Helium and Neon in Carbon-Rich Phases of Interplanetary Dust Particles
Physics and Astronomy
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.
R. L. Palma, D. J. Schlutter, R. O. Pepin, D. E. Brownlee, and D. O. Joswiak, 2001. Helium and Neon in Carbon-rich Phases of Interplanetary Dust Particles. 32nd Lunar and Planetary Science Conference, Abstract #2074, Lunar and Planetary Institute, Houston.