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Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.957...15h&link_type=abstract
Origin of the Earth and Moon, Proceedings of the Conference held 1-3 December, 1998 in Monterey, California. LPI Contribution N
Other
Atmospheric Chemistry, Cosmochemistry, Gravitational Fields, Moon, Solar System Evolution, Geochemistry, Atmospheric Composition, Isotope Ratios, Lunar Evolution, Selenology, Earth (Planet), Chondrites, Stability, Geochronology, Lunar Composition, Chemical Composition
Scientific paper
Precise measurements of the isotopic composition of K stable isotopes in the bulk Earth, Moon, eucrites, and chondrites revealed no evidence of mass fractionation. Likewise, isotopic mass fractionation has not been found in Mg, Si, and Ca between the Earth and Moon. Interpretation of these data has been the subject of much discussion, representing a significant obstacle in our understanding the use of isotopes as cosmochemical constraints. A set of simple considerations is set forth here, providing interpretational guidelines for the use of certain stable isotopes to determine the nature of cosmochemical processes that determined the composition of the Earth, Moon, and other solid solar-system materials. If these stable isotopes are to be useful in cosmochemistry, such a basic understanding must exist. The considerations are (1) certain light stable isotopes show little or no resolvable mass fractionation within planetary bodies (e.g., Mg, K, and Ca); i.e., low-temperature equilibrium isotopic fractionations for these elements is too small to resolve; (2) kinetic isotopic fractionations are sufficiently large to be clearly resolvable; (3) such kinetic isotopic fractionations occur during evaporation of condensed material; and (4) condensation of vapor produces no resolvable isotopic fractionation. If these conditions are satisfied, light stable isotopes of Mg, K, Ca, and, to a lesser extent, Si, can be used to place firm constraints on the role of high-temperature processes in determining the chemical composition of the Earth and Moon. A second set of considerations is required for applying chemical and isotopic constraints on extremely hot bodies having a significant gravitational field; i.e., the gravitational potential must be included in energetic considerations for vapor loss. For objects greater than 100 km in diameter, the gravitational potential exceeds the thermal energy required to vaporize a molecule from a condensed solid or liquid. Thus, the molecule needs significant additional energy to escape the planetary gravitational field, failing which it forms an atmosphere around the body. Loss of molecules from an atmosphere proceeds by several processes: (1) Jeans or thermal escape, (2) hydrodynamic escape (which is a limiting case of Jeans escape), and (3) impact erosion of the atmosphere. This last process is important only when an object is undergoing intense accretion, and then only for molecules that have long residence times in the atmosphere. It is unlikely to be important for extremely hot, silicate vapor atmospheres that have lifetimes of thousands of years. For the present, we will ignore impact erosion. Both Jeans escape and hydrodynamic escape are mass-dependent loss mechanisms, and fractionate both the isotopic compositions and the interelement ratios in the atmosphere in a direction of preferential loss of the lighter molecular species. This results in significantly more loss of Li over Yb, K over Rb and Cs and will not allow the loss of highly volatile elements like Tl and Pb. Such processes are excluded by the existing chemical and isotopic data for the lunar and the terrestrial compositions. The chemical differences between Earth and MOon can then only be understood in terms of inheritance of these chemical signatu8res from the proto-lunar matter, which must then be distinct from terrestrial mantle material. That having been said, the long time emphasis place by many geochemists and cosmochemists on the similarity of the Earth and Moon is unsupportable. Similarities can originate from both objects having similar nebular histories, and they do not imply an origin of lunar matter from the terrestrial mantle.
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