Xenon and krypton isotopes in extraterrestrial regolith soils and in the solar wind

Abstract

Isotopic distributions of pure solar-wind xenon and krypton are derived from an extensive data base of xenon and krypton compositions evolved from lunar and meteoritic regolith samples by acid-etching or combustion-pyrolysis experiments in several different laboratories. Regolith Xe and Kr are nonuniform mixtures of primary solar-wind components with others arising in situ from cosmic-ray spallation, neutron-capture in iodine and bromine, and, for Xe, from adsorption or shallow implantation of radiogenic isotopes generated by decay of long-extinct radionuclides and still extant in the regolith. A previously unrecognized irradiation effect in Xe has generated variable and often large excesses of 126Xe in both lunar and asteroidal regolith samples. Their most likely production mechanism appears to be solar proton reactions on dispersed surface-correlated iodine, but for most samples this requires substantially more iodine circulating in the topmost regolith than is reflected in their measured contents. We find that the pure solar-wind composition itself can be accurately modeled as a mixture of “UXe” and a heavy-isotope constituent containing primarily only 134Xe and 136Xe components which from other evidence, observed and inferred, appear to have been fundamental constituents of primordial solar-system Xe. Deconvolution of measured heavy-isotope Xe compositions according to this view reveals the presence of an additional rcgolith contribution with an isotopic distribution fully consistent with that of 244Pu fission Xe. A solar-energetic-particle (SEP) Xe component, isotopically distinct from the solar wind composition, is not apparent in this analysis. SEP-like fractionated krypton, however, is clearly a contributor to the regolith Kr system, completely in line with conclusions reached by the Zürich group, along with solar-wind Kr and the expected additions from spallation and neutron capture in dispersed regolithic bromine. A large number of Kr fractions released from a variety of samples of differing antiquities by early acid attack and low-temperature pyrolysiscombustion. and therefore considered to evolve primarily from the shallow, largely SEP- and spallation-free solar-wind implantation zones in grain surfaces, point firmly to an isotopically light and time-invariant composition for pure solar-wind Kr. This composition is essentially identical to that of an ancient Kr component :inferred, from models of atmospheric evolution on the two planets, to have been present in the primordial atmospheres of Earth and Mars.