Planetary Atmospheres, the Environment and Life (ExCos2Y)

Topic 2: Evolution of Earth’s Atmosphere

Chris Parkes

Rm 455 Kelvin Building

• 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

1. Composition of the Atmospheres of Earth, Mars and Venus

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

Too near the sun (hot)

water boils off

Too far away (cold)

water freezes

Orbit of Venus

Orbit of Mars

Orbit of Earth

Sun

BUT – significant effect of greenhouse gases on temperature, see later

A home away from home?

(liquid water)(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 CO2 removalOxygen removal

The modern O2 cycle

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.

Modern O2 cycle

Photochemistry: 2H2O O2 + 2H2 (H2 thermal escape)

2O2 O + O3 (solar ray formation of ozone)

rate: + 108 kgWeathering (chemical reactions): rate: - 1011 kg/yearVolcanism: 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/yearRecycling of sediments: rate: < - 1011 kg/year

Fossil fuel combustion (O2 CO2): rate: - 1012 kg/year

Note: rough mass of O2 in atmosphere 1017 kg

Origin of Oxygen and Ozone

• Oxygen and aerobic life linked

Uncertainty

Oxygen produced by life

first organisms anaerobic

later aerobic, plants & animals

Oxygen in oceans forms banded iron

Free Oxygen, redbeds occur

UO2 ,

FeS2 (pyrite)

Oxygen Content of atmosphere over time

The three-reservoir model of Earth

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 presentB) oxidising: enough O2 to oxidise

mineral but not enough for respirationC) aerobic: enough O2 to support aerobic

respiration

Atmosphere

Shallow Ocean

Deep Ocean

Reducing Oxygenating

volcanic gases

weathering

volcanic gases

photochemistry

photosynthesis

4 stages in the history of O2 on Earth

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)

Atmosphere(reducing)

Shallow Ocean(reducing)

Deep Ocean(reducing)

Reducing Oxygenating

volcanic gases

weathering

volcanic gases

photochemistry

Shallow Ocean(oxidising)

Reducing Oxygenating

volcanic gases

weathering

volcanic gases

photochemistry

photosynthesis

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

Atmosphere(oxidising)

Deep Ocean(reducing)

4 stages in the history of O2 on Earth

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

Atmosphere(aerobic)

Shallow Ocean(aerobic)

Deep Ocean(oxidising)

Reducing Oxygenating

weathering photosynthesis

respiration/decay

volcanic gases

4 stages in the history of O2 on Earth

Stage IV:Deep ocean becomes

aerobic

New organisms

Balance between respiration and photosynthesis

Reducing Oxygenating

respiration photosynthesis

Atmosphere(aerobic)

Shallow Ocean(aerobic)

Deep Ocean(aerobic)

respiration

respiration

As O2 increases, photochemistry creates ozone in upperatmosphere. When ozone layer is thick enough to shield from solar rays then living organisms can live out of water

4 stages in the history of O2 on Earth

Dependency of life on Oxygen

Oxygen (%) Health Effects in Humans

17 Accelerated heartbeat

16 Increased reaction time

15 Poor judgment

10 – 12 Loss of consciousness

8 – 10 Coma

< 8 Brain damage

< 6 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

• Critical for greenhouse effect (see later lecture)

Cycle driven by water

Without water CO2 stays in atmosphere as on Venus

The “Gaia” feedback mechanismSelf regulating EarthAn 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 constantCriticism:

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 modelplanet orbiting a sun & slowing getting more heat from itplanet inhabited by two types of daisy – black & whitereproduction rate of both have same dependence on T

However, white – reflect light – cooling planetblack – absorb light – heating up planet

Black hotter, reproduce moreLeave to run – reach equilibriumPlanet goes from black daisy dominating to white daisy dominating as it keep surfacetemperature 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

Height (km)

Temperature (K)

00 300 600

50

100

150

Troposphere

Stratosphere

Mesosphere

Thermosphere