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

206

Photochem

istry

of

Planetary

Atmospheres

Figure

6.8 The

principal

N,

N

+

,

and

N

2

emissions

from

Titan detected

by

Voyager UVS.

The

effects

of

instrumental scattering

and

Lyman

a

have been

removed

from

the

spectra.

A

synthetic model

based

on the

electron impact ionization

of N2 is

shown

by

heavy

lines.

After

Strobel,

D. F., and

Shemansky,

D.

E.,

1982,

"EUV

Emission

from

Titan's Upper Atmosphere: Voyager

I

Encounter."

J.

Geophys.

Res.,

87,

1361.

observed features result

from

emissions

by

various excited states

of

N

2

,

N, and

N

+

.

The

total averaged radiated power

from

the

dayside

is 2 x

10

W. In

this region

of

the

atmosphere

the

primary interaction appears

to be

between solar

EUV and

N

2

gas.

N

2

is

readily dissociated

or

ionized

by

solar EUV:

where

N(

2

D)

is the first

excited state

of the N

atom. These

processes

may

also

be

driven

by

energetic electrons

from

the

Saturnian

magnetosphere, augmenting

the

solar

driven

reactions

by

20%.

The

primary removal mechanism

for N

atoms

is

formation

of

N

2

via

where

NH is

derived

from

The N

atoms that

are not

removed

flow

downward

to the

mesosphere region, where

they

react

with

hydrocarbon radicals

to

form

HCN

(see section

6.3.3).

A

comprehensive

set of ion

reactions

is

referred

to the

literature.

The

primary

fate

of

the

ions

of

nitrogen

is

loss

by

charge transfer

to the

hydrocarbon species:

Satellites

and

Pluto

207

More complex hydrocarbon ions

can be

formed,

as in the

following examples:

Further reactions lead

to the

production

of the

terminal

ion

H

2

CN

+

by

As

shown below,

H

2

CN

+

is the

most abundant

ion in the

ionosphere

of

Titan.

A

schematic diagram summarizing

the

principal pathways that result

in the

production

of

H

2

CN

+

,

including most

of the

above reactions,

is

given

in

figure

6.9a.

Complex

hydrocarbon ions

may be

produced

as

follows:

A

schematic diagram showing

the

principal pathways

for the

production

of

complex

hydrocarbon ions

is

given

in

figure

6.9b.

All

ions containing more than three car-

bon

atoms

are

labeled

"C

n

H

m

+

."

Note that charge transfer reactions

are

capable

of

producing interesting neutral

molecules

such

as

NH

3

by

although

the

amount that

can be

produced

is

insignificant.

The

ultimate

fate

of

H

2

CN

+

and the

hydrocarbon ions

is

loss

by

dissociative

recombination

in

reactions such

as

where

p + r = n and q + s =

m.

The

number densities

of the

major ions

in the

model

are

presented

figure

6.10a.

H

2

CN

+

is the

most

abundant ion, with peak concentration

in

excess

of

10

3

cm~

3

at

about

1200

km

above

the

surface.

The

second most abundant

ion

is the sum of

complex hydrocarbon ions containing more

than

three carbon atoms

(C

B

H

m

+

).

N

2

+

is a

minor

ion

even though

the

production rate

of

N

2

+

by

(6.24a)

and

208

Photochemistry

of

Planetary Atmospheres

(a)

(b)

Figure

6.9 (a)

Ion-neutral reaction

pathways

leading

to the

formation

of

the

major

ion,

H

2

CN

+

.

(b)

Ion-neutral reaction pathways

leading

to the

formation

of the

higher

mass hydrocarbon ions,

C

n

H

m

+.

Note that

CH

3

+,

C

2

H4

+

,

and

CaH3

+

are

produced

by

photoionization

and

electron

impact ionization.

C2H5

+

is

removed primarily

by

reaction

with

HCN.

After

Keller,

C. N.

etal.,

1992,

"A

Model

of the

Ionosphere

of

Titan."

J.

Geophys.

Res.

97,

12117.

electron impact

is the

most important ionization reaction

in the

model (see

figure

6.11,

a and b). The

reason

is the

rapid charge exchange reactions such

as

(6.27a

and

6.27b)

that

remove

N2

+

.

The

total

ion

density (equal

to the

electron

density)

at the

peak

is

about

5 x

10

3

cm"

3

and is

consistent with

the

upper limit obtained

by the

Voyager

observations.

The

concentrations

of

minor ions

are

shown

in

figures

6.10,

b and c.

In

the

region

of the

main peak

CHj

+

and

C2^

+

are

abundant

due to

their

low

Figure

6.10

(a)

Density profiles

of

the

seven

major

ions,

(b)

Density

profiles

of the

minor

ions

in the

model,

(c)

Density

profiles

of the

minor ions

(of

smaller concentrations)

in the

model.

lonization

from

both

electron

impact ionization

and

photoionization

is

included.

The

solar zenith angle

is

60°.

After

Keller,

C. N. et

al.,

1992,

"A

Model

of the

Ionosphere

of

Titan."

J.

Geophys.

Res.

97,

12117.

209

210

Photochemistry

of

Planetary

Atmospheres

Figure

6.11

lonization

production

rate

profiles

for all

species. Purely photoionization

rates

are

shown here,

and the

solar

zenith

angle

is

60°.

The

ions

are

divided into groups

as

nonhydrocarbons

(a) and

hydrocarbons

(b).

After

Keller,

C. N. et

al.,

1992,

"A

Model

of

the

Ionosphere

of

Titan."

J.

Geophys.

Res.

97,

12117.

ionization

potential relative

to

other ions.

In the

upper ionosphere simple ions such

as

H2

+

,

NH

+

,

and

Hj

+

and

atomic ions such

as

C

+

and

H

+

become

important.

The

rates

of

production

of

nonhydrocarbon

and

hydrocarbon ions

in the

model

are

shown

in

figure

6.11.

Only solar

EUV

ionization

is

used

in the

model.

The

peak production

rate

of

N2

+

occurs

at an

altitude

of

1050

km,

with

a

rate

of 17

ions

cm~

3

s~'.

The

minor

ionization reactions include photoionization

of N,

CH(,

and

6.3.2

Hydrocarbon

Chemistry

The

essence

of

hydrocarbon chemistry

has

been described

in

chapter

5 and is not

repeated

here.

The

important reactions

are

summarized

in

tables

6.5, 6.6,

and 6.7 and

are

briefly

discussed

here.

A

great

difference between

the

atmosphere

of

Titan

and

those

of the

giant planets

is the

dominance

of N2 on

Titan.

The

presence

of

large

amounts

of N2

implies that

the

preferred pathway

for

'CH2

is

quenching

by

rather than

the

reaction with

H

2

to

form

CHb

by

(5.32).

However,

CHs

may

still

be

produced

by

photosensitized dissociation

via the

C2«H2

cycle

(see, e.g.,

chemical

scheme

VII in

section

5.3.1).

In

fact,

the

efficiency

of

this scheme

is

enhanced because

the

competing

scheme

(VI in

section

5.3.1)

for the

photosensitized dissociation

of H2

is

not

important

on

Titan.

Another

great

difference

between Titan

and the

giant planets

is the

ability

of hy-

drogen

(H and H2) to

escape

from

Titan.

As

pointed

out in

chapter

5, the

synthesis

of

higher hydrocarbons from

the

parent

molecule

CH4 is

accompanied

by the

production

of

H or H2

(see, e.g.,

chemical schemes

I-V

discussed

in

section

5.3.1).

The

gravity

of

Titan

is

sufficiently

low

that

H and H2 can

readily

escape.

With

the

loss

of

hydrogen

the

production

of

higher hydrocarbons becomes irreversible.

In

addition,

the

loss

of H

results

in a

much smaller concentration

of H

atoms

in the

atmosphere. Consequently,

the

cracking

of

unsaturated hydrocarbons

by H

atoms becomes

less

important when

compared

with

the

giant planets.

Satellites

and

Pluto

21

I

Table

6.5

List

of

molecular

dissociation

reactions

for the

atmosphere

of

Titan

Rl

R2

R3a

b

R4

R5a

b

c

R6a

b

R7a

b

R8a

b

c

d

e

R9

RlOa

b

c

Rlla

b

c

R12a

b

c

d

R13a

b

c

d

R14a

b

Reaction

3

N,

+e

_>

N +

N(

2

D)

+

f

N

2

+ CR

->

N +

N(

2

D)

CO

+ e

->•

C +

O

+

e

->

C +

O('D)

+

?

CH

3

+ hv

-+

'CH

2

+ H

CHi

+

hv

->

'CH

2

+

H

2

->

CH

2

+ 2H

->

CH + H +

H

2

C

2

H

2

+

Au

-*

C

2

H

+ H

->

C

2

+

H

2

C

2

H4

+ hv

->

C

2

H

2

+

H

2

->

C

2

H

2

+ 2H

CjHe

+

Au

-*

C

2

H4

+

H

2

-»•

C

2

H4

+ 2H

->

C

2

H

2

+

2H

2

-»

CH

4

+'CH

2

->

2CH

3

C

3

H

3

+

/ii>

-»

C

3

H

2

+ H

CH

3

C

2

H

+ hv

-»

C

3

H

3

+ H

-»•

C

3

H

2

+

H

2

->

CH

3

+

C

2

H

CH

2

CCH

2

+ hv

->

C

3

H

3

+ H

->

C

3

H

2

+

H

2

->

C

2

H

2

+

CH

2

C

3

H

6

+

An

->

CH

2

CCH

2

+

H

2

->

C

2

H

4

+

CH

2

->

C

2

H

2

+

CH

3

+ H

->

C

2

H

+

CH

4

+ H

CjHt

+

hv

->•

C

3

H

6

+

H

2

->•

C

2

H

6

+'CH

2

->

C

2

H

5

+

CH

3

->•

C

2

H

4

+CH

4

C

4

H

2

+

AK

-f

C

4

H

+ H

->•

2C

2

H

Rate

coefficient

11

_

—

1.0

x

10~

6

2.2

x

10~

8

2.0 x

10~

8

3.2

x

10-

9

4.5 x

10-

8

2.5 x

10~

8

1.4

x

10~

7

1.2

x

10^

7

1.1

x

10~

8

9.1

x

10~

9

9.9 x

10^

9

6.1

x

10~

9

3.5 x

10~

9

1.2

x

10-

7

9.0 x

10~

8

2.4 x

10~

8

4.5 x

10~

9

3.2 x

1Q-

7

1.2

x

I0~

7

4.9 x

10~

8

3.5 x

10~

7

1.2

x

10~

8

2.0 x

10~

7

3.0 x

10~

8

4.6 x

10~

8

7.6

x

10~

9

3.3 x

10~

8

1.8

x

10~

8

2.6 x

10~

7

1.7

x

10~

7

Reference

Strobel

and

Shemansky

(1982)

Caponeet

al.

(1980)

Lee and

McKay

(1982)

Parkes

et al.

(1973)

Walanabeet

al.

(1953)

Strobel

(1973);

Mount

and

Moos

(1978)

Nakayama

and

Watanabe

(1964)

Okabe

(1981,

1983a)

Zelikoff

and

Watanabe

(1953)

Back

and

Griffiths

(1967)

Akimotoet

al.

(1965)

Hampson

and

McNesby

(1965)

Lisaet

al.

(1970)

Mount

and

Moos

(1978)

Poole

and

Anderson

(1959);

Ramsay

and

Thistlewaithe

(1996);

Jacox

and

Milligan

(1974);

Stief

et al.

(1971)

Hamai

and

Hirayama

(1979)

Heller

and

Milne

(1978)

Sutcliffe

and

Walsh

(1952)

Rabalais

et al.

(1979)

Heller

and

Milne

(1978)

Calvert

and

Pitts

(1966)

Borrell

et al.

(1971)

Collinetal.

(1979)

Calvert

and

Pitts

(1966)

Georgieff

and

Richard

(1958)

Okabe

(1981);

Heller

and

Milne

(1978)

(continued)

Table

6.8

summarizes

the

basic assumptions

and

boundary conditions

of the

model.

The

lower boundary

of the

model

is

placed

at the

tropopause

(45

km), where

the

Cfy

mixing

ratio

is

assumed

to be 2.0 x

10~

2

.

The

upper

boundary

of the

photochemical

model

is at

1160

km for all

species,

except

H and

H

2

,

for

which

the

upper boundary

is

the

exobase,

at

1425

km

where

the

species

can

escape

at the

velocity given

by

the

Jeans

escape

formula.

The

special boundary conditions

for

nitrogen

and

oxygen

species

are

discussed

in

sections 6.3.3

and

6.3.4.

Unless otherwise stated,

for all

other

species

the

lower boundary condition

is

maximum deposition velocity (followed

by

loss

to the

troposphere)

and the

boundary condition

at the

upper boundary

is

zero

flux.

212

Photochemistry

of

Planetary

Atmospheres

Table

6.5

(continued)

Reaction

3

R15a

b

R16a

b

R17

R18

R19

R20a

b

R21a

b

R22a

b

R23

R24

CsH

2

+

hv

C

8

H

2

+hv

HCN

+ hv

HC

3

N

+

/(v

C

2

N

2

+

hv

CO

2

+ hv

H

2

O

+

hv

H

2

CO

+ hv

HCO

+ hv

CH

2

CO

+ hv

-»•

C

6

H

+ H

-*

C

4

H

+

C

2

H

->•

C

6

H

+

C

2

H

->•

2C

4

H

->

H + CN

->

C

2

H

+ CN

->

2CN

->•

CO + O

-»

CO +

O('D)

->•

H + OH

->

H

2

+0('D)

->•

H

2

+ CO

->

H + HCO

-*•

H + CO

->

CH

2

+ CO

Rate

coefficient

b

2.6 x

10~

7

1.7

x

1(T

7

2.6

x

10~

7

1.7

x

10~

7

l.l

x

10~

7

4.0 x

10~

7

2.7

x

1(T

7

7.4

x

10-"

5.5 x

10~

9

5.1

x

10~

8

2.0 x

10^

8

2.5 x

10~

7

1.8

x

10~

7

1.0

x

10~

4

1.3

x

10~

6

Reference

Kloster-Jensen

et

al.

(1974)

estimated

Kloster-Jensen

et al.

(1974)

estimated

West

(1975);

Lee

(1980)

Connors

et al.

(1974)

Connors

et al.

(1974)

Nuth

and

Clicker

(1982)

Shemansky

(1972)

Demore

and

Patapoff

(1972)

Allen

et al.

(1981)

Pinto

et al.

(1980)

Pinto

et al.

(1980)

Okabe

(1978)

"Excited

atom

or

molecule:

N(

2

D),

O('D),

'CH

2

=

CH

2

(a

]

A

l

);

CR =

cosmic rays.

b

The

values

for the

diurnally

averaged dissociation

coefficient

refer

to the top of the

atmosphere

in

units

of

s~

l

(a)

C\ and

C

2

Species,

and

H

and H2

The

primary driving force

of the

photochemistry

of

hydrocarbons

in

Titan's atmo-

sphere

is

photolysis. Figure 6.12 presents

the

diurnally averaged dissociation coef-

ficients

for

the

important hydrocarbons

in the

model.

The

major absorber

at

short

wavelengths (below 1600

A) is CTLi.

Absorption

by

C2H2,

€2^4,

and HCN

becomes

important

at

longer wavelengths. Aerosol opacity provides

significant

attenuation

of

solar radiation below

240 km.

The

mixing

ratios

of

PL^

and the

major hydrocarbon

species

CH4,

C

2

H

2

,

C

2

H4,

and

C2H

0

are

given

in figure

6.13a.

The

number densities

of the

important radicals

H,

CH,

'CH

2

,

CH

2

,

CH

3

,

C

2

H

3

,

C

2

H

5

,

C

2

H,

C

4

H,

and

C

6

H

are

shown

in figure

6.13,

b and c. The

computed mixing ratio

at

1140

km is

8.1%,

in

good

agreement

with

the

value

8 ± 3%

deduced

from

Voyager

UVS

observations.

The

increase

of CH4

at

high altitudes

is due to

diffusive

separation above

the

homopause.

The

maximum

abundances

of all

photochemically produced

species

occur near

the

homopause region

in

the

mesosphere, where

CFU

dissociation

is

greatest.

The

most abundant radical

is

H,

with

peak

concentration

equal

to 5 x

10

8

cm""

3

.

Since

H

atoms

escape

from

Titan,

their

abundance

is an

order

of

magnitude less than that

in the

Jovian atmosphere.

The

consequence

of

this

for

photochemistry

on

Titan

is

discussed later

in

this section.

The

rapid

decrease

in the

concentrations

of the

higher hydrocarbons near

the

tropopause

is

due to

condensation. Table

6.9

gives

the

saturation mixing ratio

of

selected

species

in

the

model.

It is

clear that most

of the

heavier hydrocarbons would condense

and

be

removed from

the gas

phase.

The

radicals

CH

3

,

C2H

3

,

and

C

2

H

5

all

exhibit

a

sec-

ondary

maximum

at

about

200 km.

This

is

driven

by the

photosensitized dissociation

of CH4 by

photons

at

long wavelength (see discussion

in the

following paragraph).

Satellites

and

Pluto

213

Table

6.6

List

of

essential

hydrocarbon

reactions

for the

atmosphere

of

Titan

Reaction

R25

R26

R27

R28

R29a

b

R30a

b

R31a

b

R32

R33

R34

R35

R36

R37a

b

R38

R39

R40a

b

R4la

b

R42a

b

R43

R44a

b

R45a

b

H+C

2

H

2

+ M

H+C

2

H

4

+M

CH+CHj

'CH

2

+

N

2

'CH

2

+

H

2

1

CH

2

+CH

4

CH

2

+

C

2

H

2

+ M

2CH

2

CH

5

+ H + M

CH

3

+CH

2

2CH

3

+ M

C

2

H

3

+ H

C

2

H

3

+CH

3

(+M)

2C

2

H

3

C

2

Hi

+ H

C

2

H

5

+CH

3

(+ M)

C

2

H

5

+C

2

H

3

2C

2

H

5

(+ M)

C

3

H

2

+ H + M

C

3

H

3

+ H + M

CH

3

C

2

H

+ H +

M

-

_>

—

*

-»•

—

>

-»

->

_>

->.

^

_>

->

_>

—

»

—

»

->

-»

->

_>

->

-»

_>

->

_>

->

—

>

->

C

2

H

3

+ M

C

2

H

5

+

M

C

2

H

4

+ H

CH

2

+

N

2

CH

2

+

H

2

CH

3

+ H

CH

2

+CH

4

2CH

3

CHjC

2

H

+

M

CH

2

CCH

2

+ M

C

2

H

2

+

H

2

CH

4

+ M

C

2

H

4

+ H

C

2

H

6

+ M

C

2

H

2

+

H

2

C

2

H

2

+

CH

4

C

3

H

6

+ M

C

2

H

4

+C

2

H

2

2CH

3

C

2

H

4

+CH

4

C

3

H

8

+M

C

2

H

6

+C

2

H

2

2C

2

Hj

C

2

H

6

+C

2

H

2

C

4

H

IO

+ M

C

3

H

3

+ M

CH

3

C

2

H

+

M

CH

2

CCH

2

+ M

(C

3

H^

+M)

CH,+C

2

H

2

+ M

C

3

H

5

+ M

Rate

coefficient

3

k

= 9 2 x

io-

|2

c~'

200

/

r

/t-x,

= 3.7 x

IO""

e

~

lata/T

l.Ox

l<r'

0

7.9 x

I0~'

2

l.Ox

IO-'

2

7.0 x

IQ-'

2

l.6x

I0~

l2

1

.9

x

I0-'

2

*o

= 3.8 x

IO-

25

/too

= 2.2 x

IO-'

2

/t

0

= 3.8 x

I0~

25

k.

x

= 3.7 x

IO-'

2

5.3

x

IO-"

/t

0

=

1.

7

x

I0~

27

*oo

=

l.5x

10-'°

7.0 x

IO-"

/to

=

1.

3

x

10~

23

^

=

5.5

x

10""

1.5

x

10-"

9.1

x

10~

12

/t

0

= 1.3 x

10~

22

/too

=9.1

x

10'

l2

5.3 x

I0~

12

1.9

x

IO-'

0

,?-

4

*"

7

'

1.7

x

I0-

|2

e-

200/r

to = 5.6 x

10~

22

k

— 4 2 x

10""

e~

200/rr

6.0 x

10~

12

3.0 x

10~

12

1.7

x

10-'

2

*-'

0

"'

/to

= 2.9 x

I0~

21

*«,

=

1.3

x

lO-"?-

95

'

7

/t

0

= 1.7 x

10"

26

/too

=

1-5

x

1<T

10

*o=

1.7 x

IO-

26

/too

= 1.5 x

10-'°

same

as

branch'

/to

=

8.0x

io-2

4

7-

2

f-

|225/:

A'oc,

= 9.7 x

io-

12

e

-

1550

'

7

'

same

as

branch

3

Reference

r

Payne

and

Stief

(

1976)

r

Michael

el al.

(1973);

Leeet

al.

(1978)

Butler

el al.

(1981)

Ashfold

el al.

(1980);

Laufer

(198

la)

Laufer

(198

la)

Laufer

et al.

(1983):

Laufer

(198

la)

Banyardet

al.

(1980)

Laufer

et al.

(1983);

Patrick

et al.

(1980)

Laufer

et

al.

(1981

a)

Laufer

et al.

(1983);

Callear

and

Metcalfe

(1976)

Keiletal.

(1976)

estimated

Laufer

et al.

(1983)

MacFadden

and

Currie

(1973)

Teng

and

Jones

(1972)

Teng

and

Jones

(1972)

Laufer

el al.

(1983)

Laufer

et al.

(1983)

Teng

and

Jones

(1972)

Laufer

et al.

(1983)

Laufer

el al.

(1983)

r

Whylock

et al.

(1976)

Wagner

and

Zellner

(1972a)

(continued)

A

schematic diagram showing

the

pathways

for the

interconversion

of

major

Ci

and

C

2

species

is

given

in

figure

6.13d.

The

destruction

of

CFU

is the

ultimate source

of all

higher hydrocarbons.

Fig-

ure

6.14a

presents

the

rates

of the

major reactions destroying

CFLi,

R5:

CH

4

+

hv,

R55:

CH4

+

C

2

H,

R61:

CH

4

+

C

4

H,

and

R65:

CH

4

+

C

6

H

(reaction

numbers refer

to

tables

6.5-6.7).

Direct photolysis

of

CH

4

is

important

in the

mesosphere; photo-

sensitized dissociation driven

by

C

2

Hb

and

polyynes

is

more important

in the

lower

stratosphere.

The

most important reaction

for

producing

CH

4

,

R33:

CH

3

+ H, is

also

included

in figure

6.14a.

Since there

are

fewer

H

atoms

in the

atmosphere

of Ti-

214

Photochemistry

of

Planetary Atmospheres

Table

6.6

(continued)

Reaction

R46a

CH

2

CCH

2

+ H + M

->

b

->

R47a

b

c

d

R48

R49

R50a

b

RSI

a

b

R52

R53

R54

R55

R56

R57

R58

R59

R60

R61

R62

R63

R64

R65

R66

R67

R68

R69a

b

R70

C

3

H

5

+ H

(+M)

C

3

H

6

+ H + M

C

3

H

7

+ H

C

3

H

5

+CH

3

C

3

H

7

+CH

3

(+M>

C

2

+

H

2

C

2

+

CH

4

C

2

H

+

H

2

C

2

H

+

CH4

C

2

H

+

C

2

H(,

C

2

H

+

C

3

Hg

C

2

H

+

C

2

H

2

C

2

H

+

C

4

H

2

C

2

H

+

C

6

H

2

C

4

H

+

CHi

C

4

H

+

C

2

H

6

C

4

H

+

C

2

H

2

C

4

H

+

C

4

H

2

CsH+CH,

C,,H

+

C

2

H

6

C

6

H

+

C

2

H

2

H

+

C

4

H

2

+

M

H

+

C

4

H,

H

+

CH

2

CCH

2

->

-»

->

-V

->

->.

-»

_>

—

*

->

_>

-t

->

-»

—

*

_>

-»

(C

3

H;

+ M)

CH

3

+C

2

H

2

+ M

C

3

H

5

+

M

CH

3

C

2

H

+

H

2

CH

2

CCH

2

+

H

2

C

3

H

6

+ M

CU,+C

2

H

2

C

3

H

7

+ M

C

2

H

5

+CH

3

CH

3

C

2

H

+

CH

4

CH

2

CCH

2

+Crl4

C.,H

6

+

CH

4

C

4

H,

0

+ M

C

2

H

+ H

C

2

H

+

CH

3

C

2

H

2

+

H

C

2

H

2

+CH

3

C

2

H

2

+

C

2

H

5

C

2

H

2

+C

3

H

7

C

4

H

2

+ H

C

6

H

2

+ H

C

8

H

2

+ H

C

4

H

2

+

CH3

C

4

H

2

+C

2

H

5

C

6

H

2

+ H

C

8

H

2

+ H

C

6

H

2

+CH

3

Cf,H2+C

2

H

5

CgH

2

+H

C

4

H

3

+ M

2C

2

H

2

C

4

H

2

+

H

2

CH

3

C

2

H

+ H

Rate

coefficient"

*o

=

too

1.5

kQ

-

If

"CO

1.5

t

0

=

too

1.9

4.5

2.5

to =

too

1.4

1.9

2.8

6.5

1.4

3.1

9.3

2.2

1.0

3.1

6.5

3.3

to

=

£(Xi

3.3

1.2

1.0

-

=

x

=

x

=

X

=

x

X

x

X

—

X

8.0

x

10-"

7-

2

r-

|225

'

r

8 0 x

I0~

24

y-

2

e

-'

2

^

5

/^

1

4 x

10~"

e"

1000

/

7

"

10-"

1.0

x

10~

28

1.0

x

10-"

1.5

x

10-

29

3.7

x

10-"

e

-io«o/r

10-io

e

-4

4

o/r

io-'

2

lO^

12

,

0

-12

,.-200/7

2.5

x

IO-

19

4.2

x

\o-"

e

-xx»T

io-

12

10-"

,

0

-ll

e

-l450/7-

,0-12^-250/7

io-

12

10-"

,

0

-13,

-250/7

io-

12

io-"

10-"

lO-

13

^-

250

/

7

"

io-

12

io-

12

1.0 x

IO-

28

2.0 x

IQ-'

2

io-'

2

10-"

,

0

-ii

f

-iooo/r

Reference

Wagner

and

Zellner (I972b)

Zellner(l972a,b)

Analogy

with

C

2

Ha,

+ H

Analogy

with

C

2

W

3

+ H

Analogy

with

C

2

H

3

+

CW

3

Analogy

with

C

2

H$

+

CW

3

Laufer

et al.

(1983)

Pasternack

el al.

(1979)

Pasternack

et al.

(1979)

Brown

and

Laufer

(1981)

Brown

and

Laufer

(1981)

Brown

and

Laufer

(1981)

Okabe(l983b)

Brown

and

Laufer

(1981)

Estimated

Schwanebeck

et al.

(1975)

Schwanebeck

et al.

(1975)

"Units

for

two-body

and

three-body rate

coefficients

are

cm

3

s"'

and

cm

6

s

',

respectively;

the

effective two-body rate

coefficient

for a

three-body

reaction is

given

by t =

k

e

k

x

M/(ko

+

k

x

M),

where

t

0

,

too.

and M

are,

respectively,

the

rate

coefficients

in the

low/pressure

and

high/pressure

limits,

and the

number

density

of

ambient

atmosphere

in

mole-

cules/cm

5

.

tan,

this

reaction

is

much less important

than

it is in the

giant

planets.

As

much

as

96% of the

dissociation

of CH4

results

in the

production

of

higher

hydrocarbons.

The

corresponding

efficiency

for

Jupiter

is

only

66%.

The

reason,

as

stated above,

is

the

escape

of H

from

Titan.

The net

column-integrated rate

of

destruction

of

CH

4

is

1.5 x

10

cm~

2

s~',

and

this must

be

balanced

by an

equivalent upward

flux at

the

tropopause. Figure

6.14b

gives

the

rates

of the

major

synthesis reactions, R32:

2CH

2

->

C

2

H

2

+

H

2

,

R34:

CH

2

+

CH

3

->

C

2

H

4

+ H,

R35:

2CH

3

->

C

2

H

6

,

and

R40b:

CHs

+

C

2

H_«;

->•

CsHg.

It is

clear

that

the

synthesis

of

alkenes

and

alkynes

(except

polyynes)

occurs

preferentially

in the

mesosphere, where

the

CH

2

radicals

Satellites

and

Pluto

215

Table

6.7

List

of

nitrogen

and

oxygen

reactions

for the

atmosphere

of

Titan

Reaction

R71a

b

R72

R73

R74

R75

R76

R77

R78

R79

R80

R8I

R82

R83

R84

R85

R86

R87

R88

R89a

b

R90a

b

R9I

R92

R93

R94

R95

R96

R97

R98

R99

RIOO

RI01

R102

N(

2

D)

N(

2

D)

+

N

2

N(

2

D)+CH

4

N+CH

2

N+CHi

NH

+ H

NH

+ N

2NH+

M

CN

+

CK,

CN

+

C

2

H

6

CN

+

C

2

H

2

HCN+C

2

H

CN

+ HCN

HCN

+ H + M

H,CN

+ H

HC

3

N

+ H+ M

H

2

C

3

N

+ H

C

2

N

2

+ H + M

HC

2

N

2

+ H

0('D)

+

N

2

0('D)

+

CH

4

O

+

CH

2

0 +

CHi

OH

+

CH

4

OH

+

CH

2

OH

+

CHi

OH

+ CO

OH

+

C

2

H

2

+ M

CH

2

+C0

2

H

+ CO + M

H

+ HCO

2HCO

CH

2

+ CO + M

_^

->

_>

_»

->

_>

->

_>.

_*

->

_».

->

_»

->

_>

—

*

->

_>

->

—

>

->

-*

—

>

->

-»

—

>

->

N

+AV

N

+N

2

NH

+

CH

3

HCN

+ H

HCN

+

H

2

N

+

H

2

N

2

+ H

N

2

+

H

2

+ M

HCN

+

CH

3

HCN

+

C

2

H

5

HC

3

N

+ H

HC

3

N

+ H

C

2

N

2

+ H

H

2

CN

+ M

HCN

+

H

2

H

2

C

3

N+

M

C

2

H

2

+

HCN

HC

2

N

2

+ M

2HCN

O

+

hv

0 +

N

2

OH

+

CH

3

H

2

CO

+

H

2

CO

+ 2H

H

2

CO

+ H

H

2

0

+

CHj

CO

+

H

2

+ H

CO

+

2H

2

C0

2

+ H

CH

2

CO

+

H+

M

CO

+

H

2

CO

HCO+

M

H

2

+ CO

H

2

CO

+ CO

CH

2

CO+

M

Rale

coefficient

2.3 x

10~

5

< 6 x

I0~

15

3.0 x

10-'

2

5.0 x

lO-"*'-

250

'

7

5.0 x

10""

i?"

2

-''

0

1.7

x

|o-

|2

T°-

68

<?-

950

'

r

1.1

x

10~

12

T

05

1.0

x

ID"

33

1.0

x

10-"*-

857

/

7

'

2.0

x

10-"

5.0

x

10-"

2.2 x

1Q-'

2

3.1

x

10-"

to

= 6.4 x

lo-^T-

2

?-'™/

7

1.5

x

10-

"

t

0

= 6.4 x

10-

25

T-

2

?-

1200

/

7

tno

= 9.2 x

io-

|2

f-

|200

/

r

1.5

x

10-"

to

=6.4

x

K

r

25

T-

2

<r

1200

'

7

'

toe

=9.2

x

io-'

2

e

-

|2

<W

r

1.7

x

\o~"e'"°'

T

6.7

x

10"'

1.8

x

10-"

e

lol/T

1.4

x

10-

I0

1.4

x

10""

8.3

x

10-

"

1.3

x

10-

10

2.4

x

10-'

2

e

-"

lo/r

5.0 x

10-'

2

6.7 x

10-'

2

1.4

x

10-'-

1

to = 5.8 x

IO--

11

e

l258/r

*«,

=

!.

4

x

io-<2

e

+m/T

3.9

x

10-'

4

2.0 x

io-33

e

-«so/r

3.0 x

10-'°

6.3

x

10-"

t

0

= 1.0 x

10~

28

too

= 1.0 X

IO-'

5

Reference

Okabe

(1978)

Black

el al.

(1969)

Black

el al.

(1969)

Estimated

Mayer

el al.

(1966)

Weslley(l980)

Estimated

Schackeetal.

(1977)

Estimated

Schackeel

al.

(1977)

Becker

and

Hong (1983)

Estimated

Phillips

(1978)

Albers

(1969)

Okabe

(1978)

DeMore

et al.

(1982)

DeMore

et al.

(1982)

Homann

et al.

(1981)

Hampson

(1980)

Baulch

elal.

(1980)

Estimated

Fenimore

(1969)

DeMore

el al.

(1982)

Perry

and

Williamson (1982)

Laufer

(198

la)

Pinto

et al.

(1980)

Pinto

et al.

(1980)

Pinto

et

al.

(1980)

Estimated

Laufer

(198

la)

are

abundant. Synthesis

of

alkanes occurs preferentially

in the

stratosphere, where

alkyl

radicals

are

produced

by

photosensitized dissociation.

A

comparison between

the

model computations

and

observations

of the

hydrocarbon

species

is

given

in ta-

ble

6.10.

The

agreement

is

generally

within

an

order

of

magnitude.

The

important column-integrated production rates

are

summarized

in

table

6.11.

These

rates

are

comparable

to

those

in the

Jovian atmosphere, despite

the

fact

that

the

solar irradiation

is

smaller

by a

factor

of 4. The

reason

is

partly

due to the

extended

nature

of the

atmosphere

and the

greater rate

of

photosensitized dissociation

of

CH

4

.

As

pointed

out

above,

the

removal

of H

atoms

is

crucial

for

ensuring

the

stability

of

unsaturated hydrocarbons,

which

would otherwise

be

cracked.

The

most important

reactions

for

producing

H

atoms

in the

model

are

R5b:

CH

4

+ hv

->•

CH

2

+2H,

R7b:

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

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