1966028656-953 https://ntrs.nasa.gov/search.jsp?R=19660028674 2020-03-11T17:51:16+00:00Z t ORIGIN AND EVOLUTION OF ATMOSPHERES AND OCEANS* A. G. W. CAMERON, P. J. BRAhCAZIO, AND N. W. PANAGAKOS In recent years it has become evident that the careful in applying the results thus obtained to atTnosphere and oceans have been produced by the the escape of the other rare gases from the earth. outgassing of volatile materials from the earth's L.V. Berkner of the Southwest Center for Ad- interior, principally from volcanoes. In 1950 W. vanced Studies, Dallas, Texas, discussed the origin W. Rubey presented geological evidence indieat- of oxygen in t,m earth's atmosphere. According ing that sea water has progressively accumulated to Berkner, the existence of an appreciable atmo- in this way. At about the same time, Harrison sphere of oxygen implies the presence of life. He Brown pointed out that the very low abundances pointed out that the production of oxygen in the of the noble gases in the earth's atmosphere con- primitive atmosphere from the photochemical dis- • stituted compelling evidence that at least the sociation of water would in turn lead to a layer atmospheric oxygen and nitrogen were almost of ozone in the atmosphere. The ozone layer is a _: er,tirely of secondary origin, having been out- buffer against the destruction of terrestrial life as _: gassed, possibly in different chemical form. we know it by the ultraviolet rays of the sun. If The main purpose of the 1963 conference was the amount of oxygen in the atmosphere were to consider new evidence for mechanisms which very small, the ozone layer would occur at ground ,_ might add to or subtract from the contents of the level, and the sun's ultraviolet radiation would '_ atmosphere and oceans, and to consider mecha- penetrate to the surface. In such a harmful en- nisms which could contribute to the origin of at- vironment, life would be unable to emerge frc.m !_ mospheres of other planets. H.H. Hess opened the seas onto land. the discussion with a consideration of the prob- Tracing the history of the earth's atmospllere, lems of convection currents in planetary mantles. Berkner pointed out that a sufficiently long period The important factor is th_ time between nucleo- of time had to elapse for marine life to produce i synthesis and the formation of the solid planet, sufficient oxygen to change the composition of the _ The protoplanet will contain radioactive materials atmosphere and produce high-lying ozone layers. _ which generate beat, and the rate at which it is When this took place, lifo was able to emerge-- !_ generated will decrease as the activity of the radio- first in plant form, _hen in animal form in perhaps _ isotopes decreases. A planet may or may not a few million years. have convection, depending on whether enough The helium problem wa_ examined by G. J. F. } radioactive materials remain in the mantle to MacDonald of the University of California at Los .!. cause vigorous heating at the base. Angeles. He pointed out that if it is assumed that K. K. Turekian of Yale University discussed the escape of gases from the earth's atmosphere is some models for the degussing of argon from the due only to temperate effects, then the rate of earth. He showed that at least eighty percent of influx of helium by out gassing is larger than the the argon now in the atmosphere must have come rate of escape. The helium in the earth's atmo- from the mantle. He proposed that the most use- sphere should be building up a¢ a rapid rate, but : ful degassing model appeared to be one with con- this is not the case. MacDonald noted that tinuous degassing of the earth as a whole. Ture- analyses of satellite drag data by I. Harris and i kian pointed out, however, that one should be W. Priester showed that an additional heat source derived from the solar wind is necessary to account *PubliahinedPhy_dcT,oday:.19-22F, ebruary1964. for the time variations in the temperature of the 907 ¢ 1966028656-954 908 PUBLICATIONS OF GSFC, 1964: I. SPACE SCIENCES leO0 2200 WAVE LENGTH SKETCH1.--Ultraviolet absorptionby variousatmosphericco_tituents, aspresentedby L. V. Ber]_er of the Southwest Centerfor AdvancedStudies. The absorptionbeyond2000angstromsis almostentirelydueto ozone. upper atmosphere. He also noted that if the _ " ' ' ' ' ' ' stronger, as indicated by measurementsof ancient .j ...._ AveragechondriteCOrbofloceOUSxenon earth'smagnetic fieldwas at one time significantly 600 I" ...e Richordton xenon baked clay, the interaction between the field and _ 400 _ °-_ +error solar plasma could increase the heating of the _ - -_. upper atmosphere. This could account for a ,o_ 2oo ""'".'._ greater rate of escape of helium, averaged over a loi_gtime scale, o _"'"-_ -,_.._._ R. O. Pepin discussed some of the recent work -_o I I _ , _ , ' done by J. R. Reynolds and his group at the Uni- 124 t26 _2e 130 132 134 136 versity of California, Berkeley, in the field of M "xenology"--the study of the isotopic abundances SK_.TC2.--Compa. rison betweenthegeneral anomaliesin of the xenon isotopes. These studies are provid- xenonfromthe Richardtonstone meteorite (an ordinary ing com'4derableinformation about the early his- chrondrite) andinxenonfromcarbonaceouschondrites. tory of the solar system; for example, the anomal- ously high abundance of _29Xeresulting from the In this respect, Pepin cited the work of W. B. decay of primordial 1"I allows us to obtain the Clarke, who conducted heating experiments on formation ages of meteorites. He pointed out irradiated uranium oxide samples. Clarke found that the relative abundances of the xenon isotopes .that the xenon evolved at low temperatures was differ for di,_erent types of meteorites and, in isotopically different from that evolved at high particular, the isotopic abundances of the xenon temperatures. The inference is that the isotopic isotopes in tbc earth's atmosphere differ from the anomalies may be due to temperature fractions- abundances found in meteorites. The problem is tion effects on a fissionogenic component of the to account for all these anomalies, xenon gas. 1966028656-955 _! PLANETARY ATMOSPHERES 909 Among the studies done by the Berkeley group was an analysis of some deep-seated terrestrial . _ rocks. They found that the xenon evolved from " i" ing from uranium spontaneous fission. This sug- _ • "_'_ ",_ gests the feasibility of a U-Xe dating method, to - "_" _ be used in conjunction with other established "'_ I ._._- " • _ , _ datingthese rockstechniques.contained a fission component result- ' ._' __ :I _".' _: _. Peter Signer of the University of Minnesota _i showed that the abundances of primordial gases _,_ in meteorites are generally similar to those in the _ atmosphere or in the sun only when there is a S_ETCU&--Seated in the cabin of his balloonobservatory, :_ Audouin Dollfus of the Observatoirede Paris prepares _i_ great amount uf this gas in the meteorites. If the for a flight in which he attempted to detect extrat_r- _ amount of gas is small, then a very large degree of restrial water. _ fractionation can occur among the elements. In order to understand the composition of the perature of an upper atmosphere of pure argon earth's atmosphere, one must be able to explain at well above 1400 degrees, since argon is a very i the differences between the isotopic abundances inefficient radiator of electromagnetic energy. It : of xenon isotopes in meteorites and in the earth, was agreed that much more study is needed for an _: A.G.W. Cameron of the Goddard Institute for understanding of the manner in which an argon _' Space Studies interpreted the differences in abun- atmosphere--if it exists--could be maintained on dances of the light "shielded" xenon isotopes as Mercury. ! resulting from neutron capture in the sun during Important new data concerning the presence of • the deuterium-burning stage of early solar history, water vapor on Mars and Venus was presented However, this interpretation requires that the by Audouin Dollfus. Reporting on observations : bulk of the xenon in the earth's atmosphere should made with a specially designed telescope which have once been in the sun, and that it has since he carried aloft in a balloon, and on observations been captured by the earth from the solar wind. made from an elevated mountain observatory, The c_mposition of Mercury's atmosphere Dollfus said he measured 0.01 gm/cm _ of water raised considerable discussion among participants vapor for Venus above the cloud-top level. at the conference. Mercury is the smallest planet, For Mars, Dollfus found 0.02 gm/cm 2of water and, because of its small size, has been generally vapor, enough to cover that planet to a depth of believed to have no atmosphere, or a very tenuous a fifth of a millimeter. This is five times as much _: one at best. George Field of Princeton University as had been generally estimated. The disagree- • discussed measurements of the differential polar- ment between Dollfus' observations and those of ization of scattered light from Mercury made by several other scientists who give lower values for " Audouin Dollfus of the Observatoire de Paris, the water-vapor content of the Martian atmo- Meudon, France, who had interpreted them as sphere raised sharp debate at the conference. _ showing the presence of a small atmosphere. Participants concluded that various experiments Field suggested that the atmosphere of Mercury may have been subject to unforeseen sources of : may be composed of a considerable amount of error, and that it is extremely important to make radiogenic argon which escaped from the interior new measurements.