Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres
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196
Photochemistry
of
Planetary Atmospheres
Oxygen
atoms
are the
most
abundant
species
at
high
altitudes,
and
their concen-
tration
ultimately
determines
the
level
of the
exobase. Oxygen
and
sulfur
atoms
as
well
as
atoms
of
alkali metal readily escape
from
lo.
The
neutral atoms
get
ionized
and
form
a
torus around Jupiter
in the
orbit
of
lo.
The
chemical composition
and the
dynamics
of the
torus
are
discussed
in
section
6.2.3.
Escape rates
for
major species
from
the
atmosphere
of
lo
are
summarized
in
fig-
ure
6.2b.
One
consequence
of the
higher rate
of
escape
of O
relative
to S is the
deposition
of S
atoms
on the
surface. This depositional rate
of
sulfur
is of the
order
of
10"
atoms
cm""
2
s~
l
.
This
is
equivalent
to
10~
5
to
1CT
4
cm
yr""
1
,
or
0.5-5
km of
sulfur
over
the age of the
solar system.
The
total rate
of
mass loss
from
lo
is of the
order
of 3 x
10
28
atoms
s"
1
,
or
1000
kg
s"
1
.
This
is an
interesting example
of the
profound
impact that atmospheric photochemistry
can
have
on the
evolution
of the
surface
of a
planetary body. This result
may be
compared
with
the
overall resurfacing
rate
of
10~
3
to 10 cm
yr"
1
deduced
from
the
absence
of
craters
on the
surface
of
lo.
The
loss
of
material
to the torus is
thus
a
minor
fraction
of the
average volcanic
output
over geologic time.
6.2.2
Ionosphere
A
well-developed ionosphere
of
lo
was
detected
by the
radio occultation experiment
on
Pioneer
10 in
1974
at a
local time
of
5:24 p.m.
The
corresponding solar zenith
angle equals
81°.
The
most likely ions that
can be
formed
in the
atmosphere
of
lo
are
S0
2
+
,
SO+,
0+,
and
S
+
:
Of
these ions,
SO
+
has the
lowest ionization potential,
10.2
eV,
which corresponds
to
a
photon
with
wavelength
1215
A.
Hence,
SO2
+
,
O
+
,
and
S
+
produced
in
reactions
(6.11),
(6.13),
and
(6.14)
will
all
rapidly undergo charge transfer
to
form
SO
+
:
The
ultimate fate
of
SO
+
in the
ionosphere
of
lo
is
dissociative recombination,
Table
6.2
List
of
essential
reactions
for the
ionosphere
of
lo
with
their
preferred
rate
coefficients.
Reaction
R32
R33
R34
R35
R36
R37
R38
R39
R40
R41
R42
R43
R44
R45
R46
R47
R48
R49
R50a
b
R51a
b
R52
R53
R54
R55a
b
R56
R57
R58
R59
R60
R61
R62
R63
R64
R65
R66
R67
R68
SO
2
+
hv
O
2
+hv
SO
+
hv
O
+
hv
S
+
hv
Na
+
hv
O+
+
SO
2
0++SO
0++0
2
o
+
+ s
O+
+ Na
S+
+ SO
S
+
+0
2
S+
+ Na
so
+
+ so
SO+
+ Na
0++S
0++SO
Oj
+ Na
soj
+ s
soj
+ so
soj
+ o
SOj
+
0
2
SOj
+ Na
Na+
+
Na
2
Na+
+
Na
2
O
Na+
+
Na
2
S
Na
+
+e
NaO
+
+ e
S
+
+e
O
+
+e
Oj+e
S0++e
SO^+e
Na++e
Na
2
O
+
+ e
Na
2
S
+
+ e
_>.
—
>
-».
-».
—
>•
—
>•
—
>
->.
_>.
->.
-»
->
-».
-».
_».
-».
->
->
—
>
->.
->.
—
>•
->
->.
-+
—
>•
->.
->.
->.
->•
—
>
-».
—
>
—
>
—
»
->•
—
»•
->
->
-»•
SO
2
+
+ e
O
2
+
+ e
SO
+
+e
O++e
S
+
+ e
Na
+
+e
0
2
+
+ SO
SO++O
O
2
+
+O
s
+
+ o
Na+
+O
so
+
+ s
SO++0
Na
+
+S
S
+
+
S0
2
Na
+
+ SO
S
+
+0
2
SO
+
+
O
2
Na
+
+
0
2
NaO
+
+ O
SO+
+
0
2
S
+
+
SO2
SO+
+
SO
2
SO+
+
O
2
0
2
+
+
S0
2
Na
+
+
SO
2
NaO
+
+ SO
Na
2
+
+ Na
Na
2
O+
+ Na
Na
2
S+
+ Na
Na
Na
+ O
S
O
O
+ O
S
+ O
SO
+ O
Na
+ Na
NaO
+ Na
NaS
+ Na
Rate
coefficient
2
4.2
1.8
1.8
7.6
7.6
3.6
8.0
5.0
1.1
5.0
5.0
1.0
2.3
5.0
1.0
5.0
5.0
x
x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
1.0
x
6.3
7.1
5.0
1.0
5.0
1.0
2.5
X
X
X
X
X
X
X
5.0 x
5.0
5.0
1.0
X
X
X
1.0
x
2.7
2.0
3.9
3.9
2.0
2.0
3.0
3.0
2.0
2.0
X
X
X
X
X
X
X
X
X
X
io-
8
io-
8
io-
8
io-
9
10"
9
io-
7
10-io
,0-10
,0-10
,0-10
,0-10
10"
9
10-"
,0-10
io-
11
10-io
,0-io
10
-io
10
-.o
io-
11
lO-io
10
-io
,
0
-io
,
0
-io
,0-io
,0-10
lO-io
10
-io
io-
9
io-
9
io-
|2
(^S)-
e
irj-
7
(2£g)-
5
«
10-'
2
(M»).«
,Q-12^3Q0^6
,0-7/300^.50
10-
7
(3W).5C
10-
7
(^)-
50
IQ-
7
^)-
50
10-
7
(M»)-
50
10-
7
(30°)-
M
Reference
11
Wu
&
Judge (1981)
McGuire
McGuire
(1968)
(1968)
McGuire
(1968)
AH86
assumed
A93
assumed
assumed
assumed
AH86
assumed
assumed
assumed
assumed
assumed
AH86
assumed
assumed
assumed
AH86
assumed
assumed
assumed
assumed
assumed
9
PH80
1
assumed
3
PH80
'
3
assumed
1
assumed
1
assumed
1
assumed
1
assumed
1
assumed
1
assumed
"Photoionization
coefficients
for
zero
optical
depth,
hemispheric
average,
with
units
of
s~
[
.
Ion-molecule
rate
coefficients
have
units
of
cm
3
s"
1
.
b
AH86
is
Anicich
and
Huntress
(1986),
PH80
is
Prasad
and
Huntress
(1980),
A93 is
Anicich
(1993).
197
198
Photochemistry
of
Planetary
Atmospheres
Figure
6.3
Major ions
and
electron number density.
The
data (diamonds) were
taken
by
Pioneer
10
radio occultation
on the
dayside.
After
Summers,
M. E., and
Strobel,
D.
R,
1996,
"Photochemistry
and
Vertical Transport
in
lo's
Atmosphere
and
Ionosphere."
Icarus
120,
290-316.
or
charge transfer
to an
alkali metal,
A
summary
of the
important
ion
reactions
in the
atmosphere
of
lo
is
given
in
table
6.2.
The
results
of a
simple photochemical model incorporating these reactions
are
sum-
marized
in figure
6.3.
SO
+
is the
major
ion
with
peak
concentrations
of the
order
of
5 x
10
4
cm~
3
,
followed
by
smaller amounts
of
S
+
and
SO2
+
.
6.2.3 Torus
Even
before
the
Voyager encounters, optical emissions
of Na and K
were discovered
in
a
tenuous cloud surrounding
lo,
and
S
+
ions were observed
in a
torus
in the
orbit
of
lo
around Jupiter.
The
Voyager spacecraft discovered
a
spectacular torus
of
heavy ions
that radiate profusely
in the
ultraviolet. Figure
6.4
presents
the
ultraviolet spectrum
obtained
by the
Voyager ultraviolet spectrometer (UVS).
The
strongest ultraviolet
features
have been identified
and are due to
doubly
and
triply ionized atoms
of O and
S. The
total radiation power
of the
torus
is
3-6
x
10
12
W.
The
theory
of the
torus
is
simple
in
concept, though
the
details
are
rather intricate.
The
escape velocity
for a
particle
to
leave
from
the
surface
lo
is
2.65
km
s~',
but the
velocity
is
even smaller
if the
escape
is
from
the
exosphere, which
may be
located
at
Satellites
and
Pluto
199
Figure
6.4
Extreme ultraviolet spectrum
of
the
plasma
torus
of
Jupiter observed during
the
Voyager
1
encounter. Multiplets
of
identified
species
are
indicated.
After
Broadfoot,
A. L. et
al.,
1979,
Science
204,
979-982.
a
fraction
of
lo's
radius above
the
surface.
The
atoms that
escape
from
lo
will orbit
around
Jupiter
in a
torus because
the
velocity
to
escape
from
Jupiter
at the
orbit
of
lo
is
24 km
s~'.
These
atoms eventually
get
ionized
and
corotate
with
the
magnetosphere
of
Jupiter, forming
a
plasma
of
heavy ions. Voyager observations
of the
shape
of the
ultraviolet
torus
are
shown
in figure
6.5a.
It
is now
known that
the
torus
of
lo
consists
of
three components. First there
is
the
neutral cloud
of Na
surrounding
lo.
This cloud also contains other neutral
atoms such
as K, O, and S. Due to the
short lifetime
(of the
order
of
hours)
of
the
neutral atoms against ionization,
the
neutral cloud does
not
extend
significantly
beyond
lo.
The
ionized torus consists
of a
cold plasma confined
to the
orbit
of
lo
and
a hot
plasma that extends beyond
the
orbit
of
lo.
The
mean temperatures
of the
cold
and hot
plasma
are a few eV
(10
4
K) and 80 eV
(10
6
K),
respectively.
The
total
number density
of
electrons (equal
to the
number
of
ions)
in a
cross
section
of
the
torus
is
shown
in figure
6.5b.
The
"width"
of the
torus
is of the
order
of
a
Jovian radius,
Rj.
The
maximum electron density
is
3000
cm~
3
at 5.7
Rj,
the
orbit
of
lo.
The
composition
of the
torus
of
lo
inferred
from
optical
and
ultraviolet
emissions
and
Voyager measurements
is
summarized
in
table 6.3. This composition
is
consistent
with
the
material derived
from
the
photochemistry
of
SC>2
described
in
section
6.2.1.
The
energetics
of the
plasma torus
are not
completely understood. When
the
ions
of
oxygen
and
sulfur
are
picked
up by the
magnetic
field,
each gains
260 and 520 eV of
gyro energy, respectively. Coulomb scattering
and
plasma wave interactions transfer
the
initial
ion
energy
to the
thermal plasma
in the
torus.
The
ultimate
fate
of the
particles
is to
diffuse
inward toward Jupiter
or
outward away
from
Jupiter
with
time
constants
of the
order
of 1 yr and
10-100
days, respectively.
The
enormous difference
in
the
rates
of
diffusion
is
revealed
in the
spacing
of the
contours
for
charge density
inside
and and
outside
of 5.7
Rj
(see
figure
6.5b).
200
Photochemistry
of
Planetary Atmospheres
Figure
6.5 (a)
Measured intensity
(points)
of the 685 A
feature
of the EUV
missions
as a
function
of
distance
from
Jupiter
in the
orbital plane
of the
satellites.
A
model torus used
to fit the
data
is
shown
to
scale
above
the
data;
the
intensity
predicted
by the
model
is
shown
by the
solid line.
The bar on the
left
gives
the field of
view
of the
observations.
After
Broadfoot,
A. L.
et
al.,
1979,
Science
204, 979-982.
(b) A
contour
map of
charge concentrations
in
the
vicinity
of
lo
constructed
from
measurements made along
the
inbound
trajectory
of
Voyager
1.
The
units
are
electrons/cm
3
.
Rj
=
Jupiter radius.
After
Belcher,
J. W.,
1983,
Physics
of the
Jovian
Magnetosphere
(Cambridge:
Cambridge University
Press),
p. 68.
The
plasma
in the
torus
of
lo
interacts strongly with
lo.
This
Jovian wind
can im-
pinge
on
lo
and
result
in
efficient
sputtering
of its
atmosphere.
The
sputtered material
can
produce even more ions, which
in
turn
can
contribute
to
atmospheric sputtering.
This
unstable nonlinear interaction (leading
to the
catastrophic erosion
of the
atmo-
sphere
and
surface
of
lo)
has
never been observed.
In
fact,
the
opposite
of
this—the
stability
of the
torus—has
been established
by
more than
a
decade
of
observations.
This
is
remarkable
in
view
of the
large variations
in the
rate
of
volcanic output
and
the
solar cycle variations
in
ultraviolet
flux and
solar wind.
It is now
believed that
the
interaction
is
self-regulating.
A
greatly enhanced Jovian wind would
set up a
current
loop
in the
ionosphere
of
lo,
which would generate
a
local magnetic
field
that
can
divert
the
incident plasma
of
heavy ions
at a
greater distance
from
lo.
Alternatively,
the
mass loading
of the
torus
affects
the
dynamics
and the
energetics
of the
magne-
tosphere
in
such
a way as to
remove
the
ions
at a
faster rate.
It is
beyond
the
scope
of
this
chapter
to
discuss
the
merits
of the
competing theories
of
lo's
interaction with
the
Jovian
magnetosphere.
Satellites
and
Pluto
201
Table
6.3
Composition
of
lo's
environment
Constituent
Ionosphere
electrons,
e~(dawn,
5.5h)
electrons,
e~(dusk,
17.5h)
Plasma
torus
Singly
ionized
sulfur,
SII
Doubly
ionized
sulfur,
SHI
Triply
ionized
sulfur,
SIV
Singly
ionized oxygen,
Oil
Doubly
ionized oxygen,
OIII
Total
electrons,
e~
Neutral
cloud
Sodium,
Na
Concentration
(cm-
3
)
9xl0
3
6x
10
4
1000
500
40
1000
30
2000-4000
10
Reference
Kliore
et
al.
(1974)
Bagenal
et al.
(1994)
Brown
(1974)
6.3
Titan
Titan
possesses
a
mildly reducing atmosphere
in the
outer solar system.
It has a
rather
massive atmosphere that
is
different
from
the
atmospheres
of the
giant planets
in at
least
two
aspects.
First, there
is
little
H2 in the
atmosphere
of
Titan relative
to the
major
gas
N
2
.
The
gravity
is low
enough that light
gases
like
H and H2
readily
escape
from
the
satellite.
In
contrast,
the
composition
of the
giant planets
is
dominated
by H2.
Second, unlike
the
giant planets, Titan
has a
cold
surface. Organic compounds that
are
synthesized
in the
atmosphere
are
deposited
on the
surface, resulting
in
their permanent
sequestration. This implies
an
irreversible chemical evolution
of the
atmosphere
and
the
surface.
There
is no
recycling
of
organic
species
as in the
giant planets.
The
atmosphere
is
being gradually destroyed
and
must
be
resupplied
by
primordial
ice on
the
surface
or
outgassing
from
the
interior.
Most
of our
knowledge
of
Titan
was
obtained
from
the two
Voyager
flyby
missions
to
Saturn. Table
6.4
summarizes
the
physical
parameters
and
composition measure-
ments
of
Titan. Most
of the
information
was
derived
from
Voyager UVS, IRIS,
and
RSS
(radio science).
It is
illuminating
to
display
the
compositional information
as a
comparison between Titan
and
Jupiter
in
figure
6.6.
For
future
reference,
we
display
similar
compositional information
for
Earth's
atmosphere
in
this
figure.
As
pointed
out
earlier,
the
great contrast between Titan
and
Jupiter
is in
that
N
2
dominates
in
Titan
whereas
Ha
dominates
in
Jupiter. Methane
is a
minor species
in
both atmo-
spheres
and is the
parent molecule
for the
higher hydrocarbons,
the
€2,
C^,
and
€4
species observed
in
these atmospheres.
The
detailed photochemistry
of
CH4
in
Titan's
atmosphere
and its
comparison
with
that
of
Jupiter
is
discussed
in
sections
6.3.1
and
6.3.2.
Methane
is
also
the
source
of the
small amount
of
H
2
present
in the
atmosphere
of
Titan.
H
2
is
easily lost
from
Titan
by
thermal
escape,
and its
abundance must
be
maintained
by a
constant photochemical source.
The
dominance
of
nitrogen
on
Titan gives rise
to the
rich coupled chemistry between
nitrogen
and
carbon.
The
variety
of
nitrile
species
on
Titan
appears
to be
unique
in
the
solar system.
The
detailed chemistry
is
discussed
in
section
6.3.3.
The
existence
Table
6.4
Physical
parameters
and
chemical
composition
measurements
for
Titan
Physical
or
chemical data
At
the
surface (altitude
z = 0)
ro
=
distance
to
center
=
2575
km
go
=
gravity
=
135
cm
s~
2
PO
=
total pressure
=1.5
bar
T
0
=
temperature
= 94
K
no
=
number density
= 1.2 x
10
20
cm"
3
Composition
of the
troposphere (volume mixing ratio)
N
2
>
0.97
CH,
<
0.03
H
2
=
0.002
At
the
tropopause
(z = 45 km)
P
= 130
mbar
r
=
71.4K
n
= 1.1 x
10
19
cm"
3
Composition
of the
stratosphere (volume mixing ratio)
CHt
= 1-3 x
10-
2
H
2
= 2 x
10-
3
C
2
H
6
= 2 x
10-
5
C
2
H
2
= 2 x
10-
6
C
2
Ht
=
4 x
10-
7
C
2
H
8
=
2-4
x
10-
6
CH
3
C
2
H
= 3 x
10~
8
C
4
H
2
=
10-
8
-10-
7
HCN
= 2 x
10-
7
HC
3
N
=
10-*-10-
7
C
2
N
2
=
10-
8
-10~
7
CO
= 6 x
10-
5
CO
2
= 1.5 x
10~
9
H
2
O
< 1 x
10~
9
CH
3
D
=
CH4
x 6.4 x
10~
4
Composition
of
mesosphere
and
thermosphere
N
2
= 2.7 x
10
8
cm~
3
at z
=
1280
km
CRi
= 1.2 x
10
8
cm~
3
at z =
1140
km
(mixing ratio
=
0.08)
C
2
H
2
mixing
ratio
=
1-2%
at z
=
840 km
H
atmos: disk-averaged
Lya
airglow
= 500
R
a
Haze layers
Optical
haze
at z = 300 km
UV
haze
at z = 400 km
'The
observed airglow
on the the
dayside
is
about
1
kR.
However, based
on
our
knowledge
of the Lya
nightglow
and
other emissions,
we
conclude
that
the
dayglow consists
of
roughly equal contributions
from
resonance
scattering
of
solar radiation
and
excitation
by
electron impact.
The
value
quoted
here
refers to the
component
of Lya
airglow arising
from
resonance
scattering. From
Yung,
Y. L. et
al.
(1984).
202
Satellites
and
Pluto
203
Figure
6.6
Comparison
of the
approximate
speciation
of
hydrogen,
carbon,
nitrogen,
oxygen,
sulfur,
and
other
trace
elements
in the
atmospheres
of
Earth,
Jupiter,
and
Titan.
of
COa,
a
highly oxidized
species,
was a
great surprise. However,
it can be
readily
synthesized
from
CO if we
postulate
a
source
of
extraplanetary
oxygen
in the
form
of
meteoritic
K^O.
The
question then
arises
as to the
origin
of CO. In
view
of the
ubiquitous
existence
of CO in the
outer solar system
and the
interstellar medium,
it is
conceivable that Titan's
CO is
primordial. Conversely,
it is
also possible
to
produce
CO
from
the
photochemistry
of CH4 and
oxygen
derived
from
meteoritic
H2O.
The
coupled chemistry
of
carbon
and
oxygen
species
is the
subject
of
section
6.3.4.
As
summarized
in
figure
6.6,
the
greatest contrast between
the
atmospheres
of
Earth
and
Titan
is the
large abundance
of
©2
in the
former.
The
high concentration
of
O2
gives rise
to the
Oj
layer. Like
O
2
,
most
of the
important parent molecules
in
the
terrestrial atmosphere
are of
biological origin.
These
include
species
such
as
CH
4
,
N2O,
NHs,
and
COS.
The
abundance
of CO2 in the
atmosphere
is
biologically
regulated.
H2 is not
stable
in the
atmosphere
and is
readily oxidized
to
become
the
more stable
form
of
hydrogen—H
2
O.
Biogenic
CHU
is the
ultimate source
of the
observed
H2 in the
atmosphere.
We
defer
further
discussion
of
Earth's
atmosphere
to
chapters
9 and 10.
The
photochemistry
of
Titan
is
discussed
in
some detail
in the
sections
6.3.1-6.3.5.
Figure
6.7a
shows
a
schematic
of the
thermal structure
and
various regions
of the at-
mosphere
of
Titan where
different
photochemical
and
physical
processes
are
believed
to
be
important.
A
model atmosphere
of
Titan
is
presented
in figure
6.7b.
The
eddy
and
molecular
diffusivity
profiles
are
given
in figure
6.7c.
The
former
is
derived
on
the
basis
of
trial
and
error
and by
analogy with
the
terrestrial atmosphere.
The
rea-
son for the low
eddy
mixing
near Titan's tropopause
is the
thermal inversion
in
this
204
Photochemistry
of
Planetary Atmospheres
(a)
Figure
6.7 (a)
Schematic
of the
vertical
temperature
profile
and
various
regions
of
the
atmosphere
of
Titan where
different
processes
are
believed
to be
important. After
Samuelson,
R. E.
et
al.,
1981,
"Mean
Molecular
Weight
and
Hydrogen Abundance
of
Titan's
Atmosphere."
Nature 292, 688.
part
of the
atmosphere.
At
higher altitudes eddy mixing increases
due to the
breaking
of
upward-propagating gravity waves.
The
molecular
diffusivity
profile
is
based
on
the
kinetic theory
of
gases. Absorption
of
extreme ultraviolet
(EUV)
wavelengths
in
the
thermosphere
of
Titan leads
to the
production
of N
atoms
and
ions
of
nitrogen.
These
rapidly transfer charge
to the
hydrocarbons
and
initiate
a
complex chain
of
interactions.
In the
mesosphere region, photolysis
of
CH
4
becomes
important,
leading
to the
formation
of
higher hydrocarbons
and a
high-altitude haze layer.
The
chemi-
cal
kinetics peculiar
to
Titan
are
briefly
reviewed. Nitrogen atoms derived
from
the
thermosphere
and
oxygen-containing molecules derived
from
meteoritic
HaO
interact
with
hydrocarbon radicals, resulting
in a
rich suite
of
C—N
and
C—O
compounds.
The
regions where some
of the
organic
species
are
expected
to
condense
are
indicated
in
figure
6.7a.
It is
clear that
the
upper atmosphere must
be a
source
of
complex organic
compounds.
The
tropopause
serves
as a
cold trap where
the
organic compounds
are
removed, followed
by
transport
to
their repository
on the
surface
of
Titan.
The
photo-
chemically generated aerosol layer above
the
tropopause
is
sufficiently
dense that
no
ultraviolet
photons
can
penetrate
to the
troposphere.
6.3.1
Ionosphere
Interaction
between solar
EUV
radiation
and the
thermosphere
of
Titan
is
well docu-
mented
in the
Voyager
UVS
spectrum
of
Titan shown
in figure
6.8.
The
most prominent
Figure
6.7 (b)
Altitude profiles
for the
total number density
n(z)
and
temperature
T(z).
T(z)
= 174 K for z >
1200
km.
After
Yung,
Y. L. et
al.,
1984,
"Photochemistry
of the
Atmosphere
of
Titan: Comparison between Model
and
Obser-
vations."
Astrophys.
J.
Suppl.
55,
465.
(c)
Eddy
diffusivity
profile,
K(z),
in
cm
2
s"
1
,
and
molecular
diffusivity
profile
for
CH4 in N2.
£>(z),
in
cm
2
s"
1
,
is
given
for
comparison,
(dashed
line).
After Yung,
Y. L. et
al.,
1984,
"Photochem-
istry
of the
Atmosphere
of
Titan: Comparison between Model
and
Observations."
Astrophys.
J.
Suppl.
55,
465.
205