Measuring nitrogen isotope ratios in lunar soils as a function of depth, Hashizume et al. (Reports, 10 November 2000, p. 1142) found evidence for two N sources: a 15N-depleted component associated with solar wind (SW) hydrogen, and a nonsolar, 15N-enriched "planetary" component. Hashizume et al. argued that these patterns could not be explained by isotopic fractionation, but instead required "the contribution of 15N-rich compounds," mostly of interstellar origin, "to the total nitrogen in planetary materials." Kerridge, in a comment, maintains that "rather simple arguments serve to illustrate the failure" of such a two-component model in explaining lunar regolith N isotope variability, and raises a number of other objections to the analysis of Hashizume et al. "The isotopic variability of lunar regolith N," Kerridge concludes, ". . . still lacks a viable explanation." Hashizume et al. respond by proposing a new model that relies on "variations in the flux ratio between the SW and micrometeorites" to explain the isotopic variation of lunar regolith N--a model that they hold is consistent with criteria cited by Kerridge for a hypothetical planetary component in lunar regolith N.
The full text of these comments can be found at www.sciencemag.org/cgi/content/full/293/5537/1947a.
