In situ observation of penetration process in silica aerogel: Deceleration mechanism of hard spherical projectiles

A large number of cometary dust particles were captured with low-density silica aerogels by NASA’s Stardust Mission. Knowledge of the details of the capture mechanism of hypervelocity particles in silica aerogel is needed in order to correctly derive the original particle features from impact tracks. However, the mechanism has not been fully understood yet. We shot hard spherical projectiles of several different materials into silica aerogel of density 60 mg cm⁻³ and observed their penetration processes using an image converter or a high-speed video camera. In order to observe the deceleration of projectiles clearly, we carried out impact experiments at two velocity ranges; ∼4 km s⁻¹ and ∼200 m s⁻¹. From the movies we took, it was indicated that the projectiles were decelerated by hydrodynamic force which was proportional to v² (v: projectile velocity) during the faster penetration process (∼4 km s⁻¹) and they were merely overcoming the aerogel crushing strength during the slower penetration process (∼200 m s⁻¹). We applied these deceleration mechanisms for whole capture process to calculate the track length. Our model well explains the track length in the experimental data set by Burchell et al. (Burchell, M.J., Creighton, J.A., Cole, M.J., Mann, J., Kearsley, A.T. [2001]. Meteorit. Planet. Sci. 36, 209-221).     

TOF‐SIMS analysis of Allende projectiles shot into silica aerogel

Abstract— Powdered Allende projectiles were fired into silica aerogel at 6.1 km/sec in order to evaluate particle retrieval and analysis techniques for samples from the Stardust mission. Since particles may disintegrate and ablate along the penetration paths in a high‐porosity aerogel, TOF‐SIMS analysis may be a suitable method to determine the distribution of such materials along the tracks as well as potential compositional modifications. Therefore, two ?350 μm‐sized tracks, residing at the surface of a keystone specimen that was flattened between two silicon chips, were analyzed. TOF‐SIMS allows for a detailed study of the chemical composition of particles that survived the impact mostly intact and of fine‐grained material from disintegrated projectiles. In the investigated keystone, material from light gas gun debris dominated. Besides the two tracks, a continuous, 40‐μm‐thick surface layer of implanted material—probably gun residue—was found. One of the two analyzed tracks is compositionally distinct from this surface layer and is likely to contain residual material of an Allende projectile. The analyses clearly demonstrate that tracks, resulting from impactors in the 5–10 μm size range, can be successfully analyzed with TOF‐SIMS.     

Penetration of coronal magnetic fields into solar-wind streams

The formation of solar-wind stream structure is investigated. Characteristic features of the solar and coronal magnetic-field structure, morphological features of the white-light corona, and radio maps of the solar-wind transition (transonic) region are compared. The solar-wind stream structure is detected and studied by using radio maps of the transition region, the raggedness of its boundaries, and their deviation from spherical symmetry. The radio maps have been constructed from radioastronomical observations in 1995–1997. It is shown that the structural changes in the transition region largely follow the changes occurring in regions closer to the Sun, in the circumsolar magnetic-field structure, and in the solar-corona structure. The correlations between the magnetic-field strength in the solar corona and the location of the inner (nearest the Sun) boundary of the transition region are analyzed. The distinct anticorrelation between the coronal magnetic-field strength and the distance of the transition region from the Sun is a crucial argument for the penetration of solar magnetic fields into plasma streams far from the Sun.     

In situ analysis of residues resulting from laboratory impacts into aluminum 1100 foil: Implications for Stardust crater analyses

Abstract— The encounter between the Stardust spacecraft and particles from comet 81P/Wild 2 gave impacts at a relative velocity of 6.1 km s^?1 and near perpendicular incidence to the collector surface. Such conditions are well within the performance limits of light gas gun laboratory simulations. For this study, two series of shots were conducted at the University of Kent, firing magnesium silicates (Mg end‐member forsterite, enstatite, diopside and lizardite), followed by a suite of increasingly Ferich olivines (through to Fe end‐member fayalite) into Stardust flight‐spare foils. Preserved residues were analysed using scanning electron microscopy combined with energy dispersive X‐ray analyses (SEM/EDX). X‐ray count integrals show that mineral compositions remain distinct from one another after impact, although they do show increased scatter. However, there is a small but systematic increase in Mg relative to Si for all residues when compared to projectile compositions. While some changes in Mg: Si may be due to complex analytical geometries in craters, there appears to be some preferential loss of Si. In practice, EDX analyses in craters on Stardust Al 1100 foil inevitably include contributions from Fe‐ and Si‐rich alloy inclusions, leading to further scattering of element ratios. Such inclusions have complicated Mg: Fe data interpretation. Compositional heterogeneity in the synthetic olivine projectiles also introduces data spread. Nevertheless, even with the preceding caveats, we find that the main groups of mafic silicates can be easily and reliably distinguished in EDX analyses performed in rapid surveys of foil craters, enabling access to a valuable additional collection of cometary materials.     

The penetration of soft electrons into the ionosphere

Calculations are presented of energy spectra and angular and spatial distributions of electron fluxes in the ionosphere resulting from precipitation ofmonoenergetic (E = 25, 50 and 100 eV) electrons. The incident electrons are assumed to be isotropic over the downward direction. It is found that the resulting steady-state electron fluxes above ca. 300 km are highly anisotropic, and that the pitch angle distribution is energy dependent. About 15 per cent of the incident electrons are backscattered elastically to the protonosphere. A much larger number of electrons escape after they have deposited a part of their energy in the atmosphere. The mean energy of the escaping electrons is about half that of the incident electrons. About 50% of the incident energy is absorbed in the atmosphere, the remainder being returned to the protonosphere. The rate of absorption of energy is a maximum at heights between 300 and 400 km. Most of the energy is absorbed in ionization and excitation of atomic oxygen. An appreciable amount of energy is, however, absorbed as heat by the ambient electron gas. Altitude profiles are presented of the rates of ionization, excitation, and electron heating caused by soft electron precipitation.     

The cascading of cosmic-ray nuclei in various media

More than a thousand interactions of primary heavy nuclei of the cosmic radiation with charge ≥10 and energy >1 GeV nucleon^?1 in nuclear emulsion have been studied with emphasis on how the primary nucleus fragments. It has been determined that the cases of multiple successive fragmentations that have been observed do not occur more frequently than expected. The fragmentation ofZ>20 nuclei does depend on the target nucleus to some extent so it is important to try to separate the interactions in emulsion by theirN _h (number of evaporation prongs). The fragmentation of ₈ ¹⁶ O at 2.1 GeV nucleon^?1 measured at the Bevalac shows a similar dependence on target nucleus. By using data from these new interactions combined with published data we have simulated on a computer nuclear cascades in both emulsion and air. Results on these cascades are given for both primary silicon and primary iron nuclei. These results are used to discuss the fluctuations expected in extensive air showers produced by heavy primary nuclei.     

Partial penetration of IMF By into the magnetosphere

Recent observations of partial penetration of the IMF B_y into the magnetosphere (Fairfield, 1979; Cowley and Hughes, 1983) are shown to agree with the idea of a magnetopause current K_y, induced by IMF B_y (Primdahl and Spangslev, 1983). The slow decay of K_y, caused by Joule heat losses in the cusp ionospheres is responsible for the appearance (on the average) of a fraction of IMF B_y inside the magnetosphere, and this also explains the large statistical scatter of the data. A decay time constant of 4–5 days is derived from the average fraction of IMF B_y observed inside the magnetosphere in good agreement with the 7-day time constant previously proposed.     

In situ calibration of the Martian cratering chronology

Crater densities on planetary surfaces allow assessing relative ages but so far firm calibration of so‐called cratering‐chronology models is available only for the Moon and limited to the past 4.1 billion years. Most planetary geological time scales are still model‐dependent, and essentially constrained by meteorite ages or by comparison to (dynamical) solar system evolution models. Here we describe in situ calibration of the Martian cratering chronology using cosmogenic and radiogenic isotope ages obtained by the NASA Curiosity rover. We determined the cratering‐rate ratio between Moon and Mars for recent times, and extended the calibration of cratering rates to earlier times than those based exclusively on lunar data. Our preferred interpretation supports monotonic flux decay since at least 4.24 Ga and likely since about 4.45 Ga, implying orbital migration of the giant planets, and its direct, transient, dynamical effect on the planetesimal populations was initiated early. But only Martian Sample Return will provide strongly needed capability for distinction of the different models currently available.     

Deep penetration of projectile material into the planet during the impact

A mechanism through which water could be buried inside the Moon is found. If an icy comet strikes the planetary surface and a thin natural crack exists at the site of the impact, some amount of cometary material can penetrate deep into the ground. This happens due to peculiar features of hydrodynamic flow along the crack. Numerical simulations based on the free-Lagrangian method show that the amount of water buried under the crater is several percent of the original mass of the projectile.     

Influence of the magnetosheath on solar proton penetration into the magnetosphere

The observation of solar protons (1–9 MeV) aboard HEOS-2 in the high-latitude magnetotail and magnetosheath on 9 June 1972, and their comparison with simultaneous measurements on Explorers 41 and 43, both in interplanetary space, indicate the existence of a distinct region of the inner magnetosheath (about 3 Earth radii thick) near the high-latitude magnetopause in which the solar particle flow is almost reversed with respect to the flow observed in interplanetary space. The region can also be seen by comparing magnetic field measurements on the three spacecraft. The observations in the outer layer of the magnetotail show solar protons predominantly entering the magnetosphere somewhere near the Earth, perhaps the cusp region.     

On penetration depth of the Shoemaker-Levy 9 fragments into the Jovian atmosphere

The actual penetration depth of the Shoemaker-Levy 9 fragments into the Jovian atmosphere is still an open question. From fundamental equations of meteoric physics with variable cross-section, a new analytic model of energy release of the fragments is presented. In use of reasonable parameters, a series of results are calculated for different initial mass of the fragments. The results show that the largest fragment explodes above pressure levels of 3 bars and does not penetrate into the H₂O cloud layer of the Jovian atmosphere, and that airburst of smaller fragments occur even above the upper cloud layer.     

SIMS studies of Allende projectiles fired into Stardust‐type aluminum foils at 6 km/sec

Abstract— We have explored the feasibility of C, N, and O isotopic measurements by NanoSIMS and of elemental abundance determinations by time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) on residues of Allende projectiles that impacted Stardust‐type aluminum foils in the laboratory at 6 km/sec. These investigations are part of a consortium study aimed at providing the foundation for the characterization of matter associated with microcraters that were produced during the encounter of the Stardust space probe with comet 81P/Wild‐2. Eleven experimental impact craters were studied by NanoSIMS and eighteen by TOF‐SIMS. Crater sizes were between 3 and 190 μm. The NanoSIMS measurements have shown that the crater morphology has only a minor effect on spatial resolution and on instrumental mass fractionation. The achievable spatial resolution is always better than 200 nm, and C and O isotopic ratios can be measured with a precision of several percent at a scale of several 100 nm, which is the typical size of presolar grains. This clearly demonstrates that presolar matter, provided it survives the impact into the aluminum foil partly intact, is recognizable even if embedded in material of solar system origin. TOF‐SIMS studies are restricted to materials from the crater rim. The element ratios of the major rock‐forming elements in the Allende projectiles are well‐characterized by the TOF‐SIMS measurements, indicating that fractionation of those elements during impact can be expected to be negligible. This permits chemical information on the type of impactor material to be obtained. For any more detailed assignments to specific chondrite groups, however, information on the abundances of the light elements, especially C, is crucial. This information could not be obtained in the present study due to unavoidable contamination during impact experiments.     

In situ measurements of heating parameters in the auroral ionosphere

Four sounding rocket payloads were launched in early 1977 to measure heating parameters in the auroral oval. Geophysical conditions were different for the four flights: auroral arc substorm main phase diffuse aurora, and auroral arc with negative bay. The conductivity tensor and the heating rates of particle and Joule heating are determined. The heating rates range in the order of a few tens of mWm^?2. These magnitudes accord with those determined with the aid of backscatter facilities and other sounding rocket observations.     

Impulsive penetration of filamentary plasma elements into the magnetospheres of the Earth and Jupiter

Assuming that the solar wind plasma is usually non-uniform over distances of 10,000 km or less, it is shown that filamentary plasma elements stretched out from the Sun can penetrate impulsively and become engulfed into the magnetosphere.The diamagnetic effects associated with these plasma inhomogeneities are observed in outer magnetospheres and magnetosheaths as dips or directional discontinuities in the magnetic field measurements. From the mean penetration distances of these diamagnetic plasma elements one can deduce a mean deceleration time, as well as an approximate value of the integrated Pedersen conductivity in the polar cusp of the Earth and Jupiter.     

In situ formation of palisade bodies in calcium,aluminum-rich refractory inclusions

Abstract— It has been suggested that palisade bodies—shells of spinel found within some calcium, aluminum-rich inclusions (CAIs) and the phases the shells enclose—are intact mini-CAIs that predate and were captured by their current hosts while the latter were still molten. We present new data and observations that indicate that most palisade bodies formed instead in situ while their host inclusions were crystallizing. The evidence includes observations of spinel-lined cavities and glass-filled, circular structures outlined by spinel in experimental run products crystallized from melts; a partially formed palisade body in an inclusion; a fassaite crystal that is optically continuous across a palisade wall; and similarity of unusual mineral compositions in some palisade bodies and their hosts. Our observations can be used to refute arguments for exotic origin and are most consistent with a model for in situ formation involving: (1) formation of vesicles in a largely molten inclusion; (2) nucleation of spinel upon and/or adherence to vapor-melt interfaces, forming spinel shells around vesicles; (3)leakage of vesicles and filling with melt while spinel shells remain largely intact; and (4) crystallization of melt inside shells. This model is similar to one proposed for formation of segregation vesicles, which are partially- to completely-filled vesicles found in some terrestrial basalts. In addition, we interpret framboids (i.e., dense clusters of spinel with little material between grains, found in most inclusions that contain palisade bodies) as polar or near-polar sections through palisade bodies and therefore do not make a genetic distinction between the two features.     

In situ multi-frequency measurements of magnetic susceptibility as an indicator of planetary regolith maturity

Space weathering is now generally accepted to modify the optical and magnetic properties of airless planetary regoliths such as those on the Moon and Mercury. Under micrometeorite and ion bombardment, ferrous iron in such surfaces is reduced to metallic iron spheres, found in amorphous coatings on almost all exposed regolith grains. The size and number distribution of these particles and their location in the regolith all determine the nature and extent of the optical and magnetic changes. These parameters in turn reflect the formation mechanisms, temperatures, and durations involved in the evolution of the regolith. Studying them in situ is of intrinsic value to understanding the weathering process, and useful for determining the maturity of the regolith and providing supporting data for interpreting remotely sensed mineralogy. Fine-grained metallic iron has a number of properties that make it amenable to magnetic techniques, of which magnetic susceptibility is the simplest and most robust. The magnetic properties of the lunar regolith and laboratory regolith analogues are therefore reviewed and the theoretical basis for the frequency dependence of magnetic susceptibility presented. Proposed here is then an instrument concept using multi-frequency measurements of magnetic susceptibility to confirm the presence of fine-grained magnetic material and attempt to infer its quantity and size distribution. Such an instrument would be invaluable on a future mission to an asteroid, the Moon, Mercury or other airless rocky Solar System body.     

Control of impulsive penetration of solar wind irregularities into the magnetosphere by the interplanetary magnetic field direction

Impulsive penetration of a solar wind filament into the magnetosphere is possible when the plasma element has an excess momentum density with respect to the background medium. This first condition is satisfied when the density is larger inside than outside the plasma inhomogeneity. In this paper we discuss the second condition which must be satisfied for such a plasma element to be captured by the magnetosphere: the magnetization vector (M) carried by this plasma must have a positive component along the direction of B₀, the magnetic field where the element penetrates through the magnetopause. On the contrary, when $\text{M · B}{}_0\text{< 0}$, the filament is stopped at the surface of the magnetopause. Thus the outcome of the interaction of the filament with the magnetosphere depends upon the orientation of the Interplanetary Magnetic Field. For instance, penetration and capture in the frontside magnetosphere implies that B_sw, the Interplanetary Magnetic Field, has a southward, or a small northward, component. Penetration and capture in the northern lobe of the magnetotail is favoured for an IMF pointing away from the Sun; in the southern lobe B_sw must be directed towards the Sun for capture. Finally, for capture in the vicinity of the polar cusps the magnetospheric field (B₀) assumes a wider range of orientations. Therefore, near the neutral points, it is easier to find a place where the condition $\text{M · B}{}_0\text{> 0}$ is satisfied than elsewhere. As a consequence, the penetration and capture of solar wind irregularities in the cleft regions is possible for almost any orientation of the interplanetary magnetic field direction. All observations made to date support these theoretical conclusions.     

In situ experimental formation and growth of Fe nanoparticles and vesicles in lunar soil

We report the results of the first dynamic, in situ heating of lunar soils to simulate micrometeorite impacts on the lunar surface. We performed slow‐ and rapid‐heating experiments inside the transmission electron microscope to understand the chemical and microstructural changes in surface soils resulting from space‐weathering processes. Our slow‐heating experiments show that the formation of Fe nanoparticles begins at ~575 °C. These nanoparticles also form as a result of rapid‐heating experiments, and electron energy‐loss spectroscopy measurements indicate the Fe nanoparticles are composed entirely of Fe⁰, suggesting this simulation accurately mimics micrometeorite space‐weathering processes occurring on airless body surfaces. In addition to Fe nanoparticles, rapid‐heating experiments also formed vesiculated textures in the samples. Several grains were subjected to repeated thermal shocks, and the measured size distribution and number of Fe nanoparticles evolved with each subsequent heating event. These results provide insight into the formation and growth mechanisms for Fe nanoparticles in space‐weathered soils and could provide a new methodology for relative age dating of individual soil grains from within a sample population.     

In situ analysis of platinum group elements in equilibrated ordinary chondrite kamacite and taenite

Platinum group element (PGE) concentrations have been determined in situ in ordinary chondrite kamacite and taenite grains via laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). Results demonstrate that PGE concentrations in ordinary chondrite metal (kamacite and taenite) are similar among the three ordinary chondrite groups, in contrast to previous bulk metal studies in which PGE concentrations vary in the order H < L < LL. PGE concentrations are higher in taenite than kamacite, consistent with preferential PGE partitioning into taenite. PGE concentrations vary between and within metal grains, although average concentrations in kamacite broadly agree with results from bulk studies. The variability of PGE concentrations in metal decreases with increasing petrologic type; however, variability is still evident in most type six ordinary chondrites, suggesting that equilibration of PGEs does not occur between metal grains, but rather within individual metal grains via self‐diffusion during metamorphism. The constant average PGE concentrations within metal grains across different ordinary chondrite groups are consistent with the formation of metal via nebular condensation prior to the accretion of ordinary chondrite parent bodies. Post‐condensation effects, including heating during chondrule‐formation events, may have affected some element ratios, but have not significantly affected average metal PGE concentrations.