known
chemistry
that
can
have caused such large fractionations
of
deuterium
in the
solar
nebula.
In
a
series
of
classic experiments started
in
1953, Miller
and
Urey demonstrated
prebiotic
synthesis
of
organic compounds
from
mixtures
of
simple gases
of
reducing
composition
using electric discharges.
The
detailed chemical pathways
are not
under-
Origins
99
stood,
but the
energy
from
the
discharge breaks
up the
stable bonds
of the
reactants.
The
subsequent chemistry
may be a
combination
of
thermal, ion,
and
radical chem-
istry.
It is a
weakness
of
this type
of
experiment that only
the
bulk
compositions
of
the
reactants
and the final
products
are
quantitatively analyzed.
The
detailed chemical
kinetics
and
reaction schemes
are not
known. Nevertheless, qualitative information
on
the
nature
of the
products
and the
order
of
magnitude
of the
yields
may be
derived
from
this type
of
experiment. Since
the
early work, there have been
two
innovations.
Similar
experiments have been performed with ultraviolet radiation
as a
source
of
energy,
with
high yields
of
organic compounds. Even when
the
starting
gas
mixture
is of
only mildly reducing composition such
as CO and
H2O, organic products such
as
formaldehyde, alcohol,
and
formic acids
are
readily formed.
The
main uncertainty
of
the
application
of
these laboratory results
to the
solar nebula
is the
lack
of a
quantitative
estimate
of the
disequilibrium energy source.
It
is
known from laboratory experiments that
the
synthesis
of
organic compounds
from
simple molecules such
as CO and
H
2
can
readily occur
on the
surface
of
grains
containing
Fe as a
catalyst.
The
process
is
known
as
Fischer-Tropsch
synthesis.
The
following
are
examples
of
this type
of
chemistry that might have been important
for
the
production
of
alkanes, alkenes,
and
alcohols
in the
solar nebula:
Similar reactions
may
lead
to the
formation
of
alkynes
and
aromatic compounds.
4.7
Comets
Comets
are the
most primitive bodies
in the
solar
system. Formed
in the
outer parts
of
the
solar nebula
and
composed primarily
of
ices, they preserve
as a
record
of the
physical
and
chemical environment
in
which they were formed.
We now
know that
there
are two
populations
of
comets.
The
long-period comets have periods
greater
than
200 yr.
Their orbits
are
randomly oriented
on the
celestial sphere. They have been
shown
to
come
from
the
Oort
cloud,
a
vast spherical cloud
of
comets surrounding
the
solar system
and
extending
halfway
to the
nearest star (about
10
5
AU).
The
origin
of
the
Oort cloud
is
still
debated.
The
crucial question concerns whether
the
comets
in
the
Oort
were
formed
in
situ
or
formed
in the
accretion disk followed
by
dynamical
ejection
to
large distances.
The
short-period comets have periods less
than
200 yr,
with
most
of the
periods between
5 and 20 yr.
Their orbits
are
within
30° of the
ecliptic
plane.
They
are
most
likely
from
the
Kuiper
Belt,
a
ring
of
comets beyond
the
orbit
100
Photochemistry
of
Planetary Atmospheres
Table
4.5
Comet
composition
Relative
abundance
Molecule
(by
number)
Comet
P/Halley
H
2
O
CO
H
2
CO
C0
2
CH4
NH
3
HCN
N
2
SO
2
H
2
S
CH
3
OH
Other
Comets
CO
ca,
CH
3
OH
H
2
CO
HCN
H
2
S
S
2
Data
from
Mumma
et
100
~7
~8
0-5
3
<0.2-1.2
0-2
0.1-0.3
1-2
O.I
<0.02
~0.02
<0.002
—
~1
20
2
1-3
<0.2
1.5-4.5
1-5
0.1-0.04
0.03-0.2
0.2
0.025
al.
(1993).
Comments
Remote
and in
situ
detections
Direct (native) source
Distributed
source
Variable
Infrared
(Vega
1
IKS)
Ground-based
infrared
Giotto
IMS
Variable;
based
onNH
2
Giotto
IMS
Variable; ground-based ratio
Giotto
IMS
Ground-based
Nj
emission
Ultraviolet
(IUE)
Giotto
IMS
Giotto
NMS and IMS
West
(1976
VI)
Bradfield
(1979
X)
Austin
(1990
V)
Levy
(1990
XX)
Wilson
(1987
VII)
Variable
If
a
parent
species
Several comets
Austin
(1990
V);
Levy
(1990
XX)
IRAS-Araki-Alcock
(1983 VII)
of
Neptune
(30
AU). Occasional perturbation
of
their orbits
by the
giant planets sends
them
to the
inner solar system.
From ground-based observations
and the
recent spacecraft missions
to
Comet
Hal-
ley,
we now
understand that comets were most likely formed
at
25-60
K. The
bulk
of
material comprises low-temperature condensates
and
ices. Solid materials includ-
ing
silicates
and
organics were
found
in
Halley. Table
4.5
summarizes
the
chemical
composition
of the
volatile
species
in
recent comets.
There
is a
rich variety
of
both
oxidized
and
reduced compounds
of C, N, and S. It is
important
to
distinguish
the
pho-
tochemical products
from
the
parent molecules.
For
example,
82,
C2, and C3
detected
in
comets
are
probably
not
primordial
and may be
derived
from
the
fragmentation
of
parent molecules. Even some
of the CO may not be
primordial
(see
discussion
in
section
4.7.2).
We
briefly
discuss
the
significance
of the
observed
species
shown
in
table
4.5 for our
understanding
of the
solar
system.
For
comparison,
we
also show,
in
table
4.6,
the
abundances
of
these species
in the
interstellar medium.
For
simplicity,
we
restrict
this
discussion
to
water, carbon,
and
nitrogen.
Origins
101
Table
4.6
Comet/ISM
comparison
Species
H
2
O
CH
3
OH
HCN
SO
2
NH
3
C0
2
H
2
CO
H
2
S
(CO)
CH4
CO
N
2
S
2
Temperature
(K)
152
99
95
83
78
72
64
57
(50)
31
25
22
20
Distance
(AU)
3.4-5.4
13.3-14.6
22-25
60-50
84-63
112-79
Relative abundance
Comets
100
1-5
0.02-0.1
<0.002
<0.01
0.1-0.3
3
0-5
0.1-0.04
0.2
(if
codeposited
0.2-1.2
<0.2
~7
0-20
0.02
0.025
ISM-gas
<100
0.01-0.1
0.01-0.1
0.01-0.1
0.2-2
<10
0.1-0.3
<0.01
with
water)
1
500-2000
100
7
ISM-ice
100
7^0
4
7
<5
?
<0.2
0.3
<1
0-5
7
7
Data
from
Mumma
et
al.
(1993).
4.7.1 Water
Water
is
believed
to be the
most abundant volatile
in a
comet. Indeed,
it is
generally
accepted that
a
comet
is an icy
conglomerate containing dust
and
other frozen volatiles.
H2O
molecules have been detected
in
Comet Halley using infrared
spectroscopy.
In
addition,
the
measured
ortho/para
ratio
of the
hydrogen atoms
in
water suggests that
water
was
trapped
in the
comet nucleus
at
very
low
temperatures.
4.7.2 Carbon
Carbon compounds
of
varying degree
of
oxidation from
CHi
to CO2
have been
de-
tected
in
comets.
CO
appears
to be the
dominant carbon
species.
Observations
at
Halley
indicate that there
are two
sources
of CO: a
direct source,
and a
delayed
source that
can be
modeled
as
photolytic fragmentation
of
polymeric formaldehyde.
The
CRt/CO
ratio
in
comets
provides
a key
test
of
whether
the
carbon
species
are
derived
from
the
interstellar medium
or
chemically produced
in the
solar nebula.
The
ratio
CH
4
/CO
is of the
order
of
0.1
in
comets
but
less than
10~
3
in the
interstellar
medium.
CO
2
is
within
an
order
of
magnitude
of CO in
comets,
but its
upper
limit
in
the
interstellar medium
is
several orders
of
magnitude below
the
concentration
of
CO.
Referring
to
table 4.6,
we
note that
CO is the
dominant carbon species both
for
comets
and the
interstellar medium. However,
the
other carbon species such
as
CH4,
CH3OH,
and
CO
2
are far
more abundant relative
to CO in the
comets than
in the
inter-
stellar medium. This
is
circumstantial evidence that
the
materials
in the
comets
are not
simply
condensed
from the
interstellar medium
but
have been processed chemically
in
the
solar nebula. However,
the
time needed
for
conversion
from
CO to CH4 by
equilibrium
chemistry appears
to be
longer
than
the
lifetime
of the
nebula. Processing
102
Photochemistry
of
Planetary Atmospheres
in
the
planetary subnebulae
may
have been more
efficient
and
could account
for the
observations.
4.7.3
Nitrogen
The
principal nitrogen
species
such
as
NHs
and
N
2
have never been detected
in
comets
by
direct measurements
but are
inferred
from
observations
of
molecules such
as NH2
and
N2
+
.
There
is
much more
NH3
than
N2 in
comets.
The
relative abundances
of
NHs
and
N
2
are
reversed
in the
interstellar medium. Therefore,
the
partitioning
of
nitrogen species
in the
solar nebula
was not the
same
as the
interstellar medium.
Local chemistry
had
altered
the
speciation. Similar conclusions
can be
drawn
from
the
ratios
of
HCN/N2
and
HCN/NHs.
These
ratios
are not
characteristic
of the
interstellar
medium.
Nonequilibrium
processes
in the
nebula
might
have been important
for the
production
of CN
compounds.
The
preceding discussion points
to the
possibility
of
disequilibrium chemistry
in
the
nebula. What
was the
energy source? Solar ultraviolet radiation
is the
major
source
for
disequilibrium chemistry today. According
to
section
4.5,
the
direct
solar
radiation
in
the
nebula
is
negligible
due to
grain opacity. However, recently
Shu and
colleagues
(1993)
showed that
in the
active
T-Tauri
phase
of the
sun, absorption
of
extreme
ultraviolet
by H
atoms
can
lead
to the
formation
of a
corona
as hot as
10
4
K.
Radiation
from
this
corona
due to the
radiative recombination
of
H
+
and e may
provide
a
large
source
of
diffuse
ultraviolet radiation
to the
nebula.
It
remains
to be
shown that this
source
of
energy
is
capable
of
driving photochemistry
in the
nebula
and
thereby
producing
the
chemical
species
observed
in
comets.
4.8
Formation
of
Planets
There
are two
pathways
for
matter
in the
solar nebula
to
organize.
The
bulk
of
matter
formed
the
protosun
via a
gravitational instability that occurred
in a
rarefied gaseous
medium.
The
planets formed
by a
gradual
process
of
accretion that started
from
dust
and
icy
grains.
As a
consequence
of
these
two
very
different
mechanisms,
we now
understand
why
there
are no
planetary bodies
of a
size between
the
mass
of the sun
(2
x
10
33
g) and
that
of
Jupiter
(1.9
x
10
30
g) but
there
is a
hierarchy
of
solar system
objects
with
masses between Jupiter
and a
comet, such
as
asteroids, satellites,
and the
non-Jovian
planets.
We are
primarily concerned with planetary formation.
The
essential data
on the
size
of
planetary
cores
and
gaseous envelopes
of the
giant
planets
are
summarized
in
table 4.7.
The
giant planets
all
have similar cores
of
10-30
ME
(Me
= 1
Earth mass). Jupiter
has a
massive envelope
of the
order
of
300 ME,
while
Saturn's gaseous envelope
is
somewhat smaller, about
95
ME-
The
gaseous envelopes
of
Uranus
and
Neptune
are
much smaller,
in the
range
of 1-3
ME-
The
terrestrial planets have small cores
of
size
0.1-1
ME and no
massive gaseous
envelopes.
The
small atmospheres
on
these planetary bodies
are not
captured
from
the
solar nebula
but are of
secondary origin (see discussion
in
section 4.9).
The first
step
in the
formation
of
planets
is the
formation
of
solid material
by
con-
densation.
In the
inner solar system solids consist
of
high-temperature condensates,
"rocky"
material such
as
that
in the
meteorites.
In the
outer solar system
the
temper-
ature
is
lower
and
solids
are
composed
of
"icy" material such
as
that preserved
in
Origins
103
Table
4.7
Bulk
properties
of the
giant
planets
3
Planet
Total
mass
Low Z
mass
b
High
Z
mass
Jupiter
Saturn
Uranus
Neptune
318.1
95.1
14.6
17.2
254.1-292.1
72.1-79.1
1.3-3.6
0.7-3.2
26-64
16-23
11-13.3
14-16.5
From
Pollack,
J. B., and
Bodenheimer,
P.
(1989).
a
ln
units
of
Earth masses.
b
Z
is the
atomic number.
comets.
The
initial
growth
of
dust grains
from
atomic size
to
planetesimals
(1
km) is
rapid
and
likely
is
complete
within
10
4
yr. The
subsequent growth
of the
small bodies
into
planets
is
believed
to
involve
five
steps:
1.
accretion
of
planetesimals
to the
size
of a
planetary embryo,
2.
runaway
growth
of
embryos
by
"feeding"
on the
available
planetesimals,
3. a
slower
growth
and
agglomeration
of
embryos
into
the
cores
of the
present planets,
4.
growth
of
cores
to
critical
size
(10-20
ME)
that
is
massive enough
to
accrete
a
gaseous
envelope,
and
5.
termination
of the
growth
process
due to
tidal
interactions between
the
massive planet
and
the
nebula.
Not all the
planets participate
in the
full
five
stages
of
growth.
The
terrestrial planets
complete only
the first two
steps. There
is not
enough time
to
grow bigger than
the
size
of
about
1
ME-
Step
4
explains
why all the
giant
planets have core masses
in
the
range
of
10-30
ME as
shown
in
table
4.7.
Once this critical size
is
reached
it is
easier
to
accumulate
gas
than
solids. Thus,
the
gaseous envelope grows
but the
core
remains roughly
the
same size. Step
5
places
an
ultimate limit
on the
size
of a
planet.
When
a
planet reaches
the
size
of
Jupiter,
its
tidal force
is
large enough
to
open
a gap
around
the
planet. This
inhibits
further
growth
of the
planet.
Of all the
planets, only
Jupiter
has
completed part
of
step
5.
Uranus
and
Neptune
do not
have enough time
or
material
to
accrete large gaseous envelopes. Saturn
is an
intermediate case between
Jupiter,
and
Uranus
and
Neptune.
4.9
Atmospheres
of
Terrestrial
Planets
and
Satellites
As
stated earlier,
the
terrestrial planets
and
satellites
did not
accrete
cores
large enough
to
accumulate large gaseous envelopes
from
the gas in the
nebula.
The
atmospheres
of
these planetary bodies
are
acquired later
from
degassing
of
solid material.
In
sections
4.9.1—4.9.3
we
briefly
review
the
important
processes
and
supporting evidence.
4.9.1
Noble
Gases
As
early
as
1949
Suess
and
Brown independently
recognized
the
secondary
origin
of
the
terrestrial atmosphere based
on an
analysis
of the
concentrations
of
noble
gases
in
the
atmospheres
of the sun and
Earth. Today their arguments
are
even more compelling
on
the
basis
of an
expanded
set of
data
on
noble
gases.
The
concentrations
of
noble
104
Photochemistry
of
Planetary
Atmospheres
gases
in the
atmospheres
of the
terrestrial planets
and
type
I
carbonaceous
chondrites
are
shown
in figure
4.12.
The
patterns
of the
terrestrial planets
and
meteorites
are
similar
but are
distinctly
different
from
that
of the
sun.
The
depletion factors
for the
noble
gases
in
Earth relative
to the sun are
shown
in figure
4.13.
The
depletion
is
largest
for the
lightest noble gas,
He, and the
factor
is
about
10
14
.
The
heavier noble
gases
are
depleted
by
smaller factors, about
10
6
.
The
sun's noble
gas
pattern
is
close
to
the
cosmic abundances
and is
characteristic
of the gas in the
solar nebula. Based
on
the
great
difference
in the
concentrations
and
distributions
of
noble gases,
we
conclude
that
the
atmospheres
of the
terrestrial planets were
not
captured
from
the gas in the
nebula
but
were derived
from
devolatization
from
solid material
in the
solar nebula.
4.9.2
Proto-atmospheres
From
figure
4.13
we
notice that
the
terrestrial planets
are
massively depleted
in the
noble
gases
relative
to the
sun,
but the
depletion
is
less
for
volatiles that
can be
chemically
bound. There
is
little depletion
in the
refractory material such
as the
metals. Therefore,
to first
order,
the
terrestrial planets
are
made
of the
same material
as
the
meteorites.
The
volatiles
are
derived
from
the
meteorites
by
impact volatization.
Recent experiments
by
Ahrens
and
colleagues
(1989)
have determined
the
amount
of
shock
devolatization
of
serpentine, brucite, calcite,
and
Murchison meteorite
in
high-
energy
impacts.
The
results indicate that impact devolatization occurred when Earth
and
Venus reached about
0.12
of
their present radius. Complete devolatization occurs
when
planetesimals collide with
the
protoplanets that have reached
0.3 of
their
final
radii. Thus
it is
relatively easy
for
proto-planets
to
acquire
protoatmospheres
composed
primarily
of CO2 and
H2O.
These
are
good greenhouse gases
and can
efficiently
trap
the
heat into which
the
gravitational energy
of
accretion
has
been converted.
The
surface
temperature
of the
proto-planet
rises rapidly
to
above 1500
K,
creating what
is
known
as a
magma ocean
on the
surface.
The hot
atmosphere
and
magma ocean will
further
facilitate
the
dehydration
of the
infalling
material, thereby maintaining their
existence.
As
shown
in a
model calculation
in figure
4.14a,
the
magma ocean
and the
massive
proto-atmosphere
on
Earth were created when
the
planet
was
about
0.3
times
its final
radius
and
persisted
to the end of the
accretional epoch.
The
corresponding
growth
history
of
water
is
shown
in figure
4.14b.
Note that
the
total
quantity
of
water
at
the end of
accretion
is
about
1
Earth ocean (1.5
x
10
24
g).
Ultimately
the hot
proto-atmosphere collapsed because
of the
termination
of
accretion
and the
associated
impact
energy.
On a
smaller planet like Mars, loss
of
part
of the
atmosphere
due to
impacts
is
also
possible.
The
process, known
as
atmospheric cratering,
is
capable
of
eroding away
the
growing atmosphere. What survives
is the net
result
of a
balance between
the
acquisition
and
ejection
of
gases
by
impacts. Thus, Mars might have lost 1-100 times
the
total water inventory
that
accreted onto
the
planet.
But
once
a
protoplanet grows
beyond
the
size
of
Mars, loss
by
atmospheric cratering becomes
inefficient.
4.9.3
Loss
of
Proto-atmospheres
Loss
of
proto-atmospheres
by
thermal evaporation (Jeans escape)
is
inefficient
except
for
the
lightest
gases.
The
process
likely
to be
more important
is
hydrodynamic
escape
Figure
4.12
Atmospheric abundances (per
unit
of
whole-planet mass)
of the
noble
gases
for
Venus, Earth, Mars,
and a
C3(V)
meteorite. After Cogley,
J.
G.,
and
Henderson-Sellers,
A.,
1984, "The Origin
and
the
Earliest State
of the
Earth's
Hydrosphere."
Rev.
Geophys.
Space
Phys.
22,
131.
Figure
4.13
Depletion factors
for
major elements
in
Earth with
respect
to the
sun.
After
Walker,
J. C.
G.,
1977, Evolution
of
the
Atmosphere (New York: Macmillan).
105