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

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We report mass-spectrometric measurements of light noble gases pyrolytically 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, especially given their wide mineralogic and chemical variability, but two intriguing differences from isotopic distributions observed in lunar and meteoritic regolith grains are already apparent. First are puzzling overabundances of 3He, manifested as often strikingly elevated 3He/4He ratios—up to >40x the solar-wind value—-and found primarily but not exclusively in shards of some of the larger IDPs (“cluster particles”) that fragmented on impact with the collectors carried by high-altitude aircraft. It is difficult to attribute these high ratios to 3He production by cosmic-ray-induced spallation during estimated space residence times of IDPs, or by direct implantation of solar-flare He. Minimum exposure ages inferred from the 3He excesses range from ∼50 Ma to an impossible >10 Ga, compared to Poynting-Robertson drag lifetimes for low-density 20–30 μm particles on the order of ∼0.1 Ma for an asteroidal source and ∼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 group of individual and cluster IDPs from three different collectors. Values of the SEP/SW fluence ratio in interplanetary space from a simple model utilizing these data are ∼1% of the relative SEP/SW abundances observed in lunar regolith grains, but still factors of approximately 10–100 above estimates for this ratio in low-energy solar particle emission.


Physics and Astronomy

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Meteoritics and Planetary Science