Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres

186

Photochemistry

of

Planetary

Atmospheres

5.7

Miscellaneous Topics

5.7.1

AsH

3

andGeH4

Small

concentrations (less

than

1

ppb)

of

arsine

(AsHs)

and

germane

(GeH*)

have

been

detected

in the

atmosphere

of

Jupiter

but not in any

other giant planet.

The

observed abundances

are in

agreement with

thermodynamic

models based

on

cosmic

composition

of the

elements

and

upward mixing

from

the

deep

atmosphere.

5.7.2

Sulfur

Species

Conspicuous

by its

absence

is the

detection

of any

sulfur

species

in the

giant planets.

A

serious search

was

made

for H2S in the

atmosphere

of

Jupiter, yielding

an

upper

limit

that

is

about

300

times less

than

the

cosmic abundance. There

is no

reason

to

expect such

a

large depletion

of

sulfur

in the

giant

planets during formation.

We

believe

that

most

of the

H^S

is

sequestered

in

NUjSH

clouds.

5.7.3

Isotopomers

Deuterated hydrogen (HD)

and

deuterated methane

(CHjD)

have been detected

in

all

the

giant planets. This provides valuable information

on the

D/H

ratios

in

these

planets.

The

D/H

ratios

in the

atmospheres

of

Jupiter

and

Saturn

are

close

to the

cosmic abundance value

of 1.6 x

10~

5

.

The

D/H

ratios

in

Uranus

and

Neptune show

a

significant

enhancement over

the

cosmic abundance value. This could

be

explained

as

follows.

There

were

two

distinct reservoirs

of

deuterium

in the

solar nebula: gaseous

HD and

ices such

as HDO or

CHsD.

The

former

is

characterized

by a

D/H

ratio given

by

cosmic abundance.

The

latter

has a

higher

D/H

ratio, caused

by a

hitherto uniden-

tified

mechanism.

The

hydrogen

and

deuterium

in the

giant planets

are

derived

from

a

combination

of

these

two

reservoirs.

In

Jupiter

and

Saturn

the

gaseous

contribution

dominates;

in

Uranus

and

Neptune

the

ices

are an

important component

of the

total

budget.

See

section

4.10.1

for

comparison

of

D/H

values

in the

solar system

and a

discussion

of the

mechanisms

of

isotopic fractionation.

The

13

C

and

15

N

isotopic

species

have been detected

in the

atmosphere

of

Jupiter.

The

derived isotopic ratios,

13

C/

12

C

and

15

N/

14

N,

appear

to be

close

to the

cosmic

abundance

values, except

for

that inferred

from

13

C2H2-

This last anomaly

may be

due

to the

exceedingly

low

concentration

of

€2^2

in the

atmosphere

of

Jupiter.

In the

other

giant

planets,

only

13

CH4

has

been

detected

in the

atmosphere

of

Saturn,

and

the

inferred

13

C/

12

C

appears

to be

close

to

cosmic abundance.

5.7.4 Unsolved

Problems

The

vitality

of a field is

measured

not

only

by the

number

of

solved problems

but

also

by the

number

of new

questions that

are

raised

in the

investigations

and the

problems that remain

to be

solved.

The

following list

of

unsolved problems

is

given

as

a

challenge

to

future

investigators. Unless otherwise stated,

the

problems refer

to

all

four

giant

planets.

Jovian Planets

187

(a)

Energetics

and

Dynamics

1.

What

is the

energy source

for

maintaining

the

high temperatures

in the

thermo-

spheres?

2.

What

is the

excitation mechanism

for the

excess ultraviolet emissions that cannot

be

accounted

for by

resonance

fluorescence of

sunlight?

3. Why are the

internal heat

fluxes of

Uranus

and

Neptune

so

different

for two

planets

that

are so

similar?

4. Why is the

upper atmosphere

of

Uranus

so

quiescent, whereas

the

upper atmosphere

of

Neptune

is

vigorously mixed?

(b)

Atmospheric Composition

5.

What

is the

mechanism

for

producing

the

multiple layering

in the

lower ionosphere?

6.

What

is the

reason

for an

apparent excess

of He

over

the

cosmic abundance value

in

Neptune?

7. Why are

oxygen

and

sulfur

compounds apparently deficient?

8.

What

are the

precursor molecules

of the

Axel-Danielson

dust

in the

stratosphere?

9.

What

is the

chemical nature

of the

chromophores

in the

atmosphere?

10.

Does

ion

chemistry driven

by

magnetospheric

particles

affect

the

global chemical

composition?

11.

What

is the

cause

of the

Lyman

a

bulge

in

Jupiter?

(c)

Photochemistry

and

Kinetics

These

two

areas desperately need

new

input.

In

photochemistry

we

need measurements

of

the

branching ratios

and

dissociation products

as a

function

of

wavelength.

The

measurements

are

more important near

the

thresholds

of

dissociation, where

the

cross

sections

are

smaller

but the

solar

flux is

larger.

In

chemical kinetics

a

whole

new

class

of

experiments needs

to be

done

at low

pressure

(1

mtorr

to 1

torr)

and low

temperature

(50-200

K).

12.

What

are the

photodissociation products

of

CRj

at

Lyman

a

(1216

A) and at

longer

wavelengths

(1450

A)?

13.

What

are the

quantum yields

and

branching ratios

of the

photodissociation

of

C

2

H

2

from

Lyman

a to

threshold (around 2200

A)?

14.

What

is the

fate

of

excited

C

2

H

2

in the

atmosphere?

Does

it

take part

in the

poly-

merization

Of

C2H2?

15.

What

are the

cross sections

and

branching ratios

of

photodissociation

of

C4H2

and

higher

polyynes?

16.

What

are the

rate coefficients

of the

reactions forming higher polyynes

at low

tem-

perature?

The

reactions include

R181:

C

4

H

+

C

2

H

2

and

R190:

C

6

H

+

C

2

H

2

(see

table

5.11).

17.

What

are the

low-temperature rate coefficients

for the CH

insertion reactions:

R120:

CH

+

CH

4

,

R121:

CH +

C

2

H

2

,

R122:

CH +

C

2

H4,

R123:

CH +

C

2

H

6

?

18.

What

are the

photodissociation

cross

sections, branching ratios,

and

dissociation

products

for

methyl acetylene

(CH

3

C

2

H),

allene

(CH

2

CCH

2

),

and

propene

from

Lyman

a to

threshold?

188

Photochemistry

of

Planetary

Atmospheres

19.

What

are the

low-temperature rate coefficients

for the

following reactions that

are

important

in the

synthesis

of

higher hydrocarbons: R135:

CH

3

+CH

3

,

R138:

CH

3

+

C

2

H

3

,

R141:

CH

3

+C

3

H

3

,

R143:

CH

3

+C

3

H

5

,

R167:

C

2

H

3

+

C

2

H

3

,

R171:

C

2

H

3

+

C

2

H

5

?

20.

What

are the

low-temperature rate

coefficients

for the

following reactions that scav-

enge

H

atoms: R84:

H +

C

2

H

2

,

R109:

H +

C

4

H

2>

R164:

C

2

H

3

+

H

2

?

21.

What

are the

low-temperature rate coefficients

for

cracking

of

higher hydrocarbons

by

H

atoms: R94:

H +

CH

3

C

2

H,

Rl

10:

C

4

H

3

+ H?

22.

What

is the

role

of

heterogeneous chemistry

on the

surface

of the

Axel-Danielson

dust?

23.

What

are the

optical properties

of

hydrazine

after

exposure

to

ultraviolet radiation?

24.

What

is the

combined chemistry

of CP and PN,

which

are

chemical analogs

of CN?

25.

What

are the

photochemical products

of

AsH

3

and

GeH4

photochemistry?

Satellites

and

Pluto

6.1

Introduction

The

presence

of an

atmosphere

on a

small planetary body

the

size

of the

Moon

is

surprising. Loss

of

material

by

escape would have depleted

the

atmosphere over

the

age of the

solar system. Since these objects

are not

large enough

to

possess,

or to

sustain

for

long,

a

molten core, continued outgassing

from

the

interior

is not

expected.

However,

it is now

known that

four

small bodies

in the

outer solar system

possess

substantial

atmospheres:

lo,

Titan, Triton,

and

Pluto. These atmospheres range

from

the

very tenuous

on

lo

(of the

order

of a

nanobar)

to the

very massive

on

Titan

(of

the

order

of a

bar).

The

atmospheric pressures

on

Triton

and

Pluto

are of the

order

of

10

fjLbar.

Perhaps

the

most interesting questions about these atmospheres concern

their

unusual origin

and

their chemical evolution.

lo

is the

innermost

of the

four

Galilean

satellites

of

Jupiter,

the

other three being

Ganymede, Europa,

and

Callisto.

All the

Galilean moons

are

comparable

in

size,

but

there

is no

appreciable atmosphere

on the

other moons.

The first

indications

that

lo

possesses

an

atmosphere came

in

1974

with

the

discovery

of

sodium atoms

surrounding

the

satellite

and the

detection

of a

well-developed ionosphere

from

the

Pioneer

10

radio occultation experiment.

The

Voyager encounter

in

1979

established

the

existence

of

active volcanoes

as

well

as

SOa

gas.

These

are the

only extraterrestrial

active

volcanoes discovered

to

date,

and

they

owe

their existence

to a

curious tidal

heating

mechanism associated with

the 2:1

resonance between

the

orbits

of

lo

and

Europa.

This tidal heating generates

a

total power

of

10

13

to

10

14

W, a

value

that

may

be

compared

to the

total

geothermal

heat

flux of

Earth,

3 x

10

13

W.

lo's

heat

189

6

190

Photochemistry

of

Planetary

Atmospheres

is

released over

a

surface area that

is

about

12

times smaller than that

of the

Earth.

Hence,

lo's

internal heating

per

unit

area

is an

order

of

magnitude larger

than

that

of

Earth.

The

atmosphere

of

lo

is

continuously being eroded

by

bombardment

by the en-

ergetic particles

in the

Jovian

magnetosphere,

but the

atmosphere

is

resupplied

by

material

of

volcanic origin.

The

sputtered products

escape

into

the

magnetosphere

of

Jupiter

and

create

an

extended cloud

of

neutrals around

lo

and a

torus

of

heavy

ions

in the

orbit

of

lo.

The

major ions

are

oxygen

and

sulfur

ions derived

from

the

dissociation

products

of

SC>2.

In

this chapter,

we

examine

the

bound atmosphere

of

lo:

SO2

photochemistry,

the

ionosphere, atmospheric sputtering,

and the

torus.

Titan,

the

largest satellite

of

Saturn,

is

known

to

have

an

atmosphere, since

CH-4

lines

were spectroscopically identified

in its

spectrum

in

1944. Very little

was

known

about

Titan's atmosphere

until

the

Voyager encounters, which revealed that Titan

has a

massive

N2

atmosphere,

with

surface

pressure

equal

to 1.5

bar.

A

rich variety

of

organic molecules

and

extensive

aerosol

layers were

found

to be

present

in the

atmosphere. Titan

is

believed

to

have formed

in the

Saturnian subnebula

at the

time

of

the

formation

of

Saturn.

Due to the

lower temperatures

in

this region

of the

solar

nebula,

ices were common,

and

Titan could have accreted material that

is

rich

in

ices.

As the

atmospheric constituents

are

photochemically

processed

and

converted

into

condensible

material,

the

ices

on the

surface

or

outgassing

from

the

interior must

maintain

the

supply

of gas to the

atmosphere.

The

emphasis

of

this chapter

is on

the

organic chemistry

in

Titan's atmosphere

and its

implications

for

evolution.

The

hydrocarbon chemistry

on

Titan

provides

an

interesting comparison with that

of the

giant

planets. Organic synthesis

is

greatly facilitated

in

this atmosphere

due to the

lower amount

of the

reducing

gas H2.

This, together with

the

smaller

size

of the

planetary

body, results

in a

higher rate

of

chemical evolution.

In

bulk composition

the

atmosphere

of

Titan

is

mildly reducing, with

an

oxidation

state intermediate between that

of the

giant planets

and the

terrestrial planets. This

chemical

environment

may be

similar

to

that

of

Earth

at the

time

of

formation,

an

environment

that

is

conducive

to the

synthesis

of

complex organic compounds that

may

lead

to the

spontaneous generation

of

life.

Since there

are no

preserved

records

of

the

early chemical environment

of the

Earth, Titan

offers

an

exciting analog

of the

prebiological Earth.

Triton,

the

largest satellite

of

Neptune,

is

believed

to

have formed

from

the icy

debris

in the

outer part

of the

solar nebula.

It was

subsequently captured

by

Neptune,

and

this accounts

for its

unusual retrograde orbit.

The

bulk composition

of

Triton

is

interesting because

it

provides

a

sampling

of the

condensible

species

in the

solar

nebula

such

as

CH

4

,

CO,

CO2,

NHs,

N

2

,

and

HaO.

These

species

are

also present

in

molecular clouds, precursors

of the

solar nebula.

Any

major difference

in the

inventory

of

the

major

volatiles between

the

solar nebula

and the

molecular clouds would yield

valuable

clues

on

physical

and

chemical processing that must have occurred during

the

formation

of the

solar system.

The

atmosphere

of

Triton consists primarily

of

N2,

with

trace concentrations

of

CH.4.

CO and CO2

have been detected

in the ice

on

the

surface.

There

is an

upper limit

of 1% for CO in the

atmosphere

based

on a

combination

of

observation

and

modeling.

The

vapor pressure

of CO2 is

sufficiently

low

that

the gas is

negligible

in the

atmosphere.

The

photochemistry

of N2 and

CFLj

in

the

atmosphere

of

Triton

is

similar

to

that

of

Titan, except that

the

temperature

is

Satellites

and

Pluto

191

much

lower

and the

surface pressure

of

Triton corresponds

to

that

of the

mesosphere

of

Titan.

Pluto,

the

ninth

planet

of the

solar system,

has

little

in

common

with

the

other eight

planets

but

seems

to

closely resemble Triton. Both

are

believed

to be icy

planetesimals

formed

between

30 and 50 AU in the

solar

nebula.

We

know much

less

about Pluto than

Triton.

The

bulk atmosphere

is

believed

to be

composed

of

Na,

in

vapor equilibrium

with

N2 ice on the

surface.

CH*

and CO

ices

have been identified

in the

reflection

spectrum

of

Pluto,

and we

expect

trace

amounts

of

these gases

to be

present

in the

atmosphere.

The

photochemistry

of the

atmosphere

of

Pluto

is

expected

to be

similar

to

that

of

Triton, given

our

current lack

of

knowledge

of

Pluto.

An

important

aspect

of the

atmospheres

of the

small

bodies

is

their physical extent.

In

our

experience with

the

terrestrial planets

and the

giant planets,

the

altitude

of the

exobase

of the

atmosphere

is

usually small compared

with

the

planetary radius. Thus,

the

plane parallel approximation

is

valid. However,

in the

case

of the

small

bodies,

this

approximation

is

invalid.

The

extension

of the

atmosphere

is

comparable

to the

radius

of the

solid body.

The

equations

of

continuity must

be

formulated

in

spherical

coordinates.

6.2

lo

6.2.1

Neutral

Atmosphere

Pre-Voyager

ground-based infrared reflectance measurements have shown

the

presence

of

SOa

frost

on the

surface

of

lo.

Gaseous

SO2 has

been

definitively

identified

on

lo

by

the

Voyager infrared radiometer, interferometer

and

spectrometer (IRIS) instrument.

The

original report

of

these data interpreted

the

observed

gas as

vapor

in

equilibrium

with

SC>2

frost

on the

surface

at 130 K.

However,

the

Voyager observations

may be

associated

with

volcanic plumes. Therefore, exactly

how

much

SC>2

the

atmosphere

contains

is an

unresolved issue.

The

problem

can

best

be

appreciated

by

referring

to

figure

6.

la,

which shows

the

pressure dependence

of

862

on

temperature.

In

regions

of

active volcanic activity

the

temperature

may

exceed

300 K; the

corresponding

SC>2

vapor

is

exceedingly high.

At the

subsolar

point,

the

temperature could

be as

high

as

130

K and the

vapor pressure

of

SC>2

exceeds

10~

7

bar. However,

in the

polar regions

and

on the

night side,

the

temperatures drop below

90 K and the

vapor

pressure

of

SC>2

is

less

than

10~

12

bar,

which

is

close

to the

lower

limit

of a

collisional atmosphere.

The

simplest picture

is

that

lo's

atmosphere

may be of a

transient nature: thick

and

dense atmosphere near

the

volcanic plumes

and at the

subsolar

point,

gradually

decaying

away toward

the

poles

and the

nightside.

The

best observational evidence

in

support

of

this

picture

is the

microwave observation.

The

experiment reported detection

of

4-35

x

10~

9

bar of

SC>2

covering 3-15%

of the

surface

of

lo,

a

result consistent

with

SC>2

being

in

equilibrium

with

the

surface

frost.

There

is

circumstantial evidence

for

the

presence

of

about

20 x

10~

9

bar of a

noncondensible gas, such

as

C>2

or SO, in

the

atmosphere.

02 has

been postulated

to

impede

the

lateral

flow of SO2

away

from

the

subsolar

point

and to

account

for the

warm exosphere

of

lo.

There

is no

direct

observation

to

support

this

hypothesis.

Two

models

of

lo's

atmosphere,

a

high-density

and

a

low-density model,

are

shown

in figure

6.1b.

Figure

6.1

(a)

SOi

saturation vapor pressure

as a

function

of

temperature.

The

corresponding surface number density

and

column number density

are

indicated

on

the

upper

abscissae.

After

Kumar,

S., and

Hunten,

D. M.,

1982,

The

Satellites

of

Jupiter (Tucson: University Arizona

Press),

p.

782.

(b)

Model atmosphere

of

Io,

showing

the

SC>2

number density

(N) and

temperature

(T) for

both

the

high-

density

and

low-density

cases.

The

approximate altitude location

of the

exobase

is

shown

by

short horizontal dotted lines.

The

pressure

scale

is for the

high-density

model only.

After

Summers,

M. E., and

Strobel,

D.

P.,

1996,

"Photochemistry

and

Vertical

Transport

in

lo's

Atmosphere

and

Ionosphere."

Icarus

120,

290-316.

192

Satellites

and

Pluto

193

The

photochemistry

of SO2 in the

atmosphere

is

initiated

by the

photolysis

of

SO

2

:

where

the

thresholds

of

reactions

(6. la) and

(6.1b)

are

2170

and

2084

A,

respectively.

The

products

can

undergo

further

dissociation:

Some

of

these atoms

can

diffuse

to the

exosphere, where they

are

lost

due to

sputtering

by

the

magnetospheric energetic particles. Recombination

of the

dissociation products

can

also occur:

where

the

third body

(M) is

either

the

ambient atmosphere

of

SO

2

or the

surface. For-

mation

of

more complex compounds

can

readily occur,

as in the

following

examples:

But

these compounds

are

unstable

in the

atmosphere

of

lo

and are

readily removed

by

reactions such

as

A

simple reaction

set

describing

the

photochemistry

of

SO

2

is

given

in

table 6.1.

The

main uncertainties

are the

chemical kinetics

at low

temperature near

the

surface

and

the

fate

of the

dissociation products

at the

surface.

We

adopt

the

"reasonable"

assumption

that

S

atoms stick

to the

surface

but O

atoms recombine

to

form

O

2

and

are

released back

to the

atmosphere.

The

results

of a

representative model

are

given

in figure

6.2a

for

neutral

species.

SO

2

is the

most abundant molecule

in the

atmosphere near

the

lower boundary.

At

higher altitudes

O and S

atoms

become

the

dominant

species.

The

photochemical products

SO and

O

2

are

abundance.

The

abundance

of

O

2

is

small compared with

the

other dissociation products primarily

because

of the

reaction that removes

O

2

rapidly

via

where

the S

atoms

are

derived

from

(6.1b)

and

(6.2).

Since

the

atmosphere

is so

thin,

ternary reactions that require collisional stabi-

lization

are too

slow

in the gas

phase. However,

the

surface

may

serve

as a

good

"sponge

layer"

for the

atoms

to

react

to

form

stable molecules.

The

photochemical

model predicts

a

high

abundance

of SO,

formed mainly

from

SO

2

photolysis

and by

recombination

of S and O

atoms

on the

surface

of

lo.

Since

SO

2

condenses

on the

194

Photochemistry

of

Planetary Atmospheres

Table

6.1

List

of

essential

reactions

for the

neutral

atmosphere

of

lo

with their preferred

rate

coefficients

Reaction

Rla

b

R2

R3a

b

R4

R5

R6a

b

R7a

b

R8

R9

RIO

Rll

R12

R13

R14

R15

R16

R17

R18

R19

R20

R21a

b

R22a

b

R23

R24

R25

R26

R27

R28a

b

R29a

b

R30

R31

SO

2

+ hv

SO

+

hv

O

2

+ hv

S

2

+ hv

Na

2

+ hv

NaO

2

+ hv

NaS

2

+ hv

Na

2

O

+ hv

Na

2

S

+ hv

S

+

O

2

SO

+ SO

S0

+

0

2

O

+

S

2

SO

+

SO

3

O

+ O + M

SO

+ 0 + M

SO

2

+ O + M

S

+ S + M

NaO

+ O

NaO

2

+ O

Na

2

O+O

Na

2

O

+ S

Na

+ O + M

Na

+

O

2

+ M

NaO

+

O

2

+ M

NaS

+ O

NaS

2

+ O

Na

2

S

+ O

Na

2

S

+ S

Na

2

+ O

Na

2

+S

—

>•

-».

—

>•

_»

-»

->.

—

>

-».

—

»

-»

—

»

—

>

—

>•

->.

_».

-».

—

>•

->

->.

->

-»

_*.

->

—

>•

-».

_».

—

>

—

»

—

»•

—

>

—>

-»

->

—

*

—

»

so + o

S+0

2

s + o

o + o

O('D)

+ O

s + s

Na

+ Na

NaO

+ O

Na

+

Ch

NaS

+ S

Na

+

S

2

NaO

+ Na

NaS

+ Na

SO

+ O

SO

2

+ S

S0

2

+ 0

so + s

2SO

2

Oi

+ M

S0

2

+ M

SO

3

+ M

S

2

+ M

Na

+

O

2

NaO

+

O

2

2NaO

Na

2

+

O

2

NaO

+ NaS

Na

2

+ SO

NaO

+ M

NaO

2

+ M

NaO

3

+ M

Na

+ SO

NaS

+ SO

NaS

+ NaO

Na

2

+ SO

2NaS

Na

2

+

S

2

NaO

+ Na

NaS

+ Na

Rate

coefficient

2

1.0

6.3

1.8

3.9

9.4

9.2

3.0

9.0

1.0

9.0

1.0

2.3

5.8

2.6

2.2

2.5

1.0

7.7

3.4

1.0

3.7

5.0

1.0

0Na

1.0

#Na

1.0

2.2

3.1

3.7

5.0

X

2*

X

l

k

X

l.Ox

#Na

1.0

0Na

5.0

X

^

X

io-

5

io-

7

IO-

5

10

-.o

io-

8

io-

5

10-"

io-

5

io-

5

io-

5

io-

5

io-

5

io-

5

io-

12

IO-

12

e-

17

*/

7

'

10

-i3

e

-

2

4<x>/r

10-"

e-**/

T

io-

15

IO-

26

T-

2

-

9

io-

31

io-

32

e

-

im

/

T

10

-26

j-2.9

10

-io

10-

13

10-

12

190

io-

12

20n

io-

33

,

0

-27

r

-1.2

,0-25

J--2.0

,0-io

io-

13

io-

12

26n

10-

12

27o

io-

10

-io

Reference

Okabe

(1971),

Welge

(1984)

Driscoll

&

Warneck

(1968)

Phillips (1981)

Hudson

(1971)

Brewer

&

Brabson

(1966)

Plane (1989)

0.1

branching assumed

same

as

NaO

2

+ hv

0.1

branching assumed

assumed

JPL92

HH92

JPL92

Yung

&

DeMore (1982)

Yung

&

DeMore (1982)

NIST

(1994)

Yung

&

DeMore (1982)

Yung

&

DeMore (1982)

Baulch

&

Drsydale

(1973)

JPL92

Plane

(1989)

assumed

JPL92

same

as R19

same

as R20

assumed

a

Photodissocialion

coefficients

for

zero

optical depth, hemispheric

average,

with units

of s

'.

Bimolecular

and

termolecular

rate coefficients have units

of

cm

3

s"'and

cm"

s~',

respectively.

night

side

and the

polar

regions

of Io and SO

does

not,

it is

possible that

SO

forms

a

residual

atmosphere

on Io

away from

the

sunlight

side

and in the

polar

regions.

The

predicted

pressure

is of the

order

of a

nanobar

and may

serve

to

buffer

the

atmosphere

against supersonic winds

of SO2

that would

otherwise

be

blowing from

the

dayside

to the

nightside.

Figure

6.2 (a)

Distribution

of

major

neutral constituents

in the

atmosphere

of

lo

in

the

high-density model.

The

units

are

molecules/second,

(b)

Escape

rates

for

major

species

from

the

atmosphere

of

lo

in the

high-density model.

After

Sum-

mers,

M.

E.,

and

Strobel,

D. E,

1996, "Photochemistry

and

Vertical Transport

in

lo's

Atmosphere

and

Ionosphere."

Icarus 120,

290-316.

195

Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres

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