Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres
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Photochemistry
of
Planetary
Atmospheres
Lingun
Liu, Fred
Lo,
Franklin Mills, Julie Moses, Elisabeth Moyer, Duane Muhleman,
Bruce Murray,
Hari
Nair, Mitchio Okumura,
Joseph
Pinto, Charlie
Qi,
Kelly Redeker-
Dewulf,
Mark Roulston, Ross Salawitch, Stanley Sander,
Charles
Shia,
Darrell
Strobel,
Michael
Summers,
Wing
Chi
Yung
and Hui
Zhang.
We
would like particularly
to
thank
the
staff
of
Oxford University
Press
for
their excellent editing.
Pasadena,
California
Y.L.Y.
June
1997 W.B.D.
Contents
Foreword
v
1 Introduction
3
1.1
The
Nature
of the
Problem
3
1.2
Physical State
of
Planetary Atmospheres
5
1.3
Chemical Composition
of
Planetary Atmospheres
12
1.4
Stability
of
Planetary Atmospheres against
Escape
13
2 Solar Flux
and
Molecular
Absorption
17
2.1
Introduction
17
2.2
Solar Radiation
and
Other Energy Sources
18
2.3
Absorption Cross Sections
of
Atmospheric
Gases
28
2.4
Interaction Between Solar Radiation
and the
Atmosphere
44
2.5
Sunlight
and
Life
53
3 Chemical
Kinetics
55
3.1
Introduction
55
3.2
Thermochemical
Reactions
56
3.3
Unimolecular Reactions
61
3.4
Bimolecular
Reactions
61
3.5
Termolecular
Reactions
70
xii
Photochemistry
of
Planetary
Atmospheres
3.6
Heterogeneous Reactions
72
3.7
Miscellaneous Reactions
73
4
Origins
77
4.1
Introduction
77
4.2
Cosmic Organization
79
4.3 The
Elements
of the
Periodic Table
81
4.4
Molecular Clouds
84
4.5 The
Solar Nebula
91
4.6
Meteorites
94
4.7
Comets
99
4.8
Formation
of
Planets
102
4.9
Atmospheres
of
Terrestrial Planets
and
Satellites
103
4.10
Miscellaneous Topics
107
5
Jovian
Planets
120
5.1
Introduction
120
5.2
Thermosphere
139
5.3
Hydrocarbon Chemistry
144
5.4
Nitrogen Chemistry
178
5.5
Phosphorus Chemistry
182
5.6
Oxygen Chemistry
183
5.7
Miscellaneous
Topics
186
6
Satellites
and
Pluto
189
6.1
Introduction
189
6.2
lo
191
6.3
Titan
201
6.4
Triton
234
6.5
Pluto
239
6.6
Unsolved
Problems
242
7
Mars
244
7.1
Introduction
244
7.2
Photochemistry
247
7.3
Model Results
256
7.4
Evolution
273
7.5
Terraforming
Mars
279
7.6
Unsolved Problems
280
8
Venus
282
8.1
Introduction
282
8.2
Photochemistry
287
8.3
Model Results
294
8.4
Evolution
311
Contents
xiii
8.5
Mystery
of the
Missing Water
313
8.6
Unsolved Problems
315
9
Earth: Imprint
of
Life
316
9.1
Introduction
316
9.2
Gaia
Hypothesis
319
9.3
Hydrological Cycle
327
9.4
Carbon Cycle
330
9.5
Rise
of
Oxygen
344
9.6
Nitrogen Cycle
349
9.7
Minor Elements
351
9.8
Unsolved Problems
356
10
Earth: Human Impact
357
10.1 Introduction
357
10.2
Global
Warming
363
10.3 Tropospheric Chemistry
381
10.4
Stratospheric
Chemistry
389
10.5 Unsolved Problems
411
Notes
and
Bibliography
425
Author
and
Proper
Name
Index
447
Subject
Index
450
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Photochemistry
of
Planetary Atmospheres
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I
Introduction
I.
I The
Nature
of the
Problem
It is
usual
in the
study
of
planets
to
consider
the
Earth
first, and
then
the
other
planets,
so
that
we can
better understand
how and why the
rest
of the
solar system
is
different
from
us. In
this book
the
order
of
study will
be
reversed:
we
shall
first
try
to
understand
the
solar system,
and
then
we
will
ask why
Earth
is
unique.
We
adopt this unconventional approach
for two
reasons.
First,
Earth's atmosphere today
is
the
end-point
of an
evolution that started about
4.6
billion years
ago.
The
pristine
materials have
all
been drastically altered. However,
by
examining other parts
of the
solar system that have evolved
to a
lesser
degree,
we may
deduce what
the
early
Earth might have been like. Second, Earth's atmosphere today
is
largely determined
by
the
complex biosphere, whose evolution
has
been intimately coupled
to
that
of
the
atmosphere.
In
other words, ours
is the
only atmosphere
in the
solar system that
supports
life,
and it is in
turn modified
by
life.
Therefore,
to
appreciate
the
beauty
and the
intricacy
of our
planet,
we
must start
with
simpler objects without
life.
Chemical
composition
is
intimately connected
to
evolution, which
in
turn
is
driven
by
chemical change.
In
this book
we
attempt
to
provide
a
coherent
basis
for un-
derstanding
the
planetary atmospheres,
to
identify
the
principal chemical cycles that
control
their present composition
and
past history. Figure
1.1
gives
an
illustration
of
the
intellectual framework
in
which
our field of
study
is
embedded.
The
unifying
theme
that
connects
the
planets
in the
solar system
is
"origin";
that
is, all
planets
share
a
common origin about
4.6
billion years
ago.
The
subsequent divergence
in the
solar
system
may be
partly attributed
to
evolution, driven
primarily
by
solar radiation.
3
4
Photochemistry
of
Planetary Atmospheres
Figure
I.I
Outline
of the
important concepts
and
processes addressed
in
this book.
The
bulk
of
solar radiation consists
of
photons
in the
visible spectrum
with
a
mean
blackbody radiation temperature
of
5800
K. The
part that
is
responsible
for
direct
at-
mospheric chemistry
is a
tiny
portion (less than
1%
of the
total
flux) in the
ultraviolet.
In
addition,
the sun
emits
a
steady stream
of
corpuscular particles, known
as the
solar
wind.
While
the sun
provides
the
principal source
of
energy
for
change,
the
time rate
of
change
is
crucial,
and
that
is
where chemical kinetics
and
chemical cycles play
pivotal roles.
The
fact
that
a
certain reaction proceeds
in a
microsecond rather than
in
a
billion years
may
have
the
most profound consequences
for the
observed com-
position
of
planetary atmospheres
and
their evolution.
The box in
figure
1.1
labeled
"origin"
represents
the
source
of
material
to the
atmospheres. Solar radiation, both
electromagnetic
and
corpuscular,
and
chemical kinetics determine
the
rates
of
change
of
the
material
in the
atmospheres. Most chemical reactions
in the
atmosphere
do not
result
in a net
conversion
of one
species into another,
but
rather
a
recycling among
different
compounds,
in
specific
"chemical
cycles."
Some processes
do
result
in a
net
change
of the
composition
of the
atmosphere, labeled
"Evolution"
in
figure
1.1.
This
can
happen
in two
ways: loss
of
atmospheric
species
by
escape
into space,
or
sequestering
in the
surface reservoir.
In the
latter case,
the
material
is not
perma-
nently
lost
from
the
planet
and
eventually
may be
recycled
via
tectonic
or
volcanic
processes. Material
can
also
be
gained
or
lost
from
space
by
impact
of
comets
or
meteorites.
The
purpose
of
science
is not
just
to
explain
the
present
or
reconstruct
the
past.
There
is a
more
utilitarian
motive,
to
contemplate
and
assess
the
future.
Therefore,
we
are
deeply interested
in
studying
the
response
of the
terrestrial atmosphere
to
perturbations, that
is, the
stability (both chemical
and
climatic)
of the
atmosphere.
The
exercise
is
more
than
academic, because
we now
live
on a
planet whose composition
Introduction
5
we are
seriously perturbing
in a
potentially irreversible manner.
How
robust
is our
planetary
environment? What
are the
consequences
of
overriding
one or
more
of
nature's chemical
cycles?
These
questions
are
indicated
by the two
circles
in
figure
1.1.
The
answer
to our
questions
will
have immediate impact
on our
welfare
and
future
lifestyle.
On a
more philosophical level
we ask
what
is the
proper place
for
humans
(or
whether there
is a
place)
so
that
we are in
harmony with
the
chemical environment
of
our
solar system
and the
cosmos.
It
would
be
unrealistic
to
expect complete answers
to
these questions
in the
scope
of
this
book,
but we
state these questions
as our
motivating
forces.
A
large number
of
special topics
is
included. Many concepts, some
of
which appear
unrelated,
are
synthesized.
To
help
the
reader
get an
overview
of
what this book
is
about,
we
briefly
describe
the
content
of the
various chapters.
Chapter
1
gives
a
preliminary survey
of the
most fundamental physical
and
chem-
ical
data
of
planetary atmospheres,
the
general patterns
of
speciation,
and
reasons
for
stability
and
instability. Chapter
2
discusses
the
primary driving force
for
atmospheric
chemistry
and
change—photochemistry
that
is
driven
by
solar ultraviolet radiation.
Other important drivers
of
disequilibrium chemistry include solar wind, lightning,
and
shock
waves during impacts. Chapter
3
describes chemical kinetics pertinent
to
atmo-
spheric modeling. Chapter
4
addresses
the
question
of
origins
for the
solar system;
we
discuss primitive
bodies
such
as
meteorites, asteroids
and
comets. Having prepared
the
necessary background, chapter
5
starts with
a
study
of the
Hi-dominated
atmo-
spheres
of the
giant planets, Jupiter, Saturn, Uranus,
and
Neptune.
The
giant planets
have satellites that resemble terrestrial planets
in the
inner solar system. Chapter
6 is
devoted
to
three such
satellites,
Titan, Triton,
and
lo.
(Pluto
is
also included
in
this
chapter.) Chapter
7
deals with
the
simplest atmosphere
in the
inner solar system,
the
Martian atmosphere. Chapter
8
discusses
the
atmosphere
of
Venus.
The
last
two
chap-
ters
are
devoted
to the
jewel
of the
solar
system,
the
terrestrial atmosphere. Chapter
9
discusses
the
role
of the
biosphere (excluding humans),
and the
human impact
on
global chemistry
is
discussed
in
chapter
10.
1.2
Physical State
of
Planetary
Atmospheres
All
the
planets except Mercury have substantial atmospheres.
The
giant planets have
very
extensive atmospheres that
are
major portions
of the
planets themselves.
The
giant
planets also
all
have
a
large number
of
satellites, resembling miniature solar
systems.
Three
such satellites, Titan, Triton,
and
lo,
have atmospheres.
For
reasons
that
will
become clear later,
we
group Pluto with
the
satellites.
We are
less
interested
in
the
tenuous atmospheres
of
Mercury
and the
Moon.
[The
treatment here follows
that
of
Chamberlain
and
Hunten (1987), cited
in
section
1.1.]
Tables
1.1
and 1.2
summarize
the
physical properties
of the
planetary bodies
in the
solar system that
are of
interest
to
atmospheric studies. Figure
1.2
shows
the
pressure
(P)
versus
temperature
(T)
plots
for the
atmospheres
of the
giant planets.
In
this book,
we
restrict
our
interest
to
pressures
less
than
1
kbar.
At
this
and
higher pressures,
the
atmospheric composition
is
completely controlled
by
equilibrium chemistry, which
will
not be a
primary subject
of
this book.
It is
convenient
to use
pressure rather than