Transcript The Origin of the Atmosphere - University of Illinois at

Information

Class Website


http://www.atmos.uiuc.edu/courses/atmos397g/
You are welcome to give your suggestions to
improve the site!
The 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 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
Atmosphere: 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 + hn  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
Atmosphere: 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
Atmosphere: 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
O 2 + O + M  O3 + M

Evolution of several new biochemical pathways
significant to the global biogeochemical cycles, e.g.,
nitrification
Principal geophysical and geochemical processes
contributing to the evolution of the atmosphere
Evidence for lack of free oxygen in the
Earth’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 Formations
Alternating bands of red jasper and black hematite,
about 2250 million years old (2.55 billion years old)
Jasper Knob, Ishpeming, Michigan
Red Beds
Cumulative history of O2 released by
photosynthesis through geologic time
Origin 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 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