Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres
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Earth
Imprint
of
Life
9.1
Introduction
Earth
is the
largest
of the
four
terrestrial planets, three
of
which have substantial
atmospheres.
The
astronomical
and
orbital parameters
are
summarized
in
table
9.1.
Our
planet
has an
obliquity
of
23.5°,
giving rise
to
well-known seasonal variations
in
solar insolation.
The
orbital elements
are
slightly perturbed
by
other planets
in
the
solar system (primarily Jupiter), with time
scales
from
20 to 100
kyr,
and
these
changes
are
believed
to
cause
the
advance
and
retreat
of ice
sheets.
The
last glacial
maximum
(LGM)
occurred
18 kyr
ago,
at
which time
the
planet
was
colder
by
several
degrees
centigrade
on
average.
At
present Earth
is in an
interglacial warm
period.
The
origin
of
Earth
may not be
very
different
from
that
of the
other terrestrial
bodies. However, three properties
may be
unique
to
this planet.
One is the
formation
of
the
Moon, probably
via
collision between Earth
and a
Mars-sized body. Second
is
the
release
of a
huge amount
of
water from
the
interior
(see
discussion
in
section
8.5).
Third, Earth
is
endowed with
a
large magnetic
field
that protects
it
from
direct impact
by
the
solar wind.
Seventy
percent
of
Earth's surface
is
covered
by
oceans, which have
a
mean depth
of
3 km.
There
is so
much water
that
Arthur
C.
Clarke proposed that
"Ocean"
might
be
a
better name
for our
planet than
"Earth."
The
enormous body
of
water became
the
cradle
of
life
as
early
as
3.85
Gyr
ago.
The
present terrestrial environment
is
the
end-product
of
billions
of
years
of
evolution driven
by the
hydrological cycle
316
9
Earth:
Imprint
of
Life
317
Table
9.1
Greenhouse
effects
in
planetary atmospheres
Venus
Earth
Mars
Surface
pressure
(relative
to
Earth)
90
1
0.007
Main
greenhouse
gases
> 90%
CO
2
~
0.04%CO
2
~
1%
H
2
O
> 90%
CO
2
Surface
temperature
in
absence
of
greenhouse
effect
(K)
227
255
216
Observed
surface
temperature
(K)
750
288
226
Warming
due to
greenhouse
effect
(K)
523
33
10
From
Intergovernmental Panel
on
Climale
Change
(1990).
and
global biogeochemical cycles,
in
addition
to the
slower forces
of
geodynamics
and
geochemistry.
The
massive hydrological cycle
and the
biogeochemical cycles
that
operate
on
Earth
are
absent from other planets
in the
solar system. Mars
in
the
remote past might have
had a
milder climate
with
liquid water
on the
surface,
but
the
planet dried
up a few
eons
ago.
There
is to
date
no
observational evidence
for
the
hypothetical
oceans
(composed
of
liquid
hydrocarbons)
on
Titan. Life
on a
planetary
scale
equivalent
to the
terrestrial biosphere
does
not
exist elsewhere
in the
solar system.
Perhaps
the
most distinctive character
of
Earth
is the
imprint
of
life
and the
fact
that
it has
sustained
life
for at
least
3.8
Gyr. Today
the
global planetary environ-
ment
is
genial
and
conducive
to
life.
The
surface
of the
planet
is
protected from
harmful
ultraviolet radiation
by an
ozone
layer
at an
altitude
of
about
20-30
km.
The
existence
of the
ozone
layer, which makes advanced
life
on
Earth possible,
is
caused
by the
abundance
of
atmospheric oxygen, which
is in
turn
a
product
of the
biosphere.
As
pointed
out in
chapters
7 and 8, the
abundances
of
Oj
on
Mars
and
Venus
are
about
two
orders
of
magnitude
less
than that
on
Earth. Minor constituents
of
the
atmosphere
(e.g.,
t^O
and
COz)
provide
a
greenhouse
effect
of
about
30 K,
an
amount that
is
just about
right
to
maintain
the
mean temperature
of
Earth
at a
comfortable
285 K. By
comparison
we may
note
that
the
surface temperatures
of
Mars
and
Venus
are 250 and 750 K,
respectively. Table
9.1
shows
that
Mars
has
too
little
greenhouse
effect;
Venus
has too
much.
It is
surprising that
the
Earth
has
apparently
never been completely frozen,
nor has it
been
so hot as to
destroy
life,
despite large changes
in the
solar constant
and the
composition
of the
atmosphere.
Did the
biosphere play
a
role
in
mitigating
the
excesses
of
climatic changes?
Did the
biosphere actively promote
a
physical
and
chemical environment that
is
most
felicitous
for
life?
These
questions bring
us
into
the
heart
of a
controversial theory:
the
Gaia
hypothesis.
The
theme
of
this
chapter
is the
imprint
of
life
on the
global terrestrial environment,
as
compared
to
other planets.
The
purpose
is to
prepare
the
proper background
and
perspective
for an
assessment
of the
effects
of
anthropogenic activities
on the
global
environment
(chapter 10).
Table
9.2
Chemical
composition
of the
terrestrial
troposphere
Gas
N
2
0
2
H
2
O
Ar
C0
2
Ne
4
He
CH,
Kr
H
2
N
2
0
CO
Xe
0
3
HC1
Isoprene,
etc.
C
2
H
6
,
etc.
H
2
O
2
C
2
H
2
,
etc.
C
2
Rt,
etc.
C
6
H
6
,
etc.
NH
3
HNO
3
CH
3
CI
COS
NO
r
CF
2
C1
2
(F12)
CFCIj
(Fll)
CH
3
CC1
3
CC1
4
CF
4
(F14)
CHCIF
2
(F22)
H
2
S
C
2
C1
3
F
3
(Fll
3)
CH
2
C1
2
CH
2
CICH
2
C1
CH
3
Br
SO
2
CHCI
3
CS
2
C
2
C1
2
F4
(FI14)
C
2
H
5
C1
CHCICCb
(CH
3
)
2
S
C
2
CIF
5
(Fll
5)
C
2
F
6
(F116)
Abundance"
78.084%
20.946%
<4%
9340
ppm
350 ppm
18.18
ppm
5.24
ppm
1.7
ppm
1.14
ppm
0.55
ppm
-320
ppb
125
ppb
87 ppb
-10-100
ppb
-1
ppb
-1-3
ppb
~3-80
ppb
-0.3-3
ppb
-0.2-3
ppb
-0.1-6
ppb
-0.1-1
ppb
0.1-3
ppb
~0.04-4
ppb
612 ppt
500 ppt
-30-300
ppt
300 ppt
178 ppt
157 ppt
121
ppt
69 ppt
59
ppt
30-100
ppt
30-40
ppt
30 ppt
26 ppt
12
ppt
20-90
ppt
16
ppt
-15
ppt
14 ppt
12
ppt
7.5 ppt
5-60
ppt
4 ppt
4 ppt
Source(s)
Denitrifying
bacteria
Photosynthesis
Evaporation
Outgassing
(
40
K)
Combustion,
biology
Outgassing
Outgassing
(U, Th)
Biology
and
agriculture
Outgassing
H
2
O
photolysis
Biology
Photochemistry
Outgassing
Photochemistry
Derived from
sea
salt
Foliar
emissions
Combustion,
biomass
burning,
grasslands
Photochemistry
Combustion,
biomass
burning,
oceans
Combustion, biomass
burning,
oceans
Anthropogenic
Biology
Photochemistry
(NO
r
)
Ocean,
biomass
burning
Biology
Combustion, biology
Anthropogenic
Anthropogenic
Anthropogenic
Anthropogenic
Anthropogenic
Anthropogenic
Biology
Anthropogenic
Anthropogenic
Anthropogenic
Ocean,
marine biota
Combustion
Anthropogenic
Anthropogenic
Anthropogenic
Anthropogenic
Anthropogenic
Biology
Anthropogenic
Anthropogenic
Sink(s)
Nitrogen-fixing
bacteria
Respiration
and
decay
Condensation
—
Biology
—
Escape
Reaction with
OH
—
H
atom
escape
Photolysis (stratosphere)
Photochemistry
—
Photochemistry
Rainout
Photooxidation
Photooxidation
Photochemistry
Photooxidation
Photooxidation
Photooxidation
Wet
and dry
deposition
Rainout
Reaction
with
OH
Photodissociation
Photooxidation
Photolysis (stratosphere)
Photolysis (stratosphere)
Reaction with
OH
Photolysis (stratosphere)
Photolysis (upper
atmosphere)
Reaction with
OH
Photooxidation
Photolysis
(stratosphere)
Reaction with
OH
Reaction with
OH
Reaction with
OH
Pholooxidation
Reaction with
OH
Photooxidation
Photolysis (stratosphere)
Reaction
with
OH
Reaction
with
OH
Photooxidation
Photolysis (stratosphere)
Photolysis (upper
atmosphere)
Comments
variable
+
l%/yr
+5.1%/yr
+5.1%/yr
+4.4%/yr
+
1.3%/yr
+2.0%/yr
+
10.9%/yr
+
1 1
.5%/yr
marine
air
marine
air
318
Earth:
Imprint
of
Life
319
Table
9.2
(continued)
Gas
Abundance
Source(s)
Sink(s)
Comments
CC1F.,
(F13)
CH
3
[
CHCI
2
F(F21)
CClF
2
Br
3.3 ppt
~2ppt
1
.6
ppt
1.2
ppt
Anthropogenic
Ocean,
marine
biota
Anthropogenic
Anthropogenic
Photolysis (stratosphere)
Photolysis
(troposphere)
Reaction with
OH
Photolysis (stratosphere)
+20%/yr
From
Fegley,
B. Jr.
(1995).
"Abundances
by
volume
in dry
air.
ppm
=
parts
per
million,
ppb
=
parts
per
billion,
ppt
=
parts
per
trillion.
Table
9.3
Isotopic composition
of the
noble gases
in
the
terrestrial atmosphere
and of
terrestrial standards
for
isotopic analyses
Isotopic ratio
Observed
value
Comments
3
D/H
He/
4
He
I2
C/
13
C
14
N/
I5
N
16
0/
17
Q
16
O/
18
O
20
Ne/
22
Ne
2l
Ne/
22
Ne
35
CI/
37
C1
36
Ar/
38
Ar
40
Ar/
36
Ar
^Kr/^Kr
80
Kr/
84
Kr
82
Kr/
84
Kr
"Kr/^Kr
S'Kr/^Kr
124
Xe/
132
Xe
l26
Xe/
132
Xe
128
Xe/
132
Xe
129
Xe/
132
Xe
13
»Xe/
132
Xe
13l
Xe/
l32
Xe
134
XE/
132
Xe
136
Xe/'
32
Xe
(1.5576
±
0.0005)
x
10~
4
(1.399
±
0.013)
x
10~
6
89.01
±
0.38
272.0
± 0.3
2681.80
±
1.72
498.71
±
0.25
9.800
±
0.080
(2.899
±
0.025)
x
10~
2
3.1273
±
0.1990
5.320
±
0.002
296.0
± 0.5
(6.087
±
0.002)
x
1<T
3
3.960
±
0.002%
20.217
±0.021%
20.136
±
0.021%
30.524
±
0.025%
(3.537
±
0.0011)
x
10-
3
(3.300
±
0.017)
x
10~
3
7.136±
0.009%
98.32
±
0.12%
15.
136
±
0.012%
78.90
±
0.11%
38.79
±
0.06%
32.94
±
0.04%
SMOW
air
SMOW
SMOW
From
Fegley,
B. Jr.
(1995).
"SMOW
=
standard mean ocean
water.
9.2
Gaia Hypothesis
9.2.1
Gaia:
Imprint
of
Life
The
chemical composition
of the
terrestrial atmosphere
and its
isotopic composition
are
summarized
in
tables
9.2 and
9.3, respectively.
The
chemical composition
of the
oceans
is
given
in
table 9.4.
The
composition
of the
terrestrial atmosphere departs
Table
9.4
Chemical composition
of
seawater
Atomic
No.
1
2
3
4
5
6
7
8
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
35
36
37
38
39
40
41
42
44
45
46
47
48
49
50
Element
H
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
Rb
Sr
Y
Zr
Nb
Mo
Ru
Rh
Pd
Ag
Cd
In
Sn
Dissolved
form
H
2
Dissolved
He
Inorganic
boron
Total
CC>2
N
2
N0
3
Dissolved
O
2
Fluoride
Dissolved
Ne
Silicate
Reactive
phosphate
Sulfate
Chloride
Dissolved
Ar
As(V)
Total
Se
Bromide
Dissolved
Kr
Mean
concentration
1
.9
nmole/kg
178
Mg/kg
0.2
ng/kg
4.4
mg/kg
2200
Aimole/kg
590
^mole/kg
30
/Lfmole/kg
150
Aimole/kg
1.3
mg/kg
8
nmole/kg
10.781
g/kg
1.28g/kg
1
microg/kg
110
//mole/kg
2
/^mole/kg
2.712
g/kg
19.353
g/kg
15.6
Mmole/kg
399
mg/kg
415
mg/kg
<1
ng/kg
<1
ng/kg
<1
Mg/kg
330
ng/kg
10
ng/kg
40
ng/kg
2
ng/kg
480
ng/kg
120
ng/kg
390
ng/kg
7-60
ng/kg
5
ng/kg
2
Mg/kg
1
70
ng/kg
67
mg/kg
3.7
nmole/kg
124
microg/kg
7.8
mg/kg
13
ng/kg
<1
Mg/kg
1
ng/kg
11
Mg/kg
~1
ng/kg
0.2-0.7
pmole/kg
3
ng/kg
70
ng/kg
0.2
ng/kg
0.5
ng/kg
Comments
biogenic
or
hydrothermal origin
non-nutrient
dissolved
gas
conservative
increases with depth
conservative
nutrient
non-nutrient
dissolved
gas
nutrient
biologically
controlled profile
conservative
non-nutrient
dissolved
gas
conservative
conservative
nutrientlike
profile
nutrient
nutrient
conservative
conservative
non-nutrient
dissolved
gas
conservative
conservative
(1st. approx.)
conservative
nutrientlike
profile
nutrientlike profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike profile
correlated with silicate
nutrientlike
profile
correlated
with
phosphate
conservative
non-nutrient
dissolved
gas
conservative
correlated
with
phosphate
conservative
(1st. approx.)
conservative
correlated with phosphate
anthropogenic
320
Earth:
Imprint
of
Life
321
Table
9.4
(continued)
Atomic
No.
51
52
53
54
55
56
57
58
59
60
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
90
92
Element
Dissolved
form
Sb
Te
Total
Te
I
Xe
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
W
Re
Os
lr
Pt
Au
Hg
Tl
Pb
Bi
Th
U
Mean
concentration
0.2
ng/kg
0.6-1.3
pmole/kg
59
/.tg/kg
0.5
nmole/kg
0.3
ng/kg
11.7/zg/kg
4
ng/kg
4
ng/kg
0.6
ng/kg
4
ng/kg
0.6
ng/kg
0.1
ng/kg
0.8
ng/kg
O.I
ng/kg
1
ng/kg
0.2
ng/kg
0.9
ng/kg
0.2
ng/kg
0.9
ng/kg
0.2
ng/kg
<8
ng/kg
<2.5
ng/kg
<1
ng/kg
1.2-1
A
ng/kg
50-150
fmole/liter
6
ng/kg
12
ng/kg
1
ng/kg
10
ng/kg
<0.7
ng/kg
3.2
/ig/kg
Comments
conservative
correlated
with
phosphate
non-nutrient
dissolved
gas
conservative
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
nutrientlike
profile
conservative
correlated
with
silicate
conservative
anthropogenic
conservative
From
Fegley,
B. Jr.
(1995).
radically
from
that
of a
lifeless planet. This
can
best
be
appreciated
from
figure 9. la by
comparing
the
composition
of the
present
atmosphere
with
a
hypothetical atmosphere
on an
abiotic Earth. Note
that
in the
absence
of
life,
CO
2
would
be the
dominant
gas in the
atmosphere, with total pressure
of
about
200
mbar.
The
higher abundance
(by
more than three orders
of
magnitude compared
to the
present)
is the
result
of
the
slower removal rate
of
atmospheric
CO2 by
geochemical
processes
in the
absence
of
life.
Therefore,
the
composition
of the
terrestrial atmosphere would,
to first
order,
resemble
the
sister planets Mars
and
Venus.
N
2
and
62
would
be
minor constituents,
with
abundances orders
of
magnitudes below those
at
present.
The
disappearance
of
most
of the
C>2,
an
extremely reactive gas,
is not
surprising. However,
the
loss
of
most
of the N2, a
very unreactive gas,
is
somewhat surprising.
The
reason
is
that
the
322
Photochemistry
of
Planetary
Atmospheres
Figure
9.1
(a)
Partial
pressure
of
reactive
gases
of
Earth's
atmosphere
at
present
(with
life)
and
those
calculated
to
result
from
reactions
were
life
extinguished.
The
concentrations
are
expressed
as log
base
10 of
quantities
in
parts
per
billion
by
volume.
After
Margulis,
L.,
and
Lovelock,
J. E.,
1989,
"Gaia
and
Geognosy,"
in
Rambler
et
al.
(1989;
cited
in
section
9.1),
and
Lovelock,
J.
E.,
and
Margulis,
L.,
1974, "Atmospheric Homeostasis,
by and for
the
Biosphere:
The
Gaia Hypothesis."
Tellus
26, 1.
stable form
of
nitrogen
on an
abiotic Earth
is
nitrate
ion in the
oceans.
The
trace
gases
such
as
CH
4
,
N
2
O,
NHs,
HCI,
and H2
would either vanish from
the
atmosphere
or
have their abundances greatly
reduced.
Only
the
concentrations
of CO
would greatly
increase,
due to the
greater source from
CO
2
photolysis.
The
time constant
for the
complete obliteration
of the
imprint
of
life
from
Earth's
atmosphere
is of the
order
of
millions
of
years,
a
geologically
insignificant
length
of
time.
The
intense disequilibrium
of the
present atmosphere with
respect
to an
abiotic
Earth,
as
illustrated
in figure
9.
la,
suggests
that
the
biosphere must
be
continuously
producing prodigious amounts
of
gases
that
are out of
thermodynamic
equilibrium
with
the
chemistry
and
geochemistry
of the
rest
of the
planet. Figure
9.
Ib
presents
estimates
of the
biologically produced
fluxes of
these
gases.
Note
the
simultaneous
production
of an
oxidizing gas,
O
2
,
as
well
as a
highly reducing gas,
CH^.
In the
absence
of
life,
all
production
rates
will
drop
by
many
orders
of
magnitude
to the
level
that
can be
sustained
by
photochemical
and
geochemical reactions.
For
gases
such
as
CH
4
and
NH
3
the
production rates
on an
abiotic Earth
are
close
to
zero
because
it is
very
difficult
to
synthesize such complex molecules
by
atmospheric
chemistry
in a
CO2-dominated atmosphere.
We may
take
the
absence
of
these
gases
Earth:
Imprint
of
Life
323
Figure
9.1
(b)
Fluxes
of
reactive
gases
of
Earth's
atmosphere
at
present
(with life)
and
those
calculated
to
result from
reactions
were
life
extinguished.
The
fluxes
are
expressed
as log
base
10 of
fluxes
in
10
9
moles
per
year.
After
Margulis,
L.,
and
Lovelock,
J. E.,
1989,
"Gaia
and
Geognosy,"
in
Rambler
et
al.
(1989;
cited
in
section
9.1),
and
Lovelock,
J.
E.,
and
Margulis,
L.,
1974,
"Atmospheric
Homeostasis,
by and for the
Biosphere:
The
Gaia
Hypothesis."
Tellus
26,
1.
in
the
atmospheres
of
Mars
and
Venus
as
proof that
CFLt
and
NHi
would
be
absent
on
an
abiotic Earth.
The
most
important reaction
in the
biosphere
is
photosynthesis,
represented
schemat-
ically
as
where
(CH2O),,
represents
carbohydrate.
The
global photosynthesis rate
is 3 x
10
15
moles
of
carbon
per
year,
or 142
Gt-C
yr~'
(1 Gt
=
10
15
g). The
mean rate
of
destruction
of
CCb
is
equivalent
to 4.4 x
10
13
molecules
cm'
2
s~~'.
For
comparison,
we may
note
that
the
photolysis rates
of
CC>2
on
Mars
and
Venus
are 1 x
10
12
and
7 x
10
12
cm"
2
s~',
respectively.
The
biosphere
is
much
more
efficient
at
removing
CC>2
than
ultraviolet
photolysis, because
(9.1)
is a
double photon process
that
can use
visible
light.
At
this
rate
all the
CO
2
in the
terrestrial atmosphere
can be
removed
by
photosynthesis
in 5 yr. Of
course, most
of the
CC>2
consumed
in
(9.1)
is
restored
by
respiration
and
decay,
a
process
that
nearly balances
photosynthesis.
Photosynthesis
is the
heartbeat
of the
entire biosphere.
It is the
ultimate
source
of
nearly
all
organic
matter
as
well
as
oxygen. Reaction (9.1) stores
up
124.2
kcal
of
chemical energy
for
each mole
of
carbon synthesized
into
organic matter.
The
total
biospheric
power
is
1.7
x
10
14
W, or
about
0.1%
of the
solar energy
incident
on
Earth.
For
comparison,
324
Photochemistry
of
Planetary Atmospheres
Table
9.5
Biogeochemical
cycles
in
prokaryotes
Element
Process
Examples
of
organisms
and
summary
of
reactions
Carbon
Sulfur
CO2 fixation
Methanogenesis
Methylotrophy
Fermentation
Respiration
Sulfur
reduction
Sulfur
oxidation
Nitrogen
N
2
fixation
Nitrification
Denitrification
C0
2
+
H
2
-*
(CH
2
0)
n
+
A
2
(A = O, S)
photoautotrophs:
cyanobacteria,
purple
and
green
sulfur
bacteria
chemoautotrophs:
sulfur-
and
iron-oxidizing bacteria
coo-
+
H
2
->
cat
methanogenic
bacteria
CH
4
+
0
2
-»
C0
2
methylotrophic
bacteria
(CH
2
0)«
->
C0
2
aerobic
heterotrophic
bacteria
(CH
2
0)
n
+
0
2
-c
C0
2
aerobic heterotrophic bacteria
S0
4
2
-
+
H
2
->
H
2
S
sulfur
reducing bacteria
H
2
S
->
S°
purple
and
green
sulfur
phototrophs
S°
+
O
2
->
SO
4
sulfur-oxidizing
bacteria
N
2
+
H
2
->
NH4
phototrophic
bacteria,
nitrogen-fixing
heterotrophic
bacteria
NKr
+
Oi
->
NO
2
,
NO)
nitrifying
bacteria
NO
2
,NO
3
->
NO
2
,N
2
denitrifying
heterotrophic bacteria
From Rambler,
M.
B. et
al.
(1987).
the
power that drives
all
human activities
in
1980
is 1 x
10
12
W. We
discuss
in
sections
9.4 and 9.5 the
impact
of
photosynthesis
on the
chemical composition
and
climate
of
Earth.
The
biota
may be
classified into three categories
by
their functions: producers,
consumers,
and
decomposers
of
organic compounds.
The
primary producers
are
plants,
algae,
and
photosynthetic
bacteria. They synthesize organic compounds from
CC>2
and
H2O
using solar energy
(9.1)
and
provide
the
ultimate source
of
energy
to
fuel
the
biosphere.
The
consumers, such
as
animals, protozoans,
and
some bacteria,
do not
synthesize organic compounds from inorganic compounds,
but
they
can
transform
organic
compounds into other organic
or
inorganic compounds.
Decomposers
feed
on
dead organisms
by
transforming
the
organic compounds back
to
inorganic compounds,
usually
CO2 and
HaO.
Examples
of
decomposers include
fungi
and
many bacteria.
The
activities
of the
biosphere have
a
major impact
on the
cycling
of the
biogeo-
chemical
elements:
H, O, C, N, S, and P.
Table
9.5
lists some
of the
important
net
reactions
in the
biosphere
for
transforming carbon, nitrogen,
and
sulfur
compounds.
Hydrogen
and
oxygen
are
involved
in
these reactions. Phosphorus appears
to
have
no
important
linkage
to the
atmosphere.
The
details
of the
hydrogen, oxygen, carbon,
nitrogen,
and
sulfur
cycles
will
be
discussed
in
separate sections
in
this chapter.
Earth:
Imprint
of
Life
325
9.2.2 Gaia: Planetary
Homeorrhesis
We first
explain
the
difference
between homeostasis
and
homeorrhesis.
A
homeostatic
system
maintains
specified variables
at
relatively constant levels despite perturbing
influences.
If,
however,
the
specified variables
are not fixed but
change progressively
with
time,
the
system
is
said
to be
homeorrhetic.
The
planetary environment
of
Earth
has
progressively changed
with
time. There
are at
least three driving forces. First,
the
solar constant
is
believed
to
have increased
by
about
40%
since
the
origin
of the
solar
system. Second,
substantial
escape
of
hydrogen
from
the
planet
has
occurred
and is
still
occurring. Third, there
has
been
a
profound biological evolution from simple
to
complex organisms,
and
from marine
to
terrestrial ecosystems.
A key
question about
the
global environment
is why it has
remained
so
well regulated
for the
benefit
of
the
biosphere.
Has the
biosphere played
the
role
of
homeorrhesis
for the
planetary
environment?
The
interaction between
the
environment
and the
biosphere
is a
problem
of
such
enormous complexity that
a
fundamental
understanding based
on the
rigorous prin-
ciples
of
physics
and
chemistry
is
probably
very difficult,
if not
impossible.
We use
an
idealized model
by
Lovelock
to
illustrate
the
role
of the
biosphere
in
homeorrhe-
sis. Consider
the
problem
of the
"faint
sun."
According
to
accepted
theories
of the
evolution
of
stars,
in the
main sequence
of
which
the sun is a
typical member,
the
solar luminosity
has
been steadily increasing
by
about
40%
since
the
sun's origin.
The
mean surface temperature
(T) of a
planet
like
the
Earth
is
determined
by the
energy
balance equation
where
e is the
emissivity,
a is the
Stefan-Boltzmann constant,
a is the
planetary
albedo,
and F is the
solar
constant. Consider
a
particularly simple model
of
Earth
with
no
greenhouse
effect
(e
=
1). The
surface temperature
is,
from
(9.2),
This planet
has no
biology.
The
only feedback
on the
change
of
climate
is
physical,
via
the
change
of
phase
of
water
and its
impact
on the
albedo (a).
The
results
are
presented
in
figure
9.2a.
The
dotted
line
represents
the
monotonic
increase
in
temperature
as the
solar luminosity increases
for a
planet
with
constant albedo.
The
line ABCDE
is
from
a
model that allows
for
feedback
due to
phase changes
of
water. From
A to B, the
planet
is
completely covered
by
ice.
Between
B and C the ice is
removed
as the
planet
becomes
warmer.
The
lower albedo results
in a
rapid warming
of the
planet. However,
as the
planet
gets
even warmer, there
is
more water vapor
in the
atmosphere
and
clouds
can
form.
The
higher albedo
of
clouds slows
the
rate
of
warming between
C and D.
Beyond
D,
there
is no
further
albedo change. This simple model
illustrates
the
relation
between
the
surface temperature
of the
planet
and the
solar constant when there
is no
albedo feedback
(AB and
DE),
positive feedback
(BC),
and
negative feedback
(CD).
Now let us
consider
a
planet
on
which
life
is
possible.
For
simplicity,
let
"life"
consist
of two
kinds
of
daisies:
a
light
species
(with
high albedo)
and a
dark
species
(with
low
albedo).
The
feedback between
the
environment
and
this
biosphere
is now
governed
by the
laws
of
biology.
The
growth rate
of
living
organisms
is
optimum
in a