Helium and Neon in Carbon-Rich Phases of Interplanetary Dust Particles

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Interplanetary dust particles (IDPs) are exposed to solar wind (SW) and solar energetic particle (SEP) radiation as they spiral sunward by Poynting-Robertson drag. SW ions penetrate particle surfaces to depths of 10s of nanometers; implantation depths of SEP ions are poorly known but are probably on the order of ~1 μm. Saturation doses of SW-He are incident on grain surfaces in just a few centuries of exposure near 1 AU. One would therefore expect the He inventories of IDPs impacting the top of the Earth's atmosphere to be dominated by SW-SEP mixtures residing largely in surficial and thermally labile sites. Measured 3He/4He ratios do indeed fall between the SW and SEP compositions for the majority of IDPs (others, however, display intriguing and as yet unexplained elevations of 3He/4He that cannot be due to solar corpuscular radiation). Flash heating of IDPs during atmospheric drag deceleration depletes these SW-SEP reservoirs and shifts laboratory 4He release profiles toward higher temperatures, to extents that depend on the intensities of drag-heating. These profile shifts have been used as relative measures of IDP atmospheric entry speeds and thus as a way to distinguish between probably asteroidal and probably cometary particles.


Physics and Astronomy