XENON AND KRYPTON ISOTOPES IN EXTRATERRESTRIAL REGOLITH SOILS AND IN THE SOLAR-WIND

Isotopic distributions of pure solar-wind xenon and krypton are derive d from an extensive data base of xenon and krypton compositions evolve d from lunar and meteoritic regolith samples by acid-etching or combus tion-pyrolysis experiments in several different laboratories. Regolith Xe and Kr are nonuniform mixtures of primary solar-wind components wi th others arising in situ from cosmic-ray spallation, neutron-capture in iodine and bromine, and, for Xe, from adsorption or shallow implant ation of radiogenic isotopes generated by decay of long-extinct radion uclides and still extant in the regolith. A previously unrecognized ir radiation effect in Xe has generated variable and often large excesses of Xe-126 in both lunar and asteroidal regolith samples. Their most l ikely production mechanism appears to be solar proton reactions on dis persed surface-correlated iodine, but for most samples this requires s ubstantially more iodine circulating in the topmost regolith than is r eflected 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 Xe-134 and X e-136, components which from other evidence, observed and inferred, ap pear 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 regolith contributi on with an isotopic distribution fully consistent with that of Pu-244 fission Xe. A solar-energetic-particle (SEP) Xe component, isotopicall y distinct from the solar wind composition, is not apparent in this an alysis. SEP-like fractionated krypton, however, is clearly a contribut or to the regolith Kr system, completely in line with conclusions reac hed by the Zurich group, along with solar-wind Kr and the expected add itions from spallation and neutron capture in dispersed regolithic bro mine. A large number of Kr fractions released from a variety of sample s of differing antiquities by early acid attack and low-temperature py rolysis-combustion, 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 tim e-invariant composition for pure solar-wind Kr. This composition is es sentially identical to that of an ancient Kr component inferred, from models of atmospheric evolution on the two planets, to have been prese nt in the primordial atmospheres of Earth and Mars.