Other
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003trgeo...4..319p&link_type=abstract
Treatise on Geochemistry, Volume 4. Editor: Ralph F. Keeling. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp. 3
Other
Scientific paper
One of the salient characteristics of the composition of the Earth is the depletion in volatiles compared to parental solar-nebula relative abundances, and this is most pronounced in the noble gases. These chemically unreactive species, concentrated in the atmosphere, have retained many characteristics established early Earth history. A comparison between noble gases on the terrestrial planets and other solar system objects reveals significant differences in both elemental ratios and isotopic compositions and indicates that complex processes were involved in accumulating planetary volatiles from the nebula. Therefore, the atmosphere is not primary (i.e., directly acquired entirely from the solar nebula without modification). Atmophile elements have been added to the Earth in material that has contributed into the growing solid Earth and has subsequently degassed into the atmosphere, in late-accreting materials that degassed upon impact, and possibly directly to the atmosphere from the nebula as well. However, the importance of each of these sources, and the processes that modified these volatiles after initial capture, are still debated.While considerations of the origin of planetary noble gases have been predominantly focused on those presently found in the atmosphere, noble gases still within the Earth provide further constraints about volatile trapping during planet formation. A wide range of noble-gas information for the Earth's mantle has been obtained from mantle-derived materials, and indicates that there are separate reservoirs within the Earth that have distinctive characteristics that were established early in Earth history. These must be included in comprehensive models of Earth volatile history. Also, data are now available for the atmospheres of both Venus and Mars, as well as from the interior of Mars, so that the evolution of Earth volatiles can be considered within the context of terrestrial-planet formation across the solar system.The origins of the noble-gas features of the terrestrial-planet atmospheres and interiors have defied simple explanations and are clearly the result of a combination of acquisition and subsequent loss processes that have generated unique solar system compositions. The relevant data for noble gases in the atmospheres and interiors of the terrestrial planets, and the constraints these provide, are summarized below first. The emphasis is on the information provided by noble gases, since the signatures of volatile origin would be expected to survive most clearly in these species, although the implications for the major volatiles - nitrogen, carbon, and hydrogen - are also covered. Acquisition and loss processes are then discussed separately. Models for each planet are then described that are consistent with the available data and involve both acquisition episodes and subsequent modifications during partial loss.The numerous debates regarding the origin of noble gases on the terrestrial planets have inevitably engendered explanations that are no longer viable, as well as more controversial viewpoints that, as of early 2000s, have not received sufficient support or substantial acceptance. These are discussed further by Pepin and Porcelli (2002).
Pepin Robert O.
Porcelli Don
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