Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres
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266
Photochemistry
of
Planetary Atmospheres
Figure
7.11
(e)
Mixing ratios
of
N
2
O,
HNO
2
,
HNO
3
,
and
HO
2
NO
2
.
From
Nair,
H.,
Allen,
M.,
Anbar,
A. D.,
Yung,
Y. L., and
Clancy,
R.
T,
1994,
"A
Photochemical
Model
of
the
Martian
Atmosphere."
Icarus 111, 124.
in
the
lower atmosphere. This accounts
for the
decrease
in
their mixing ratios
at low
altitudes.
The
mixing ratios
of the odd
oxygen species,
O,
O('D),
and 03, in the
lower
atmosphere
are
shown
in
figure
7.lib.
O
atoms
are
shown here again
for
comparison
with
the
other
odd
oxygen species.
The
model predicts
a
maximum ozone mixing ratio
of
10~
7
.
The
column-integrated
03
abundance
is
1.1
/^atm
(or
0.11
Dobson units).
A
comparison
of the
model results
and
observations
is
summarized
in
table 7.6.
The
agreement
is
good
for O2 and
03,
but for CO the
prediction
of the
standard model
is
somewhat
low. This implies
that
the
HO.
V
chemistry
is too
efficient
in
removing
CO
catalytically.
The
result
is an
ironic
twist
of the
early models.
In
these early models,
for
schemes
(la)
and
(Ib)
to be
effective,
an
excessively large
K =
10
8
cm
2
s""
1
in the
lower atmosphere
had to be
postulated.
For
scheme (II)
to be
effective,
a
large amount
Table
7.6
Comparison between observed
and
model
CO,
O
2
,
and
Observed
Model
/CO
(x
10
4
)
6±
1.5
4.9
/0
2
(x
10
3
)
1.2±
0.2
1.3
/C0//0
2
0.50
±
0.15
0.40
/[0
3
]A
(jum-amagat)
a
1.5
± 0.5
1.1
From
Nair
el
al.
(1994).
/
denotes
mixing
ratio
and
/[
]dz
denotes column density.
"One
^m-amagat
is
equivalent
to a
column density
of
2.69
x
10
15
cm
2
.
Mars
267
Figure
7.12
Reaction rates
of
destruction
and
formation
of
CC>2.
From Nair,
H.,
Allen,
M.,
Anbar,
A. D.,
Yung,
Y. L., and
Clancy,
R.
T,
1994,
"A
Photochemical
Model
of
the
Martian
Atmosphere."
Icarus
111, 124.
of
H
2
O
in the
atmosphere must
be
assumed.
We
should point
out
that
the low CO
is
a
recent model result caused
by the use of the
temperature-dependent
CO
2
cross
sections.
These
are
lower than those used
in the
early models,
and as a
result they
allow
greater penetration
of
ultraviolet radiation
to the
lower atmosphere where
H
2
O
is
dissociated.
A
judicious adjustment
of the
rate
coefficients
(within
1 a
error bars)
of key
reactions
can
resolve
this
problem,
but we
shall
not go
into
the
details here.
Alternatively,
the
amount
of
H
2
O
in the
atmosphere
may be
lower
than
that adopted
in
the
model.
The
mixing
ratios
of the
HO^
species,
H, OH,
HO
2
,
and
H
2
O
2
,
are
presented
in
figure
7.1
Ic.
The
bulk
of
HO*
catalytic chemistry takes place
in the
lower atmosphere
below
50 km. The
most reactive radicals
are H, OH, and
HO
2
.
It is
surprising that
the
stability
of the
Martian atmosphere
is
controlled
by
trace constituents
with
abundances
less
than
1
ppb.
H
2
O
2
is a
temporary reservoir
of
HO
r
.
Its
abundance
is
higher because
it
is
less reactive
than
the
other
HO
X
species.
The
altitude
profiles
of the
most abundant
NO^
species,
N,
N(
2
D),
NO, and
NO
2
,
are
shown
in figure
7.lid.
The
principal source
of odd
nitrogen
is the
ionosphere,
where
odd
nitrogen
is
most
abundant.
NO is the
major carrier
of
NO^
from
the
upper
atmosphere
to the
lower atmosphere.
The
concentrations
of the
other
odd
nitrogen
species,
N
2
O,
HNO
2
,
HNO.i,
and
HO
2
NO
2
,
are
given
in figure
7.lie;
We
reproduce
NO and
NO
2
in
this
figure for
comparison
with
the
other
odd
nitrogen species.
The
important
reaction rates
in the
model
are
briefly
discussed. Figure
7.12
presents
the
rates
of
destruction (photodissociation)
and
formation
of
CO
2
.
The
dissociation
of
268
Photochemistry
of
Planetary
Atmospheres
Figure
7.13
Reaction
rates
of key
reactions
in the
catalytic
oxidation
of CO.
From
Nair,
H.,
Allen,
M.,
Anbar,
A. D.,
Yung,
Y. L., and
Clancy,
R.
T.,
1994,
"A
Photochemical
Model
of the
Martian
Atmosphere."
Icarus
111, 124.
CO2
consists
of two
parts:
a
high-altitude branch that produces
CO and the
excited
O('D),
and
another branch that produces
CO and O in the
ground state
at
lower
,-2
,.-1
altitudes.
The
column-integrated rate
for CO2
destruction
is 1.1 x
10
cm
i
s
The
destruction
of CO2 in the
model
is
restored
by
only
one
reaction,
R42:
CO + OH,
which
occurs mainly between
the
surface
and 40 km. The
rates
of the key
reactions
in
catalytic
cycles
(I)-(III)
are
given
in figure
7.13.
Note that
due to the
tight balance
in
the
HO
V
catalytic cycle (see
figure
7.4),
the
following
rates
are
approximately equal:
The
contributions
of the five
catalytic chemical
schemes
to the
oxidation
of CO to
CO
2
may be
deduced
from
the
reaction rates
presented
in
table 7.7.
The
bulk
of
the
oxidation
is
carried
out by
schemes
(la)
(77%)
and
(Ib)
(8%), giving
a
total
of
85%. Scheme
(II)
(the
H
2
O
2
path) contributes
8%. The
NO
V
schemes
(Ilia)
and
(Illb)
contribute
5% and
1.5%, respectively.
The
principal source
of
HO
V
is the
decomposition
of
H
2
O
by
photolysis
and the
reaction
with
O('D).
Figure 7.14 presents
the
rates
of
these reactions.
The
rates
of
Mars
269
Table
7.7
Column
rates
of
some
important
reactions
in the
model
Reaction
From
Nair
et
al.
(1994).
Rate
(cm"
CO
production
HO<
cycling
R13
R14
R42
R68
R12
R77
R27
R28
R34
CO
2
+
hv
CO
+ OH
O
+
NO
2
H2O
2
+
hv
NO
+
HO
2
H
+
O
2
+
M
H
+
0
3
O
+
HO
2
—
>•
->
_>
->
—
>
->
-»
->.
—>
co + o
CO
+
O('D)
CO
2
+ H
O
2
+ NO
2
OH
NO
2
+ OH
HO
2
+ M
OH
+
O
2
OH
+
O
2
9.7
1.3
1.1
2.1
4.8
8.9
1.2
9.7
9.8
x
10"
x
10"
x
IO'
2
x
10'°
x
10'°
x
10'°
x
IO
12
x
10'°
x
10"
recycling
of
HO^
radicals have been discussed
in
section 7.2.2
in
connection
with
the
catalytic
oxidation
of CO to
COa-
The
principal
reaction
that
destroys
HO*
is
R40:
OH + HO2
—>
H2O + 02; its
rate
is
given
in
figure
7.14.
Note
that
the
column-
integrated
rate
of
destruction
of H2O is of the
order
of
10'°
cm
2
s
',
whereas
the
corresponding rate
of
production
of CO2 is of the
order
of
IO
12
cm~
2
s"
1
.
Thus,
every
HO^
radical derived
from
H2O is
used about
100
times
in the
catalytic schemes
for
the
restoration
of CO2
before
it is
terminated
by
returning
to H2O via
R40:
OH +
HO
2
-»
H
2
O
+
O
2
.
Figure
7.14
Production
and
loss
rates
of
HO
t
.
From
Nair,
H.,
Allen,
M.,
Anbar,
A. D.,
Yung,
Y. L., and
Clancy,
R. T.,
1994,
"A
Photochemical
Model
of the
Martian Atmo-
sphere."
Icarus
111,
124.
270
Photochemistry
of
Planetary
Atmospheres
Figure
7.15
Reaction rates
for
production
(a) and
loss
(b) of the
O-
O
bond. From Nair,
H.,
Allen,
M.,
Anbar,
A. D.,
Yung,
Y. L., and
Clancy,
R. T.,
1994,
"A
Photochemical Model
of the
Martian Atmo-
sphere."
Icarus
111,
124.
The
rates
of the
reactions that lead
to the
formation
and the
breaking
of the O-O
bond
are
shown
in figure
7.15,
a and b,
respectively.
The
most important reactions that
form
O
2
are
R33:
O + OH and
R15:
O + O + M.
Note that
R33 is the
rate-limiting
step
in
scheme
(IV),
forming
O
2
from
CO2
(7.17).
This
reaction
is
about
an
order
of
magnitude more important than
the
direct recombination
of O
atoms
(R15:
2O +
M
-»•
Oa
+ M).
Reaction
R68:
NO2 + O and the
Chapman reaction,
R18:
O +
03, are
relatively unimportant.
The
main reactions that destroy
the
O—O
bond
are
photolysis
of
O
2
,
H
2
O
2
,
and
HO
2
and
reaction
R77:
NO +
HO
2
.
Of
these reactions,
the
photolysis
of
O
2
is the
most important.
Mars
271
Figure
7.16
Reaction rates
for
production
(a) and
loss
(b) of H2.
From
Nair,
H.,
Allen,
M,
Anbar,
A. D.,
Yung,
Y. L., and
Clancy,
R. T.,
1994,
"A
Photochemical Model
of the
Martian Atmosphere." Icarus 111,
124.
One
by-product
of the
HO*
chemistry
is
H
2
produced
by the
reaction R30:
H +
HO
2
;
the
rate
of
this
reaction
is
given
in figure
7.16a.
The
main reactions
that
destroy
H
2
are
R39:
H
2
+ OH and
R23:
H
2
+
O('D),
whose rates
are
shown
in figure
7.16b.
H
2
is a
long-lived molecule
and can be
transported
to the
upper
atmosphere,
where
it
is
decomposed
by
scheme (VI).
The
rate
of the
initiation
reaction
R101:
CO
2
+
+
H
2
is
given
in figure
7.16b.
This reaction provides
the
main source
of
hydrogen
to
drive
its
escape
from Mars.
In the
ionosphere (about
130 km)
H
2
is
broken down into
H
atoms. Some
of it flows
upward
and
eventually
escapes
from Mars.
The
other portion
flows
downward
and
eventually
becomes oxidized
to
H
2
O.
Figure
7.17
Production
and
loss
rates
of odd
nitrogen.
From
Nair,
H.,
Allen,
M.,
Anbar,
A.
D.,
Yung,
Y. L., and
Clancy,
R. T.,
1994,
"A
Photochemical
Model
of the
Martian
Atmosphere."
Icarus 111, 124.
Figure
7.18
Production rates
of
more
complex
odd
nitrogen
species.
From
Fox,
J.
L.,
1993,
"Production
and
Escape
of
Nitrogen from
Mars."
J.
Geophys.
Res.
98,
3297.
272
Mars
273
The
major sources
and
sinks
of odd
nitrogen
in the
model
are
given
in figure
7.17.
The
production
of N and
N(
2
D)
in the
upper atmosphere
is
taken from
the
ionospheric
model
of Fox
(1993).
In the
lower atmosphere
odd
nitrogen
is
produced
by
cosmic
ray
impact.
The
only reaction that destroys
odd
nitrogen
by
conversion
to N2 is
reaction
R61:
N + NO
—>•
N2 + O.
Figure 7.18 presents
the
rates
of the
main reactions
forming
more complex nitrogen compounds from
NO: NO2
formed
by
R77:
NO +
HO
2
;
NO
3
formed
by
R69:
NO
2
+ O + M;
N
2
O
5
formed
by
R90:
NO
2
+
NO
3
+
M;
N
2
O
formed
by
R62:
N +
NO
2
;
HNO
2
formed
by
R81:
NO + OH + M;
HNO
3
formed
by
R82:
OH +
NO
2
+ M; and
HO
2
NO
2
formed
by
R87:
HO
2
+
NO
2
+ M.
The
principal loss mechanism
for all
these
odd
nitrogen
species
is
photolysis,
and
these rates
are not
shown.
7.4
Evolution
There
are at
least
two
rather profound questions concerning
the
evolution
of the
Mar-
tian
atmosphere. First,
why is the
present atmosphere
so
thin?
The
total atmospheric
pressure
is
only
6
mbar.
The
abundances
of the
noncondensible
gases,
N
2
and Ar, are
0.16
and
0.1
mbar, respectively.
These
abundances
are
much
less
than
the
correspond-
ing
terrestrial values
of 780 and 9.3
mbar
for
N
2
and Ar.
Second,
the
surface
of
Mars
is
covered
by
ancient
fluvially
generated channels, strongly suggesting
the
existence
of
a
warmer climate
that
could sustain
flow of
water. That
is
very
different
from
the
cur-
rent
cold
and
arid climate. What
are the
causes
of
such drastic environmental change?
Both
of
these questions suggest
that
the
Martian atmosphere
was
denser
and
warmer
in
the
past.
The
density
of the
atmosphere
and its
climate
are of
course related.
In the
current
atmosphere
the
greenhouse
effect
due to
CO
2
is
only
6 K. To
sustain
a
warm
climate with
fluid flow on the
surface,
a
greenhouse
effect
of 30 K is
needed;
a
CO
2
atmosphere
of the
order
of a bar
would
be
required.
It
is now
generally accepted
that
the
present Martian atmosphere
is the
end-product
of
planetary evolution over
the age of the
solar system. There
are
three mechanisms
for
removing
a
substantial
fraction
of the
Martian atmosphere.
The
gravity
of
Mars
is
sufficiently
small
that
impacts
by
small bodies (planetesimals) would have been
effective
in
blowing much
of the
atmosphere away. This
is
known
as
atmospheric
cratering,
a
process
that
is
believed
to be
more important during
the
period
of
heavy
bombardment
in the
early history
of the
solar system. (This
process
is not
believed
to
be
important
for the
larger terrestrial planets Earth
and
Venus.) Another mechanism
is
the
sequestering
of
volatiles
in the
polar
and
subsurface reservoirs.
The
polar regions
of
Mars
are
known
to
hold large deposits
of
CO
2
and
H
2
O
ice.
The
regolith
of
Mars
(up
to 1 km
deep)
may
contain large amounts
of
adsorbed
CO
2
or
carbonates.
The
exact amount
is
difficult
to
estimate.
A
third
mechanism
is the
escape
of
gases
from
the
exosphere
of the
planet
by
thermal
or
nonthermal
processes.
This last mechanism
can be
quantitatively
modeled,
at
least
in the
current epoch.
In
addition,
the
exospheric
escape
leaves behind
a
cumulative isotopic signature that
can be
measured.
In
this
chapter
we are
primarily concerned
with
the
third mechanism.
The
rates
of
escape
of
gases
from
the
exosphere
of the
planet
and the
subsequent isotopic
fractionation
of the
planetary volatile reservoir
may be
calculated
from
the
model
described
in
section 7.3.
274
Photochemistry
of
Planetary
Atmospheres
Figure
7.19
Time
history
of the
response
of the
steady
state
atmosphere
to an
abrupt
reduction
of
oxygen
flux by a
factor
of 2 at
time
=
0. (a)
Oi
(solid
line)
and CO
(dashed
line)
mixing
ratios,
(b)
hh
mixing
ratio
(solid
line)
and H2
production
rate
(dashed
line),
(c) H
escape
flux
(solid
line)
and H2
escape
flux
(dashed
line).
From
Nair,
H.,
Allen,
M.,
Anbar,
A.
D.,
Yung,
Y. L., and
Clancy,
R. T.,
1994,
"A
Photochemical
Model
of
the
Martian
Atmosphere."
Icarus
111, 124.
7.4.1
Escape
of
Gases
Spacecraft observations
of
Mars
at
Lyman
a
have revealed that
the
planet
is
surrounded
by
a
corona
of H
atoms.
The
escape
flux has
been
deduced
to be 1-2 x
10
8
cm
""
2
s~'.
In the
model described
in
section 7.3,
a
mean
flux of
total hydrogen
(H +
1.8
x
10
8
cm"
2
s~',
has
been adopted. This
is
equivalent
to the
loss
of 2.6 x
10
26
H
atoms
s~'
from
Mars.
According
to the
hypothesis
of
McElroy
(1972),
there
is a
corresponding
escape
of O
atoms
at
half
the
rate
of
hydrogen
escape,
so the net
result
is
the
loss
of H2O
from
Mars.
As a
demonstration
of
this remarkable relation,
we
performed
a
numerical experiment.
In the
steady state model with
the
oxygen
escape
Mars
275
Figure
7.20
Interaction
between
the
solar
wind
and the
upper
atmosphere
of a
nonmagnetic
planet.
From
Luhmann,
J. G., and
Kozyra,
J. U.,
1991,
"Dayside
Pickup
of
Oxygen
Ions
Precipitation
at
Venus
and
Mars:
Spatial
Distribution,
Energy
Deposition
and
Consequences."
/
Geophys.
Res.
96
5457.
flux
given
by
table
7.5,
we
reduced,
at
time zero,
this
flux to
half.
The
subsequent
evolution
of the
model atmosphere
is
shown
in figure
7.19.
Note that after about
10
5
yr, the
model converges
to a new
steady state characterized
by
half
the
rate
of
hydrogen
escape
(figure
7.19c).
The new
steady state atmosphere
has
more
O
2
,
less
CO
(figure
7.19a),
less
H2, and a
lower
H2
production rate.
If we
assume that this
reduced rate
of
oxygen
escape
has
been
uniform
over
the age of the
solar system
(4.6
Gyr),
the
total amount
of
H
2
O
lost
to
space
is 1.3 x
10
25
molecules/cm
2
.
This
amount
is
equal
to 362
mbar
in the
atmosphere
or 3.9 m of
water
uniformly
spread
over
the
Martian surface. This
is
probably
a
lower
limit.
For
comparison,
we
note
that
the
column-integrated
H
2
O
in the
atmosphere today
is 8.8
/urn,
four
orders
of
magnitude
less
than
the
amount
of
water
that
could have escaped.
The
atmosphere
of
Mars
is too
cold
to
contain much water vapor.
The
bulk
of
water
on
Mars must
be
sequestered either
at the
poles
or in the
ground.
A
quantitative
estimate based
on the
analysis
of the
isotopic composition
of
H
2
O
is
deferred section
7.4.2.
The
escape
of
nitrogen
is
driven
by the ion
recombination reaction (7.37).
The
rate
of
loss
is 7.5 x
10
5
N
atoms
cm~
2
s"
1
in the
current epoch.
At
this
rate
of
loss
(assumed
to be
uniform),
the
total atmospheric
inventory
of
N
2
would
be
exhausted
in
0.51 Gyr,
geologically
a
very short time. This also implies
that
N
2
must have
been
more
abundant
in the
past.
A
quantitative
analysis
may be
carried
out
using
the
isotopic
data obtained
by
Viking,
as
discussed
in
section 7.4.2.
There
is the
possibility
that
greater amounts
of
material might have been lost
from
Mars
by
solar wind-induced sputtering. This mechanism
is
highly
speculative
and
poorly
quantified
at
present.
Due to the
lack
of an
intrinsic
magnetic
field, the
solar
wind
impinges
on the
upper atmosphere
of
Mars
(see
figure
7.20).
Direct
sputtering
is
efficient
for
light
species
(such
as H and D).
However,
for the
loss
of
heavier