Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres
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Table
7.3
Isotopic composition
of the
atmosphere
of
Mars
Species
D/H
12
C/
13
C
I4
N/
15
N
16
0/
17
0
I6
0/
18
Q
36
Ar/
38
Ar
40
Ar/
36
Ar
1M
Xe/
132
Xe
Ratio
9
± 4 x
10-"
7.8
± 0.3 x
1(T
4
90
± 5
170
± 15
2655
± 25
490
± 25
545
± 20
5.5
± 1.5
3000
± 500
2.5
±*
Reference
(1)
(2)
(3)
(3)
(2)
(3)
(2)
(4)
(5)
(5)
(1)
Owen
et
al.
(1988);
(2)
Bjoraker
et
al.
(1989);
(3)
Nier
and
McEl-
roy
(1977);
(4)
Biemann
et
al.
(1976).
Figure
7.1
Mass spectrum showing
various
mass peaks
(amu)
as
measured
by the
neutral mass
spectrometer
on
board Viking
1.
From
Nier,
A. O. et
al.,
1976,
Isotopic composition
of the
Martian
atmosphere
Science,
194,
68.
Figure
7.2
Surface pressures recorded
by
Viking
Landers
1 and 2. The
years
refer
to
Martian
years. From Tillman,
J. E.,
1988,
Mars global atmospheric
oscillations—annually
synchronized,
transient
normal-mode oscillations
and the
triggering
of
global dust storms
J.
Ceophys.
Res.,
93,
9433.
SOL =
Martian
day.
246
Mars
247
by
photochemistry
and
escape
of
hydrogen
and
oxygen from
the
planet.
Mars
is the
first
planet
in the
solar system whose global chemical environment
was
shown
to
be
sensitively controlled
by
trace constituents that
can act as
catalysts
for
important
chemical cycles. Subsequently, similar catalytic effects were shown
to be
important
in
other planetary atmospheres,
including
that
of
Earth.
7.2
Photochemistry
7.2.1
Pure
CO2
atmosphere
The
major constituent
of the
Martian atmosphere,
CO
2
,
is
readily photolyzed
by
solar
ultraviolet
radiation under
2050
A:
where near
the
threshold
the O
atom
is in the
ground state,
O(
3
P),
but at
shorter
wavelengths
the
atom could
be in
excited
states
O('D)
and
O('S).
However,
the
primary
fate
of the
excited
atoms
is
quenching
to the
ground state
by
CO
2
(a
small
fraction
of the
excited
atoms
reacts
with
H
2
O).
Once
CO
2
is
converted into
CO and
O(
3
P),
it is
difficult
to
restore
it. The
reverse
of
reaction
(7.1),
is
spin-forbidden.
The
three
body rate coefficient
at 200 K is 3 x
10~
37
cm
3
s"
1
,
a
value that
is
many
orders
of
magnitude smaller than
the
corresponding rate coefficient
for
a
typical three-body reaction such
as
with
rate coefficient equal
to 2.8 x
10~
32
cm
3
s~
l
at 200 K.
Hence,
the net
results
of
CO2
photodissociation
are
Thus,
a
pure
CO
2
atmosphere
exposed
to
solar ultraviolet radiation would have large
amounts
of CO and
O
2
at a
ratio
of 2 : 1. Of
course,
O
2
cannot build
up
indefinitely,
and
eventually
it
would dissociate:
We can
construct
a
self-consistent model with reactions
(7.1)-(7.5),
assuming that
the
only
path
for
reversing
the
result
of
photodissociation
(7.4)
is the
slow reaction
(7.2),
with
the
oxygen atoms being supplied
by
(7.5).
Such
a
model predicts that
the
mixing
ratios
of CO and
O
2
are
7.72
x
10~
2
and
3.87
x
10~
2
,
respectively. However,
the
predictions
of CO and
O
2
are
greater
than
the
observed abundances summarized
in
table
7.2 by
factors
of
110
and 30,
respectively.
In
addition,
the
model
CO/O
2
ratio
of 2 is
significantly
larger than
the
observed ratio
of
0.5.
It is of
interest
to
point
out
that
in
this hypothetical model
of
Mars,
the
predicted
O.i
mixing ratio
at the
surface
is
10~
5
and the
column-integrated
O
3
is 3.4 x
10
18
cm~
2
or 126 DU (1
Dobson
unit
=
2.69
x
10
16
cm"
2
).
This amount
of
O_i
is
sufficient
for
shielding
the
surface
of
Mars
from
harmful ultraviolet radiation.
248
Photochemistry
of
Planetary
Atmospheres
Figure
7.3
Contours
of
water vapor abundance
as a
function
of
latitude
and
season
as
measured
by the
Viking
Orbiter.
The
Earth
calendar years
are
given
at
the top of the figure. (Ls =
areocentric
longitude
of the
sun;
LS = 0 is
at the
vernal
equinox
for the
northerm
hemisphere)
The
units
are
precipitable
microns
of
water. From
Jakosky,
B.
M.,
and
Farmer,
C. B.,
1982, "The seasonal
and
global behavior
of
water-vapor
in the
Mars
atmosphere—complete
global
results
of the
Viking
atmospheric water detector experiment."
J.
Geophys.
Res.
87,
2999.
7.2.2
HO*
Catalytic
Chemistry
The
remarkable stability
of
CC>2
in the
Martian atmosphere
appears
to be a
contra-
diction
of the
known chemical kinetics
for a
pure
CO2
atmosphere.
This
outstanding
problem,
the
lack
of
high
amounts
of CO and
C>2
in the
Martian atmosphere,
is
known
as the
COa
stability problem. This problem
was first
independently solved
in two
clas-
sic
papers
of
McElroy
and
Donahue
(1972)
and
Parkinson
and
Hunten
(1972).
The
crucial idea
is the
recognition that
the
small amount
of
water vapor
in the
atmosphere
could play
a
fundamental role
in the
photochemistry
of the the
atmosphere.
As
listed
in
table
7.2 the
amount
of
H
2
O
in the
atmosphere
of
Mars
is of the
order
of a few
hundred
parts
per
million
and is
highly variable both
as a
function
of
altitude, latitude,
and
season.
A
convenient
measure
of the
column-integrated
F^O
amount
in the
atmo-
sphere
is the
precipitable micron
(pr-/zm);
1
pr-/im
equals 3.34
x
10
18
molecules/cm
2
,
corresponding
to a
mixing ratio
of 15
ppm
if
uniformly mixed.
The
distribution
of
the
H
2
O
column abundances
on
Mars over
a
Martian year
is
shown
in figure
7.3.
The
complexity
of the
Martian hydrological cycle
is
comparable
to
that
of the
terrestrial
hydrological cycle.
The
northern polar
cap of
Mars
seems
to be a
source
of
water
vapor
in
spring.
The
dominant
flow
of
volatiles
on
Mars
is
from pole
to
pole
as a
function
of
season, whereas
on
Earth atmospheric water
(in the
form
of
vapor
or
cloud)
flows
from
low
latitudes
to the
high latitudes.
H2O is
photolyzed
in the
atmosphere
by the
absorption
of
solar ultraviolet photons:
The OH
radical readily attacks
CO to
form
CO
2
:
Mars
249
The
recycling
of H may be
accomplished
by the
following reactions:
The
sequence
of
reactions
may be
summarized
as
The net
result
of
chemical scheme
(la)
is the
same
as
(7.2),
but it is
orders
of
magnitude
faster
because each
of the
reactions that constitute
the
cycle
in
(la)
is an
allowed
reaction.
Note that
the
HO^
radicals
(H, OH,
HO
2
)
are
used
as a
catalyst
in the
recombination
of CO and O.
Since
the
HO^
radicals
are not
consumed
in the
chemical
scheme, very
few
molecules
are
needed.
As we
show
in
section
7.3.2,
the
mole
fraction
of the sum of the
HO*
radicals needed
to
ensure
the
stability
of the
bulk atmosphere
of
Mars
is
less than
10~
9
.
All the
profoundness
and
subtlety
of
photochemistry
is
contained
in the
statement that
tiny
amounts
of a
reactive
species
control
the
bulk
composition
of the
atmosphere. This
has the
obvious consequence that
the
composition
of the
atmosphere
is
easily perturbed
by
altering
the
concentrations
of key
radical
species.
There
is a
variant
of
chemical scheme
(la)
that
can be
summarized
as
follows:
In
this
scheme
the OH
that
is
consumed
in
(7.7)
is
restored
by
reaction with
O
3
formed
in a
three-body reaction
(7.11).
Both chemical
schemes
(la)
and
(Ib)
share
a
common property that they recombine
CO and O to
form
CO
2
.
These
schemes cannot
use
O
2
formed
in
(7.3)
for the
oxidation
of CO.
O
2
must
first be
decomposed
by
(7.5).
However,
the
following scheme
is
capable
of
directly using
O
2
for the
oxidation
of
CO:
Note
that
in
chemical scheme (II)
the
O—O
bond
is
broken
by the
photolysis
of
hydrogen
peroxide
(7.13).
Chemical
schemes
(la)
and
(Ib)
are
more important
for a dry
atmosphere. Scheme
(II)
is
more important when
H
2
O
is
abundant.
The
rate-limiting step
of
this scheme,
reaction
(7.12),
is
quadratic
in the
concentration
of
HO
2
radicals.
The
relative impor-
tance
of the two
schemes
in the
current atmosphere
is
discussed later
in
section
7.3.2.
Early
work showed that
the
three
HO
A
schemes provide
a
satisfactory solution
to the
250
Photochemistry
of
Planetary Atmospheres
stability
problem.
The
only
significant
addition since
the
1972
classic papers
is the
new
reaction
that
breaks
the
O—O
bond
by
where
the
O
2
in
HOT
is
derived
from
(7.8)
and NO is
derived
from
the
dissociation
of
N
2
(see
section 7.2.7). This introduces
two new
chemical schemes
for the
oxidation
of
CO:
Note
that
this
scheme uses
HO*
and
oxides
of
nitrogen
(NO*)
as
catalysts. Chemical
schemes
(Ilia)
and
(Hlb)
are
examples
of the
synergistic coupling between
two
kinds
of
catalysts. There
are
many examples
this
kind
of
coupled scheme
in the
terrestrial
atmosphere
(see
chapter
9). As we
show
in
section
7.3.2,
the
coupled
HO^-NO*
scheme
is
less important
than
the two
classical
HO*
schemes
for the
oxidation
of CO
to
C0
2
.
7.2.3
O
2
and
O
3
We
have shown
that
HO*
and
NO*
species
can
catalytically break
the
O—O
bond
in
the
atmosphere.
We now
show
that
the
O—O
bond
can
also
be
formed using
HO
X
as
a
catalyst,
as in the
following
scheme:
where
the
crucial reaction
is
(7.17)
in
which
the
O—O
bond
is
formed.
The net
result
of
scheme (IV)
is the
same
as
(7.4). Thus,
the
presence
of
HO*
radicals promotes
the
formation
of
O
2
.
As
pointed
out in
section
7.2.1,
the
column ozone concentration
in a
hypothetical
pure
CO
2
atmosphere
on
Mars could
be as
high
as 126 DU. In a
realistic atmosphere
with
trace
quantities
of
H
2
O,
the
amount
of 03 is
smaller
by
three orders
of
magnitude.
Part
of the
reason
is the
loss
of
oxygen atoms
in
(7.17)
and
(7.9),
resulting
in
scheme
IV
that
converts
CO
2
to CO and
O
2
rather
than
CO and O. The
other reason
is
that
Mars
251
Figure
7.4
Schematic diagram showing
the
principal pathways
for
reac-
tions involving
hydrogen
species.
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.
(7.10)
removes
O.i
directly.
It is
unlikely
that
the O
atom transferred
to OH in
(7.10)
returns
to
63
because
the
most probable
fate
of
this
OH
molecule
is to
react
with
CO
to
form
CO
2
(7.7).
We
should note that
the
photolysis
of
Oi,
only leads
to a
recycling
of
03 and not a net
loss
of odd
oxygen (odd oxygen
O
x
= O +
03).
7.2.4
Hydrogen
Escape
A
schematic showing
the
principal pathways
of the
hydrogen
species
in the
Martian
atmosphere
is
presented
in figure
7.4.
H2O is the
ultimate source
of all
hydrogen
species
in the
atmosphere.
The
production
of the
reactive radical
species
(HO^)
is
initiated
by
photolysis (7.6)
with
a
minor contribution from
where
the
excited oxygen atom
is
derived
from
the
photolysis
of 03
[formed
by
(7.11)]:
Once they
are
produced,
the
HO
V
species participate
in the
catalytic cycles
for the
oxidation
of CO to
CO
2
.
Ultimately,
the
HO,
species
are
destroyed
by the
reaction
that
turns
the
radicals back
to
H
2
O:
This ultimately limits
the
amount
of
HO
A
radicals that
can
exist
in the
atmosphere.
There
is,
however,
a
minor path
to
remove
HO
X
by the
formation
of
H
2
via the
reaction
The
result
may be
summarized
by the
following chemical scheme
252
Photochemistry
of
Planetary
Atmospheres
Most
of the
H
2
produced
in
scheme
(V) is
removed
by the
reactions
and the
ultimate
fate
of the
HO*
radicals produced
in
(7.22)
and
(7.23)
is to
return
to
H
2
O.
However,
a
small portion (about
20%)
of the
H
2
formed
in
scheme
(V)
will
escape chemical destruction
in the
lower atmosphere
and get
transported
to the
upper
atmosphere, where
the
molecule
can be
decomposed
by
ionospheric reactions:
We
defer
the
detailed discussion
of ion
chemistry
to
section
7.2.6.
Reaction
(7.6)
produces
an H
atom
in the
lower atmosphere
of
Mars.
Its
chemical
lifetime
is
short,
and it
cannot readily
diffuse
to the top of the
atmosphere where
it can
escape.
How-
ever,
the
H
2
molecule produced
by
scheme
(V) is
relatively
unreactive.
It can
readily
diffuse
to the
upper atmosphere, where
it is
decomposed
into
H
atoms
by the ion
reactions
(scheme
VI).
The
hydrogen atoms produced
in the
ionosphere
can
diffuse
to
the
exobase
and
escape thermally
from
Mars.
H
2
can
also
escape,
but its flux is a
minor
fraction
of the
total escape
flux. The
escape
of
hydrogen from Mars
is a
well-
documented
phenomenon. Ultraviolet observations
at
Lyman
a
indicate that Mars
is
surrounded
by a
corona
of
escaping
H
atoms.
In
summary,
the
escape
of
hydrogen
from
Mars consists
of five
steps:
(a)
source
of
hydrogen from
the
dissociation
of
(b)
production
of the
long-range carrier
of
hydrogen,
H2, by
scheme
(V);
(c)
diffusion
of H2
from
the
lower atmosphere
to the
upper atmosphere;
(d)
decomposition
of
HI
into
H
atoms
by
scheme
(VI),
and
(e)
thermal escape
of H
atoms.
These
five
steps show
that
the
photochemistry
of
HO*,
transport, ionospheric chem-
istry,
and
exospheric dynamics
are
intimately
coupled.
The
consequence
of
hydrogen
escape
for the
oxidation state
of the
atmosphere
of
Mars
is
discussed
in
section
7.2.5.
7.2.5 Oxygen
Escape
The
escape
of
hydrogen
from
Mars
has one
important implication
for the
chemistry
of
the
atmosphere.
The
ultimate source
of the
escaping hydrogen
is
H
2
O,
decomposed
according
to
scheme
(V)
Mars
253
Note that
the
loss
of
every
two
molecules
of H2
would leave behind
one
molecule
of
C>2.
If
there were
no
permanent sink
for 02, the
oxygen concentration
in the
Martian
atmosphere would continue
to
build
up. It can be
shown that
at the
current
escape
rate
of
hydrogen,
it
takes only
on the
order
of
10
5
yr to
double
the
amount
of
O
2
in
the
Martian atmosphere. Thus,
if the
Martian atmosphere
has
reached
a
steady state
for
its
state
of
oxidation, there must
be a
sink
for
oxygen comparable
in
magnitude
to the
escape
rate
of
hydrogen, that
is,
where
<t>(O)
is the
escape
rate
of
oxygen atoms
(in any
form)
and
0(H)
is the
escape
rate
of
hydrogen atoms
(in any
form).
Unlike
hydrogen, oxygen
is too
heavy
to
escape
from Mars thermally. However,
there
is a
nonthermal
mechanism associated
with
ion
chemistry that
can
drive
the es-
cape
of
oxygen from Mars. Oxygen atoms produced
in the
dissociative recombination
reaction
may
have
as
much
as 2.5 eV per
atom,
or 7 km
s~'.
This
is
higher
than
the
escape
velocity
of 5 km
s~'
for
Mars. Hence, exothermic oxygen atoms produced above
the
exobase
can
escape.
From
scheme
(V),
the
oxygen
escape
rate must
be
half
that
of
hydrogen
so
that
the
state
of
oxidation
of the
atmosphere remains unchanged.
The
question then arises
of how
these
two
vastly
different
mechanisms
of
escape,
one via
Jeans
escape
and the
other
via
dissociative recombination,
can
affect
each
other. This problem
is
known
as
self-regulation,
and we
briefly
describe
it as
follows.
As
discussed
in
section 7.2.4,
the
escape
rate
of
hydrogen
is
controlled
by five
steps.
One of the
rate-limiting steps
is the
production
of H2 in the
HO*
chemistry,
This rate
is
proportional
to the
product
of two
HO*
radicals,
H and
HO2-
HO2 is
the
most abundant
of the
HO
r
radicals
in the
region
of
active photochemistry.
Its
concentration
is
larger than
OH and H, and it is
insensitive
to O2.
However, reaction
(7.8) converts
H
into HO2,
and
this
reaction determines
the
concentration
of H
atoms
in
the
atmosphere.
At
high
O2
concentrations
[H] and
P(H2)
are
small,
and
vice versa.
Consider
a
hypothetical situation
in
which
the
balance
in
(7.27)
is not
satisfied.
For
example,
we can
imagine
a
decrease
in
0(O)
such that
Initially
hydrogen
will
continue
to
escape
at the
same rate.
But
this
would result
in a
rapid buildup
of
O
2
.
By
(7.8)
and
(7.29)
the
rate
of
production
of
H
2
,
P(H
2
),
would
fall
as a
result
of the
increase
of O2.
Eventually
a new
steady state
is
reached
in
which
<f>(H)
would
be
smaller,
so
(7.27)
is
again satisfied. Similar arguments
can be
applied
to the
case
in
which
In
this
case,
the O2
concentration
of the
atmosphere
will
decrease
due to its
higher
rate
of
loss. This causes
an
increase
in
P(H
2
)
by
(7.8)
and
(7.29), resulting
in a
greater
254
Photochemistry
of
Planetary
Atmospheres
escape rate
of
hydrogen,
0(H).
Again,
a new
steady state would
be
reached that would
satisfy
(7.27).
The
time-dependent response
of the
atmosphere
in
which
the
escape
rates
of
hydrogen
and
oxygen
are
initially
perturbed
from
the
stoichiometric ratio
of
(7.27)
are
presented
in
section 7.4.1.
7.2.6
Ionosphere
The
ionosphere
of
Mars plays
a
fundamental role
in at
least three important aspects
of
the
Martian atmosphere.
Ion
chemistry
is a
source
of odd
nitrogen that
is
important
for
the
CO2
stability (schemes
Ilia
and
Illb).
Ion
reactions
are
responsible
for
breaking
H2
into
H
atoms
and
supply
a
source
of
escaping atoms. Ionic reactions produce energetic
oxygen
atoms that
can
escape
from Mars.
We
briefly
describe
the
essential features
of
the
ionosphere
of
Mars
in
this section.
Ion
production occurs
by
ionization
of CO2 at
wavelengths below
900 A
(7.24).
However,
the
major
ion in the
ionosphere
is not
CO
2
+
but
O
2
+
formed
by the
exchange
reaction
O2
+
may
also
be
formed
in two
steps
The
most important pathway
for the
removal
of
ions
is
dissociative recombination
by
(7.28)
and
A
minor
loss
process
is via
charge transfer
to NO:
followed
by
dissociative recombination
of
NO
+
:
The
source
of NO in the
upper atmosphere
of
Mars
is
discussed
in
section
7.2.7.
Another
minor reaction
for
removing
CO
2
+
is
(7.25),
which results
in
chemical cycle
(VI),
the
breaking
up of
H
2
into
H
atoms.
Note that (7.28)
is
capable
of
producing exothermic oxygen atoms that
can es-
cape
from
Mars. Another reaction that
is
important
for the
evolution
of the
Martian
atmosphere
is
in
which
the N
atoms produced have velocities
in
excess
of the
escape
velocity from
Mars.
Mars
255
7.2.7
Nitrogen
Chemistry
Although
nitrogen amounts
to
2.7%
of the
atmosphere,
it is
relatively unreactive unless
its
strong
N—N
bond
is
broken, forming
odd
nitrogen.
It is
convenient
to
group
the
oxides
of
nitrogen
as
NO,
(N + NO +
NO
2
+
NO
3
+
2N
2
O
5
)
and
NO,,
(NO*
+
HNO
2
+
HNO
3
+
HO
2
NO
2
).
Odd
nitrogen
is
formed primarily
by the
absorption
of
extreme ultraviolet solar radiation
or
photoelectron
in the
ionosphere
or the
absorption
of
comic rays
in the
lower atmosphere.
N
2
is
dissociated
by EUV
solar
radiation:
where
N
denotes
the
nitrogen atom
in the
ground
[N(
4
S)J
state
and
N(
2
D)
is the first
excited state
of the
nitrogen atom.
For our
purpose,
we
adopt
the
common assumption
that
the
yields
of N and
N(
2
D)
in
(7.38)
are 0.5 for
each branch.
The
primary fate
of
N(
2
D)
and
N+
is to
react
with
CO
2
to
form
NO:
The
primary fate
of
N
2
+
is the
charge
transfer reaction with
CO
2
and O:
and
dissociative recombination
to
form
N
atoms
(7.37).
The
N—N
bond
can
also
be
broken
by
impact dissociation
by
photoelectrons [produced
in
reactions such
as
(7.24)]
The
above energetic
processes
all
ultimately
lead
to the
production
of odd
nitrogen
(N
and
NO).
NO is
readily removed
by
dissociation:
The
permanent sink
for odd
nitrogen
in the
upper atmosphere
is the
reaction
This reaction destroys
two odd
nitrogen species
with
the
formation
of a new
N—N
bond.
Odd
nitrogen that survives
the
destruction
by
(7.46)
is
transported
to the
lower
atmosphere where higher oxides
of
nitrogen
are
formed: