LLOYD V. BERKNER MEMORIAL SYMPOSIUM

Evolution of the Earth's Atmosphere

AAAS Annual Meeting26-3 1 December 1967

New York City

We living things are a late outgrowthof the metabolism of our Galaxy. Thecarbon that enters so importantly intoour composition was cooked in the re-

mote past in a dying star. From it atlower temperatures nitrogen and oxy-gen were formed. These, our indis-pensable elements, were spewed out in-to s.pace in the exhalations of red giantsand such stellar catastrophes as super-novae, there to be mixed with hydro-gen, to form eventually the substanceof the sun and planets, and ourselves.The waters of ancient seas set the pat-tern of ions in our blood. The ancientatmospheres molded our metabolism.

-GEORGE WALD

The present composition of theearth's atmosphere is 78 percent nitro-gen, 21 percent oxygen, and 1 percentargon, with traces of carbon dioxide,water vapor, and ozone. The atmo-spheres of Mars and Venus, on theother hand, are predominantly com-

posed of carbon dioxide, while those ofJupiter and Saturn contain mainly hy-drogen, helium, methane, and am-

monia. Such a wide variety in the com-

position of the atmospheres of theplanets appears most intriguing whenone considers that all nine planets were

probably formed at the same time andout of the same chemically homoge-neous mixture of gas and dust; that is,the primitive solar nebula.The most likely explanation for this

diversity in composition seems to bethat the planetary atmospheres haveundergone important evolutionarychanges during their long history ofabout 4.5 billion years. On the earth,the evidence for such changes is mani-fold. Ever since Louis Pasteur dem-onstrated that spontaneous generationof life does not take place at the pres-ent time, it has become obvious that if

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life originated on the earth, as it prob-ably did, the atmospheric conditions inthe early history of the planet musthave been quite different in order foLthe chemical evolution to have takenplace. Also, it appears that relativeto the sun, the earth is deficient by sev-eral orders of magnitude, not only inhydrogen and helium, which being thelightest can be assumed to have escapedfrom the gravitational field of the earth,but also in carbon, nitrogen, and thecosmically abundant rare gases (neon,argon-36, krypton, and xenon). (SeeTable 1.) A comparison of the de-ficiency. factors of these elements withthose of nonvolatiles such as sodium,magnesium, aluminum, and silicon,strongly suggests that the earth lost allelements which could have been gas-eous at temperatures of a few hundreddegrees during the very early history ofthe planet. The relatively small amountsof carbon, hydrogen, nitrogen, and oxy-gen now present in the crust, hydro-sphere, and atmosphere were probablyoutgassed from the interior and ac-

Table 1. Deficiency of elements in the earth.

Atoms/10,000 atoms of siliconEle- Whole Solar Deficiencyment earth* systemt factor

(a) (b) log (b/a)

HHeC

N0

NeNaMgAlSiAlKrXe

2503.5 X 10-7

140.21

35,0001.2 x 10-'

4608,900940

10,0005.9 x 10-46 x 10-B5 x 109-

2.6 X 10I2.1 X 107135,00024,400

236,00023,000

63210,500

85110,0002,280

0.690.07

6.013.84.05.10.8

10.3

0

6.67.17.1

* After Rubey, Ringwood, and Mason. t AfterCameron.

cumulated slowly during the lifetime ofthe earth.

Both biochemical arguments and geo-logical evidence, therefore, strongly sug-gest that the atmosphere of the earthhas undergone major evolutionarychanges during its long history. Wheth-er the atmospheres of Mars and Venushave also gone through similar stagesof evolution is a question which cannotbe answered without more knowledgeabout their composition. Future in situexploration of these planets and asearch for the rare gases, especiallyneon which is deficient on earth by afactor of 1010, will provide importantclues to the origin of these atmospheres.

For the earth there is convincingevidence that the present atmosphereand hydrosphere arose largely from theinterior by volcanic emanations. Butthe sequence of events which led topresent-day composition of N2 and °2has yet to be established. What is thehistory of volatiles now present at thesurface of the earth? Has the carbon,nitrogen, oxygen, and hydrogen alwaysbeen in the form of C20, N2, H20, andH0, or did carbon and nitrogen com-bine with hydrogen early in the earth'shistory to form CH4 and NH3? Underwhat atmospheric conditions did lifeoriginate on earth and how did the ap-pearance of life change the atmo-sphere? These are some of the basicquestions which must be answered inorder to paint a coherent picture of theevolution of the earth's atmosphere.Opinions on these questions are manyand varied. Sometimes they are almostdiametrically opposed. The Oparin-Urey theory of the origin of life on theearth, supported by the laboratory ex-periments of Miller and, more recently,of Ponnamperuma, suggest a primitiveatmosphere composed mainly of CH4with small amounts of NH3, H2, andH20 vapor. On the other hand, theschool of thought led by Abelson, andsupported also by laboratory experi-ments on the synthesis of amino acids,holds that the early atmosphere of theearth was made up of C02, CO, H2,N2, and H20 vapor.

Geologists are also divided on thesubject. Holland has presented a modelfor the evolution of the atmosphere inwhich, during the very early stage afterthe formation of the earth and at thecommencement of outgassing, the dom-inant gases could have been CH4 andH2, provided free hydrogen did not "es-cape" as rapidly as it does today.Rubey, on the other hand, believes that

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Photographs taken by the Applications Technology Satellite-I from an altitude of 23,000 statute miles aid in studies of the earth'satmosphere. [Goddard Space Flight Center]

the early atmosphere was probablymade of CO., and N., because notenough hydrogen was available to keepCH4 from converting into CO.. Hol-land's model is supported by the calcu-lations of Rasool and McGovern, whohave investigated the thermal proper-ties of model primitive atmospheres ofthe earth. They find that in an atmo-sphere with a ratio percent of 99 CH4to 1 percent H., the aiverage exospherictemperature may be as low as 650°K(compared with the present-day valueof 1500°K), thus making the escape ofhydrogen a relatively slow phenomenon.However, Abelson has argued that ifmethane were abundant in the primitiveatmosphere, the earliest rocks shouldcontain unusual amounts of organicmatter, which apparently is not thecase.

Despite the disagreement over thecomposition of the primitive attmo-sphere, it is almost certain that it wasdevoid of free oxygen. How and whendid free oxygen then become a majorconstituent of the atmosphere? The lateLloyd V. Berkner and L. C. Marshallhave presented detailed calculations of

22 SEI'TEMBER 1967

the photochemistry of H.,O, indicatingthat free oxygen was limited to about0.1 percent of the present atmosphericlevel during the entire prebiological his-tory and accumulated slowly to thepresent amount since the start of photo-synthesis about 800 million years ago.This theory is probably the most widelyaccepted explanation for the growth ofoxygen in the earth's atmosphere. How-ever, difficulties arise when one attemptsto construct an evolutionary model ofthe atmosphere which would be con-sistent from the prebiological period tothe present. For example, it is not wellunderstood whether the small amount ofO., suggested to be present in the primi-tive atmosphere would not be sufficientto rapidly oxidize CH, and NH., intoCO., and N. It may be that the pro-duction of even this smnall amount offree oxygen in the upper atmosphere bythe photodissociation of water vaporwas inhibited by the presence of NH.,itself.Some of these important questions

will be discussed in the interdiscipli-ncary symposium on the Evolution ofthe Earth's Atmosphere to be held 27

December 1967 during the AnnualMeeting of the AAAS in New YorkCity under the chairmanship of Har-rison Brown (California Institute ofTechnology).

Because of the diversity of disciplinesinvolved, an attempt has been made tobring together geologists, biologists, me-teorologists, and astrophysicists, withthe hope of obtaining fresh ideas anda broader perspective on the subject.The program includes the followingpresentations: -Nature of the early at-mosphere and ocean." P. Cloud; "Grav-itational escape of hydrogen frommodel primitive atmospheres," S. I.Rasool and W. E. McGovern; "Simu-lated atmospheres and models of thesynthesis of the first organismns," S. W.Fox: "Evolution of N.,-O., atmosphereon earth." L. C. Marshall; "Atmospher-ic CO.,. past and present." H. D. Hol-land: and summiiary and critique, Har-rison Brown.

S. I. Rxssoot.In.stitute for Space Stiudies,v NationalAeronaultics a1nd(1 Spaice Admini/istration,Newv York, New York, and Newv Yor-kU/liversit)'

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Evolution of the Earth's AtmosphereS. I. Rasool

DOI: 10.1126/science.157.3795.1466 (3795), 1466-1467.157Science 

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