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The Origin of the Atmosphere Lecture Objective: l To identify how the earth’s atmospheric constituents have evolved by outlining the principal events in the evolution of the earth’s atmosphere l To identify the factors that determine the capability of a planet to maintain an atmosphere
The Early Earth n Hot: l Formed in the inner portion of solar nebula l bulk of the mass comes from collision and compression of planetesimals during accretion l heat generated from radiaoactive decay and collision of planetesimals n Devoid of atmosphere l Gravitational field too weak to retain gaseous elements
Stages in the Evolution of the Earth’s Atmosphere: Stage I n Primitive Atmosphere l Produced as a result of volcanic outgassing l 4.4 – 4.0 billion years ago with a time span of one million years l A “reducing atmosphere” primarily consisting of H 2 and He, with trace levels of CO, CH 4, H 2 O (v), N 2, H 2 S, NH 3, HCl, Ar, and HCN l Lighter gases (H 2 and He) escaped to space l CH 4 CO CO 2 (oxidation) 2NH 3 + h N 2 + 2H 2
n Reduced substance: electron-rich tendency to lose electrons l H 2, NH 3, CH 4 n Oxidized substance: electron-poor tendency to gain electrons l O 2
Stages in the Evolution of the Earth’s Atmosphere: Stage II n Secondary Atmosphere l Continued outgassing from the Earth’s interiors l 4.0 to 3.3 billion years ago l H 2 O, N 2, and CO 2 predominant constituents, with trace levels of CO, SO 2, Ar, He l Cooling of earth resulted in condensation of water vapor and the appearance of oceans (3.8 billion years ago) l Water soluble gases (CO 2, SO 2, HCl) dissolved in the primitive ocean l Appearance of chemosynthetic bacteria about 3.5 bya l First appearance of Oxygen (O 2 ) in the prebiotic atmosphere
l No accumulation of O 2 at this stage – used up for oxidation of reduced species
Stages in the Evolution of the Earth’s Atmosphere: Stage III n Living Atmosphere l 3.3 bya to present l Accumulation of O 2 to its present day atmospheric level of 21% as early as 430 million years ago l Development of the “ozone” layer responsible for shielding the earth’s surface from UV rays O 2 + O + M O 3 + M l 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 n Banded Iron Formations (BIF) l Fe 2+ oxidized to Fe 2 O 3 in the sediments of the primitive ocean l Peak occurrence in rocks of 2.5 to 3.0 billion years ago n Red Beds l Oxidation of exposed reduced minerals, such as FeS 2, on the barren land resulted in alternating layers of Fe 2 O 3 with sediments of land origin. l Earliest occurrence not before 2.0 bya n Oxygen poisoning of methanogenic bacteria and sulfur bacteria n Chemical building blocks of life could not have been formed in the presence of atmospheric O 2
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 O 2 released by photosynthesis through geologic time
Origin of Life n First sign of single-celled life 3.5 bya n Abiotic synthesis aided by exogenous source of organic molecules n Traditional viewpoint: life arose in the sea l Important biochemical elements also abundant in seawater n Methanogenesis, sulfate-reduction, and N-fixation: primitive pathways of anaerobic metabolism n O 2 production by photosynthesis and the subsequent formation of O 3 layer paved way for colonization of land by higher organisms
Chemical Evidence for Origin of Life n Miller-Urey Experiment l Synthesis of simple, reduced organic molecules from constituents of primitive atmosphere and ocean (CH 4, NH 3, H 2, H 2 O) l Experiment successful in abiotic conditions l Building blocks of life (amino acids) could be synthesized in primitive secondary atmospheric conditions