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

106

Photochemistry

of

Planetary

Atmospheres

Figure

4.14

(a) The

evolution

of

surface temperature during accretion

for

standard

Earth

(solid

line)

and

higher water (dot-dashed line) models.

The

dashed

line traces

the

surface temperature

of the

model without

an

impact-generated atmosphere.

RQ

is

the

final

radius,

(b) The

total mass

of the

impact-generated

P^O

atmosphere

is

plotted

against

the

normalized radius.

After

Ahrens,

T.

J.,

O'Keefe,

J. D., and

Lange,

M.

A.,

1989,

"Formation

of

Atmospheres during Accretion

of the

Terrestrial

Planets,"

in

Atreya

et

al.

(1989;

cited

in

section 4.1),

pp.

328-385.

fueled

perhaps

by the

production

of

large amounts

of

H

2

in the

iron-water reac-

tion

The

presence

of

large amounts

of a

light

gas

like

H

2

in a

terrestrial planet

is

grav-

itationally

unstable

and

results

in a

hydrodynamic

flow. An

important consequence

of

this mechanism

of

escape

is

that heavier

species

will

be

"dragged

off"

with

a

characteristic mass fractionation described

as

follows. Consider

the

simple case

of an

atmosphere with

two

components

with

masses

mi

and

m

2

and

escape

fluxes

FI

and

F

2

,

respectively.

Let

Xi

and

X

2

be the

mole fractions

of the

species

such that

Xj

+X

2

= 1. The fluxes

FI

and

F

2

are

where

a is a

parameter that

is

independent

of m, X, and F,

mi

is the

lighter mass

(of

H

2

),

and

FI

is the flux of

H

2

.

Expression (4.52) suggests that

to first

order

the

flux

of

the

heavier

gas is

proportional

to

that

of the

lighter

gas

plus

a

mass-dependent

correction term. This provides

a

beautifully

simple explanation

of the

fractionation

pattern

of the

isotopes

of

heavier noble gases such

as Xe in

planetary atmospheres

relative

to

meteorites,

as

shown

in figure

4.15. Note

the

approximately linear depen-

dence

of the Xe

isotopes

on the

mass

of the

species.

A

similar fractionation pattern

is

also observed

for

Kr.

The

hydrodynamic

escape

may be

further facilitated

by the

enhanced energy output

from

a

more active phase (T-Tauri)

of the

protosun.

Origins

107

Figure

4.15

Isotopic

composition

of

xenon

in

meteorites

and in

planetary atmospheres, plotted

relative

to

terrestrial atmospheric

xenon. Units

are

permil

deviation

from

the

terrestrial composition.

After

Pepin,

R. O.,

1989,

"Atmospheric

Composition:

Key

Similarities

and

Differences,"

in

Atreya

et

al.

(1989;

cited

in

section

4.1),

pp.

291-305.

4.10

Miscellaneous Topics

4.10.1

Deuterium

in the

Solar

System

As

discussed

in

section 4.3.1, deuterium

was

synthesized

in the Big

Bang during

the

first

minutes

of the

origin

of the

universe. Subsequent nuclear reactions

may

have

destroyed deuterium

by

converting

it to

heavier elements

but

would

not be a

significant

source

of

fresh

deuterium. This

fact,

in

addition

to

deuterium's chemical reactivity

and

larger mass relative

to

hydrogen, makes deuterium

one of the

most

useful

tracers

for

studying

the

origin

and

evolution

of

solar system objects.

The

overall picture

of

the D/H

ratio

in the

solar system

and the

interstellar medium

is

summarized

in

figure

4.16.

For the

bulk

of

hydrogen

in the

universe,

the

principal reservoir

is H or

H2,

and the D/H

ratio

is

about

10~

5

.

The

interstellar medium (bulk),

the

protosun,

and

the

giant planets Jupiter

and

Saturn

all

share

the

same reservoir

of

hydrogen

and

this

D/H

value.

In

solar system objects smaller than

the

giant planets,

the

dominant hydrogen-

containing

species

is not H or H2 but the

condensible molecules such

as

CFLj,

H2O,

and

NHs.

Earth's reservoir

of

hydrogen

is

ocean water.

Its D/H

ratio

is

known

as

SMOW (standard mean ocean water)

and has the

value

1.5576

±

0.0005

x

10~

4

.

The

small bodies

in the

solar system, meteorites, Comet Halley,

and

Titan, have

D/H

ratios close

to

SMOW.

The

meteorites also contain organic compounds that have much

higher

D/H

ratio (labeled meteorite organics

in figure

4.16).

These

organic compounds

are

believed

to

have originated

from

the

interstellar medium, where organic synthesis

via

ion-molecule reactions

has a

distinct

D/H

signature

(see

section

4.4.2).

It is

clear

that

the

hydrogen

in the

small bodies

is not

derived directly

from

the

bulk

of the

hydrogen

reservoir,

but is

formed

in

ices

at low

temperature

in the

outer part

of the

108

Photochemistry

of

Planetary

Atmospheres

Figure

4.16

D/H

ratios

in the

solar system

and

interstellar molecular clouds.

The

uncertainties

in

the

measurements

are

enclosed

by

vertical bars. SMOW

=

standard mean ocean water,

ISM

=

interstellar medium.

After

Yung,

Y. L., and

Dissly,

R. W.,

1992, "Deuterium

in the

Solar System,"

in

/sotope

Effects

in

Gas-P/ww

Chemistry,

J. A.

Kaye, editor (Washington,

D.C.: American Chemical Society),

pp.

369-389.

solar nebula.

The

question arises

as to

what determines

the D/H

ratio

in the

ices

at

the

time

of

formation.

Since equilibrium chemistry

is

believed

to

dominate

in the

solar nebula, there

is

an

obvious possibility

of

exchange between

the

condensible species

(CELj,

H2O,

and

bulk

hydrogen,

as in the

following reactions:

At

low

temperatures

(T < 500 K) the

equilibrium favors

the

right side

of

reactions

(4.53)-(4.55)

and

deuterium tends

to

become more concentrated

in

CHsD,

HDO,

and

NH

2

D

with

respect

to HD. As

shown

in figure

4.17,

the

fractionation

is as

much

as a

factor

of 10 and

becomes even larger

at

lower temperatures.

Thus,

if all

ices

in the so-

lar

nebula formed

at 200 K or

colder,

exchange reactions

(4.53)-(4.55)

would explain

the

observed large enrichment

in

deuterium.

But

this beautifully simple mechanism

has an

intrinsic self-limitation. Reactions

(4.53)-(4.55)

all

have high activation ener-

gies.

To

achieve

high

enrichment factors

the

temperature must

be

sufficiently

low,

but

at

low

temperatures

the

kinetic rates

of the

reactions

are too

slow.

The

time constants

for

the

reactions

to

reach equilibrium

are

excessively long compared

with

the

duration

of

the

solar nebula.

The

efficiency

of the

reactions

may be

enhanced

by

catalysis

on

the

surface

of

grains,

but

there

may not be

enough metallic grains

in the

solar nebula

Origins

109

Figure

4.17

Deuterium abundances galactic

reservoirs,

the

pro-

tosolar nebula, Earth,

and

meteorites.

The

curves give

the D/H

ratios

expected

from

isotopic

exchange between neutral

species

and

ion-molecule reactions

as a

function

of

temperature.

IDP

=

Interplanetary dust

particles;

UOC =

Unequilibrated ordinary

chondrites;

C.C.

=

Carbonaceous

chondrites.

After Zinner,

E.,

1988,

"Interstellar

Cloud Material

in

Meteorites,"

in

Kerridge

and

Matthews

(1988;

cited

in

section 4.1),

pp.

956-983.

for

this purpose (Grinspoon

and

Lewis, 1987).

The

failure

of

this theory

to

account

for

deuterium

fractionation

in the

solar system serves

to

enhance

our

appreciation

of

the

beauty

of the

ion-exchange mechanism that successfully accounts

for the

huge

deuterium fractionation

in the

molecular clouds. Exchange reactions

(4.21)

and

(4.22)

are

similar

to

(4.53)-(4.55),

except that, being ion-molecule reactions,

the

former have

no

activation energy

and can

proceed rapidly

at low

temperatures where

the

fraction-

ation

is

greatest. Were there ion-molecule reactions

or

other disequilibrium reactions

in

the

solar nebula?

In

view

of the

characteristic speciation

in

comets discussed

in

section 4.7,

we

believe that this

is

possible. However,

no

quantitative modeling

has

been performed.

If

the

hydrogen-bearing ices

in the

nebula

are

enriched

in

deuterium

by

some

mechanism

that remains

to be

identified,

we can

understand

the

enhanced values

of

D/H in all the

small bodies

and

Earth, whose volatiles

are

derived

from

planetesimals.

The

cores

of

giant planets presumably contain ices

of

similar origin,

but due to the

enormous

amounts

of

H

2

in

these planets,

the

enrichment

in the

ices

is

greatly diluted.

The

lesser giant planets, Uranus

and

Neptune, have relatively larger cores

and

smaller

gaseous envelopes compared

with

Jupiter

and

Saturn;

the

dilution

effect

is

less.

As

I

1

0

Photochemistry

of

Planetary Atmospheres

a

consequence,

the D/H

ratios

of

Uranus

and

Neptune

are

intermediate between

the

interstellar

medium value

and the

SMOW value (see

figure

4.16).

Now to

explain

the

enrichment factors

in

deuterium

for

Mars

and

Venus. Relative

to

SMOW,

D/H

ratios

on

Mars

and

Venus

are

enriched

by

factors

of 5 and

150,

respectively.

This

is

most

likely

due to

atmospheric evolution. Hydrogen

is

known

to

escape

from

the

terrestrial planets today.

The

efficiency

of

escape

for

deuterium

is

less

than

that

for

hydrogen. Hence, over

the age of the

solar system

D/H

values tend

to

increase

with

time. This enrichment

is

greatly diluted

on

Earth

by the

large reservoir

of

hydrogen

in the

terrestrial ocean.

But

Mars

and

Venus

do not

have oceans,

and the

enrichment

is

preserved

in the

atmospheric water vapor.

These

topics

are

pursued

in

greater detail

in

chapters

7 and 8.

A

summary

of our

current understanding

of the

origin

and

fractionation

of

deu-

terium

is

presented

in figure

4.18.

The

vertical arrow indicates increasing values

of

the

D/H

ratio,

with

the

cosmic abundance

of D to H (in HD in

molecular clouds)

as

the

lower limit,

and

chemically

processed

material

in

molecular clouds

as the

upper

limit.

Thus,

the

precursor

of the

solar nebula,

the

molecular cloud,

is the

source

of

both unfractionated

and

highly fractionated deuterated material.

The

horizontal arrow

indicates

the

direction

of

time.

The D/H

values

in the

present solar system

are the

end-product

of

cumulative chemical, physical,

and

evolutionary

processes

in the

solar

nebula

and

planetary atmospheres.

4.10.2

Oxygen

Isotopes

in the

Solar

System

Oxygen

has

three isotopes,

16

O,

I7

O,

and

18

O,

with abundances roughly

in the

propor-

tions

1 : 4 x

10~

4

: 2 x

10~

3

.

The

standard

isotopic

composition

of

oxygen

is

that

of

SMOW.

The

variations

in the

isotopic composition

in

solar system material

are

small,

and

it is

convenient

to

define

a

fractional variation relative

to

SMOW:

where

O =

16

O

and Q =

17

O

or

18

O.

It is

often

useful

to

plot

the

variations

of

<5

17

O

with

that

of

<5

18

O

and

study

the

anomalies. Figure 4.19 summarizes

the

<5

17

O

and

<S

18

O

data

for

solar

system

objects.

Note

that

the

bulk

of

isotopic

variations

falls

on

the

terrestrial line.

The

slope

of

this line

is

close

to

0.5, consistent with equilibrium

fractionation.

Most material

in the

solar system,

for

example, meteorites

and

lunar

and

SNC

meteorites (from Mars),

also

fall

on a

line with this

slope.

However,

the

oxy-

gen

isotopes

in the

calcium-aluminum inclusions

(CAIs)

in the

Allende carbonaceous

chondrite

are

fundamentally

different.

The

Allende mixing

line

has a

slope

of 1

instead

of

0.5. Discovered

in

1971

by

Clayton

and

colleagues, there

is no

generally accepted

explanation

for

this anomaly.

We

should point

out

that only oxygen appears

to be

unusual

in

this

respect.

A

similar

study

for the

three

isotopes

of

silicon,

28

Si,

29

Si,

and

30

Si,

in

Allende chondrules reveals

the

expected

0.5

slope

between

the

variations

of

29

Si

and

30

Si

relative

to

28

Si.

There

is no

anomaly

for

silicon.

There

are at

least three hypotheses proposed

to

explain

the

oxygen anomaly.

One

proposal

is

that

the

solar nebula

was

subject

to an

injection

of

pure

I6

O

from

a

nearby

supernova explosion.

The net

addition

of

16

O

would obviously cause

the

<S

17

O

and

5

18

O

values

to

decrease

in a

mass-independent

way

consistent with

the

Allende observations.

Figure

4.18

Simplified schematic showing

the

origin

of

deuterium

and

important frac-

tionation

processes.

The D/H

values

of the

rectangular boxes near

the top are

higher than

those below.

Time

evolution

is

from

left

to right.

After

Yung,

Y. L., and

Dissly,

R. W.,

1992,

"Deuterium

in the

Solar

System,"

in

Isotope

Effects

in

Gas-Phase Chemistry,

J.

A.

Kaye,

editor (Washington, D.C.: American Chemical Society),

pp.

369-389.

Ill

112

Photochemistry

of

Planetary

Atmospheres

Figure

4.19

Oxygen isotopic compositions

of

carbonaceous chon-

drites.

Terrestrial fractionation

(TF)

and

refractory inclusion

(CAI)

lines

are

included

for

reference.

After

Clayton,

R. N.,

1993, "Oxy-

gen

Isotopes

in

Meteorites."

Ann. Rev. Earth Planet. Sci.

21,

112-

149.

Note that

in

this case

the

ultimate source

of the

oxygen anomaly

is

extra-solar system.

Another

class

of

explanation

is

based

on the

assumption that

the

origin

of the

anomaly

is

within

the

solar system.

The

mechanisms

fall

into

two

categories:

nuclear

and

chemical.

An

explanation

of the

first

type invokes

a

large

flux of

energetic particles

from

the

protosun,

resulting

in the

preferential production

of

16

O

and the

destruction

of

I7

O

and

18

O.

Quantitative modeling shows that

an

enormous proton

flux and a

special distribution

of

matter

in the

solar nebula

are

needed.

The

chemical explanation

is

based

on

laboratory experiments

on the

mass-independent isotopic fractionations

between

Oz

and

Os

when

the

former

is

subject

to

dissociation

in an

electric discharge

experiment.

However,

the

laboratory results have

not

been used

to

model

the

observed

effects

in the

meteorites.

We

conclude

by

pointing

out

that there

is a

mass-independent

isotopic anomaly

in

atmospheric ozone

in the

terrestrial stratosphere.

Appendix

4.1

Cosmic abundances

of the

elements

z

1

2

Element

Hydrogen

Helium

A

H

1

2

He

3

4

Atoms

(%)

~100

0.002

0.0142

~100

Process

3

U

U,

H

U,

H

Abundance

6

at./10

6

Si

2.72

x

10'°

5.40

x

10

s

3.10

x

10

s

2.18

x

10

9

(continued)

Appendix

4.1

(continued)

z

3

4

5

6

7

g

9

10

11

12

13

14

15

16

17

IS

19

20

21

22

Element

Lithium

Beryllium

Boron

Carbon

Nitrogen

Oxygen

Fluorine

Neon

Sodium

Magnesium

Aluminum

Silicon

Phosphorus

Sulfur

Chlorine

Argon

Potassium

Calcium

Scandium

Titanium

Li

Be

B

C

N

O

F

Ne

Na

Mg

Al

Si

P

S

Cl

AT

K

Ca

Sc

Ti

A

6

7

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

36

35

37

36

38

40

39

40

41

40

42

43

44

46

48

45

46

47

48

49

50

Atoms

(%)

7.5

92.5

100

19.8

80.2

98.89

1.11

99.634

0.366

99.76

0.038

0.204

100

92.99

0.226

6.79

100

78.99

10.00

11.01

100

92.23

4.67

3.10

100

95.02

0.75

4.21

0.017

75.77

24.23

84.2

15.8

0.00061

93.258

0.01167

6.730

96.94

0.647

0.135

2.09

0.0035

0.187

100

8.2

7.4

73.7

5.4

5.2

Process"

X

X, H, U

X

He

H

He

H

He, N

N

Ex

He, N

Ex

Ex,

HeS

Ex, HeS

Ex,

HeS

Ex

Ex,

E

E, Ex

Abundance

6

at./10

6

Si

4.48

55.52

x

10'

7.80

x

10-'

4.80

1.92

x

10'

1.20

x

10

7

1.34

x

10

5

2.47

x

10

6

9.08

x

10

3

2.01

x

10

7

7.64

x

10

3

4.10

x

10

4

8.43

x

10

2

3.25

x

10

6

7.91

x

10

3

2.38

x

10

5

5.70

x

10

4

8.49

x

10

5

1.07

x

10

s

1.18

x

10

5

8.49

x

10

4

9.22

x

10

5

4.67

x

10

4

3.10

x

10

4

1.04

x

10

4

4.89

x

10

5

3.86

x

10

3

2.17

x

10

4

8.80

x

10'

3.97

x

10

3

1.27

x

10

3

8.76

x

10

4

1.64

x

10"

5.50

x

lO"

1

2.00

x

10-

2

3.516

x

10

3

4.40

x

10-'

5.48

2.537

x

10

2

5.92

x

10

4

3.95

x

10

2

8.25

x

10'

1.277

x

10

3

2.14

1.44

x

10

2

3.38

x

10'

1.97

x

10

2

1.78

x

10

2

1.769

x

10

3

1.3

x

10

2

1.25

x

10

2

(continued)

113

Appendix

4.1

(continued)

z

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

Element

Vanadium

Chromium

Manganese

Iron

Cobalt

Nickel

Copper

Zinc

Gallium

Germanium

Arsenic

Selenium

Bromine

Krypton

Rubidium

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ga

Ge

As

Se

Br

Kr

Rb

A

50

51

50

52

53

54

55

54

56

57

58

59

58

60

61

62

64

63

65

64

66

67

68

70

69

71

70

72

73

74

76

75

74

76

77

78

80

82

79

81

78

80

82

83

84

86

85

87

Atoms

(%)

0.25

99.75

4.35

83.79

9.50

2.36

100

5.8

91.8

2.15

0.29

100

68.3

26.1

1.13

3.59

0.91

69.1

30.8

48.6

27.9

4.10

18.8

0.62

60.1

39.9

20.5

27.4

7.8

36.5

7.8

100

0.87

9.0

7.6

23.5

49.8

9.2

50.69

49.31

0.339

2.22

11.45

11.47

57.11

17.42

72.17

27.83

Process

0

E

E, HeS

E,

HeS

E,

HeS

E,

HeS

E, HeS

E,

HeS

E, HeS

HeS,

?

P

HeS

HeS,

?

HeS,

?

HeS,

?

HeS,

?

HeS,

?

P

HeS,

P

HeS

HeS,

?

HeS,

?

R

S, R

R

Abundance

11

at./10

6

Si

7.4

x

10-'

2.94

x

10

2

5.83

x

10

2

1.12

x

10

4

1.28

x

10

3

3.16

x

10

2

9.51

x

10

3

5.22

x

10"

8.26

x

10

s

1.94

x

10

4

2.61

x

10

3

2.25

x

10

3

3.37

x

10

4

1.29

x

10

4

5.57

x

10

2

1.77

x

10

3

4.49

x

10

2

3.56

x

10

2

1.58

x

10

2

6.12

x

10

2

3.52

x

10

2

5.17

x

10'

2.34

x

10

2

7.81

2.27

x

10'

1.51

x

10

1

2.42

x

10'

3.23

x

10'

9.20

4.31

x

10

1

9.20

6.79

5.4

x

10-'

5.59

4.72

1.46

x

10'

3.09

x

10'

5.71

5.98

5.82

1.54

x

10-'

1.01

5.19

5.20

2.59

x

10

1

7.89

5.12

1.97

2.10

114

Appendix

4.1

(continued)

z

38

39

40

41

42

44

45

46

47

48

49

50

Element

Strontium

Yttrium

Zirconium

Niobium

Molybdenum

Ruthenium

Rhodium

Palladium

Silver

Cadmium

Indium

Tin

Sr

Y

Zr

Nb

Mo

Ru

Rh

Pd

Ag

Cd

In

Sn

A

84

86

87

88

89

90

91

92

94

96

93

92

94

95

96

97

98

100

96

98

99

100

101

102

104

103

102

104

105

106

108

110

107

109

106

108

110

111

112

113

114

116

113

115

112

114

115

116

117

118

Atoms

(%)

0.56

9.82

7.41

82.22

100

51.5

11.2

17.1

17.4

2.80

100

14.8

9.3

15.9

16.7

9.6

24.1

9.6

5.5

1.86

12.7

12.6

17.0

31.6

18.7

100

1.0

11.0

22.2

27.3

26.7

11.8

51.83

48.17

1.25

0.89

12.5

12.8

24.1

12.2

28.7

7.5

4.3

95.7

1.01

0.67

0.38

14.8

7.75

24.3

Process

2

P

S

S, R

R

S, R

P

S, R

S

S, R

R

P

S, R

S

S, R

R

S, R

P

S

S, R

R

S, R

P

S

S, R

R

P,

S, R

P

P,

S, R

S

S, R

Abundance

1

"

at./10

6

Si

1.32

x

10-'

2.34

1.76

1.63

1.957

x

10'

4.64

5.51

1.20

1.83

1.86

3.00

x

10-'

7.1

x

10-'

3.73

x

10-'

2.34

x

10-'

4.01

x

10-'

4.21

x

10-'

2.42

x

10-'

6.07

x

10-'

2.42

x

10-'

1.02

x

I0~

l

3.46

x

10~

2

2.36

x

10-'

2.34

x

10-'

3.16

x

10-'

5.88

x

10-'

3.48

x

10-'

3.44

x

10~'

1.39

x

10~

2

1.53

x

10-'

3.09

x

10-'

3.79

x

10-'

3.71

x

10-'

1.64

x

10-'

2.74

x

10-'

2.55

x

10-'

1.99

x

10-

2

1.42

x

10~

2

1.99

x 10-

2.04

x 10-

3.83

x 10-

1.94

x

10~

4.56

x 10-

1.19

x 10-

7.9

x

10-

3

1.76

x

10-'

3.86

x

10-

2

2.56

x

10~

2

1.45

x

10~

2

5.65

x

10-'

2.96

x

10-'

9.29

x

10-'

(continued)

115

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

Подождите немного. Документ загружается.