Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres
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76
Photochemistry
of
Planetary
Atmospheres
In
general, radiative association
of
neutral atoms
is too
slow
to
compete
with
ter-
molecular
reaction
and
recombination
on the
surface
of
aerosols
and
grains. However,
radiative
association between
an ion and a
neutral
is
expected
to be
faster
and the
reaction
is
expected
to
have
a
rate
coefficient
of
10~
15
to
10~
16
cm
3
s"
1
.
At
this rate
(3.90)
may be
important
in the
interstellar medium.
4
Origins
4.1
Introduction
Cosmology
is a
subject that
borders
on and
sometimes merges with philosophy
and
religion. Since antiquity,
the
deep
mysteries
of the
universe have intrigued mankind.
Who
are we?
Where
do we
come from? What
are we
made
of? Is the
development
of
advanced intelligence capable
of
comprehending
the
grand design
of the
cosmos,
the
ultimate
purpose
of the
universe?
Is
there
life
elsewhere
in the
universe?
Is
ours
the
only
advanced intelligence
or the
most advanced intelligence
in the
universe?
These
questions have motivated great thinkers
to
pursue what Einstein called
"the
highest
wisdom
and the
most radiant
beauty."
In the
fourth
century B.C.,
the
essence
of the
cosmological
question
was
formulated
by the
philosopher Chuang
Tzu:
If
there
was a
beginning,
then there
was a
time before that beginning.
And a
time before
the
time which
was
before
the
time
of
that beginning.
If
there
is
existence, there must
have been non-existence.
And if
there
was a
time when nothing existed, then there must
be a
time before
that—when
even nothing
did not
exist.
Suddenly,
when nothing came
into
existence, could
one
really
say
whether
it
belonged
to the
category
of
existence
or
of
nonexistence? Even
the
very words
I
have just uttered,
I
cannot
say
whether they have
really been uttered
or
not.
There
is
nothing under
the
canopy
of
heaven
greater
than
the
tip
of an
autumn hair.
A
vast mountain
is a
small thing. Neither
is
there
any age
greater
than
that
of a
child
cut off in
infancy.
P'eng
Tsu [a
Chinese Methuselah] himself died
young.
The
universe
and I
came into being together;
and I, and
everything therein,
are
one.
77
78
Photochemistry
of
Planetary
Atmospheres
Figure
4.1
Schematic
diagram
showing
the
principal pathways
of the
origin
and
evolution
of the
solar system.
After
Yung,
Y. L., and
Dissly,
R.
W.,
1992, "Deu-
terium
in the
Solar
System,"
in
Isotope
Effects
in
Gas-Phase Chemistry,
J. A.
Kaye,
editor (Washington, D.C.: American Chemical Society),
pp.
369-389.
Fortunately,
our
subject matter, solar system chemistry,
is
less
esoteric than
the
questions
asked
by
Chuang Tzu.
A
schematic diagram showing
the
principal pathways
by
which
our
solar
system
is
formed
is
given
in
figure 4.1.
The
great
triumphs
of
modern
science have been summarized
in
this
figure as
fundamental contributions
to
the
five
"origins":
(a)
origin
of the
universe,
(b)
origin
of the
elements,
(c)
origin
of
the
solar system,
(d)
origin
of
life,
and (e)
origin
of
advanced intelligence. Note that
almost
all the
concrete advances have been made during
the
last hundred
years.
We now
Origins
79
stand
at the
epochal point
in the
history
of the
solar
system, when
the
development
of the
human consciousness arrives
at the
level
of a
fundamental understanding
of
the
planetary
and
cosmic environments that give rise
to it. It is an
exhilarating
and
intoxicating
experience
to
recognize that
the
causal sequence
from
creation (Big Bang)
to
planets
is a
logical
and
inevitable consequence
of the
known laws
of
physics.
It is
not
necessary
to
invoke supernatural
or
unnatural assumptions.
In
this
chapter
we
briefly
outline
the
part
of
cosmochemistry
that
is
important
for
determining
the
chemical environment
of the
origin
of the
solar system. What
was
the
source
of
solar
system material? What were
the
principal events leading
to the
formation
of the
solar system?
How did
planets form?
How did the
terrestrial planets
acquire their atmospheres? Most
of the
evidence
from
the
time
of the
formation
of the
solar system
is
lost.
We
have
to
reconstruct
the
events
from
the few
fragments that
survive
to the
present.
The
topics
of
this
chapter have been chosen
not on the
basis
of
what
we
would
like
to
know,
but on the
basis
of
what limited information
is
available. Observations
of
the sun and the
local interstellar medium provide
us
with
fundamental
data
on
the
"cosmic
abundances"
of the
elements. Molecular clouds
are
precursors
of
solar
system
material.
There
is a
considerable body
of
data
on the
chemical speciation,
processing,
and
evolution
of
molecular clouds. Within
the
solar system,
it is
convenient
to
divide
the
materials into three types:
(a)
gas,
(b)
rock,
and (c)
ice.
The
giant planets
contain
gases
that were remnants
of the
solar nebula.
In the
inner solar system
we
have
more
"rocky"
material. Meteorites
are the
best samples
of
this type
of
material.
The
materials that
are in the
terrestrial planets
are not
"primitive."
The
planets
are
large enough that extensive
differentiation
has
occurred,
and it is
difficult
to
infer
the
original composition
by
studying
the
present surface composition.
In the
outer
solar system
we
have more "icy" material,
of
which comets
are
examples. Thus,
the
giant planets
are
representative
of the
bulk
of the
material
in the
solar system.
The
meteorites
and
comets
are
probes
to the
inner
and
outer parts, respectively,
of the
solar
nebula that
has
since dissipated.
In
addition,
the
study
of
radioactive nuclei
found
in
meteorites yields important time constraints
for the
duration
of the
solar nebula. Noble
gases
and
isotopic fractionations provide clues
to the
origin
of
atmospheres.
4.2
Cosmic
Organization
When
one
contemplates
the
wonders
of the
universe
from
the
large
astronomical scales
to
the
small atomic scales, nothing
is
more wonderful than
the
hierarchy
of
scales
of
organization.
In
other words,
the
mass
of the
universe
is
neither uniformly
nor
randomly
distributed over
all
sizes,
but
seems
to be
concentrated
in
certain preferred
ranges.
For
example,
in the
large
scale
we
have well-known
highly
organized units
such
as
planets, stars, molecular clouds,
and
galaxies.
In the
small scale
we
have
atoms,
molecules, proteins, aerosols,
and
cells.
Our
wonder
at
this intricate pattern
of
organization
is not
diminished
by the
fact
that
the
fundamental laws
of
physics
that
govern
the
distribution
of
matter
are
known.
On the
large
scale,
the
agent
of
organization
is
gravity;
on the
small scale,
the
corresponding agent
is the
Coulomb
force.
Both
are
long-range forces whose intensity
falls
off as the
inverse square
of
distance.
Of the
four
fundamental
forces
of
nature,
only
these
two are
mediated
by
80
Photochemistry
of
Planetary
Atmospheres
Figure
4.2
Observed objects
in
the
universe plotted
in a
size-
mass diagram.
After
Barrow,
J. D., and
Tipler,
F. J.,
1986,
The
Anthropic
Cosmological
Principle
(New York: Oxford
University
Press).
massless particles (photon
and
graviton), which account
for
their
infinite
range. Even
though
the
electromagnetic force
is
many times stronger than that
of
gravity,
it can
be
both attractive
and
repulsive
and can be
canceled. Gravity
is
always attractive
and
will
dominate
at
large distances.
The
human being occupies
an
intermediate position
in
this hierarchy
of
scales. Structurally
we are
built
by
electromagnetic forces,
but
our
mobility
is
severely limited
by
gravity.
The
terrestrial planets
are
unusual
in
that
the
gravitational binding energy
of a
molecule
to the
planet
is of the
same order
of
magnitude
as the
chemical bond energy (see section 1.4). This determines
a
rate
of
chemical evolution that
is
neither
too
fast
(with catastrophic consequences)
nor too
slow
(with
no
evolution).
The
masses
and
sizes (linear dimension)
of the
important organized units
in the
universe
are
shown
in
figure
4.2.
The
mass
of the
known universe
is
very large,
estimated
to be of the
order
of
10
56
g. The
bulk
of the
universe, stretched over
10
28
cm,
is
empty,
and in
this vast monotony
of
space
are
scattered concentrated populations
of
stars, known
as
galaxies.
The
mass
of a
galaxy
is of the
order
10
45
g. The
stars
have
masses
in the
range
of
0.1-100
solar masses
(1
solar mass
= 2 x
10
33
g). In
the
intermediate mass range, between
a
galaxy
and a
star,
we
have giant molecular
clouds,
of the
order
of
10
s
to
10
6
solar masses.
As we
shall show,
the
cosmic chemical
environment
is
largely determined
by
processes
at
this
scale.
In
our
solar system,
the
bulk
of the
mass
is
concentrated
in the
sun.
The
mass
of
Earth
is 6 x
10
27
g. The
mass
of
Jupiter,
the
largest planet,
is 318
Earth masses.
Origins
81
The
giant
planets consist mostly
of
gases,
with
small cores made
of
rocky material.
The
terrestrial planets
are
composed primarily
of
rocky material, with
thin
envelopes
of
gases.
The
mass
of
Earth's atmosphere
is 5 x
10
21
g, and
that
of the
ocean
is
1.5
x
10
24
g. All
life
on
Earth
is in
10
18
g of
carbon
(living
biomass). This hierarchy
of
masses suggests that
the
various
'
'origins''
discussed
above
are
closely connected
to
each other.
The
origin
of the
planets
is
part
of the
origin
of the
solar system.
The
origin
of
planetary atmospheres
is
part
of the
origin
of the
planets.
The
ultimate source
of all
energy
and
matter
in our
universe
is the Big
Bang, which
occurred about
15
billion years ago.
Why
should
the
initially
hot and
nearly homo-
geneous
gaseous universe evolve into highly organized structures such
as
galaxies,
molecular clouds (with organic molecules), stars, solar systems, planets,
and
even-
tually
(on at
least
one
planet)
life
and
advanced intelligence? This
is all the
more
surprising
because according
to the
second
law of
thermodynamics, matter
in a
closed
box
tends
to
evolve toward equilibrium,
a
state
of
maximum entropy known
to the
nineteenth-century
cosmologists
as the
"heat
death
of the
universe."
That
the
universe
has
evolved along
a
more interesting alternative path,
as
shown
in
figure
4.1,
rather
than
toward heat death
is now
understood. There
are at
least three reasons:
(a) the
expansion
of the
universe,
(b)
gravity,
and (c)
relaxation time.
We
briefly
comment
on
these reasons. First,
the
universe
is
expanding.
It is a
dynamic,
not a
static (equilib-
rium),
system. Second, gravity
is a
long-ranged force that
is
capable
of
creating
and
maintaining
complex structures
on all
macroscopic
scales
from
galaxies
to
planetary
systems.
An
extreme example
of the
effect
of
gravity
is the
formation
of a
black hole,
the
ultimate compact
and
superdense
form
of
matter. Third,
as
pointed
out in
chapter
3
on
chemical kinetics,
the
time needed
for a
chemical system
to
reach
thermodynamic
equilibrium
may be too
long
for the
process
to be
relevant
in an
evolving system.
For
example,
in the first few
minutes
of the
expansion
of the
universe,
the
most
thermo-
dynamically
stable
form
of the
nucleons
was Fe.
However,
the
neutron
flux
during
this
period
was too low for the
equilibrium composition
to be
attained.
As
discussed
in
section 4.3, this
is the
ultimate source
of the
nuclear
fuel
that
was
left
over
from
the
origin
of the
universe.
It
would
be a
major source
of
energy
for
stars
later.
4.3
The
Elements
of the
Periodic
Table
A
major triumph
of
astrophysics
is its
ability
to
account
for the
origin
of the
elements
as
part
of the
origin
of the
universe
and
stellar evolution.
The
cosmic abundances
of
the
elements determine
the
"initial
conditions"
for the
chemistry
of the
solar system.
The
distribution
of the
elements
in the
solar system
and the
planets depends
on
their
"geochemical
tendencies,"
and on
this
basis
they
are
classified.
A
most important
property,
for
example,
is
volatility.
The
noble gases
are
very volatile
and
always
remain
in the gas
phase. Iron,
on the
other hand,
is
nonvolatile
and
prefers
to be in
the
solid state.
The
classical scheme
of
Goldschmidt divides
all
elements into
four
groups:
lithophile,
siderophile, chalcophile,
and
atmophile (meaning, respectively, rock
loving,
iron loving,
sulfide
loving,
and
atmosphere loving).
We
adopt
the
more recent
classification
scheme
of
Larimer
(1987).
82
Photochemistry
of
Planetary
Atmospheres
Figure
4.3
Relative
abundance
of the
elements
in our
sun. ABS.
=
absent.
The
abundance
of Si is
taken
to be
IO
6
.
After
Broecker,
W. S.,
1985,
How to
Build
a
Habitable
Planet (New York: Eldigio
Press).
4.3.1
Cosmic
Abundances
of the
Elements
In
the first few
minutes
of the
origin
of the
universe,
the
temperature
of the fireball
exceeded
IO
9
K
and the
simplest nuclei
H, D, He, and Li
were synthesized.
The
nascent
universe
cooled
rapidly
due to
adiabatic
expansion,
and
there
was not
enough
time
for
the
synthesis
of the
heavier elements beyond
Li.
Matter consisted
of 90% H and 10%
He
by
mass, with smaller amounts
of D
(10~
4
)
and Li
(10~
9
).
The
conversion
of the
simplest elements into
the
approximately
100
heavier elements
of the
periodic table
took place
in the
interior
of
stars, where
the
heating (due
to
gravitational compression)
is
intense enough
to
ignite
the
nuclear
fuel.
The
details
of how the
originally
diffuse
matter
of the
universe organized into dense self-gravitating balls
of gas is not
pursued
here.
In the
interior
of
stars, hydrogen
and
helium
are
"burned,"
resulting
in the
formation
of
heavier elements
in a
process known
as
nucleosynthesis, which
is in
fact
the
principal source
of
energy
for
stars.
Stars
have
finite
lifetimes,
and on
death, eject
the
synthesized heavy
elements
into
the
interstellar
medium.
The
material
of
which
the
bulk
of the
universe
is
made
is
thus
a
mixture
of
primor-
dial
material
and
material
of
secondary origin. This distribution
of
matter
is
known
as the
cosmic abundances
of
elements presented
in figure
4.3. Appendix
4.1
gives
the
same
information
in a
table
and
provides data
on the
relative abundances
of
impor-
tant
isotopes
of the
elements. This
is the
ultimate chemical yardstick
on
which
all
partitioning
and
speciation
is
based.
The sun and the
interstellar medium
in the
solar
neighborhood have roughly
this
composition.
Origins
83
Figure
4.4
Cosmochemical
classification
of the
elements. Elements with
crosses
are
lithophiles.
After
Larimer,
J. W.,
1987, "The
Cosmochemical
Classification
of the
Elements,"
in
Kerridge
and
Matthews (1988; cited
in
section
4.1),
pp.
375-389.
4.3.2 Classification
of
Elements
It
is
convenient
in
cosmochemistry
to
divide
the
elements into
four
groups: refractory,
siderophile,
moderately volatile,
and
highly volatile,
as
shown
in
figure
4.4.
This
scheme
is
proposed
by
Larimer
(1987)
on the
basis
of
Urey's
(1959)
recognition
that
the
most important fractionation
process
in the
formation
of the
solar system
is
the
separation
of
dust
and
gas.
Thus,
the
primary motivation
of
this classification
is
the
equilibrium condensation sequence
in the
solar
nebula
as it
cools.
The
refractory
group consists
of
most
of the
elements
to the
left
of the
periodic table, plus lanthanides
and
actinides.
The
refractory elements
are the first to
condense
in the
solar
nebula
at
temperatures above
1000
K. The
siderophile elements
as a
group
all
occur
as
metals
usually
alloyed
with
Fe and Ni. As a
result
of
this
affinity,
they
are
concentrated
in the
cores
of
differentiated
planetary bodies.
The
elements
in the
moderately volatile group
include
most
of the first
column
(the
alkali metals)
and the
right
side
of the
periodic
table (figure
4.4).
These
elements condense
at
significantly
lower temperatures.
The
last group,
the
highly volatile elements, condense
at low
temperatures, forming
ices
in
the
outer solar system.
The
noble
gases,
which belong
to
this group, will
not
condense even
in the
outer solar system. However, they
can be
trapped
in
ices
and in
carbonaceous material.
This classification
is not
mutually
exclusive
and
depends
on
ambient pressure,
temperature,
and
oxidation state.
A key
variable
in the
solar nebula
is the C/O
ratio,
which
is of the
order
of
unity.
A
slight
excess
of C
over
O
would imply that
the
nebula
favors
the
production
of
"refractories"
such
as
graphite,
SiC,
A1N,
and
TiN.
The
alternative possibility,
with
an
excess
of O
over
C,
would
imply
the
dominance
of
CO, a
highly volatile
gas,
and the
instability
of
organics
against
oxidation.
84
Photochemistry
of
Planetary Atmospheres
To
the
Larimer scheme
we can add
another group,
the
biogeochemical elements
C,
H, O, N, S, and P.
Interest
in the
latter group
is
connected
to the
origin
of
life
and the
history
of
Earth.
We
note that biology
has the
ability
to
mobilize oxygen
and
sulfur
as
gaseous compounds.
The
geochemistry
and
atmospheric chemistry
of
Earth
are
profoundly
affected
by the
biosphere.
The
synthesis
of
complex organic compounds
is
not,
however,
the
monopoly
of
living
organisms.
As we
show
in
section 4.4, organic
synthesis
is
prevalent
on a
cosmic scale.
4.4
Molecular
Clouds
From
figure
4.2 we
note
that
on a
size scale much smaller than
a
planet,
the
agent
of
organization
is the
Coulomb force,
and the
most remarkable product
is
life.
The
fundamental
unit
of
matter
is an
atom, with mass
of the
order
of
10~
24
g and
size
10~
8
cm. A
primitive living organism
(a
cell) consists
of
10
10
atoms.
A
complex form
of
life
(a
human
being)
is
about
10
5
g. The
basis
of
this organizational hierarchy
is
the
organic chemistry based
on
carbon that allows
the
construction
of
molecules
of
arbitrary complexity, given
a
suitable source
of
energy. Today,
the
rate
of
organic
synthesis
on
Earth
is
greater than that
on any
other planet
in the
solar system
by a
factor
of
10
3
.
This
is, of
course,
due to the
remarkable machinery
of
photosynthesis,
itself
a
product
of
life
(see discussion
in
section 2.5).
A
question then naturally arises:
What
was the
prebiological organic synthesis that
led to the
spontaneous generation
of
life?
In a
series
of
classic experiments, Miller
and
Urey demonstrated
the
synthesis
of
complex organic compounds
from
mixtures
of
simple gases
of
reducing composition
(such
as
CELt
and
NHj)
using
electric
discharges. Subsequent observations showed
that, indeed, organic synthesis
is
fairly
common without
the
presence
of
life
in the
molecular clouds
and in the
atmospheres
of the
planets
in the
outer solar system.
In
section
4.4.1
we
discuss
the
organic chemistry
of
molecular clouds.
A
most spectacular achievement
of
modern radio astronomy
is the
discovery
of
large numbers
of
complex organic molecules
in
giant molecular clouds.
The richness
of
the
rotational spectra observed
at
millimeter wavelengths
is
shown
in
figure
4.5
from
a
recent survey
of the
Orion Molecular Cloud. Table
4.1
lists
the
molecular
species
identified
in the
interstellar medium
and
circumstellar
envelopes
as of
1996.
These molecules include amino acids, which
are the
building blocks
of
proteins.
There
is
no
doubt that even more complex organic molecules exist,
but
these molecules have
many
quantum states that make their detection
difficult.
The
existence
of
this
rich
variety
of
organic compounds implies that
the
basic chemicals essential
for the
origin
of
life
are
produced
in
abundance
on a
cosmic scale.
Of
course, there
is
still
the
problem
of
delivering this preprocessed material intact
to the
solar system.
There
is
evidence that
at
least some
of the
organic material present during
the
formation
of the
solar system have survived
in the
meteorites.
4.4.1 Organic Synthesis
The
basic chemistry that gives rise
to the
large number
of
species
in
table
4.1
is
understood.
The
molecular clouds have
low
number densities,
in the
range
of
10
3
to
10
6
molecules/cm
3
.
The
temperatures
are
also
low, about
10-50
K.
Under these
Figure
4.5
Emission lines observed
from
gas
phase molecules
in the
Orion Molecular Cloud
during
a
spectral scan
in
1986. Over
800
resolved spectral lines
are
present
and
have been
assigned
to
some
29
molecular species. From Black,
G.
A.,
et
al.,
1986, "The Rotational
Emission-Line Spectrum
of
Orion-A between 247-GHz
and
263-GHz."
Astrophys.
J.
Suppl.
Ser.
60,
357.
85