Micrometeorite collections: a review and their current status.

Micrometeorites are estimated to represent the main part of the present flux of extraterrestrial matter found on the Earth's surface and provide valuable samples to probe the interplanetary medium. Here, we describe large and representative collections of micrometeorites currently available to the scientific community. These include Antarctic collections from surface ice and snow, as well as glacial sediments from the eroded top of nunataks-summits outcropping from the icesheet-and moraines. Collections extracted from deep-sea sediments (DSS) produced a large number of micrometeorites, in particular, iron-rich cosmic spherules that are rarer in other collections. Collections from the old and stable surface of the Atacama Desert show that finding large numbers of micrometeorites is not restricted to polar regions or DSS. The advent of rooftop collections marks an important step into involving citizen science in the study of micrometeorites, as well as providing potential sampling locations over all latitudes to explore the modern flux. We explore their strengths of the collections to address specific scientific questions and their potential weaknesses. The future of micrometeorite research will involve the finding of large fossil micrometeorite collections and benefit from recent advances in sampling cosmic dust directly from the air. This article is part of the theme issue 'Dust in the Solar System and beyond'.

Multiple generations of grain aggregation in different environments preceded solar system body formation.

The solar system formed from interstellar dust and gas in a molecular cloud. Astronomical observations show that typical interstellar dust consists of amorphous (a-) silicate and organic carbon. Bona fide physical samples for laboratory studies would yield unprecedented insight about solar system formation, but they were largely destroyed. The most likely repositories of surviving presolar dust are the least altered extraterrestrial materials, interplanetary dust particles (IDPs) with probable cometary origins. Cometary IDPs contain abundant submicron a-silicate grains called GEMS (glass with embedded metal and sulfides), believed to be carbon-free. Some have detectable isotopically anomalous a-silicate components from other stars, proving they are preserved dust inherited from the interstellar medium. However, it is debated whether the majority of GEMS predate the solar system or formed in the solar nebula by condensation of high-temperature (>1,300 K) gas. Here, we map IDP compositions with single nanometer-scale resolution and find that GEMS contain organic carbon. Mapping reveals two generations of grain aggregation, the key process in growth from dust grains to planetesimals, mediated by carbon. GEMS grains, some with a-silicate subgrains mantled by organic carbon, comprise the earliest generation of aggregates. These aggregates (and other grains) are encapsulated in lower-density organic carbon matrix, indicating a second generation of aggregation. Since this organic carbon thermally decomposes above ∼450 K, GEMS cannot have accreted in the hot solar nebula, and formed, instead, in the cold presolar molecular cloud and/or outer protoplanetary disk. We suggest that GEMS are consistent with surviving interstellar dust, condensed in situ, and cycled through multiple molecular clouds.

Oxygen isotopic composition of an enstatite ribbon of probable cometary origin.

Filamentary enstatite crystals are found in interplanetary dust particles (IDPs) of likely cometary origin but are very rare or absent in meteorites. Crystallographic characteristics of filamentary enstatites indicate that they condensed directly from vapor. We measured the O isotopic composition of an enstatite ribbon from a giant cluster IDP to be δ18O = 25 ± 55, δ17O = -19 ± 129, Δ17O = -32 ± 134 (2σ errors), which is inconsistent at the 2σ level with the composition of the Sun inferred from the Genesis solar wind measurements. The particle's O isotopic composition, consistent with the terrestrial composition, implies that it condensed from a gas of nonsolar O isotopic composition, possibly as a result of vaporization of disk region enriched in 16O-depleted solids. The relative scarcity of filamentary enstatite in asteroids compared to comets implies either that this crystal condensed from dust vaporized in situ in the outer solar system where comets formed or it condensed in the inner solar system and was subsequently transported outward to the comet-forming region.

Constraints on the formation environment of two chondrule-like igneous particles from Comet 81P/Wild 2.

Using chemical and petrologic evidence and modeling, we deduce that two chondrule-like particles named Iris and Callie, from Stardust cometary track C2052,12,74, formed in an environment very similar to that seen for type II chondrules in meteorites. Iris was heated near liquidus, equilibrated, and cooled at ≤ 100 °C/hr and within ≈ 2 log units of the IW buffer with a high partial pressure of Na such as would be present with dust enrichments of ≈ 103. There was no detectable metamorphic, nebular or aqueous alteration. In previous work Ogliore et al. (2012) reported that Iris formed late, > 3 Myr after CAIs, assuming 26Al was homogenously distributed, and was rich in heavy oxygen. Iris may be similar to assemblages found only in interplanetary dust particles and Stardust cometary samples called Kool particles. Callie is chemically and isotopically very similar but not identical to Iris.

Correlated isotopic and chemical evidence for condensation origins of olivine in comet 81P/Wild 2 and in AOAs from CV and CO chondrites.

Magnesium stable isotope ratios and minor element abundances of five olivine particles from comet 81P/Wild 2 were examined by secondary ion mass spectrometry (SIMS). Wild 2 olivine particles exhibit only small variations in δ25Mg values from -1.0 +0.4/-0.5 ‰ to 0.6 +0.5/- 0.6 ‰ (2σ). This variation can be simply explained by mass-dependent fractionation from Mg isotopic compositions of the Earth and bulk meteorites, suggesting that Wild 2 olivine particles formed in the chondritic reservoir with respect to Mg isotope compositions. We also determined minor element abundances, and O and Mg isotope ratios of olivine grains in amoeboid olivine aggregates (AOAs) from Kaba (CV3.1) and DOM 08006 (CO3.01) carbonaceous chondrites. Our new SIMS minor element data reveal uniform, low FeO contents of ~0.05 wt% among AOA olivines from DOM 08006, suggesting that AOAs formed at more reducing environments in the solar nebula than previously thought. Furthermore, the SIMS-derived FeO contents of the AOA olivines are consistently lower than those obtained by electron microprobe analyses (~1 wt% FeO), indicating possible fluorescence from surrounding matrix materials and/or Fe,Ni-metals in AOAs during electron microprobe analyses. For Mg isotopes, AOA olivines show more negative mass-dependent fractionation (-3.8 ± 0.5‰ ≤ δ25Mg ≤ -0.2 ± 0.3‰; 2σ) relative to Wild 2 olivines. Further, these Mg isotope variations are correlated with their host AOA textures. Large negative Mg isotope fractionations in olivine are often observed in pore-rich AOAs, while those in compact AOAs tend to have near-chondritic Mg isotopic compositions. These observations indicate that pore-rich AOAs preserved their gas-solid condensation histories, while compact AOAs experienced thermal processing in the solar nebula after their condensation and aggregation. Importantly, one 16O-rich Wild 2 LIME olivine particle (T77/F50) shows negative Mg isotope fractionation (δ25Mg = -0.8 ± 0.4‰, δ26Mg = -1.4 ± 0.9‰; 2σ) relative to bulk chondrites. Minor element abundances of T77/F50 are in excellent agreement with those of olivines from pore-rich AOAs in DOM 08006. The observed similarity in O and Mg isotopes, and minor element abundances suggest that T77/F50 formed in an environment similar to AOAs, probably near the proto-Sun, and then was transported to the Kuiper belt, where comet 81P/Wild 2 likely accreted.

Origin of crystalline silicates from Comet 81P/Wild 2: Combined study on their oxygen isotopes and mineral chemistry.

In order to explore the link between comet 81P/Wild 2 and materials in primitive meteorites, seven particles 5 to 15 µm in diameter from comet 81P/Wild 2 have been analyzed for their oxygen isotope ratios using a secondary ion mass spectrometer. Most particles are single minerals consisting of olivine or pyroxene with Mg# higher than 85, which are relatively minor in 81P/Wild 2 particles (~1/3 of the 16O-poor cluster). Four particles extracted from Track 149 are 16O-poor and show Δ17O (= δ17O - 0.52 × Î´18O) values from -2%0 to +1%0, similar to previous studies, while one enstatite (En99) particle shows lower Δ17O value of -7±4%o (2σ). This compositional range has not been reported among 16O-poor particles in 81P/Wild 2, but is commonly observed among chondrules in carbonaceous chondrites and in particular in CR chondrites. The distribution in Δ17O indicates that 16O-poor 81P/Wild 2 particles are most similar to chondrules (and their fragments) in the CR chondrites and Tagish Lake-like WIS91600 chondrite chondrule silicate grains, which indicates that they likely come from a reservoir with similar dust/ice ratios as CR chondrites and WIS91600. However, differences in the Mg# distribution imply that the 81P/Wild 2 reservoir was comparatively more oxidized, with a higher dust enrichment. Two nearly pure enstatite grains from track 172 are significantly enriched in 16O, with δ18O values of -51.2 ± 1.5%0 (2σ) and -43.0 ± 1.3% (2σ), respectively, and Δ17O values of -22.3 ± 1.9% (2σ) and -21.3 ± 2.3%0 (2σ), respectively. They are the first 16O-rich pyroxenes found among 81P/Wild 2 particles, with similar Δ17O values to those of 16O-rich low-iron, manganese-enriched (LIME) olivine and CAI (calcium and aluminum-rich inclusions) -like particles from 81P/Wild 2. The major element and oxygen isotopic compositions of the pyroxenes are similar to those of enstatite in amoeboid olivine aggregates (AOAs) in primitive chondrites, in which 16O-rich pyroxenes have previously been found, and thus suggest a condensation origin.

Interstellar dust. Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft.

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.

LIFE: Life Investigation For Enceladus A Sample Return Mission Concept in Search for Evidence of Life.

Life Investigation For Enceladus (LIFE) presents a low-cost sample return mission to Enceladus, a body with high astrobiological potential. There is ample evidence that liquid water exists under ice coverage in the form of active geysers in the "tiger stripes" area of the southern Enceladus hemisphere. This active plume consists of gas and ice particles and enables the sampling of fresh materials from the interior that may originate from a liquid water source. The particles consist mostly of water ice and are 1-10 µ in diameter. The plume composition shows H(2)O, CO(2), CH(4), NH(3), Ar, and evidence that more complex organic species might be present. Since life on Earth exists whenever liquid water, organics, and energy coexist, understanding the chemical components of the emanating ice particles could indicate whether life is potentially present on Enceladus. The icy worlds of the outer planets are testing grounds for some of the theories for the origin of life on Earth. The LIFE mission concept is envisioned in two parts: first, to orbit Saturn (in order to achieve lower sampling speeds, approaching 2 km/s, and thus enable a softer sample collection impact than Stardust, and to make possible multiple flybys of Enceladus); second, to sample Enceladus' plume, the E ring of Saturn, and the Titan upper atmosphere. With new findings from these samples, NASA could provide detailed chemical and isotopic and, potentially, biological compositional context of the plume. Since the duration of the Enceladus plume is unpredictable, it is imperative that these samples are captured at the earliest flight opportunity. If LIFE is launched before 2019, it could take advantage of a Jupiter gravity assist, which would thus reduce mission lifetimes and launch vehicle costs. The LIFE concept offers science returns comparable to those of a Flagship mission but at the measurably lower sample return costs of a Discovery-class mission.

Impact features on Stardust: implications for comet 81P/Wild 2 dust.

Particles emanating from comet 81P/Wild 2 collided with the Stardust spacecraft at 6.1 kilometers per second, producing hypervelocity impact features on the collector surfaces that were returned to Earth. The morphologies of these surprisingly diverse features were created by particles varying from dense mineral grains to loosely bound, polymineralic aggregates ranging from tens of nanometers to hundreds of micrometers in size. The cumulative size distribution of Wild 2 dust is shallower than that of comet Halley, yet steeper than that of comet Grigg-Skjellerup.

Organics captured from comet 81P/Wild 2 by the Stardust spacecraft.

Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples.

Modeling the nucleus and jets of comet 81P/Wild 2 based on the Stardust encounter data.

We interpret the nucleus properties and jet activity from the Stardust spacecraft imaging and the onboard dust monitoring system data. Triangulation of 20 jets shows that 2 emanate from the nucleus dark side and 16 emanate from sources that are on slopes where the Sun's elevation is greater than predicted from the fitted triaxial ellipsoid. Seven sources, including five in the Mayo depression, coincide with relatively bright surface spots. Fitting the imaged jets, the spikelike temporal distribution of dust impacts indicates that the spacecraft crossed thin, densely populated sheets of particulate ejecta extending from small sources on the rotating nucleus, consistent with an emission cone model.

Surface of young Jupiter family comet 81P/Wild 2: view from the Stardust Spacecraft.

Images taken by the Stardust mission during its flyby of 81P/Wild 2 show the comet to be a 5-kilometer oblate body covered with remarkable topographic features, including unusual circular features that appear to be impact craters. The presence of high-angle slopes shows that the surface is cohesive and self-supporting. The comet does not appear to be a rubble pile, and its rounded shape is not directly consistent with the comet being a fragment of a larger body. The surface is active and yet it retains ancient terrain. Wild 2 appears to be in the early stages of its degradation phase as a small volatile-rich body in the inner solar system.

Dust measurements in the coma of comet 81P/Wild 2 by the Dust Flux Monitor Instrument.

We present measurements of the dust particle flux and mass distribution from the Stardust Dust Flux Monitor Instrument (DFMI) throughout the flyby of comet 81P/Wild 2. In the particle mass regime from 10(-14) to 10(-7) kilograms, the spacecraft encountered regions of intense swarms of particles, together with bursts of activity corresponding to clouds of particles only a few hundred meters across. This fine-scale structure can be explained by particle fragmentation. We estimate that 2800 +/- 500 particles of diameter 15 micrometers or larger impacted the aerogel collectors, the largest being approximately 6 x 10(-7) kilograms, which dominates the total collected mass.