Investigations on a Large Collection of Cosmic Dust From the Central Indian Ocean

We collected 1,245 spherules from the Central Indian Ocean basin by Magnetic cosmic dust collection (MACDUC) experiment raking the deep sea floor. This collection ranks among the large deep sea collections of cosmic dust. For this study, 168 particles are analyzed with SEM-EDS to characterise their cosmic nature and identify the processes that their morphological features, textures and chemical compositions reveal. All the three basic types of cosmic spherules have been identified: I-type, S-type and the G-type. The silicate or the S-type spherules are dominant in this collection. In all, 115 spherules were sectioned, polished and analyzed for major elements. I-type spherules are mainly composed of Fe and Ni oxides, some have metallic cores where appreciable amounts of Co is observed in addition to glassy phases with lithophile elements are also observed in these spherules. These evidences are supportive of the view that the I-type spherules could be metal grains from carbonaceous/unequilibrated chondritic bodies. The S-type spherules show elemental composition of Mg, Al, Si, Ca, Fe, and Ni approximately similar to chondritic compositions. In addition, some other rare particles such as an S-type sphere which contains a large zoned relict chromite crystal, other spheres with a semi-porphyritic/barred olivine texture are also observed. While most the S-type spherules appear to have carbonaceous chondrites as their parent bodies, the relict grain bearing spherule shows distinctly an ordinary chondritic parent body.     

Impact ejecta in upper Eocene deposits at Massignano,Italy

Abstract— Previous workers have shown that an impact ejecta layer at Massignano, Italy contains a positive Ir anomaly, flattened spheroids (pancake spherules), Ni‐rich spinel crystals, and shocked quartz with multiple sets of planar deformation features. Because of sample sizes and work by different investigators, it was not clear if the shocked quartz is associated with the Ir anomaly and pancake spherules or if it belongs to a separate impact event. To address this problem, we carried out a high‐resolution stratigraphic study of this ejecta layer. The ejecta layer was sampled continuously at 1 cm intervals in two adjacent columns. The carbonate was removed with dilute HCl, and the non‐carbonate fraction was gently sieved. Pancake spherules were recovered from the 250–500 μm size fraction and counted. At the peak abundance, the number of pancake spherules in the 250–500 μm size fraction is about 6–7/g of sample. The pancake spherules removed from the 250–500 μm size fraction are mostly translucent to opaque pale green, but some have a grey color or dark opaque patches due to a coating of Ni‐ and Cr‐rich spinel crystals. Energy‐dispersive X‐ray analysis and X‐ray diffraction data indicate that the green spherules are composed of iron‐rich smectite, probably nontronite. Black opaque spinel stringers (dark spinel‐rich pancake spherules), usually <200 μm across, can be seen in a polished section of a block that includes the ejecta layer. None of the dark spinel‐rich pancake spherules were recovered from the sieved non‐carbonate fraction due to their fragile nature, but we believe that they are from the same impact event as the green pancake spherules. The <250 μm size fractions from both columns were disaggregated using ultrasonics and re‐sieved. The 63–125 μm size fractions were then searched for shocked quartz using a petrographic microscope. At the peak‐abundance level, the number of shocked quartz grains in the 63–125 μm size fraction is about 7/g of sample. Some of the shocked quartz grains have a “toasted” appearance. These grains have a brownish color and contain a patchy distribution of faint, densely spaced planar deformation features (PDFs). Polymineralic fragments containing one or two shocked quartz grains with one or two sets of PDFs were observed. They appear to have an organic matrix and are probably fragments of agglutinated foraminiferal tests. We searched for, but did not find, coesite or shocked zircons. We found that the peak abundance of the shocked quartz is within a centimeter of the peak abundance of the green pancake spherules. We conclude that the pancake spherules are diagenetically altered clinopyroxene‐bearing spherules and that the shocked quartz, green (and presumably the dark spinel‐rich) pancake spherules, and Ir anomaly all belong to the same impact event. This conclusion is consistent with previous suggestions that the cpx spherule layer may be from the 100 km‐diameter Popigai impact crater in northern Siberia.     

New insights on petrography and geochemistry of impactites from the Lonar crater,India

The Lonar impact crater, India, is one of the few known terrestrial impact craters excavated in continental basaltic target rocks (Deccan Traps, ~65 Ma). The impactites reported from the crater to date mainly include centimeter‐ to decimeter‐sized impact‐melt bombs, and aerodynamically shaped millimeter‐ and submillimeter‐sized impact spherules. They occur in situ within the ejecta around the crater rim and show schlieren structure. In contrast, non–in situ glassy objects, loosely strewn around the crater lake and in the ejecta around the crater rim do not show any schlieren structure. These non–in situ fragments appear to be similar to ancient bricks from the Daityasudan temple in the Lonar village. Synthesis of existing and new major and trace element data on the Lonar impact spherules show that (1) the target Lonar basalts incorporated into the spherules had undergone minimal preimpact alteration. Also, the paleosol layer as preserved between the top‐most target basalt flow and the ejecta blanket, even after the impact, was not a source component for the Lonar impactites, (2) the Archean basement below the Deccan traps were unlikely to have contributed material to the impactite parental melts, and (3) the impactor asteroid components (Cr, Co, Ni) were concentrated only within the submillimeter‐sized spherules. Two component mixing calculations using major oxides and Cr, Co, and Ni suggest that the Lonar impactor was a EH‐type chondrite with the submillimeter‐sized spherules containing ~6 wt% impactor components.     

The crystalline lunar spherules: Their formation and implications for the origin of meteoritic chondrules

Abstract— Crystalline lunar spherules (CLS) from three thin sections of Apollo 14 regolith breccias (14318,6; 14318,48 and 14315,20) have been examined. The objects have been classified and their abundances, size distributions, bulk compositions, and (where possible) plagioclase compositions determined. By number, 64% consist predominantly of very fine-grained equant plagioclase grains but can also contain larger (～50 μm) feldspar crystals (type X), while 22% contain plagioclase lathes in a fine-grained mafic mesostasis (type Y). Plagioclase in both spherule types displays bright yellow cathodoluminescence that is conspicuous among the blue CL of the normal feldspar of the breccias. Type Z spherules (5%) contain feldspar with blue CL and minor amounts of olivine and pyroxene. Type Q spherules (4%) contain feldspar with yellow CL but in a luminescent mesostasis (of quartz or feldspar?). A few spherules are mixtures of type Y and type X textures. Most type X spherules, and a few type Y spherules, have fine-grained opaque rims. Compound objects were also found and consist of two or more CLS that appear to have collided while still plastic or molten. The CLS are thought to be impact spherules that crystallized in free flight, their coarse textures suggesting fairly slow cooling rates (～ <1 °C/s). The abundance of the CLS resembles that of chondrules in the CM chondrite Murchison, and their cumulative size-frequency distributions are very similar to those of the chondrules in several meteorite classes. The bulk compositions of the CLS do not resemble regoliths at any of the Apollo sites, including Apollo 14, or any of the common impact glasses, but they do resemble the bulk compositions of several lunar meteorites and the impact glasses they contain. The Apollo 14 site is located on a region containing Imbrium ejecta, and we suggest that the CLS derive from the Imbrium impact. Ballistic calculations indicate that only impact events of this size on the Moon are capable of producing melt spherules with the required free flight times and slow cooling rates. Smaller impacts produce glassy spherules and agglutinates. As has been pointed out many times, the CLS have many properties in common with meteoritic chondrules. While much remains unclear, difficulties with a nebular origin and new developments in chondrule chronology, studies of asteroid surfaces and impact ejecta behavior, and the present observations indicate that meteoritic chondrules could have formed by impact.     

Textural constraints on the formation of impact spherules: A case study from the Dales Gorge BIF,Paleoproterozoic Hamersley Group of Western Australia

Abstract— Impact ejecta (about 2.5 Gyr old) in the DS4 layer of the Dales Gorge BIF (Hamersley Group, Western Australia) are so well preserved that many original textures such as vesicles and microlites are faithfully preserved. About 65% of the particles in the layer originated as impact ejecta, of which 81% are splash forms. The remaining 19% are angular, but the splash forms and angular particles have the same composition (mainly diagenetic stilpnomelane and K‐feldspar) and share a common suite of internal textures. Some particles contain randomly oriented microlites texturally identical to plagioclase in basalts. Most splash forms have rims of inward‐growing crystals that may have formed from the melt (perhaps nucleated by impinging dust) or via thermal devitrification. The rims clearly formed in flight because in broken particles (which make up about 13% of the splash forms) they are generally not present on broken surfaces. The origin of the angular particles is uncertain, but they may represent solid ejecta. Given the large sizes and variable shapes of the splash forms, they are probably droplets of impact melt emplaced ballistically. This is largely by analogy to the K‐T boundary layer, but DS4 splash forms differ from K‐T spherules in important ways suggesting the K‐T model is not universal. The occurrence of basaltic ejecta from a large impact highlights its scarcity in the stratigraphic record despite the areal abundance of oceanic crust. The diverse textures formed via in‐flight crystallization suggest particle paths in the plume are more complex than is generally appreciated.     

Upper Eocene tektite and impact ejecta layer on the continental slope off New Jersey

Abstract— During Leg 150 of the Ocean Drilling Project (ODP), two sites (903C and 904A) were cored that have sediments of the same biostratigraphic age as the upper Eocene tektite-bearing ejecta layer at Deep Sea Drilling Project (DSDP) Site 612. Core 45X from ODP Site 904A (～4 km north of Site 612) contains a 5 cm thick tektite-bearing ejecta layer, and Core 56 from Site 903C (～8 km north-northwest of Site 904) contains a 2 cm thick layer of impact ejecta without any tektite or impact glass. Shocked quartz and feldspar grains, with multiple sets of planar deformation features (PDFs), and abundant coesite-bearing grains are present at both sites. The major oxide contents, trace element compositions, and rare earth element (REE) patterns of the Site 904 tektites are similar to those of the Site 612 tektites and to North American tektites (especially bediasites). The ?_Sr and ?_Nd values for one composite tektite sample from Site 904 fall within the range previously obtained for the Site 612 tektites, which defines a linear trend that, if extrapolated, would intersect the values obtained for North American tektites. The water contents of eight tektite fragments from Site 904 range from 0.017 to 0.098 wt%, and, thus, are somewhat higher than is typical for tektites. The heavy mineral assemblages of the 63–125 μm size fractions from the ejecta layers at Sites 612, 903, and 904 are all similar. Therefore, we conclude that the ejecta layer at all three sites is from the same impact event and that the tektites at Sites 904 and 612 belong to the North American tektite strewn field. Clinopyroxene-bearing (cpx) spherules occur below, or in the lower part of, the main ejecta layer at all three sites. At all three sites, the cpx spherules have been partly or completely replaced with pyrite that preserved the original crystalline textures. Site 612, 903, and 904 cpx spherules are similar to those found in the Caribbean Sea, Gulf of Mexico, central equatorial Pacific, western equatorial Pacific, and eastern Indian Ocean. The cpx event appears to have preceded the North American tektite event by 10–15 ka or less. The fining-upward sequence at all three sites and concentration of the denser, unmelted impact ejecta at the top of the tektite layer at Sites 612 and 904 suggest that the tektite-bearing ejecta layers are not the result of downslope redeposition and that the unmelted ejecta landed after the glass. Geographic variations in thickness of the tektite-bearing ejecta layer, the lack of carbonate clasts in the ejecta layer, and the low CaO content of the tektite glass suggest that the ejecta (including the tektite glass) were derived from the Chesapeake Bay structure rather than from the Toms Canyon structure. A sharp decline in microfossil abundances suggests that local environmental changes caused by the impact may have had adverse effects on benthic foraminifera, radiolaria, sponges, and fish as well as the planktic foraminifera.     

Petrography and geochemistry of ejecta from the Sudbury impact event

Ejecta from the Connors Creek site in Michigan (500 km from the Sudbury Igneous Complex [SIC]), the Pine River site in western Ontario (650 km from the SIC), and the Coleraine site in Minnesota (980 km from the SIC) were petrographically and geochemically analyzed. Connors Creek was found to have approximately 2 m of ejecta, including shocked quartz, melt droplets, and accretionary lapilli; Pine River has similar deposits about 1 m in thickness, although with smaller lapilli; Coleraine contains only impact spherules in a 20 cm‐thick layer (impact spherules being similar to microkrystites or microtektites). The ejecta transition from chaotic deposits of massively bedded impactoclastic material with locally derived detritus at Connors Creek to a deposit with apparently very little detrital material that is primarily composed of melt droplets at Pine River to a deposit that is almost entirely composed of melt spherules at Coleraine. The major and trace element compositions of the ejecta confirm the previously observed similarity of the ejecta deposits to the Onaping Formation in the SIC. Platinum‐group element (PGE) concentrations from each of the sites were also measured, revealing significantly elevated PGE contents in the spherule samples compared with background values. PGE abundances in samples from the Pine River site can be reproduced by addition of approximately 0.2 wt% CI chondrite to the background composition of the underlying sediments in the core. PGE interelement ratios indicate that the Sudbury impact event was probably caused by a chondritic impactor.     

Sr and Nd analyses of upper Eocene spherules and their implications for target rocks

Abstract— Upper Eocene impact ejecta has been discovered all over the world. The number of upper Eocene impact layers and the geographic distribution of each layer, based on major chemical composition and biostratigraphic data, are not agreed upon. We have performed four Sr‐Nd isotopic analyses of clinopyroxene‐bearing spherules (cpx spherules) and three Sr‐Nd analyses of microtektites from five Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) sites in the South Atlantic and Indian Oceans. Our data support the hypothesis that there is only one cpx spherule layer in upper Eocene sediments. We also find that the microtektites associated with the cpx spherule layer in the South Atlantic and Indian Oceans are not part of the North American tektite strewn field, but belong to the same event that produced the cpx spherules. The microtektites, together with cpx spherules, are more heterogeneous than microtektites/tektites from other strewn fields. No direct link has been established between the microtektites from this study and possible target rock at the Popigai crater.     

The Leonard Award Address Presented 1996 July 25, Berlin,Germany: The elemental composition of stony cosmic spherules

Abstract— Five hundred stony cosmic spherules collected from deep-sea sediments, polar ice, and the stratosphere have been analyzed for major and some minor element composition. Typical spherules are products of atmospheric melting of millimeter sized and smaller meteoroids. The samples are small and modified by atmospheric entry, but they are an important source of information on the composition of asteroids. The spherules in this study were all analyzed in an identical manner, and they provide a sampling of the solar system's asteroids that is both different and less biased than provided by studies of conventional meteorites. Volatile elements such as Na and S are depleted due to atmospheric heating, while siderophiles are depleted by less understood causes. The refractory nonsiderophile elements appear not to have been significantly disturbed during atmospheric melting and provide important clues on the elemental composition of millimeter sized meteoroids colliding with the Earth. Typical spherules have CM-like composition that is distinctively different than ordinary chondrites and most other meteorite types. We assume that C-type asteroids are the primary origin of spherules with this composition. Type S asteroids should also be an important source of the spherules, and the analysis data provide constraints on their composition. A minor fraction of the spherules are melt products of precursor particles that did not have chondritic elemental compositions. The most common of these are particles that are dominated by olivine. The observed compositions of spherules are inconsistent with the possibility that an appreciable fraction of the spherules are simply chondrules remelted during atmospheric entry.     

Vesicle dynamics during the atmospheric entry heating of cosmic spherules

Cosmic spherules are unique igneous objects that form by melting due to gas drag heating during atmospheric entry heating. Vesicles are an important component of many cosmic spherules since they suggest their precursors had finite volatile contents. Vesicle abundances in spherules decrease through the series porphyritic, glassy, barred, to cryptocrystalline spherules. Anomalous hollow spherules, with large off‐center vesicles occur in both porphyritic and glassy spheres. Numerical simulation of the dynamic behavior of vesicles during atmospheric flight is presented that indicates vesicles rapidly migrate due to deceleration and separate from nonporphyritic particles. Modest rotation rates of tens of radians s^?1 are, however, sufficient to impede loss of vesicles and may explain the presence of small solitary vesicles in barred, cryptocrystalline and glassy spherules. Rapid rotation at spin rates of several thousand radians s^?1 are required to concentrate vesicles at the rotational axis and leads to rapid growth by coalescence and either separation or retention depending on the orientation of the rotational axis. Complex rapid rotations that concentrate vesicles in the core of particles are proposed as a mechanism for the formation of hollow spherules. High vesicle contents in porphyritic spherules suggest volatile‐rich precursors; however, calculation of volatile retention indicates these have lost >99.9% of volatiles to degassing prior to melting. The formation of hollow spherules, by rapid spin, necessarily implies preatmospheric rotations of several thousand radians s^?1. These particles are suggested to represent immature dust, recently released from parent bodies, in which rotations have not been slowed by magnetic damping.     

The Cretaceous/Paleogene (K‐Pg) boundary at the J Anomaly Ridge,Newfoundland (IODP Expedition 342, Hole U1403B)

We present results of an in‐situ geochemical study using laser‐ablation inductively coupled plasma–mass spectrometry (LA‐ICP‐MS) analyses along a ~4.3 cm long section across the K‐Pg event bed, drilled during IODP Expedition 342 at J Anomaly Ridge south of St. John's, Newfoundland. This section comprises the Maastrichtian with a sharp boundary to the graded, between 1.5 and 1.8 cm thick ejecta layer with totally altered impact glass spherules, which in turn is topped by Danian sediments. The porous and clayey material required elaborate preparation in order to yield reliable data. The ejecta bed shows a highly variable depletion in rare earth elements that even results in strongly subchondritic concentrations. The Ce/Ce* varies strongly (0.81–34), Ni/Cr ranges from 0.38 to 2.79. The maximum platinum group elements (PGE) concentrations are located in one LA‐spot exactly at the basis of the ejecta layer; they amount (in μg g^?1) to 0.35 (Rh), 1.64 (Pd), 2.79 (Pt), and 0.86 (Au). The Nb/Ta ratio increases in the Ma from ~10 to 35.9 toward the ejecta horizon, which itself has higher Nb, Ta, Zr, and Hf concentrations than the background sedimentation, combined with low Nb/Ta (~5–10), and low Zr/Hf (~20–30). The overall result is that alteration processes changed totally the original geochemical characteristics of this K‐Pg spherule bed. To explain the exorbitant element mobility at distances of hundreds of μm, we discuss a combination of mostly reducing redox processes and interaction with organic compounds. This study demonstrates the high potential of in‐situ analyses with high spatial resolution at complex geological materials. Moreover, our results indicate that some caution is necessary in determining the projectile type in impactites via PGE ratios.     

A model of the origin of the Jovian ring

Starting with the assumption that the micron-sized particles which make up the bright Jovian ring are fragments of erosive collisions between micrometeoroid projectiles and large parent bodies, a physical model of the ring is calculated. The physics of high-velocity impacts leads to a well-defined size distribution for the ejecta, the optical properties of which can be compared with observation. This gives information on the ejecta material (very likely silicates) and on the maximum size of the projectiles, which turns out to be about 0.1 μm. The origin of these projectiles is discussed, and it is concluded that dust particles ejected in volcanic activity from Io are the most likely source. The impact model leads quite naturally to a distribution in ejecta sizes, which in turn determines the structure of the ring. The largest ejecta form the bright ring, medium-sized ejecta form a disk extending all the way to the Jovian atmosphere, and the small ejecta form a faint halo, the structure of which is dominated by electromagnetic forces. In addition to the Io particles, interaction with interplanetary micrometeoroids is also considered. It is concluded that μm-sized ejecta from this source have ejection velocities which are several orders of magnitude too large, and thus cannot contribute significantly to the observed bright ring. However, the total mass ejection rate is significant. Destruction of these ejecta by the Io particles may provide additional particles for the halo.     

Geochemical identification of impactor for Lonar crater,India

Abstract— The only well‐known terrestrial analogue of impact craters in basaltic crusts of the rocky planets is the Lonar crater, India. For the first time, evidence of the impactor that formed the crater has been identified within the impact spherules, which are ?0.3 to 1 mm in size and of different aerodynamic shapes including spheres, teardrops, cylinders, dumbbells and spindles. They were found in ejecta on the rim of the crater. The spherules have high magnetic susceptibility (from 0.31 to 0.02 SI‐mass) and natural remanent magnetization (NRM) intensity. Both NRM and saturation isothermal remanent magnetization (SIRM) intensity are ?2 Am²/Kg. Demagnetization response by the NRM suggests a complicated history of remanence acquisition. The spherules show schlieren structure described by chains of tiny dendritic and octahedral‐shaped magnetite crystals indicating their quenching from liquid droplets. Microprobe analyses show that, relative to the target basalt compositions, the spherules have relatively high average Fe₂O₃ (by ?1.5 wt%), MgO (?1 wt%), Mn (?200 ppm), Cr (?200 ppm), Co (?50 ppm), Ni (?1000 ppm) and Zn (?70 ppm), and low Na₂O (?1 wt%) and P₂O₅ (?0.2 wt%). Very high Ni contents, up to 14 times the average content of Lonar basalt, require the presence of a meteoritic component in these spherules. We interpret the high Ni, Cr, and Co abundances in these spherules to indicate that the impactor of the Lonar crater was a chondrite, which is present in abundances of 12 to 20 percent by weight in these impact spherules. Relatively high Zn yet low Na₂O and P₂O₅ contents of these spherules indicate exchange of volatiles between the quenching spherule droplets and the impact plume.     

Carbon Stable Isotope Analyses of Individual Deep-Sea Spherules

Abstract— A preliminary investigation into the carbon isotopic composition of deep-sea spherules has been undertaken. A variety of particles have been analysed including both melted and unmelted samples of type S (stony) and type I (iron) spherules, emphasis being placed on surveying the carbon in different sorts of particles rather than analysing large numbers of samples. Some general observations can be made: there appear to be four different sorts of carbonaceous materials in the spherules. Melted and unmelted spherules of either type I or S, apparently contain two forms of low temperature combustible carbon distinguished, not by combustion temperature, but by isotopic composition. The low temperature of combustion is commensurate with these forms of carbon being organic in nature. The most likely explanation for this carbon is terrestrial biogenic contamination although there exists the possibility that there are some indigenous organic materials. Unmelted type S spherules contain a high temperature carbon component, characterised by a very minor ¹³C-enrichment, which is considered to be indigenous to the sample. All melted samples contain only small amounts of high temperature carbon with an isotopic composition suggestive of handling blank.     

Geochemistry of a confirmed Precambrian impact ejecta deposit: The Grænsesø spherule layer,South Greenland

Samples from a single outcrop of the Graenseso spherule layer, Midternaes, South Greenland, consist of a spherule‐bearing dolomixtite with matrix‐supported intraclasts up to 1 m in size. In addition to field observations, we performed mineralogical and whole rock geochemical analysis, including electron microprobe, neutron activation analysis, X‐ray fluorescence, and mass spectrometry of the horizon and the overlying and underlying strata. We show that the spherules are petrographically similar to those in the Zaonega spherule layer, Karelia, Russia. Our petrographic and chemical results are consistent with the previous suggestion that the Grænsesø layer correlates with the Zaonega layer, and it is possible that both layers are related to the Vredefort impact event. The samples containing spherules, as well as the overlying rocks, have elevated REEs compared to the underlying pre‐impact layer, suggestive of a new continental source of sediment that may be coincident with the impact event. Zircons separated from the lower part of the Grænsesø spherule layer display complex age patterns suggesting that they have genetically different origins based on distinctly different Th/U ratios. Crystallization ages of all groups are ≥ 2.8 Ga, with ~2.8 Ga representing a time of major crustal growth globally. Therefore, we cannot conclusively determine in this study if the zircons are locally derived or if they are transported with the ejecta. The spherule layer was deposited by a high‐energy, subaqueous debris flow, an origin that is consistent with the hypothesis that the layer was deposited by impact‐induced waves and/or currents.     

Moderate velocity oblique impact sliding: Production of shocked meteorite textures and palaeomagnetically important metallic spherules in planetary regoliths

We detail the production of metallic spherules in laboratory oblique shock impact experiments, and their applicability (1) to textures in a partly shock‐melted chondritic meteorite and (2) to the occurrence of palaeomagnetically important fine iron or iron alloy particles in the lunar regolith. Samples recovered from 29–44 GPa, 800 ns, experiments revealed melting and textures reminiscent of metallic spherules in the Yanzhuang H‐chondrite, including “dumbbell” forms and other more complex morphologies. Our experiments demonstrate that metallic spherules can be produced via oblique impact sliding at lower velocities (1.85 km s^?1) than are generally assumed in previous work associated with bulk‐shock melting, and that oblique impact sliding is a viable mechanism for producing spherules in shock‐induced veins in moderately shocked meteorites. Significantly, our experiments also produced fine metallic (iron alloy) spherules within the theoretical narrow size range (a few tens of nanometers for slightly ellipsoidal particles) for stable single‐domain (SSD) particles, which are the most important palaeomagnetically, since they can record lunar and planetary magnetic fields over geological time periods. The experiments also produced spherules consistent with superparamagnetic (SP) and multidomain (MD) particle sizes. The fine SSD and SP particles on the lunar surface are currently thought to have been formed predominantly by space weathering processes. Our experiments suggest that oblique shock impact sliding may be a further means of producing the SSD and SP iron or iron alloy particles observed in the lunar regolith, and which are likely to occur in the regoliths of Mercury and other planetary bodies.     

Micrometeorites from the northern ice cap of the Novaya Zemlya archipelago,Russia: The first occurrence

Abstract— Glacial deposits at the margins of the ice cap of the northern island of the Novaya Zemlya archipelago, Russia, contain numerous spherules and rare scoriaceous particles thought to be extraterrestrial. The 1 Kyr old glacier has decreased in volume and coverage during the last 40 years, leaving the spherules contained in the ice at the margins of the glacier where they can be easily collected. The spherules are similar in their appearance, texture, and mineralogy to cosmic spherules found in deep‐sea sediments in Greenland and Antarctica. Silicate spherules have typical bar‐like textures (75%) or porphyritic textures (15%), while other spherules are glassy (7%). The spherules from Novaya Zemlya are altered only slightly. There are spherules consisting of iron oxides, metal cores with iron oxide rims, a continuous network of iron oxide dendrites in a glass matrix, and particles rich in chromite (3%). Some spherules contain metal droplets and relict forsterite and low‐Ca pyroxene. Silicate spherule compositions match compositions of other cosmic spherules. Both Nova Zemlya and other cosmic spherules are close to carbonaceous chondrite matrices in patterns of variations for Ca, Mg, Si, and Al, which might suggest that their predecessor was similar to carbonaceous chondrite matrices. Unmelted micrometeorites are generally depleted in Ca and Mg and enriched in Al relative to cosmic spherules. The depletion of the micrometeorites in Ca and Mg can be connected with their terrestrial alteration (Kurat et al. 1994), while the Al enrichment seems to be primary.     

The entry heating and abundances of basaltic micrometeorites

Basaltic micrometeorites (MMs) derived from HED‐like parent bodies have been found among particles collected from the Antarctic and from Arctic glaciers and are to date the only achondritic particles reported among cosmic dust. The majority of Antarctic basaltic particles are completely melted cosmic spherules with only one unmelted particle recognized from the region. This paper investigates the entry heating of basaltic MMs in order to predict the relative abundances of unmelted to melted basaltic particles and to evaluate how mineralogical differences in precursor materials influence the final products of atmospheric entry collected on the Earth's surface. Thermodynamic modeling is used to simulate the melting behavior of particles with compositions corresponding to eucrites, diogenites, and ordinary chondrites in order to evaluate degree of partial melting and to make a comparison between the behavior of chondritic particles that dominate the terrestrial dust flux and basaltic micrometeroids. The results of 120,000 simulations were compiled to predict relative abundances and indicate that the phase relations of precursor materials are crucial in determining the relative abundances of particle types. Diogenite and ordinary chondrite materials exhibit similar behavior, although diogenite precursors are more likely to form cosmic spherules under similar entry parameters. Eucrite particles, however, are much more likely to melt due to their lower liquidus temperatures and small temperature interval of partial melting. Eucrite MMs, therefore, usually form completely molten cosmic spherules except at particle diameters <100 μm. The low abundance of unmelted basaltic MMs compared with spherules, if statistically valid, is also shown to be inconsistent with a low velocity population (12 km s^?1) and is more compatible with higher velocities which may suggest a near‐Earth asteroid source dominates the current dust production of basaltic MMs.     

A dust cloud of Ganymede maintained by hypervelocity impacts of interplanetary micrometeoroids

A dust cloud of Ganymede has been detected by in situ measurements with the dust detector onboard the Galileo spacecraft. The dust grains have been sensed at altitudes below five Ganymede radii (Ganymede radius=2635 km). Our analysis identifies the particles in the dust cloud surrounding Ganymede by their impact direction, impact velocity, and mass distribution and implies that they have been kicked up by hypervelocity impacts of micrometeoroids onto the satellite's surface. We calculate the radial density profile of the particles ejected from the satellite by interplanetary dust grains. We assume the yields, mass and velocity distributions of the ejecta obtained from laboratory impact experiments onto icy targets and consider the dynamics of the ejected grains in ballistic and escaping trajectories near Ganymede. The spatial dust density profile calculated with interplanetary particles as impactors is consistent with the profile derived from the Galileo measurements. The contribution of interstellar grains as projectiles is negligible. Dust measurements in the vicinities of satellites by spacecraft detectors are suggested as a beneficial tool to obtain more knowledge about the satellite surfaces, as well as dusty planetary rings maintained by satellites through the impact ejecta mechanism.