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.     

Dynamic pyrometamorphism during atmospheric entry of large (˜10 micron) pyrrhotite fragments from cluster IDPs

Abstract— Petrological changes in Ni‐free and low‐Ni pyrrhotite, and much less in pentlandite, during atmospheric entry flash‐heating of the sulfide IDPs L2005E40, L2005C39, and L2006A28 support 1) ferrous sulfide oxidation with vacancy formation and Fe³⁺ ordering; and 2) Fe‐oxide formation and sulfur vapor loss through abundant vesicles. Melting of metastable chondritic aggregate materials at the IDP surface has occurred. All changes, e.g., formation of a continuous maghémite rim, proceeded as solid‐state reactions at a peak heating temperature of ?700 °C. This temperature in combination with particle size and density suggest a ?10 km/s^?1 entry velocity. The IDPs probably belonged to cluster IDPs that entered the atmosphere with near‐Earth or Earth‐crossing asteroid velocities. They could be debris from extinct or dormant comet nuclei, which is consistent with shock comminution of pyrrhotite in these IDPs.     

The atmospheric entry of fine‐grained micrometeorites: The role of volatile gases in heating and fragmentation

The early stages of atmospheric entry are investigated in four large (250–950 μm) unmelted micrometeorites (three fine‐grained and one composite), derived from the Transantarctic Mountain micrometeorite collection. These particles have abundant, interconnected, secondary pore spaces which form branching channels and show evidence of enhanced heating along their channel walls. Additionally, a micrometeorite with a double‐walled igneous rim is described, suggesting that some particles undergo volume expansion during entry. This study provides new textural data which links together entry heating processes known to operate inside micrometeoroids, thereby generating a more comprehensive model of their petrographic evolution. Initially, flash heated micrometeorites develop a melt layer on their exterior; this igneous rim migrates inwards. Meanwhile, the particle core is heated by the decomposition of low‐temperature phases and by volatile gas release. Where the igneous rim acts as a seal, gas pressures rise, resulting in the formation of interconnected voids and higher particle porosities. Eventually, the igneous rim is breached and gas exchange with the atmosphere occurs. This mechanism replaces inefficient conductive rim‐to‐core thermal gradients with more efficient particle‐wide heating, driven by convective gas flow. Interconnected voids also increase the likelihood of particle fragmentation during entry and, may therefore explain the rarity of large fine‐grained micrometeorites among collections.     

Heating experiments simulating atmospheric entry heating of micrometeorites: Clues to their parent body sources

Abstract— Depending on their velocity, entry angle and mass, extraterrestrial dust particles suffer certain degrees of heating during entry into Earth's atmosphere, and the mineralogy and chemical composition of these dust particles are significantly changed. In the present study, pulse-heating experiments simulating the atmospheric entry heating of micrometeoroids were carried out in order to understand the mineralogical and chemical changes quantitatively as well as to estimate the peak temperature experienced by the particles during entry heating. Fragments of the CI chondrites Orgueil and Alais as well as pyrrhotites from Orgueil were used as analogue material. The experiments show that the volatile elements S, Zn, Ga, Ge, and Se can be lost from 50 to 100 μm sized CI meteorite fragments at temperatures and heating times applicable to the entry heating of similar sized cosmic dust particles. It is concluded that depletions of these elements relative to CI as observed in micrometeorites are mainly caused by atmospheric entry heating. Besides explaining the element abundances in micrometeorites, the experimentally obtained release patterns can also be used as indicators to estimate the peak heating of dust particles during entry. Using the abundances of Zn and Ge and assuming their original concentrations close to CI, a maximum heating of 1100–1200 °C is obtained for previously analyzed Antarctic micrometeroites. Thermal alteration also strongly influenced the mineralogy of the meteorite fragments. While the unheated samples mainly consisted of phyllosilicates, these phases almost completely transformed into olivine and pyroxene in the fragments heated to ≥800 °C. Therefore, dust particles that still contain hydrous minerals were probably never heated to temperatures ≥800 °C in the atmosphere. During continued heating, the grain size of the newly formed silicates increased and the composition of the olivines equilibrated. Applying these results quantitatively to Antarctic micrometeorites, typical peak temperatures in the range of 1100–1200 °C during atmospheric entry heating are deduced. This temperature range corresponds to the one obtained from the volatile element concentrations measured in these micrometeorites and points to an asteroidal origin of the particles.     

The origins of I‐type spherules and the atmospheric entry of iron micrometeoroids

The Earth's extraterrestrial dust flux includes a wide variety of dust particles that include FeNi metallic grains. During their atmospheric entry iron micrometeoroids melt and oxidize to form cosmic spherules termed I‐type spherules. These particles are chemically resistant and readily collected by magnetic separation and are thus the most likely micrometeorites to be recovered from modern and ancient sediments. Understanding their behavior during atmospheric entry is crucial in constraining their abundance relative to other particle types and the nature of the zodiacal dust population at 1 AU. This article presents numerical simulations of the atmospheric entry heating of iron meteoroids to investigate the abundance and nature of these materials. The results indicate that iron micrometeoroids experience peak temperatures 300–800 K higher than silicate particles explaining the rarity of unmelted iron particles which can only be present at sizes of <50 μm. The lower evaporation rates of liquid iron oxide leads to greater survival of iron particles compared with silicates, which enhances their abundance among micrometeorites by a factor of 2. The abundance of I‐types is shown to be broadly consistent with the abundance and size of metal in ordinary chondrites and the current day flux of ordinary chondrite‐derived MMs arriving at Earth. Furthermore, carbonaceous asteroids and cometary dust are suggested to make negligible contributions to the I‐type spherule flux. Events involving such objects, therefore, cannot be recognized from I‐type spherule abundances in the geological record.     

Density,porosity, mineralogy,and internal structure of cosmic dust and alteration of its properties during high‐velocity atmospheric entry

X‐ray microtomography (XMT), X‐ray diffraction (XRD), and magnetic hysteresis measurements were used to determine micrometeorite internal structure, mineralogy, crystallography, and physical properties at μm resolution. The study samples include unmelted, partially melted (scoriaceous), and completely melted (cosmic spherules) micrometeorites. This variety not only allows comparison of the mineralogy and porosity of these three micrometeorite types but also reveals changes in meteoroid properties during atmospheric entry at various velocities. At low entry velocities, meteoroids do not melt and their physical properties do not change. The porosity of unmelted micrometeorites varies considerably (0–12%) with one friable example having porosity around 50%. At higher velocities, the range of meteoroid porosity narrows, but average porosity increases (to 16–27%) due to volatile evaporation and partial melting (scoriaceous phase). Metal distribution seems to be mostly unaffected at this stage. At even higher entry velocities, complete melting follows the scoriaceous phase. Complete melting is accompanied by metal oxidation and redistribution, loss of porosity (1 ± 1%), and narrowing of the bulk (3.2 ± 0.5 g cm^?3) and grain (3.3 ± 0.5 g cm^?3) density range. Melted cosmic spherules with a barred olivine structure show an oriented crystallographic structure, whereas other subtypes do not.     

The fusion crust of the Winchcombe meteorite: A preserved record of atmospheric entry processes

Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred during deceleration in the atmosphere. The fusion crust of the Winchcombe meteorite closely resembles that of other stony meteorites, and in particular CM2 chondrites, since it is dominated by olivine phenocrysts set in a glassy mesostasis with magnetite, and is highly vesicular. Dehydration cracks are unusually abundant in Winchcombe. Failure of this weak layer is an additional ablation mechanism to produce large numbers of particles during deceleration, consistent with the observation of pulses of plasma in videos of the Winchcombe fireball. Calving events might provide an observable phenomenon related to meteorites that are particularly susceptible to dehydration. Oscillatory zoning is observed within olivine phenocrysts in the fusion crust, in contrast to other meteorites, perhaps owing to temperature fluctuations resulting from calving events. Magnetite monolayers are found in the crust, and have also not been previously reported, and form discontinuous strata. These features grade into magnetite rims formed on the external surface of the crust and suggest the trapping of surface magnetite by collapse of melt. Magnetite monolayers may be a feature of meteorites that undergo significant degassing. Silicate warts with dendritic textures were observed and are suggested to be droplets ablated from another stone in the shower. They, therefore, represent the first evidence for intershower transfer of ablation materials and are consistent with the other evidence in the Winchcombe meteorite for unusually intense gas loss and ablation, despite its low entry velocity.     

Response of sandstone to atmospheric heating during the STONE 5 experiment: Implications for the palaeofluid record in meteorites

A 1 cm thick sandstone disk exposed to atmospheric re-entry on the heat shield of a spacecraft (the STONE 5 experiment) shows alteration of fluid inclusions compared to a control sample. The sandstone contained inclusions in quartz grains, feldspar grains and calcite cement before flight. After flight, inclusions in the feldspar were all decrepitated, few inclusions in calcite survived intact and they yielded widely varying microthermometric data, and the quartz inclusions also yielded disturbed microthermometric data. The quartz becomes less affected with depth below the surface, and extrapolation suggests would be unaffected at a depth of about 2 cm. These data show that fluid inclusion data from meteorites must be treated with caution, but that a genuine fluid record may survive in the interior portions. The possibility of thermal sterilization to 2 cm depth also implies that small meteorites may be unsuitable vehicles for the transfer of microbial life from one planetary body to another. As the interiors of larger meteorites tend to have very low porosity and permeability, microbial colonization would be difficult, and the potential for panspermia is accordingly low.     

Explicating the behavior of Mn‐bearing phases during shock melting and crystallization of the Abee EH‐chondrite impact‐melt breccia

Abstract— –Literature data show that, among EH chondrites, the Abee impact‐melt breccia exhibits unusual mineralogical characteristics. These include very low MnO in enstatite (<0.04 wt%), higher Mn in troilite (0.24 wt%) and oldhamite (0.36 wt%) than in EH4 Indarch and EH3 Kota‐Kota (which are not impact‐melt breccias), low Mn in keilite (3.6–4.3 wt%), high modal abundances of keilite (11.2 wt%) and silica (～7 wt%, but ranging up to 16 wt% in some regions), low modal abundances of total silicates (58.8 wt%) and troilite (5.8 wt%), and the presence of acicular grains of the amphibole, fluor‐richterite. These features result from Abee's complex history of shock melting and crystallization. Impact heating was responsible for the loss of MnO from enstatite and the concomitant sulfidation of Mn. Troilite and oldhamite grains that crystallized from the impact melt acquired relatively high Mn contents. Abundant keilite and silica also crystallized from the melt; these phases (along with metallic Fe) were produced at the expense of enstatite, niningerite and troilite. Melting of the latter two phases produced a S‐rich liquid with higher Fe/Mg and Fe/Mn ratios than in the original niningerite, allowing the crystallization of keilite. Prior to impact melting, F was distributed throughout Abee, perhaps in part adsorbed onto grain surfaces; after impact melting, most of the F that was not volatilized was incorporated into crystallizing grains of fluor‐richterite. Other EH‐chondrite impact‐melt breccias and impact‐melt rocks exhibit some of these mineralogical features and must have experienced broadly similar thermal histories.     

GEMS,hydrated chondritic IDPs,CI‐matrix material: Sources of water in 81P/comet Wild 2

So far there is no conclusive evidence for water in the nucleus of 81P/comet Wild 2. Recently magnetite in collected Wild 2 samples was cited as proxy evidence for parent body aqueous alteration in this comet (Hicks et al. 2017 ). A potentional source for water of hydration would be layer silicates but unfortunately there is no record, neither texturally nor chemically, for hydrated layer silicates that survived hypervelocity impact in the Wild 2 samples. This paper reports large vesicles in the matrix of allocation C2044,2,41,2,5 from a volatile‐rich type B/C Stardust track. These vesicles were probably caused by boiling water that were generated when hydrated Wild 2 silicates impacted the near‐surface silica aerogel layer. Potential water sources were partially and fully hydrated GEMS (glass with embedded metal and sulfides) and CI carbonaceous chondrite materials among the earliest dusts that experienced hydration and icy‐body formation and long‐range transport and mixing with materials from across the solar system.     

The formation of rims on calcium‐aluminum‐rich inclusions: Step I—Flash heating

Abstract— Wark‐Lovering rims of six calcium‐aluminum‐rich inclusions (CAIs) representing the main CAI types and groups in Allende, Efremovka and Vigarano were microsurgically separated and analysed by neutron activation analysis (NAA). All the rims have similar ～4x enrichments, relative to the interiors, of highly refractory lithophile and siderophile elements. The NAA results are confirmed by ion microprobe and scanning electron microscope (SEM) analyses of rim perovskites and rim metal grains. Less refractory Eu, Yb, V, Sr, Ca and Ni are less enriched in the rims. The refractory element patterns in the rims parallel the patterns in the outer parts of the CAIs. In particular, the rims on type B1 CAIs have the igneously fractionated rare earth element (REE) pattern of the melilite mantle below the rim and not the REE pattern of the bulk CAI, proving that the refractory elements in the rims were derived from the outer mantle and were not condensates onto the CAIs. The refractory elements were enriched in an Al₂O₃‐rich residue <50 μm thick after the most volatile ～80% of the outermost 200 μm of each CAI had been volatilized, including much Mg, Si and Ca. Some volatilization occurred below the rim, and created refractory partial melts that crystallized hibonite and gehlenitic melilite. The required “flash heating” probably exceeded 2000 °C, but for only a few seconds, in order to melt only the outer CAI and to unselectively volatilize slow‐diffusing O isotopes which show no mass fractionation in the rim. The volatilization did, however, produce “heavy” mass‐fractionated Mg in rims. In some CAIs this was later obscured when “normal” Mg diffused in from accreted olivine grains at relatively high temperature (not the lower temperature meteorite metamorphism) and created the ～50 μm set of monomineralic rim layers of pyroxene, melilite and spinel.     

Fe‐bearing phases in a ureilite fragment from the asteroid 2008 TC3 (= Almahata Sitta meteorites): A combined Mössbauer spectroscopy and X‐ray diffraction study

The iron‐bearing phases in a ureilite fragment (AS#051) from the Almahata Sitta meteorite are studied using Mössbauer spectroscopy, X‐ray diffraction (XRD), and electron microprobe analysis (EMPA). AS#051 has a typical ureilite texture of medium‐ to coarse‐grained silicates (olivine, orthopyroxene, and pigeonite) with minor opaques (Fe‐Ni metal, troilite, and graphite). The silicate compositions, determined by EMPA, are homogeneous: olivine (Fo_90.2), orthopyroxene (En_86.3Fs_8.6Wo_5.1), and pigeonite (En_81.6Fs_8.9Wo_9.5), and are similar to those of magnesian ureilites. The modal abundance of mineral phases was determined by Rietveld refinement of the powder XRD data. The Mössbauer spectra at 295 K and 78 K are composed of two sharp well‐defined paramagnetic doublets superimposed on a well‐resolved magnetic sextet and other weak absorption features. The two paramagnetic doublets are assigned to olivine and pyroxene (orthopyroxene and pigeonite), and the ferromagnetic sextet to kamacite (magnetic hyperfine field ≈ 33.2 T), in agreement with the XRD characterization. The Mössbauer results also show the presence of small amounts of troilite (FeS) and cohenite ([Fe,Ni,Co]₃C). Using the Mössbauer data, the relative abundance of each Fe‐bearing phase is determined and compared with the results obtained by XRD.     

Kosmochloric Ca‐rich pyroxenes and FeO‐rich olivines (Kool grains) and associated phases in Stardust tracks and chondritic porous interplanetary dust particles: Possible precursors to FeO‐rich type II chondrules in ordinary chondrites

Abstract— Terminal particles and mineral fragments from comet 81P/Wild 2 were studied in 16 aerogel tracks by transmission and secondary electron microscopy. In eight tracks clinopyroxenes with correlated Na₂O and Cr₂O₃ contents as high as 6.0 wt% and 13.0 wt%, respectively, were found. Kosmochloric (Ko) clinopyroxenes were also observed in 4 chondritic interplanetary dust particles (IDPs). The Ko‐clinopyroxenes were often associated with FeO‐rich olivine ± Cr‐rich spinel ± aluminosilicate glass or albitic feldspar, assemblages referred to as Kool grains (Ko = kosmochloric Ca‐rich pyroxene, ol = olivine). Fine‐grained (submicron) Kool fragments have textures suggestive of crystallization from melts while coarse‐grained (>1 μm) Kool fragments are often glass‐free and may have formed by thermal metamorphism in the nebula. Average major and minor element distributions between clinopyroxenes and coexisting FeO‐rich olivines are consistent with these phases forming at or near equilibrium. In glass‐bearing fine‐grained Kool fragments, high concentrations of Na in the clinopyroxenes are inconsistent with existing experimentally determined partition coefficients at equilibrium. We speculate that the availability of Cr in the melt increased the clinopyroxene Na partition coefficient via a coupled substitution thereby enhancing this phase with the kosmochlor component. The high temperature minerals, fine‐grain sizes, bulk compositions and common occurrence in the SD tracks and IDPs support the idea that Kool grains could have been precursors to type II chondrules in ordinary chondrites. These grains, however, have not been observed in these meteorites suggesting that they were destroyed during chondrule formation and recycling or were not present in the nebula at the time and location where meteoritic chondrules formed.     

Crystallization of calcium‐aluminum‐rich inclusions: Experimental studies on the effects of repeated heating events

Abstract— The observed textures and chemistry of calcium‐aluminum‐rich inclusions (CAIs) are presumed to be the culmination of a series of repeated heating and cooling events in the early history of the solar nebula. we have examined the effects of these heating/cooling cycles experimentally on a bulk composition representing an average type b cai composition by varying the nature of the starting material. Although the most recent and/or highest temperature event prior to incorporation into the parent body dominates the texture and chemistry of the CAI, prior events affect the phase compositions and texture. We have determined that a prior low‐temperature event (simulated by heating the sample to 1275 °C and cooling slowly) increases the likelihood of anorthite crystallization in subsequent higher temperature events. A prior high‐temperature event that produced dendritic melilite (simulated by heating the sample to 1550 °C and cooling rapidly) results in melilite that shows evidence of rapid crystallization in subsequent lower temperature events. The addition of Pt powder to the starting material appears to enhance the ability of anorthite to nucleate from this composition and increases the number of melilite crystals present in samples that crystallize euhedral melilite.     

High‐pressure phases in shock‐induced melt veins of the Umbarger L6 chondrite: Constraints of shock pressure

Abstract— We report a previously undocumented set of high‐pressure minerals in shock‐induced melt veins of the Umbarger L6 chondrite. High‐pressure minerals were identified with transmission electron microscopy (TEM) using selected area electron diffraction and energy‐dispersive X‐ray spectroscopy. Ringwoodite (Fa₃₀), akimotoite (En₁₁Fs₈₉), and augite (En₄₂Wo₃₃Fs₂₅) were found in the silicate matrix of the melt vein, representing the crystallization from a silicate melt during the shock pulse. Ringwoodite (Fa₂₇) and hollandite‐structured plagioclase were also found as polycrystalline aggregates in the melt vein, representing solid state transformation or melting with subsequent crystallization of entrained host rock fragments in the vein. In addition, Fe₂SiO₄‐spinel (Fa₆₆‐Fa₉₉) and stishovite crystallized from a FeO‐SiO₂‐rich zone in the melt vein, which formed by shock melting of FeO‐SiO₂‐rich material that had been altered and metasomatized before shock. Based on the pressure stabilities of the high‐pressure minerals, ringwoodite, akimotoite, and Ca‐clinopyroxene, the melt vein crystallized at approximately 18 GPa. The Fe₂SiO₄‐spinel + stishovite assemblage in the FeO‐SiO₂‐rich melts is consistent with crystallization of the melt vein matrix at the pressure up to 18 GPa. The crystallization pressure of ?18 GPa is much lower than the 45–90 GPa pressure one would conclude from the S6 shock effects in melt veins (Stöffler et al. 1991) and somewhat less than the 25–30 GPa inferred from S5 shock effects (Schmitt 2000) found in the bulk rock.     

Survival of refractory presolar grain analogs during Stardust‐like impact into Al foils: Implications for Wild 2 presolar grain abundances and study of the cometary fine fraction

We present results of FIB–TEM studies of 12 Stardust analog Al foil craters which were created by firing refractory Si and Ti carbide and nitride grains into Al foils at 6.05 km s^?1 with a light‐gas gun to simulate capture of cometary grains by the Stardust mission. These foils were prepared primarily to understand the low presolar grain abundances (both SiC and silicates) measured by SIMS in Stardust Al foil samples. Our results demonstrate the intact survival of submicron SiC, TiC, TiN, and less‐refractory Si₃N₄ grains. In small (<2 μm) craters that are formed by single grain impacts, the entire impacting crystalline grain is often preserved intact with minimal modification. While they also survive in crystalline form, grains at the bottom of larger craters (>5 μm) are typically fragmented and are somewhat flattened in the direction of impact due to partial melting and/or plastic deformation. The low presolar grain abundance estimates derived from SIMS measurements of large craters (mostly >50 μm) likely result from greater modification of these impactors (i.e., melting and isotopic dilution), due to higher peak temperatures/pressures in these crater impacts. The better survivability of grains in smaller craters suggests that more accurate presolar grain estimates may be achievable through measurement of such craters. It also suggests small craters can provide a complementary method of study of the Wild 2 fine fraction, especially for refractory CAI‐like minerals.     

The non‐igneous genesis of angrites: Support from trace element distribution between phases in D'Orbigny

Abstract— D'Orbigny is an exceptional angrite. Chemically, it resembles other angrites such as Asuka‐881371, Sahara 99555, Lewis Cliff (LEW) 87051, and LEW 86010, but its structure and texture are peculiar. It has a compact and porous lithology, abundant glasses, augite‐bearing druses, and chemical and mineralogical properties that are highly unusual for igneous rocks. Our previous studies led us to a new view on angrites: they can possibly be considered as CAIs that grew to larger sizes than the ones we know from carbonaceous chondrites. Thus, angrites may bear a record of rare and special conditions in some part of the early solar nebula. Here we report trace element contents of D'Orbigny phases. Trace element data were obtained from both the porous and the compact part of this meteorite. We have confronted our results with the popular igneous genetic model. According to this model, if all phases of D'Orbigny crystallized from the same system, as an igneous origin implies, a record of this genesis should be expressed in the distribution of trace elements among early and late phases. Our results show that the trace element distribution of the two contemporaneous phases olivine and plagioclase, which form the backbone of the rock, seem to require liquids of different composition. Abundances of highly incompatible elements in all olivines, including the megacrysts, indicate disequilibrium with the bulk rock and suggest liquids very rich in these elements (>10,000 x CI), which is much richer than any fractional crystallization could possibly produce. In addition, trace element contents of late phases are incompatible with formation from the bulk system's residual melt. These results add additional severe constraints to the many conflicts that existed previously between an igneous model for the origin of angrites and the mineralogical and chemical observations. This new trace element content data, reported here, corroborate our previous results based on the shape, structure, mineralogy, chemical, and isotopic data of the whole meteorite, as well as on a petrographic and chemical composition study of all types of glasses and give strength to a new genetic model that postulates that D'Orbigny (and possibly all angrites) could have formed in the solar nebula under changing redox conditions, more akin to chondritic constituents (e.g., CAIs) than to planetary differentiated rock.     

Classification of the Didwana‐Rajod meteorite: A Mössbauer spectroscopic study

Abstract— Mössbauer spectroscopic studies of the Didwana‐Rajod chondrite, which fell on 1991 August 12 in western Rajasthan, India, are presented. The results are compared with the Mössbauer data of several enstatite and ordinary chondrites including the Dhajala chondrite for which Mössbauer data were acquired during the present study. The Didwana‐Rajod chondrite's iron phases and its oxidation states strongly suggest that it should be classified as an H‐type ordinary chondrite instead of the earlier suggestion (based on petrographic studies) that it could be an enstatite chondrite. The present study demonstrates that Mössbauer spectroscopy is a very powerful technique for aiding in the classification of meteorites.     

Four‐colour photometry of EY Dra: A study of an ultra‐fast rotating active dM1‐2e star

We present more than 1000‐day long photometry of EY Draconis in BV (RI)_C passbands. The changes in the light curve are caused by the spottedness of the rotating surface. Modelling of the spotted surface shows that there are two large active regions present on the star on the opposite hemispheres. The evolution of the surface patterns suggests a flip‐flop phenomenon. Using Fourier analysis, we detect a rotation period of P_rot = 0.45875 d, and an activity cycle with P ≈ 350 d, similar to the 11‐year long cycle of the Sun. This cycle with its year‐long period is the shortest one ever detected on active stars. Two bright flares are also detected and analysed (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High‐pressure phases in shock‐induced melt of the unique highly shocked LL6 chondrite Northwest Africa 757

Northwest Africa 757 is unique in the LL chondrite group because of its abundant shock‐induced melt and high‐pressure minerals. Olivine fragments entrained in the melt transform partially and completely into ringwoodite. Plagioclase and Ca‐phosphate transform to maskelynite, lingunite, and tuite. Two distinct shock‐melt crystallization assemblages were studied by FIB‐TEM analysis. The first melt assemblage, which includes majoritic garnet, ringwoodite plus magnetite‐magnesiowüstite, crystallized at pressures of 20–25 GPa. The other melt assemblage, which consists of clinopyroxene and wadsleyite, solidified at ~15 GPa, suggesting a second veining event under lower pressure conditions. These shock features are similar to those in S6 L chondrites and indicate that NWA 757 experienced an intense impact event, comparable to the impact event that disrupted the L chondrite parent body at 470 Ma.