Seven particles captured by the Stardust Interstellar Dust Collector and returned to Earth for laboratory analysis have features consistent with an origin in the contemporary interstellar dust stream. More than 50 spacecraft debris particles were also identified. The interstellar dust candidates are readily distinguished from debris impacts on the basis of elemental composition and/or impact trajectory. The seven candidate interstellar particles are diverse in elemental composition, crystal structure, and size. The presence of crystalline grains and multiple iron-bearing phases, including sulfide, in some particles indicates that individual interstellar particles diverge from any one representative model of interstellar dust inferred from astronomical observations and theory.
Abstract Here, we report analyses by synchrotron X‐ray fluorescence microscopy of the elemental composition of eight candidate impact features extracted from the Stardust Interstellar Dust Collector (SIDC). Six of the features were unambiguous tracks, and two were crater‐like features. Five of the tracks are so‐called “midnight” tracks—that is, they had trajectories consistent with an origin either in the interstellar dust stream or as secondaries from impacts on the Sample Return Capsule (SRC). In a companion paper reporting synchrotron X‐ray diffraction analyses of ISPE candidates, we show that two of these particles contain natural crystalline materials: the terminal particle of track 30 contains olivine and spinel, and the terminal particle of track 34 contains olivine. Here, we show that the terminal particle of track 30, Orion, shows elemental abundances, normalized to Fe, that are close to CI values, and a complex, fine‐grained structure. The terminal particle of track 34, Hylabrook, shows abundances that deviate strongly from CI , but shows little fine structure and is nearly homogenous. The terminal particles of other midnight tracks, 29 and 37, had heavy element abundances below detection threshold. A third, track 28, showed a composition inconsistent with an extraterrestrial origin, but also inconsistent with known spacecraft materials. A sixth track, with a trajectory consistent with secondary ejecta from an impact on one of the spacecraft solar panels, contains abundant Ce and Zn. This is consistent with the known composition of the glass covering the solar panel. Neither crater‐like feature is likely to be associated with extraterrestrial materials. We also analyzed blank aerogel samples to characterize background and variability between aerogel tiles. We found significant differences in contamination levels and compositions, emphasizing the need for local background subtraction for accurate quantification.
Abstract A recrystallized band of pale feldspathic impact melt in a gneissic impact breccia from the approximately 10 km Paasselkä impact structure in southeast Finland was dated via 40 Ar/ 39 Ar step‐heating. The newly obtained plateau age of 228.7 ± 1.8 (2.2) Ma (2σ) ( MSWD = 0.32; p = 0.93) is equal to the previously published pseudoplateau age of 228.7 ± 3.0 (3.4) (2σ) for the impact event. According to the current international chronostratigraphic chart and using the most recent published suggestions for the K decay constants, a Carnian (Late Triassic) age for the Paasselkä impact structure of 231.0 ± 1.8 (2.2) Ma (2σ) is calculated and considered the most precise and accurate age for this impact structure. The new plateau age for Paasselkä confirms the previous dating result but is, based on its internal statistics, much more compelling.
As part of an ongoing initiative to constrain mantle noble gas budgets, we evaluate both recently published high‐quality noble gas data and report new data in order to rationalize the use of three‐dimensional (3‐D) modeling techniques. Modeling of these data shows that traditional 2‐D mixing plots are not adequate tools to fully constrain mixing systems and mantle end‐member compositions. We show that these mantle noble gas analyses are compromised to varying degrees by the addition of a fractionated atmospheric contaminant, irrespective of eruption setting. This component can also be present in analyses with mantle‐like neon or argon isotope ratios. Previous estimates for the mantle end‐member 40 Ar/ 36 Ar and 129 Xe/ 130 Xe compositions of most of these samples are likely to be minimum values. In fact it may well be that there are no analyses in the present literature that have completely unadulterated mantle volatiles present.
Understanding the compaction and differentiation of the planetesimals and protoplanets from the Asteroid Belt and the terrestrial planet region of the Solar System requires a reliable modeling of their internal thermal evolution. An important ingredient for this is a detailed knowledge of the heat conductivity of the chondritic mixture of minerals and metal in planetesimals. The temperature dependence of the heat conductivity is evaluated here from the properties of its mixture components by a theoretical model. This allows to predict the temperature dependent heat conductivity for the full range of observed meteoritic compositions and also for possible other compositions. For this purpose, published results on the temperature dependence of heat conductivity of the mineral components found in chondritic material are fitted to the model of Callaway for heat conductivity in solids by phonons. For the Ni,Fe-alloy published laboratory data are used. The heat conductivity of chondritic material then is calculated by means of mixing-rules. The role of micro-cracks is studied which increase the importance of wall-scattering for phonon-based heat conductivity. The model is applied to published data on heat conductivity of individual chondrites. The experimental data for the dependence of the heat conductivity on temperature can be reproduced rather well by the model if the heat conductivity is calculated for the composition of the meteorites. It is found that micro-cracks have a significant impact on the temperature dependence of the heat conductivity because of their reduction of phonon scattering length.
Abstract The processes of alteration of airless bodies exposed to the space environment are referred to be as “space weathering.” Multiple agents contribute generally to space weathering, to an extent that depends on the specific location of the surface within the solar system. Typical space weathering agents encountered in the solar system are solar radiation, solar wind and cosmic rays, magnetospheric plasma (e.g., at Jupiter or Saturn), and cosmic dust. The effect of space weathering is generally assessed by measuring the surfaces optical properties, for example, by near‐infrared spectroscopy. The alteration of the surfaces is due to a cumulative effect over time of all agents. We investigate in this paper the contribution of micrometeoroid (dust) bombardment on different asteroids, by using the Interplanetary Micrometeoroid Environment Model for the interplanetary dust populations and a simplified model of interstellar dust dynamics. We quantify, for different representative asteroids (main belt and Near Earth Objects [NEOs]), the particle cumulative flux, mass flux, impact velocity, and the kinetic impact energy deposited. This work is primarily intended to support laboratory work investigating the effect of energy deposition onto sample surfaces, as well as astronomical observations of optical properties of asteroid surfaces.
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