Noble gases in interplanetary dust particles, I: The excess helium-3 problem and estimates of the relative fluxes of solar wind and solar energetic particles in interplanetary space

We report mass-spectrometric measurements of light noble gases pyrolyticall y extracted from 28 interplanetary dust particles (IDPs) and discuss these new data in the context of earlier analyses of 44 IDPs at the University of Minnesota. The noble gas database for IDPs is still very sparse, especiall y given their wide mineralogic and chemical variability, bur two intriguing differences from isotopic distributions observed in lunar and meteoritic r egolith grains are already apparent. First are puzzling overabundances of H e-3, manifested as often strikingly elevated He-3/He-4 ratios-up to >40x th e solar-wind value-and found primarily but not exclusively in shards of som e of the larger IDPs ("cluster particles") that fragmented on impact with t he collectors carried by high-altitude aircraft. It is difficult to attribu te these high ratios to He-3 production by cosmic-ray-induced spallation du ring estimated space residence times of IDPs, or by direct implantation of solar-flare He. Minimum exposure ages inferred from the He-3 excesses range from similar to 50 Ma to an impossible >10 Ga, compared to Poynting-Robert son drag lifetimes for low-density 20-30 mu m particles on the order of sim ilar to 0.1 Ma for an asteroidal source and similar to 10 Ma for origin in the Kuiper belt. The second difference is a dominant contribution of solar- energetic-particle (SEP) gases, to the virtual exclusion of solar-wind (SW) components, in several particles scattered throughout the various datasets but most clearly and consistently observed in recent measurements of a gro up of individual and cluster IDPs from three different collectors. Values o f the SEP/SW fluence ratio in interplanetary space from a simple model util izing these data are similar to 1% of the relative SEP/SW abundances observ ed in lunar regolith grains, but still factors of approximately 10-100 abov e estimates for this ratio in low-energy solar particle emission.