Planetary Atmospheres, the Environment
and Life (ExCos2Y)
Topic 2: Evolution of Earth’s Atmosphere
Chris Parkes
Rm 455 Kelvin Building
1. Composition of the Atmospheres
of Earth, Mars and Venus
• Physical characteristics of planets
• Atmospheric composition of Planets
– Earth: Nitrogen, Oxygen Mars/Venus: CO2
– Mars: low pressure, Venus: high pressure
• Gain & Loss mechanisms
• Thermal Escape:
– Temperature
– Gravity, Mass & Radius of planet – Escape Velocity
Revision
Lecture 2: Evolution of the Earth’s atmosphere
“..the thickness of the Earth's atmosphere, compared with the size of the
Earth, is in about the same ratio as the thickness of a coat of varnish on a
schoolroom globe is to the diameter of the globe.” Carl Sagan
Skeptical Enquirer, Volume 19, Issue 1, January-February 1995
The presence of water – the life zone
Orbit of Venus
Too near the sun (hot)
water boils off
Sun
Too far away (cold)
water freezes
Orbit of Mars
BUT – significant effect of
greenhouse gases on temperature, see later
Orbit of Earth
A home away from home?
(liquid water)
Far from equilibrium
• Atmospheres on Mars and Venus
– CO2 rich and in chemical equilibrium
• Earth’s atmosphere
– not in chemical equilibrium
– held in a precarious state by the biosphere
O2 not naturally free e.g. rusting
Oxygen production
Oxygen removal
CO2 removal
1. Plants release O2 - photosynthesis.
2. Animals/plants respiration use O2 to break down sugars.
3. CO2 is released by respiration & used in photosynthesis.
4. O2 cycles between oceans & atmosphere, maintaining equilibrium.
The modern O2 cycle
Modern O2 cycle
2H2O  O2 + 2H2 (H2 thermal escape)
2O2  O + O3 (solar ray formation of ozone)
rate: + 108 kg
Weathering (chemical reactions): rate: - 1011 kg/year
Volcanism:
emits CO, Sulphur, react; rate: - ~1010 kg/year
water vapour (as above) gives a source of oxygen
Photosynthesis:
CO2  O2; rate: + 1014 kg/year
Respiration & decay: O2  CO2; rate: - ~1014 kg/year (balancing)
Burial of Carbon: (no longer reacts with O2) rate: + 1011 kg/year
Recycling of sediments: rate: < - 1011 kg/year
Fossil fuel combustion (O2  CO2): rate: - 1012 kg/year
Photochemistry:
Note: rough mass of O2 in atmosphere 1017 kg
Origin of Oxygen and Ozone
• Oxygen and aerobic life linked
Oxygen Content of atmosphere over time
UO2 ,
FeS2 (pyrite)
Uncertainty
Oxygen produced by life
first organisms anaerobic
later aerobic, plants & animals
Oxygen in oceans forms banded iron
Free Oxygen, redbeds occur
The three-reservoir model of Earth
Reducing
Oxygenating
Atmosphere
volcanic
gases
photochemistry
Shallow Ocean
weathering
photosynthesis
volcanic
gases
Deep Ocean
Evolution of Atmosphere
- Treat earth as covered with ocean
- 3 reservoir of O2 
atmosphere
shallow ocean
deep ocean
- Each has a combination of processes
which are grouped into
- O2 reducing (R) & Oxygenating (O).
3 different states:
A) reducing: very little O2 present
B) oxidising: enough O2 to oxidise
mineral but not enough for respiration
C) aerobic: enough O2 to support aerobic
respiration
4 stages in the history of O2 on Earth
Reducing
volcanic
gases
Oxygenating
Atmosphere
(reducing)
photochemistry
Shallow Ocean
weathering
(reducing)
volcanic
gases
Deep Ocean
(reducing)
Stage I:
After water is established
Photochemistry  O2
Reach balance with
weathering & volcanism
Very little O2 in atmosphere
~between 10-8 to 10-14 PAL
(present atmospheric level)
4 stages in the history of O2 on Earth
Stage II:
photosynthesising organism
spread new source of O2
possible increase burial rate
due to tectonic activities
O2 level at 10-2
~2 billion years ago
Reducing
Oxygenating
Atmosphere
volcanic
gases
(oxidising)
photochemistry
Shallow Ocean
weathering
(oxidising)
photosynthesis
volcanic
gases
Deep Ocean
(reducing)
4 stages in the history of O2 on Earth
Reducing
Oxygenating
Atmosphere
(aerobic)
respiration/decay
Shallow Ocean
weathering
(aerobic)
photosynthesis
Deep Ocean
volcanic
gases
(oxidising)
Stage III:
Abundance of photosynthesising
organism
O2 level limited because primitive
anaerobes can’t tolerate high
O2 level
Organisms had to evolve to cope
with high O2 level
4 stages in the history of O2 on Earth
Reducing
Oxygenating
Atmosphere
Stage IV:
Deep ocean becomes
aerobic
New organisms
Balance between
respiration and
photosynthesis
respiration
(aerobic)
Shallow Ocean
respiration
(aerobic)
respiration
photosynthesis
Deep Ocean
(aerobic)
As O2 increases, photochemistry creates ozone in upper
atmosphere. When ozone layer is thick enough to shield
from solar rays then living organisms can live out of water
Dependency of life on Oxygen
Oxygen (%)
17
16
15
10 – 12
8 – 10
<8
<6
Health Effects in Humans
Accelerated heartbeat
Increased reaction time
Poor judgment
Loss of consciousness
Coma
Brain damage
Death
Water Cycle
• Clouds form by convection in high, cold regions of
troposphere (see next lecture)
• Stronger convection more clouds
– Thunderstorms on summer afternoons
– Lush jungle regions at equator
– Desert at 20-30o, depleted of moisture (see lecture on wind)
CO2 cycle
Cycle driven by water
Without water CO2 stays
in atmosphere as on Venus
• Critical for greenhouse effect (see later lecture)
The “Gaia” feedback mechanism
Self regulating Earth
An hypothesis
“a complex entity involving the Earth's biosphere, atmosphere,
oceans, and soil; the totality constituting a feedback or cybernetic
system which seeks an optimal physical and chemical environment
for life on this planet.” James Lovelock
Arguments:
• Earth’s surface temperature remained roughly constant, despite
change of 30% in solar energy input
• Even though out of equilibrium, atmospheric composition remains
constant
• Ocean salinity is constant
Criticism:
What mechanism drives self-regulation ?
“there was no way for evolution by natural selection to lead to altruism on a Global scale”
Richard Dawkins, Extended Phenotype
The “Gaia” feedback mechanism
Daisy world - A computer model
planet orbiting a sun & slowing getting more heat from it
planet inhabited by two types of daisy – black & white
reproduction rate of both have same dependence on T
However,
white – reflect light – cooling planet
black – absorb light – heating up planet
Black hotter, reproduce more
Leave to run – reach equilibrium
Planet goes from black daisy dominating
to white daisy dominating as it keep surface
temperature constant
Stable and Self-regulating – within temperature limits
Example exam questions
Q1. Name three processes which add oxygen to the
Earth’s atmosphere?
Q2. Describe the main features of Daisy world?
What is its significance?
Q3. What will happen to oxygen in the earth’s
atmosphere if living organisms were to die off?
Next lecture – structure of planetary atmosphere
O2 & CO2 cycles
150
Height (km)
Thermosphere
100
Mesosphere
50
Stratosphere
Troposphere
0
0
300
600
Temperature (K)