Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres
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16
Photochemistry
of
Planetary Atmospheres
Table
1.6
Jeans
escape
parameters
for
hydrogen
and
oxygen
atoms
Planetary
body
Jupiter
Saturn
Uranus
Neptune
Titan
Triton
Pluto
lo
Mars
Venus
Earth
Moon
Mercury
Temperature"
(K)
1200
400
810
540
185
95
100
500
365
400
1200
390
700
**
b
165
162
33
61
2.3
1.3
0.9
0.8
4.1
16.2
6.3
0.9
1.6
*o
b
2665
2614
531
977
36
22
14
13
67
260
100
14
25
V
H
C
(cm
s~')
0
0
1.6x
10~
8
1.7x
10-
20
1.6 x
10"
2.2
x
10
4
2.8 x
10
4
6.6 x
10
4
5.9 x
10
3
0.11
1.7x
10
3
5.5 x
10"
5.0 x
10
4
V
0
C
(cm
s-')
0
0
0
0
1.1
x
10~
10
5.7
x
10~
5
0.11
0.64
0
0
0
0.22
2.4
x
10~
5
"Representative
exospheric
temperature.
"Jeans
escape parameters
for
hydrogen
and
oxygen atoms, computed using (1.15).
°Jeans
escape velocity
for
hydrogen
and
oxygen atoms.
The
product
of
this velocity
and the
density
of the
escaping species equals
the
escape
flux
(1.16).
We
should point
out
that
the
Jeans formula
is not
rigorously
correct
for
large
escape
rates, because
it is
derived
for an
equilibrium distribution
of gas
velocities.
Nor is it
the
only
process
for
escape
from
an
atmosphere;
nonthermal
escape
can be
important
in
many atmospheres. Limiting factors such
as
diffusion
also
play
a
fundamental role
in
regulating
the
rate
of
escape.
But for the
purpose
of
this chapter, Jeans
escape
gives
us
a
preliminary view
of the
stability
of
planetary atmospheres.
Solar
Flux
and
Molecular
Absorption
2.1
Introduction
In
this book
we are
concerned primarily with disequilibrium chemistry,
of
which
the
sun
is the
principal driving force.
The sun is
not,
however,
the
only
source
of
disequi-
librium
chemistry
in the
solar
system.
We
briefly
discuss other minor energy sources
such
as the
solar
wind, starlight, precipitation
of
energetic particles,
and
lightning.
Note that these sources
are not
independent.
For
example,
the
ultimate energy source
of
the
magnetospheric particles
is the
solar wind
and
planetary rotation;
the
energy
source
for
lightning
is
atmospheric winds powered
by
solar
irradiance.
Only starlight
and
galactic cosmic rays
are
completely independent
of the
sun.
While
the sun is the
energy source,
the
atoms
and
molecules
in the
planetary atmo-
spheres
are the
receivers
of
this energy.
For
atoms
the
interaction
with
radiation results
in
three possibilities:
(a)
resonance scattering,
(b)
absorption followed
by fluorescence,
and
(c)
ionization.
lonization
usually requires photons
in the
extreme ultraviolet.
The
interaction
between molecules
and the
radiation
field
is
more complicated.
In
addition
to the
above (including Rayleigh
and
Raman scattering)
we can
have
(d)
dissociation,
(e)
intramolecular conversion,
and (f)
vibrational
and
rotational excitation. Note that
processes
(a)-(e)
involve electronic excitation;
process
(f)
usually involves infrared
radiation
that
is not
energetic enough
to
cause electronic excitation.
The
last
process
is
important
for the
thermal budget
of the
atmosphere,
a
subject that
is not
pursued
in
this
book. Scattering
and fluorescence are a
source
of
airglow
and
aurorae
and
provide valuable tools
for
monitoring detailed atomic
and
molecular
processes
in the
atmosphere.
Processes
(c) and (d) are
most
important
for
determining
the
chemi-
17
2
18
Photochemistry
of
Planetary
Atmospheres
cal
composition
of
planetary atmospheres. Interesting chemical reactions
are
initiated
when
the
absorption
of
solar energy leads
to
ionization
or the
breaking
of
chemical
bonds.
In
this chapter
we
provide
a
survey
of the
absorption cross sections
of
selected
atoms
and
molecules.
The
selection
is
based
on the
likely importance
of
these species
in
planetary atmospheres.
2.2
Solar
Radiation
and
Other
Energy Sources
2.2.1
Solar
Energy
Flux
The sun
radiates
with
a
total luminosity
of 3.8 x
10
26
W,
with
its
energy
source
ultimately
derived
from
nuclear
fusion
in the
core.
The
nuclear
fuel
consists
of the
light
elements hydrogen
and
helium, material that
was
created
in the Big
Bang,
the
origin
of the
universe, about
15
billion years ago.
The sun is a
typical main sequence
star
of
type G2V, formed about
5
billion years ago,
and has
enough
fuel
for
another
5
billion years. This
is the
largest source
of
energy
in the
solar system, since
the
next
nearest star
is
270,000
AU
away.
The
contribution
of
stellar radiation
is
generally
small, except perhaps
for
brief periods
following
a
supernova explosion.
The
sun's
average energy
flux per
unit area incident
on
Earth
is
1373
W
m~
2
,
and
this
is
known
as the
solar constant,
FO,
which
is
actually
not
constant with time (see section 2.2.2).
The
sun's
energy
has a
characteristic spectral distribution, which
to first
order
is
blackbody
radiation with emission temperature
T =
5785
K. The
expression
for
this
distribution
is the
Planck function,
where
v =
frequency (Hz),
h —
Planck's constant,
k =
Boltzmann's
constant,
and c
=
velocity
of
light.
B
v
is
known
as the
specific intensity
of
radiation, with units
of
energy
per
unit area
per
second
and per
hertz
per
steradian. There
is a
straightforward
integral
over
v,
where
a is the
Stefan-Boltzmann constant.
It can be
shown that
the
energy
flux
from
the
surface
of the sun is
just
n
times integral (2.2):
The
solar constant
FO is
related
to
F
sun
by
where
R
sun
is the
radius
of the sun and d is the
distance from
the sun to
Earth
(1
AU).
We
sometimes prefer
to
express
the
blackbody radiation
as a
function
of
wavelength
A.
=
c/v.
In
this
case
we
have
Solar
Flux
and
Molecular
Absorption
19
Figure
2.1
The
solar
irradiance
from
10~
7
to
10
3
cm.
After
Allen,
C. W.,
1958,
"Solar
Radiation."
Quart.
J.
Roy.
Meteorol.
Soc.
84,
307.
where
C\ =
2jr/zc
2
and
C
2
=
/zc/fc.
The
observed
spectral distribution
of
solar
flux is
shown
in
figure
2.1,
along with
a
theoretical Planck curve
for
comparison.
A
broad-band smoothing
has
been applied
to
the
observed spectrum
to
remove
the
sun's Fraunhofer absorption features. Note
that
the
bulk
of
radiation under
the
main peak
from
3000
A
(soft ultraviolet)
to
1
cm
(microwave)
is
very well approximated
by a
blackbody radiation
at
5785
K.
The
agreement
is
good
over
an
impressive
five
orders
of
magnitude
in
wavelength.
However, under
3000
A, the
agreement with
the
Planck curve breaks down.
The sun
radiates
far
more energy
in the
ultraviolet than
that
predicted
by the
Planck distribution,
because
the
bulk
of the
solar energy originates
from
the
visible disk
of the
sun,
the
photosphere, where
the
solar atmosphere
is in
thermodynamic
equilibrium
and the
assumptions
of the
blackbody
are
valid. However,
the
ultraviolet radiation
is
derived
from
the
upper atmosphere
of the
sun,
the
corona,
which
radiates
at
10
6
K and is out
of
equilibrium
with
the
photosphere.
We
should point
out
that
the
mechanism
that
sustains
the
extremely
hot
corona
of the sun is not
well understood. This problem
is
20
Photochemistry
of
Planetary Atmospheres
Table
2.1
Solar
spectral
irradiance
at 1 AU
A
(A)
1200
2000
2500
3000
3500
4000
5000
8000
Energy flux
(fraction
of
solar constant
<
X)
4.4
x
10-
6
8.1
x
10-
5
1.9x
10~
3
1.2
x
10-
2
4.5
x
10-
2
8.7
x
10~
2
2.3
x
10-'
5.6
x
10-'
Photon flux
(photons
cm~
2
s~')
1.97x
10"
1.14x
10
13
2.49
x
10
14
2.26
x
10
15
9.14
x
10
15
1.98
x
10
16
6.44
x
10
16
2.15
x
10
17
reminiscent
of
that
of the hot
thermospheres
of the
giant
planets
discussed
in
chapter
1.
The
variation
in the
corona
is
largely
responsible
for
variations
of the
solar
ultraviolet
flux
over
a
solar
cycle.
We
also
note
the
large
excess
emission
of the sun in the
radio region compared
with
blackbody emission (see
figure
2.1).
The
reason
for the
excess
is the
same
as
that
of the
ultraviolet excess: Both emissions
are
from
the
corona. Hence, even though
the
radio emissions play
no
role
in the
photochemistry
of
planetary atmospheres, they
are
easily observed
from
the
ground
and the
data
can
serve
as an
indicator
of
coronal
activity.
Since there
is no
simple
way to
monitor
the
variability
of
solar ultraviolet
radiation
from
the
ground,
the
10.7
cm
emission
from
the sun
serves
as a
useful
proxy.
A
coarse
resolution tabulation
of the
solar
flux is
given
in
table
2.1.
For
convenience
we
also list
the
photon
flux,
which
is a
particularly
useful
quantity when
we
study
photochemistry.
The
total number
of
chemical bonds broken
is
equal
to the
total
number
of
photons absorbed times
the
probability
of
dissociation. Table
2.1
shows
that
the
bulk
of
solar energy
is in the
visible
and
near infrared region. Ultraviolet
radiation, which
is
crucial
for
photochemistry, amounts
to a
tiny
fraction
of the
solar
constant. Thus,
all the
solar
flux
below
3000
A
contributes about
1% to the
integrated
flux,
or
10
15
photons
cm"
2
s~'.
The
portion
of
solar energy that
is
capable
of
ionizing
simple molecules,
at
Lyman
a and
shorter wavelengths,
is
only
10~
5
of the
solar
constant;
the
total photon
flux is
10
12
cm~
2
s"
1
.
Hence,
the
total column-integrated
rate
of
ionization
in the
terrestrial atmosphere cannot exceed this value
(10
12
cm~
2
s"
1
),
which determines
the
maximum state
of
ionization
in
Earth's ionosphere.
2.2.2
Solar
Variability
The
bulk
of
solar energy originates
from
the
photosphere, where
the
ambient pressure
of
the
solar atmosphere
is
high
and
variability
is
low.
It is
difficult
to
detect
from
the
ground
such small variations
in the
solar constant, despite
a
century
of
such
effort.
The
only
reliable data were taken
from
space-borne platforms
in the
last decade. Figure
2.2
shows
the
variability
of
solar irradiance
from
1980
to
1989.
The
short-term variability
could
be as
high
as
0.3%,
but the
long-term variability
is
less
than
0.1%
over this
period. Variations
of
this magnitude cannot account
for
substantial climatic changes
based
on
known mechanisms.
Solar Flux
and
Molecular
Absorption
21
Figure
2.2
Time
series
of
total
solar
irradiance
taken
by
satellites
studies:
ERB
(top),
ACRIM
(middle),
and
ERBE
(bottom).
The
lines
are 81 day
running
means.
After
Frohlich,
C. et
al.,
1991, "Solar
Irradiance
Variability
from
Modern
Measurements,"
in
Sonett
et al.
(1991;
cited
in
section
2.2),
p. 11.
The
most striking evidence
of the
sun's
variability
is the
visual disturbance
of the
photosphere caused
by
sunspots. There
is a
continuous record
of the
observed number
of
sunspots since their discovery
by
Galileo.
The
record
from
1749
to
1991,
as
shown
in
figure
2.3, reveals
a
sunspot cycle
with
an
average period
of
11
yr. The
amplitude
of
each cycle
is not
constant
and can
vary between zero
and 200
sunspots. There
appears
to be
another modulation
of the
amplitude with period
of
about
88 yr. In the
seventeenth
century (not shown), there
was a
period
of
over
50 yr
during which
the
sunspots
disappeared. This interval, known
as the
Maunder Minimum,
corresponds
to
an
unusually
cold period
in the
northern hemisphere.
There
are
speculations that
the
solar irradiance
was
reduced
at
that time,
but
this
has not
been proved.
A
number
of
other
phenomena
are
related
to or
modulated
by the
sunspot
cycle.
The
solar magnetic
field
reverses polarity every
11
yr,
completing
a
full
cycle
in 22 yr.
Since
galactic cosmic rays
are
deflected
by the
solar magnetic
field,
this results
in a
modulation
of the
galactic cosmic
ray flux
incident
on the
planets. Galactic cosmic
rays
are the
primary source
of
14
C
in the
atmosphere,
so the
ancient
record
of
14
C
(e.g.,
in
tree rings) provides
a
useful
proxy
for the
sunspot cycle
in
prehistoric times.
This record shows that
in
addition
to the
11
and 88 yr
periods evident
in figure
2.3,
there
are
other prominent periods
at 208 and
2300
yr.
Another solar output
that
varies
strongly
with
the
solar cycle
is the
solar
wind
(see section 2.2.4).
By far the
most
important
for the
atmospheres
of the
planets,
the
ultraviolet radiation varies with
the
sunspot
cycle.
The
ultraviolet portion
of the
solar radiation originates
from
the
corona
of the
sun,
where
the
pressure
is low and the
temperature
is
high.
This region
is
strongly disturbed
22
Photochemistry
of
Planetary
Atmospheres
Figure
2.3
Time series
of the
monthly averaged sunspot num-
bers
from
1749
to
1991.
The
solar cycle number
is
given under
each peak.
After
Donahue,
R.
A.,
and
Baliunas,
S.
L.,
1992,
"Periodogram
Analysis
of 240
Years
of
Sunspot
Records."
So-
lar
Physics 141, 181.
by
solar magnetic activity,
of
which
the
visible sunspots
are
only
one
manifestation.
Variability
of
ultraviolet radiation over
a
solar cycle
is
shown
in
figure
2.4. Note that
there
is
little
variation above
2000
A. At
Lyman
a, the
variations
are
about
a
factor
of
2; in the
extreme ultraviolet (EUV) region,
the
variations approach
a
factor
of 10.
Most
of the
solar energy
in the EUV is
absorbed
in the
thermospheres
of
planets.
The
large
variability
in the
solar
EUV
results
in
great
variations over
a
solar
cycle
in the
temperature
of
planetary thermospheres,
the
chemistry
of the
ionosphere,
and
even
the
dynamics
of the
upper atmosphere.
Although
the
influence
of the
solar cycle
on
the
upper atmosphere
is
well documented, there
is no
convincing evidence that
the
troposphere
is
affected
by the
solar cycle, despite occasional claims
to the
contrary.
The
sun's variability over
a
longer time period
is not
known
from
observations
but
may be
inferred
from
the
theory
of
evolution
of
stars
that
are
similar
to the
sun.
There seems
to be
general agreement
in the
astrophysical community that
the sun was
Solar Flux
and
Molecular
Absorption
23
Figure
2.4
Comparison
of UV
irradiance
variability between
solar
maximum
and
solar minimum using satellite data
from
SUSIM
(solid
line),
SBUV-
2
(short dashed line),
and SME
(long dashed line). After Lean,
J. et
al.,
1992,
"SUSIM/UARS
Observations
of the 120 to 300 nm
Flux Variations
during
the
Maximum
of the
Solar
Cycle:
Inferences
for the
11-Year
Cycles."
Geophys. Res.
Lett.
19,
2203.
smaller
in
radius
and
cooler
in the
past.
The
estimated solar irradiance
5
billion years
ago was 40%
less
than
the
present value,
and it
will
continue
to
increase beyond
the
present value
in the
next
few
billion years. This magnitude
of
change over
the age of
the
solar system must have
had a
profound impact
on the
climate
of the
planets.
How
the
surface
of
Earth
could
maintain
conditions
suitable
for the
continuation
of
life
in
face
of
such substantial changes
in the
solar radiation
is a
puzzle.
The
composition
of
the
terrestrial atmosphere must have been
different.
Did
Gaia (the biosphere) play
a
pivotal
role
in the
planetary transformation?
We
return
to
these questions
in
chapter
9.
Recent astronomical observations show that stars like
the sun go
through
an
initial
phase
of
enormous activity accompanied
by
enhanced ultraviolet emissions.
The en-
hancement
factor over
the
current brightness
in the
EUV
could
be as
high
as
10
4
but
decreases
rapidly
with
time. This period,
the
T-Tauri phase, occurred
at
about
the
time
of
the
formation
of the
solar system.
The
primitive atmospheres
of the
planets could
have been subjected
to
this intense ultraviolet radiation (see
figure
2.5).
The
resulting
photochemistry
and
rate
of
evolution would have been
different
from
any
subsequent
period
in the
solar
system.
One
interesting possibility
is
that
the
huge
EUV flux
could
have induced hydrodynamic escape
of
gases
from
planetary atmospheres,
a
process
that
differs
from
the
gentler Jeans
escape
described
in
chapter
1.
Heavy inert gases
such
as
xenon could have escaped
from
Earth
via
this mechanism. However,
it is not
known
how
transparent
the
solar system
was for the
transmission
of EUV
radiation
during
this epoch.
24
Photochemistry
of
Planetary Atmospheres
Figure
2.5
Enhancement
of UV flux in
T-Tauri stars relative
to the sun
derived
from
14
spectra
of
three T-Tauri stars. After Canuto,
V. M.
et
al.,
1983, "The
Young
Sun and the
Atmosphere
and
Photochemistry
of
the
Early
Earth."
Nature
305,
281.
2.2.3 Solar
Entropy
Flux
The
total solar power intercepted
by
Earth
is
10
17
W, of
which about
30% is
reflected
into
space.
The
remaining
70% is
absorbed
by the
planet.
But
Earth does
not
store
this
energy
from
the
sun,
and an
equal amount
of
thermal energy
is
radiated
back
to
space.
There
is a
fundamental
difference
in the
quality
of the
incident radiation
with
radiation temperature
at
5785
K and
that
of the
emergent radiation
from
Earth
with
radiation
temperature
of
about
255 K.
There
is a
quantitative
measure
of the
quality
of
energy
flux, the
entropy
flux. The
mean entropy
flux
emitted
from
the top
of
the
atmosphere
is
about
22
times larger than
the
mean entropy
flux of the
incident
solar radiation.
The sun is
thus
an
enormous source
of
negative entropy
based
on its
higher radiation temperature alone. This source
is
required
to
maintain
the
chemistry
of
the
atmosphere
so far out of
equilibrium
with
thermodynamics. Since entropy
and
information
are
different
only
by a
negative constant,
we can
regard
the sun as the
source
of
10
37
bits
of
information
per
second
to
Earth.
To first
order,
this
is
just
the
total number
of
photons intercepted
by our
planet. This
may be
regarded
as the
ultimate
source
of
life
and
intelligence,
the
ability
to
create
and
sustain
a
high degree
of
organization.
We
should point
out
that there
are,
in
addition
to the
radiation entropy,
two
other
"geometric"
sources
of
negative entropy.
Due to its
large distance
from
the
sun, sun-
light
reaches Earth
in an
almost parallel beam.
The
thermal radiation
of the
planet
is
roughly isotropic. Thus,
the
solar
beam
can be
focused
(e.g.,
using
mirrors),
whereas
the
planetary radiation cannot.
The
former obviously
has
less entropy. Finally, Earth
being
a
sphere, there
is
more solar power
per
unit
area incident
on the
tropics than
on
the
higher latitudes. This unevenness
in
incident
solar
flux is a
source
of
negative
en-
tropy
that
maintains highly organized atmospheric motions (e.g., Hadley cells) against
dissipation
(entropy production).
Solar
Flux
and
Molecular
Absorption
25
Table
2.2
Disequilibrium
energy
fluxes
for
Earth
Physical
process
Energy
flux
(W/m
2
)
Electromagnetic radiation
Total solar
irradiance
1370
Irradiance
<
3000
A 16
Solar
wind
3.0 x
10~
4
Visible
aurorae (variable)
International
brightness
coefficient
I 6.0 x
10~
4
International
brightness
coefficient
IV 0.6
Auroral
particles (variable)
Bright
auroral
arc 1.0
Proton
aurora, very disturbed conditions
3.6 x
10~
2
Polar
cusp electrons
1.0
x
10~
3
Polar
cusp
protons
1.0 x
lO"
4
Joule heating
of the
thermosphere
(E
= 100
mV/m)
0.14
Solar protons (polar
cap
absorption, variable)
2.0 x
10~
3
Galactic cosmic rays
7.0 x
10~
6
Downward
conduction
of
heat
from
magnetosphere
3.2 x
10~
5
Lightning
3.0 x
10~
3
Starlight
2.0 x
10~
5
Internal
heat
of
earth
8.7 x
10~
2
The
above discussion
is
directed
at
Earth
but can
obviously
be
generalized
to all
planetary
atmospheres
with
minor modifications.
2.2.4
Other
Sources
of
Energy
Table
2.2
lists
the
major energy sources
for
Earth's atmosphere. Note that
all
other
sources
are
very small compared
with
solar radiation. However,
in the
outer solar
system,
the
sun's
influence
falls
off
rapidly
with
distance,
but the
galactic
sources
such
as
starlight
or
galactic cosmic rays remain constant
and
become relatively more
important.
We
briefly
discuss
the
characteristics
of
each
of the
energy sources.
The
solar wind, discovered
in the
1950s,
is a
stream
of
corpuscular radiation ema-
nating
from
the
sun.
It is an
ionized plasma
and
carries
with
it its own
magnetic
field.
At
1 AU the the
speed
of the
solar wind
is
approximately
500
km/sec
and the
mean
density
is 5
particles/cm
3
.
The
energy
flux and
particle
flux are 3 x
10~
4
W
m~
2
and
2.5 x
10
8
cm~
2
s"
1
,
respectively.
The
most abundant elements
in the
solar wind
are
H
(~0.96),
He
(~0.04),
and O (~5 x
10~
4
),
where
the
numbers refer
to
mole
fraction.
Though
the
energy
flux
carried
by the
solar wind
is
negligible compared
with
the
photon
flux, the
momentum
flux is
higher
than
that
due to
photons. Thus,
in
planetary atmospheres that
are not
shielded
by
magnetic
fields,
the
solar
wind
in-
teracts directly
with
the
exosphere, causing sputtering
of the
atmospheres. Secondary
processes
involve
local acceleration
of
ions
by the
solar
wind
magnetic
field and may
enhance
the
direct sputtering process. Direct
and
induced solar
wind
sputtering
may
have
played
an
important role
in the
evolution
of the
atmospheres
of
Mars
and
Venus
and
may
have contributed also
to the
loss
of
primordial atmospheres
on
Mercury
and
the
Moon.