Evidence for a Dominant Component of Solar-Energetic-Particle (SEP) Helium and Neon in a Suite of Interplanetary Dust Particles

Most of the 12 IDPs analyzed in this study are surprisingly rich in SEP gases, unaccompanied by a significant component due to solar-wind irradiation. Estimates of the SEP/SW fluence ratio from these data are 10–3 to 10–4

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 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 \u3e40x 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 \u3e10 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

Noble Gases in Interplanetary Dust Particles, II: Excess Helium-3 in Cluster Particles and Modeling Constraints on Interplanetary Dust Particle Exposures to Cosmic-Ray Irradiation

Measurements of He isotopes in cluster interplanetary dust particles (IDPs) from stratospheric dust collector L2009 reveal anomalous 3He/4He ratios comparable to those seen earlier, up to ∼40x the solar wind ratio, in particles from the companion collector L2011. These overabundances of 3He in the L2009 samples are masked by much higher 4He contents compared to the L2011 particles, and are visible only in minor gas fractions evolved by stepwise heating at high temperatures. Cosmic-ray induced spallogenic reactions are efficient producers of 3He. The majority of this paper is devoted to a detailed assessment of the possible role of spallation in generating the 3He excesses in these and other cluster IDPs. A model of collisional erosion and fragmentation during inward transit through the interplanetary dust environment is used to estimate space lifetimes of particles from asteroidal and Edgeworth–Kuiper Belt sources. Results of the modeling indicate that Poynting–Robertson orbital evolution timescales of IDPs small enough to elude destruction on their way to Earth from either location are far shorter than the cosmic-ray exposure ages required to account for observed 3He overabundances. Grains large enough to have sufficiently long space residence times are fragmented close to their sources. An alternative to long in-space exposure could be prolonged irradiation of particles buried in parent body regoliths prior to their ejection as IDPs. A qualitative calculation suggests, however, that collisional erosion of asteroidal upper-regolith materials is likely to occur on timescales shorter than the \u3e 1 Ga burial times needed for accumulation of spallogenic 3He to the levels seen in several cluster particles. In contrast, regoliths on Edgeworth–Kuiper Belt objects may be stable enough to account for the 3He excesses, and delivery of heavily pre-irradiated IDPs to the inner solar system by short-period Edgeworth–Kuiper Belt comets remains a possibility. A potential problem is that the expected associated abundances of spallation-produced 21Ne appear to be absent, although here the present IDP data base is too sparse and for the most part too imprecise to rule out a spallogenic origin. Relatively short periods of pre-ejection residence in asteroidal regoliths may be responsible for the curiously broad exposure age distributions reported for micrometeorites extracted from Greenland and sea-floor sediments

Excess 3He in Cluster Interplanetary Dust Particles (IDPs) from Collectors L2009 and L2011

The 3He excesses found by Nier & Schlutter (1993) in cluster IDPs from collector L2011 are also present at about the same levels in particles from companion collector L2009. If these are produced by spallation, inferred exposure ages are very long

Helium and Neon Isotopic Compositions from Stardust Aerogel Particle Tracks

Helium and neon isotopic compositions were measured in flight spare aerogel, flight aerogel without apparent cometary material, and three aerogel fragments from a melted Stardust particle entry track

Irradiation Records in Regolith Materials, II: Solar-Wind and Solar-Energetic-Particle Components in Helium, Neon, and Argon Extracted from Single Lunar Mineral Grains and from the Kapoeta Howardite by Stepwise Pulse-Heating

High-resolution stepped heating has been used to extract light noble gases implanted in a suite of 13 individual lunar ilmenite and iron grains and in the Kapoeta howardite by solar wind (SW) and solar energetic particle (SEP) irradiation. Isotopic analyses of gases evolved at low temperatures from the lunar grains confirm the neon and argon compositions obtained by Pepin et al. (Pepin R. O., Becker R. H., and Schlutter D. J., “Irradiation records in regolith materials, I: Isotopic compositions of solar-wind neon and argon in single lunar regolith grains”, Geochim. Cosmochim. Acta63, 2145–2162, 1999) in an initial study of 11 regolith grains, primarily ilmenites. Combination of the data sets from both investigations yields 20Ne/22Ne = 13.85 ± 0.04, 21Ne/22Ne = 0.0334 ± 0.0003, and 36Ar/38Ar = 5.80 ± 0.06 for the lunar samples; the corresponding 36Ar/38Ar ratio in Kapoeta is 5.74 ± 0.06. The neon ratios agree well with those measured by Benkert et al. (Benkert J.-P., Baur H., Signer P., and Wieler R., “He, Ne, and Ar from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes”, J. Geophys. Res. (Planets)98, 13147–13162, 1993) in gases extracted from bulk lunar ilmenite samples by stepped acid etching and attributed by them to the SW. The 36Ar/38Ar ratios, however, are significantly above both Benkert et al.’s (1993) proposed SW value of 5.48 ± 0.05 and a later estimate of 5.58 ± 0.03 from an acid-etch analysis of Kapoeta (Becker R. H., Schlutter D. J., Rider P. E., and Pepin R. O., “An acid-etch study of the Kapoeta achondrite: Implications for the argon-36/argon-38 ratio in the solar wind”, Meteorit. Planet. Sci.33, 109–113, 1998). We believe, for reasons discussed here and in our earlier report, that 5.80 ± 0.06 ratio most nearly represents the wind composition. The 3He/4He ratio in low-temperature gas releases, not measured in the first particle suite, is found in several grains to be indistinguishable from Benkert et al.’s (1993) SW estimate. Elemental ratios of He, Ne, and Ar initially released from grain-surface SW implantation zones are solar-like, as found earlier by Pepin et al. (1999). Gases evolved from these reservoirs at higher temperatures show evidence for perturbations from solar elemental compositions by prior He loss, thermal mobilization of excess Ne from fractionated SW components, or both. Attention in this second investigation was focused on estimating the isotopic compositions of both the SW and the more deeply sited SEP components in regolith grains. Several high-temperature “isotopic plateaus”—approximately constant isotopic ratios in gas fractions released over a number of consecutive heating steps—were observed in the close vicinities of the SEP ratios for He, Ne, and Ar reported by Benkert et al. (1993). Arguments presented in the text suggest that these plateaus are relatively free of interferences from multicomponent mixing artifacts that can mimic pure component signatures. Average SEP compositions derived from the stepped-heating plateau measurements are in remarkable agreement with the Zürich acid-etch values for all three gases

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

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\u27s 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

Solar Wind and Spallation Neon in Small Dark Fragments Separated from the Kapoeta Howardite: Evidence for Early GCR Irradiation?

The Kapoeta howardite contains abundant noble gases in the dark phases of its brecciated structure, implanted during exposure of the parent body regolith to the solar wind. These gases have been studied extensively over the years, most recently by the modern closed-system stepped etching technique, in efforts to determine the elemental and isotopic composition of the solar wind at the time of regolith exposure. Kapoeta is interesting as well for another aspect of its irradiation history: several investigations, most recently by Rao et al., have shown that the dark, solar-wind-irradiated phases contain excesses of spallation-produced Ne above the levels expected to be generated by galactic cosmic rays (GCR) during the meteorite\u27s space exposure age of ~3 Ma. These excesses have been attributed to production by GCR, and by a solar cosmic ray (SCR) flux substantially enhanced over current levels, during an early ~3-6 Ma irradiation of the parent-body regolith prior to compaction, burial, and ultimate ejection of the Kapoeta object to space

Carbonaceous Meteor Ash - A Significant Carrier of Carbon, Organic Material and Noble Gas tot he Surfaces of Terrestrial Planets?

Meteor ash — a significant carrier of carbon, noble gas and organic matter to terrestrial planet surfaces

A Consortium Investigation of Possible Cometary IDPs

We have formed a consortium to study the Messenger and Walker hypothesis that cluster IDPs from June/July 1991 originate from comet SW-3. Initial results from this study do not confirm the anomalous He previously reported as ubiquitous in these IDPs