earth’s evolving atmosphere

8/8/2019 Earth’s Evolving Atmosphere

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Earth·s EvolvingAtmosphere



First Atmosphere Composition - Probably H2, He

These gases are relatively rare on Earth

compared to other places in the universe andwere probably lost to space early in Earth'shistory because ± Earth's gravity is not strong enough to hold lighter

gases

± Earth still did not have a differentiated (layered) core(solid inner/liquid outer core) which creates Earth'smagnetic field which deflects solar winds.

Once the core differentiated the heavier gases

could be retained



Second AtmosphereSecond Atmosphere

Volcanic out gassingVolcanic out gassing

Gases produced were probably similar to those creatGases produced were probably similar to those creat

by modern volcanoes (Hby modern volcanoes (H22O, COO, CO

22, SO, SO

22, CO, S, CO, S

22, Cl, Cl

22, N, N

22,,

HH22) and NH) and NH33 (ammonia) and CH(ammonia) and CH44 (methane).(methane).

No free ONo free O22 at this time (not found in volcanic gases).at this time (not found in volcanic gases).



Comet impactsComet impacts

Sun was only at 70% of its current energySun was only at 70% of its current energy

level so Earth was cooler.level so Earth was cooler.

Water from volcanic eruptions would haveWater from volcanic eruptions would have

been frozen unless atmosphere was verybeen frozen unless atmosphere was very

thickthick

As comets hit the Earth the water would haveAs comets hit the Earth the water would have

vaporized into the atmosphere from the heatvaporized into the atmosphere from the heat

of friction creating the greenhouse effect toof friction creating the greenhouse effect to



Addition of O2 to the Atmosphere

Today, the atmosphere is ~21%Today, the atmosphere is ~21%

free oxygen. How did oxygen reachfree oxygen. How did oxygen reachthese levels in the atmosphere?these levels in the atmosphere?

Revisit the oxygen cycle:Revisit the oxygen cycle:



Oxygen ProductionOxygen Production

± ± PhotochemicalPhotochemical dissociationdissociation -- breakup of breakup of water molecules by ultraviolet rayswater molecules by ultraviolet rays

Produced OProduced O22 levels approx. 1levels approx. 1--2% current levels2% current levels

At these levels O At these levels O33 (Ozone) can form to shield(Ozone) can form to shieldEarth surface from UVEarth surface from UV



Water Hit With Young

Star's Best Shot

Pasadena CA (SPX) Sep 22, 2008

Water is being blasted to pieces by a young star's laser-like jets,according to new observations from NASA's Spitzer SpaceTelescope. The discovery provides a better understanding of how water -- an essential ingredient for life as we know it -- is

processed in emerging solar systems. The shock from collidingatoms and molecules generates ultraviolet radiation, which willbreak up water molecules, leaving extremely hot hydroxylmolecules.



Photosynthesis CO2 + H2O + sunlight = organic compounds

+ O2

produced by cyanobacteria, and eventually

higher plants - supplied the rest of O2 toatmosphere.



Oxygen Consumers

± Chemical Weathering - through oxidation of

surface materials (early consumer)

± Animal Respiration (much later)

± Burning of Fossil Fuels (much, much later)



3.8 to 2.5 billion years ago3.8 to 2.5 billion years ago

Throughout the Archean there was little toThroughout the Archean there was little tono free oxygen in the atmosphere (<1% of no free oxygen in the atmosphere (<1% of

present levels).present levels).

What little was produced by cyanobacteria,What little was produced by cyanobacteria,

was probably consumed by the weatheringwas probably consumed by the weathering

process.process.

Once rocks at the surface were sufficientlyOnce rocks at the surface were sufficiently

oxidized, more oxygen could remain free inoxidized, more oxygen could remain free in

the atmosphere.the atmosphere.



2.5 billion to 543 million years ago

During the Proterozoic the amount of free

O2 in the atmosphere rose from 1 - 10 %.

Most of this was released by

cyanobacteria, which increase inabundance in the fossil record 2.3 Bya.

Present levels of O2 were probably not

achieved until ~400 Mya.

earth’s evolving atmosphere

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