Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres
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Photochemistry
of
Planetary Atmospheres
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Photochemistry
of
Planetary Atmospheres
Yuk
L
Yung
William
B.
DeMore
New
York
Oxford
Oxford University
Press
1999
Oxford
University Press
Oxford
New
York
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Copyright
©
1999
by
Oxford University Press, Inc.
Published
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No
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Oxford University Press.
Library
of
Congress
Cataloging-in-Publication
Data
Yung,
Y. L.
(Yuk Ling)
Photochemistry
of
planetary atmospheres
/
Yuk
L.
Yung,
William
B.
DeMore.
p. cm.
Includes
bibliographical references
and
index.
ISBN
0-19-510501-X
1.
Planets—Atmospheres.
2.
Earth—Environmental
aspects.
3.
Atmosphere.
4.
Photochemistry.
5.
Atmospheric chemistry.
I.
DeMore, William
B.
H.
Title.
QB603.A85Y86
1998
551.51'1'09992—dc21
97-23575
987654321
Printed
in the
United
States
of
America
on
acid-free paper
Foreword
One of the
greatest triumphs
of
scientific
research
during
the
past several
decades
has
been
our
achievement
of
understanding complex macroscopic phenomena,
of
which
combustion
and
atmospheric chemistry
are
notable examples.
In the field of
atmospheric chemistry,
our
detailed knowledge
of
elementary chemical reactions
and
photochemical
processes
has
given
us an
understanding
of
phenomena such
as the de-
pletion
of the
ozone layer
and
creation
of the
ozone hole. While there
are
still
areas—
for
example, polyatomic
radicals—where
our
knowledge
is
incomplete,
the use of
powerful
computers
for
model calculations allows
us to
make intelligent predictions.
Data
from
solar
system
probes,
meanwhile, provide
a
context
in
which Earth-based
observations
can be
interpreted.
The
breadth
of
material relevant
to the field
means,
however, that there
is a
dire need
for a
monograph
in
which this information
is
sorted
and
woven into
a
comprehensive volume.
To fill
this void,
two of the
leading experts
in
the
photochemistry
of
planetary atmospheres, Drs.
Yuk
Ling Yung
and
William
B.
DeMore, have written this well-researched treatise.
The
book consists
of a
com-
prehensive
survey
of the
principal chemical cycles that control
the
compositions
of
planetary
atmospheres, emphasizing
the
"big
picture"
at
planetary
scales.
It
gives
a
detailed account
of the
history
of the
various planetary atmospheres, ranging from
the
comparatively primitive atmospheres
of the
giant planets
to the
highly evolved
atmospheres
of
small bodies.
It
also contains
an
extensive tabulation
of
physical con-
stants, astronomical
and
atmospheric data,
and
various parameters associated
with
the
eleven
planetary atmospheres covered
in the
book, including material with much sig-
nificance
for
evaluating
the
ecological
impact
of
life
on
Earth. With
its
self-contained
and
lucid
writing,
this book
is
useful
as a
textbook;
it
also serves well
as a
reference
of
source material
for
experts,
containing
the
basic information needed
to
understand
vi
Photochemistry
of
Planetary
Atmospheres
the
complex chemistry
of
planetary atmospheres.
The
triumph
of
understanding that
has
allowed
us to
unravel
the
evolution
of
planetary
atmospheres
does
ultimately
beg
the
question
the
authors take pains
to
avoid: whether
Lovelock's
controversial Gaia
hypothesis, that
the
Earth
is a
living organism playing
an
active role
in
maintaining
its
own
equilibrium,
can be
proven
or
refuted.
In
spite
of the
progress
made
by
this
book, this
will
likely remain
a
mystery. Fortunately,
in
Einstein's words, mystery,
complemented
by
beauty,
is a
quintessential element
of
science.
Dr.
Yuan
T. Lee
Academia Sinica
Taipei, Taiwan
August, 1997
Preface
This
book
is
based
on a
course that
one of the
authors (Y.L.Y.)
has
taught
at the
California
Institute
of
Technology since 1977.
It is
intended
as an
introduction
to
photochemistry
in the
atmospheres
of the
planets (including Earth)
for
seniors
and
first
year graduate students
in
atmospheric
science,
chemistry, physics, astronomy,
environmental
science,
or
engineering.
Most
of the
material
may be
traced back
to the
inspiring
course
taught
by
Prof.
M. B.
McElroy
at
Harvard
and
taken
by
Y.L.Y.
in
1972.
At
that time
the
scientific
community
was first
arriving
at a
fundamental understanding
of the
chemical pro-
cesses
in the
atmospheres
of the
planets (including
Eanh)
and
their implications
for
secular change
and
atmospheric evolution.
It was a
time
of
unparalleled excitement
and
discovery.
The
theoretical foundations
of
atmospheric chemistry were laid earlier
by the pi-
oneers
James
Jeans, Sidney Chapman, Harold Urey, David Bates,
and
Marcel Nico-
let.
The
dawn
of the
space age, heralded
by the
launching
of
Sputnik, provided
the
much-needed data
to
test these ideas,
and to
refine
and
modify
some
of
them.
For
the
first
time,
we
achieved
a first-order
understanding
of
Earth's
upper atmosphere,
space
environment,
and
interactions
with
solar radiation
and
solar wind,
and how
these
processes
operate
on the
other planets
in the
solar system.
In
retrospect,
it is
hard
to
recapture
the
vibrant
"atmosphere"
of the
period,
so
full
of new
possibil-
ities, grand syntheses, previously unsuspected connections,
and
sheer speculations.
The
stability
and
evolution
of the
thin
CC>2
atmosphere
of
Mars were explained
by
McElroy, Hunten,
and
Donahue. Great progress
in
understanding
the
chemistry
of
the
atmosphere
of
Venus
was
made
by
Lewis, Prinn,
and
McElroy
and his
students,
while
Strobel
did
pioneering work
on the
chemistry
of
hydrocarbons
in the
giant
viii
Photochemistry
of
Planetary Atmospheres
planets.
Meanwhile,
fundamental
discoveries were made
in
Earth's atmosphere.
Ox-
ides
of
nitrogen
(a
potential
pollutant
from
proposed supersonic
aircraft
flying in
the
stratosphere) were shown
by
Crutzen
and
Johnston
to be
capable
of
destroying
the
ozone layer. Molina
and
Rowland demonstrated
a
similar
adverse
environmental
impact
due to the
release
of
chlorofluorocarbons.
McElroy
proposed
that human-
induced
acceleration
of the
nitrogen cycle
may
increase
the
production
of
N2O,
with
negative impact
on
stratospheric ozone. Levy
first
showed that photochemistry
in
the
terrestrial troposphere could
be
initiated
by
photolysis
of
63
at
long wave-
length
in the
Huggins bands, providing
an
explanation
for the
oxidizing power
of the
troposphere.
It
is
hardly possible that
all
these exciting discoveries occurred
in
isolation. Indeed,
the
chlorine chemistry
on
Earth
was
anticipated
by
that
on
Venus,
and the
sulfur
work
on
Venus
by
Prinn
led
Crutzen
to
propose
a
similar theory
for the
origin
of the
Junge
layer
of
sulfate aerosols
in the
terrestrial stratosphere.
The
sudden wealth
of
data
on
the
atmospheres
of
Earth
and
other planets provided
a
synergy that allowed
a
more
fundamental
understanding
of all
atmospheres
in our
solar system.
For
example, this
kind
of
"cosmic
vision" created
by
viewing
the
Earth
in the
reference frame
of the
solar system inspired
at
least
one
grand synthesis, Lovelock's Gaia hypothesis.
Noteworthy planetary missions
to the
inner planets
in the
1970s
included
the
Mariners
and
Vikings
to
Mars,
and the
Mariners, Pioneer Venus,
and
Soviet Ven-
era
series
to
Venus.
In the
outer solar system,
the
early Pioneers
10 and
11
to
Jupiter
and
Saturn were followed
by the
spectacular Voyagers
1 and 2 to
all
the
giant plan-
ets and
most
of
their moons.
In the
1980s, comet Halley
was
studied
by
spacecraft
launched
by the
European
Space
Agency (Giotto),
the
Soviet Union (VEGA
2), and
Japan (Sakigate
and
Suisei).
In
addition, great contributions were made throughout
the
last
few
decades
by
ground-based
telescopes,
balloon-
and
aircraft-borne instru-
ments,
and
Earth-orbiting
satellites.
Together,
these
activities
resulted
in an
explosion
of
knowledge about Earth
and its
neighbors,
and
provided
a
proper perspective
for
viewing
Earth
as
part
of the
solar system
and the
cosmos
at
large.
Three
important ideas guided
the
development
of the field of
atmospheric chemistry.
The first is the
idea
of the
stability
of a
planetary atmosphere against thermal
escape
of
its
constituents
to
space,
as
formulated
at the
beginning
of the
century
by
Jeans.
The
second
is the
role
of
equilibrium chemistry
in
determining
the
partitioning
of
chemical
species,
as
developed
by
Urey. Third
is the
role
of
photochemistry, which produces
drastic departures from equilibrium chemistry;
a
classic example
of
this
is
Chapman's
theory
of the
UV-driven formation
of the
Earth's ozone layer.
It is now
understood that
escape
of
lighter molecules
is
important
for the
terrestrial planets
and for
satellites,
but
it is
unimportant
for the
helium-
and
hydrogen-rich giant planets. Equilibrium
chemistry dominates
in the
deep
atmospheres
of the
giant planets, near
the
surface
of
Venus,
and in the
interior
of
terrestrial planets,
and may
also
have been important
in the
solar nebula. Photochemical
processes,
by
contrast, govern atmospheric composition
and
evolution
in the
middle atmosphere
of all
planets, producing phenomena such
as
the
Earth's ozone layer
or the
hydrocarbon haze layers
in the
atmospheres
of the
outer
solar system.
To
these three ideas
we
must
add a
fourth,
the
role
of
biochemistry
at
Earth's sur-
face.
While Earth
is a
part
of the
solar system
and the
cosmochemical
environment,
Preface
ix
its
atmospheric chemistry appears radically
different.
Only
in
Earth's atmosphere
do
strong
reducing
and
oxidizing
species
coexist
to
such
a
degree: nitrogen species
in
Earth's
atmosphere,
for
example, span eight oxidation
states
from ammonia
to
nitric
acid. Consequently, much
of the
Earth's atmospheric chemistry consists
of
reactions
to
relax
the
disequilibrium caused
by
such biologically produced molecules. Consid-
eration
of the
effect
of
life
on the
atmosphere brings
us
inevitably
to the
question
posed
by
Lovelock
in his
Gaia hypothesis,
of
whether
life
also regulates atmospheric
composition
in a
more proactive biologically
beneficial
way.
The
idea
is
controver-
sial,
and it is not the
purpose
of
this book
to
argue
for or
against
it.
However, there
is
no
doubt that
the
biosphere plays
a key
role
in
determining Earth's atmospheric
composition.
Life
provides
a
means
for
using solar energy
to
drive chemical reactions
that
would otherwise
not
occur;
it
represents
a
kind
of
photochemistry that
is, at
least
within
our
solar system, unique
to
Earth.
We
briefly
explain
the
unusual structure
of the
book.
It is
common
in the
study
of
planets
to
study Earth
first, and
then
the
other planets.
In
this
way we may
better
understand
why the
rest
of the
solar system
is
different
from
us. In
this book
the
order
of
study
is
reversed.
We first try to
understand
the
solar system,
and
then
ask why
Earth
is
unique. With this viewpoint,
we can
begin
to
address
the
issue
of the
place
of
life
in our
(and other) solar system(s).
Are
there planets around other stars with
atmospheres similar
to
those
of our
solar system? What
are the
conditions that
can
give
rise
to
life
and
sustain
its
development? What human perturbations might change
our
own
planet, making
it
less habitable,
or
even unhabitable?
The
diversity
of
planetary
atmospheres
in our
solar system provides
a
natural testing ground
for our
ideas,
and
the
evolutionary history
of
Earth provides clues about
our
future.
It is
clear that
our
planet
and its
environment represent
a
dynamic balance
of
physical
and
chemical
driving
forces. Earth
has
evolved from
initial
conditions that
are
profoundly
different
from
those
today,
and it
will undoubtedly continue
to
evolve (with
or
without human
intervention)
into physical
and
chemical conditions that
may be
drastically
different
from
those
at
present.
Yet
life
has
been sustained
and has
continued
to
thrive
for at
least 3.85 billion years.
Is
there another place
in the
universe where these conditions
are
duplicated, even partially?
It is a
measure
of the
vitality
of a
civilization
to ask
such profound questions,
so
that posterity
may be
inspired
to
search
for the
answers.
Is
there
a
more
beautiful
vision
or a
greater technological challenge than
this
to
pass
on to the
next millennium?
Y.L.Y.
is
extremely grateful
to
Academia Sinica
in
Taiwan, Republic
of
China, where
much
of the
writing
was
done.
He
thanks former President
Ta-You
Wu of the
Academy
for
his
kindness
and
hospitality,
and
President
Yuan
T. Lee of the
Academy
for his
interest
in
this book
and for
writing
the
Foreword.
He is
especially
grateful
to his
colleagues, Typhoon Lee, Francis
Wu, and
Leon Teng,
for
arranging
the
sabbatical
visit
and to
former Director Yeong Tein
Yeh of the
Institute
of
Earth Sciences
for
hosting
the
visit.
We
thank
the
following
colleagues
for
valuable help, suggestions,
and
comments:
Mark Allen,
Ariel
Anbar, Geoffrey Blake,
Josh
Cheng,
James
Cho,
Lucien Froidevaux,
Mimi
Gerstell, Randy Gladstone, Andrew Ingersoll, Robert Herman,
Heinrich
Holland,
Kenneth
Hsu, Yibo Jiang, David
Kass,
Zhiming
Kuang,
Anthony Lee, Ming-Taun Leu,