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Register a new userComparing the reflectivity of ungrouped carbonaceous chondrites with that of short period comets like 2P/Encke
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Aims. The existence of asteroid complexes produced by the disruption of these comets suggests that evolved comets could also produce high-strength materials able to survive as meteorites. We chose as an example comet 2P/Encke, one of the largest object of the so-called Taurid complex. We compare the reflectance spectrum of this comet with the laboratory spectra of some Antarctic ungrouped carbonaceous chondrites to investigate whether some of these meteorites could be associated with evolved comets. Methods. We compared the spectral behaviour of 2P/Encke with laboratory spectra of carbonaceous chondrites. Different specimens of the common carbonaceous chondrite groups do not match the overall features and slope of comet 2P/Encke. Trying anomalous carbonaceous chondrites, we found two meteorites, Meteorite Hills 01017 and Grosvenor Mountains 95551, which could be good proxies for the dark materials forming this short-period comet. We hypothesise that these two meteorites could be rare surviving samples, either from the Taurid complex or another compositionally similar body. In any case, it is difficult to get rid of the effects of terrestrial weathering in these Antarctic finds, and further studies are needed. Future sample return from the so-called dormant comets could be also useful to establish a ground truth on the materials forming evolved short-period comets. Results. As a natural outcome, we think that identifying good proxies of 2P/Encke-forming materials might have interesting implications for future sample-return missions to evolved, potentially dormant or extinct comets. To understand the compositional nature of evolved comets is particularly relevant in the context of the future mitigation of impact hazard from these dark and dangerous projectiles.
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We measured 3-micron reflectance spectra of 21 meteorites that represent all carbonaceous chondrite types available in terrestrial meteorite collections. The measurements were conducted at the Laboratory for Spectroscopy under Planetary Environmental Conditions (LabSPEC) at the Johns Hopkins University Applied Physics Laboratory (JHU APL) under vacuum and thermally-desiccated conditions (asteroid-like conditions). This is the most comprehensive 3-micron dataset of carbonaceous chondrites ever acquired in environments similar to the ones experienced by asteroids. The 3-micron reflectance spectra are extremely important for direct comparisons with and appropriate interpretations of reflectance data from ground-based telescopic and spacecraft observations of asteroids. We found good agreement between 3-{mu}m spectral characteristics of carbonaceous chondrites and carbonaceous chondrite classifications. The 3-{mu}m band is diverse, indicative of varying composition, thus suggesting that these carbonaceous chondrites experienced distinct parent body aqueous alteration and metamorphism environments.
The valence of iron has been used in terrestrial studies to trace the hydrolysis of primary silicate rocks. Here, we use a similar approach to characterize the secondary processes, namely thermal metamorphism and aqueous alteration, that have affected carbonaceous chondrites. X-ray absorption near-edge structure spectroscopy at the Fe-K- edge was performed on a series of 36 CM, 9 CR, 10 CV, and 2 CI chondrites. Among the four carbonaceous chondrites groups studied, a correlation between the iron oxidation index (IOI = [2 ((Fe2+) + 3(Fe3+))/FeTOT) and the hydrogen content is observed. However, within the CM group, for which a progressive alteration sequence has been defined, a conversion of Fe3+ to Fe2+ is observed with increasing degree of aqueous alteration. This reduction of iron can be explained by an evolution in the mineralogy of the secondary phases. In the case of the few CM chondrites that experienced some thermal metamorphism, in addition to aqueous alteration, a redox memory of the aqueous alteration is present: a significant fraction of 3+ 2+ 0 Fe is present, together with Fe and sometimes Fe. From our data set, the CR chondrites show a wider range of IOI from 1.5 to 2.5. In all considered CR chondrites, the three oxidation states of iron coexist. Even in the least-altered CR chondrites, the fraction of Fe3+ can be high (30% for MET 00426). This observation confirms that oxidized iron has been integrated during formation of fine-grained amorphous material in the matrix. Last, the IOI of CV chondrites does not reflect the reduced/oxidized classification based on metal and magnetite proportions, but is strongly correlated with petrographic types. The valence of iron in CV chondrites therefore appears to be most closely related to thermal history, rather than aqueous alteration, even if these processes can occur together .
We report microscopic, cathodoluminescence, chemical and O isotopic measurements of FeO-poor isolated olivine grains (IOG) in the carbonaceous chondrites Allende (CV3), Northwest Africa 5958 (C2-ung), Northwest Africa 11086 (CM2-an), Allan Hills 77307 (CO3.0). The general petrographic, chemical and isotopic similarity with bona fide type I chondrules confirms that the IOG derived from them. The concentric CL zoning, reflecting a decrease in refractory elements toward the margins, and frequent rimming by enstatite are taken as evidence of interaction of the IOG with the gas as stand-alone objects. This indicates that they were splashed out of chondrules when these were still partially molten. CaO-rich refractory forsterites, which are restricted to $Delta^{17}O < -4permil$ likely escaped equilibration at lower temperatures because of their large size and possibly quicker quenching. The IOG thus bear witness to frequent collisions in the chondrule-forming regions.
We present results of imaging polarimetry of comet 2P/Encke performed on January 23, 2017 at the heliocentric (1.052 au) and geocentric (1.336 au) distances and phase angle 46.8 deg, 46 days before perihelion. Observations were made through the medium-band SED500 ({lambda}5019/246 {AA}) and broadband r-sdss ({lambda}6200/1200 {AA}) filters with the multimode focal reducer SCORPIO-2 at the 6-m BTA telescope of the Special Astrophysical Observatory (Russia). Dust in comet 2P/Encke was mainly concentrated in the near-nucleus region of the coma: the maximum dust/gas ratios were 1.5 and 2.9 in the SED500 and the r-sdss filters near the nucleus but dropped sharply to ~0.2 and ~1 at the distance ~2500 km, respectively. Then these ratios began to increase at distances ~12000 km from the nucleus, the ratio was ~0.3 (SED500) and ~1.3 (r-sdds). There were significant variations of polarization over the coma, which correlated with the variations in the dust color and dust/gas ratio. Changes in polarization and color across the 2P/Encke coma indicate changes in physical properties of the dust particles with the distance from the nucleus. Our Sh-matrix computer simulations of light scattering by Gaussian particles allow us to suggest that the observed trends in color and polarization are mainly result from changing particle size.
We perform a search for dormant comets, asteroidal objects of cometary origin, in the near-Earth asteroid (NEA) population based on dynamical and physical considerations. Our study is based on albedos derived within the ExploreNEOs program and is extended by adding data from NEOWISE and the Akari asteroid catalog. We use a statistical approach to identify asteroids on orbits that resemble those of short-period near-Earth comets using the Tisserand parameter with respect to Jupiter, the aphelion distance, and the minimum orbital intersection distance with respect to Jupiter. From the sample of NEAs on comet-like orbits, we select those with a geometric albedo $p_V leq 0.064$ as dormant comet candidates, and find that only $sim$50% of NEAs on comet-like orbits also have comet-like albedos. We identify a total of 23 NEAs from our sample that are likely to be dormant short-period near-Earth comets and, based on a de-biasing procedure applied to the cryogenic NEOWISE survey, estimate both magnitude-limited and size-limited fractions of the NEA population that are dormant short-period comets. We find that 0.3-3.3% of the NEA population with $H leq 21$, and $9^{+2}_{-5}$% of the population with diameters $d geq 1$ km, are dormant short-period near-Earth comets.
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