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The Origin of the AtmosphereThe Origin of the Atmosphere

Lecture Objective:

To identify how the earth’s atmospheric constituents have evolved by outlining the principal events in the evolution of the earth’s atmosphere

To identify the factors that determine the capability of a planet to maintain an atmosphere

The Early EarthThe Early Earth

Hot:

Formed in the inner portion of solar nebula bulk of the mass comes from collision and compression

of planetesimals during accretion heat generated from radiaoactive decay and collision of

planetesimals

Devoid of atmosphere

Gravitational field too weak to retain gaseous elements

Stages in the Evolution of the Earth’s Stages in the Evolution of the Earth’s Atmosphere: Stage IAtmosphere: Stage I

Primitive Atmosphere Produced as a result of volcanic outgassing

4.4 – 4.0 billion years ago with a time span of one million

years

A “reducing atmosphere” primarily consisting of H2 and

He, with trace levels of CO, CH4, H2O (v), N2, H2S, NH3,

HCl, Ar, and HCN

Lighter gases (H2 and He) escaped to space

CH4 CO CO2 (oxidation)

2NH3 + h N2 + 2H2

Reduced substance: electron-rich tendency to

lose electrons

H2, NH3, CH4

Oxidized substance: electron-poor tendency to

gain electrons

O2

Stages in the Evolution of the Earth’s Stages in the Evolution of the Earth’s Atmosphere: Stage IIAtmosphere: Stage II

Secondary Atmosphere Continued outgassing from the Earth’s interiors

4.0 to 3.3 billion years ago

H2O, N2, and CO2 predominant constituents, with trace

levels of CO, SO2, Ar, He

Cooling of earth resulted in condensation of water vapor

and the appearance of oceans (3.8 billion years ago)

Water soluble gases (CO2, SO2, HCl) dissolved in the

primitive ocean

Appearance of chemosynthetic bacteria about 3.5 bya

First appearance of Oxygen (O2) in the prebiotic

atmosphere

No accumulation of O2 at this stage – used up for oxidation of reduced species

Stages in the Evolution of the Earth’s Stages in the Evolution of the Earth’s Atmosphere: Stage IIIAtmosphere: Stage III

Living Atmosphere 3.3 bya to present

Accumulation of O2 to its present day atmospheric level

of 21% as early as 430 million years ago

Development of the “ozone” layer responsible for

shielding the earth’s surface from UV rays

O2 + O + M O3 + M

Evolution of several new biochemical pathways

significant to the global biogeochemical cycles, e.g.,

nitrification

Principal geophysical and geochemical processes Principal geophysical and geochemical processes contributing to the evolution of the atmospherecontributing to the evolution of the atmosphere

Evidence for lack of free oxygen in the Evidence for lack of free oxygen in the Earth’s atmosphere until 2 byaEarth’s atmosphere until 2 bya

Banded Iron Formations (BIF) Fe2+ oxidized to Fe2O3 in the sediments of the primitive

ocean Peak occurrence in rocks of 2.5 to 3.0 billion years ago

Red Beds Oxidation of exposed reduced minerals, such as FeS2, on

the barren land resulted in alternating layers of Fe2O3 with sediments of land origin.

Earliest occurrence not before 2.0 bya

Oxygen poisoning of methanogenic bacteria and sulfur bacteria

Chemical building blocks of life could not have been formed in the presence of atmospheric O2

Banded Iron FormationsBanded Iron Formations

Alternating bands of red jasper and black hematite,

about 2250 million years old (2.55 billion years old)Jasper Knob, Ishpeming, Michigan

Red BedsRed Beds

Cumulative history of OCumulative history of O22 released by released by photosynthesis through geologic timephotosynthesis through geologic time

Origin of LifeOrigin of Life

First sign of single-celled life 3.5 bya

Abiotic synthesis aided by exogenous source of organic molecules

Traditional viewpoint: life arose in the sea Important biochemical elements also abundant in

seawater

Methanogenesis, sulfate-reduction, and N-fixation: primitive pathways of anaerobic metabolism

O2 production by photosynthesis and the

subsequent formation of O3 layer paved way for

colonization of land by higher organisms

Chemical Evidence for Origin of LifeChemical Evidence for Origin of Life

Miller-Urey Experiment

Synthesis of simple, reduced organic molecules from constituents of primitive atmosphere and ocean (CH4, NH3, H2, H2O)

Experiment successful in abiotic conditions

Building blocks of life (amino acids) could be synthesized in primitive secondary atmospheric conditions