Polarimetry of main belt asteroids: Wavelength dependence

New UBVRI polarimetric observations of ten asteroids, including space mission targets 1 Ceres and 21 Lutetia, are presented. These observations were obtained with the 1.25-m telescope of the Crimean Astrophysical Observatory and have been used to study the wavelength dependence of polarization for a sample of asteroids belonging to the M and low albedo classes. A more general analysis including also a larger data set of UBVRI polarimetric observations available in the literature for more than 50 main belt asteroids belonging to different taxonomic classes shows that the variation of the polarization degree Pr as a function of wavelength is generally well described by a linear trend. It typically does not exceed 0.2% in the studied spectral range 0.37-0.83 microns and tends to increase for increasing phase angle. Asteroids belonging to the S and M classes are found to exhibit a deeper negative branch and smaller positive polarization for increasing wavelength (negative sign of the slope of ΔPr/Δλ). Since the objects belonging to these classes are known to exhibit reddish reflectance spectra, the observed wavelength behavior of negative polarization contradicts the well-known inverse correlation of Pmin and albedo. Low albedo asteroids show larger dispersion of spectral slopes, but the overall trend is characterized by a shallower negative branch and a larger positive polarization for increasing wavelength (positive sign of the slope of ΔPr/Δλ). A few exceptions from this general trend are discussed. The observed variety in the wavelength dependence of asteroid polarization seems to be mainly attributed to surface composition.     

Positive branch of asteroid polarization: Observational data and computer modeling

Observations of near-Earth asteroids at large phase angles made it possible to obtain a more complete (for ground-based observations) phase dependence of the polarization of the E-type asteroids’ radiation including the maximum of the positive branch of the linear polarization degree. It is shown that the position of the polarization maximum of high-albedo asteroids is noticeably shifted to the decrease of phase angles compared with S-type asteroids. Model calculations of polarimetric properties of random Gaussian particles that simulate dust particles on the regolith surface are carried out. Model calculations show a qualitatively similar behavior pattern of parameters of the positive polarization branch. The influence of the refractive index of individual scattering particles on the size and position of the maximum of the positive branch of the linear polarization degree is investigated within the considered model.     

Composition of cometary dust from polarization spectra

The wavelength dependence of the polarization (“polarization spectra”) of cometary dust is discussed. It is shown that, in the case of large phase angles, the wavelength dependence of the polarization is mainly controlled by the complex refractive index of the particle material, whereas the spectral dependence of the intensity is also sensitive to the size of the particles. This suggests that observations of “polarization spectra” may determine the composition of cometary dust. An attempt is made to find the composition of the cometary dust material by comparing the observed polarimetric data with laboratory measurements of complex refractive indices of possible cometary constituents. Silicates, graphite, metals, organics, water ice and their mixtures are considered. It is shown that astronomical silicate must be the most abundant constituent of cometary dust in the range of heliocentric distances from 0.8 to 1.8 AU, whereas the volume fraction of pure graphite or pure metals is less then 1%. A substance similar to that of F-type asteroids may be present in comets. There is evidence for an organic material that is being destroyed between heliocentric distances of 0.8–1.8 AU.     

Space Weathering of Small Solar System Bodies

Micrometeorite bombardment and irradiation by solar wind and cosmic ions cause variations in the optical properties of the small Solar System bodies surface materials. These space weathering processes are reasonably well understood for the Moon and S-type asteroids. The research is based on laboratory experiments performed by several groups on meteorites and minor bodies surface analogues, whose results have been applied to the spectral modeling and interpretation of observations from large surveys and space missions. Recent results from young asteroidal families, and the relation between spectral slopes and dynamical properties, have stressed the role of the solar wind exposure timescale. Space weathering processes remain poorly investigated in the case of other types of asteroids, and they are still unclear in the case of outer Solar System bodies, due to a strong dependence of the weathering process on the original composition.     

The material of the interstellar dust and the surface of asteroids: A comparison of the spectral properties in the 0.3–1.1-μm range

From the data on bright stars of different spectral and luminosity classes from the 13-color photometry catalog, the selective extinction of light by the interstellar dust has been studied. The stars from the 1000-pc vicinity of the Sun were investigated. In the optical spectral range, the interstellar extinction curves show systematic deviations from the “λ^?1” law, which allows one to sort them into three types. The observed curves of the interstellar dust extinction were compared with the theoretical curves calculated from the reflectance spectra of the asteroids under the approximation of the Rayleigh particles. The calculated extinction curves for the surface material of D-type asteroids and the Tagish Lake carbonaceous chondrite agree rather well with the observed curves of the interstellar extinction of the first type.     

Polarization of the reflected light of asteroid 433 Eros

Linear polarizations measured for asteroid 433 Eros at various wavelengths and at solar phase angles ranging from 9° to 53° are presented. The polarization results are entirely typical of main-belt S asteroids, and indicate a dusty surface with geometric albedo 0.20. The derived effective diameter at photometric maximum is 21 km. Eros is quite uniform polarimetrically; no dependence on aspect is detected, and the polarization is shown to be constant during a single rotation with a precision of one part in forty.     

Spectropolarimetry of comets Austin and Churyumov-Gerasimenko

Spectropolarimetric observations from 5000 to 8000 Å have been obtained for comets P/Austin (1982g) and P/Churyumov-Gerasimenko (1982f). The observations were spaced over phase angles of 50–125° for comet Austin and 10–40° for comet Churyumov-Gerasimenko. The use of spectropolarimetry allowed an evaluation of continuum polarization without molecular line contamination. Especially for comet Churyumov-Gerasimenko, the curve of polarization versus phase angle resembles curves for asteroids, where the polarization is negative (electric vector maximum parallel to the scattering plane) for phase angles less than 20° and the most negative polarization is from ?1 to ?2%. The negative polarization at backscattering angles may be due to multiple scattering in agglomerated grains, as assumed for asteroids, or to Mie scattering by small dielectric particles. If multiple scattering is important in comet dust, polarization measurements may imply a low albedo, less than 0.08. The polarization of comet Austin remained steady during a large change in the dust production rate. Both comets increased continuum flux by a factor of 2 near perihelion. The continuum of comet Churyumov-Gerasimenko had the shape of the solar spectrum with derivations less than 5%. The equivalent width of spectral features of C₂, NH₂, and O varied as r^?2.     

Identification of Asteroids in Two-Body Resonances

A catalog of asteroids in two-body orbital resonances with the planets of the Solar System has been created. The AstDyS database was a source of the input data, and all the numbered objects (467303 objects at the time of the analysis) were considered. The orbits were integrated in the framework of a pure gravitational problem considering all the planets of the Solar System and Pluto. From the analysis of the behavior of the resonant argument and the semimajor axis on the 100-kyr interval, the resonance membership and the libration type (pure or transient) were verified for each of the asteroids. A more accurate method to identify the resonant argument librations was developed on the basis of the analysis of mutual periodograms. We found 23251 resonant asteroids, ~36% of which (8397 objects) are in pure resonances.     

Generalized Calibration of the Polarimetric Albedo Scale of Asteroids

Six different calibrations of the polarimetric albedo scale of asteroids have been published so far. Each of them contains its particular random and systematic errors and yields its values of geometric albedo. On the one hand, this complicates their analysis and comparison; on the other hand, it becomes more and more difficult to decide which of the proposed calibrations should be used. Moreover, in recent years, new databases on the albedo of asteroids obtained from the radiometric surveys of the sky with the orbital space facilities (the InfraRed Astronomical Satellite (IRAS), the Japanese astronomical satellite AKARI (which means “light”), the Wide-field Infrared Survey Explorer (WISE), and the Near-Earth Object Wide-field Survey Explorer (NEOWISE)) have appeared; and the database on the diameters and albedos of asteroids obtained from their occultations of stars has substantially increased. Here, we critically review the currently available calibrations and propose a new generalized calibration derived from the interrelations between the slope h and the albedo and between P_min and the albedo. This calibration is based on all of the available series of the asteroid albedos and the most complete data on the polarization parameters of asteroids. The generalized calibration yields the values of the polarimetric albedo of asteroids in the system unified with the radiometric albedos and the albedos obtained from occultations of stars by asteroids. This, in turn, removes the difficulties in their comparison, joint analysis, etc.     

A Critical review of theoretical models of negatively polarized light scattered by atmosphereless solar system bodies

About a dozen physical mechanisms and models aspire to explain the negative polarization of light scattered by atmosphereless celestial bodies. This is too large a number for the reliable interpretation of observational data. Through a comparative analysis of the models, our main goal is to answer the question: Does any one model have an advantage over the others? Our analysis is based on new laboratory polarimetric and photometric data as well as on theoretical results. We show that the widely used models due to Hopfield and Wolff cannot realistically explain the phase-angle dependence of the degree of polarization observed at small phase angles. The so-called interference or coherent backscattering mechanism is the most promising model. Models based on that mechanism use well-defined physical parameters to explain both negative polarization and the opposition effect. They are supported by laboratory experiments, particularly those showing enhancement of negative polarization with decreasing particle size down to the wavelength of light. According to the interference mechanism, pronounced negative branches of polarization, like those of C-class asteroids, may indicate a high degree of optical inhomogeneity of light-scattering surfaces at small scales. The mechanism also seems appropriate for treating the negative polarization and opposition effects of cometary dust comae, planetary rings, and the zodiacal light.     

Physicomechanical properties and gravitational deformation of metallic asteroids

A review is given of the mineralogical and chemical composition of iron meteorites and the problems associated with their origin. A detailed discussion is presented of the physical and mechanical properties of iron meteorites and their dependence on the structure, chemical composition, and temperature. Iron meteorites are shown to characterize, with no distortions, the physical and mechanical properties of their parent bodies (metallic asteroids). The population of M-type asteroids and the main characteristics of the identified metallic asteroids are examined. Compared with iron meteorites, metallic asteroids have a different shape and are not fragments of larger metallic parent bodies. The estimates for the current deviatoric stress in metallic asteroids show that, since their formation, asteroids have not been heated to over 600°C and certainly have not been subjected to partial or complete melting. An empirical dependence is found of the critical dimensions of small metallic objects (which allow for gravitational deformation) on the yield strength at temperatures below 300 K. It is shown that the physical and mechanical data are also a strong argument against the hypothesis of the origin of iron meteorites and metallic asteroids from the iron core of a differentiated parent body.     

Near-infrared spectroscopy of 3:1 Kirkwood Gap asteroids: Mineralogical diversity and plausible meteorite parent bodies

The 3:1 Kirkwood gap asteroids are a mineralogically diverse set of asteroids located in a region that delivers meteoroids into Earth-crossing orbits. Mineralogical characterizations of asteroids in/near the 3:1 Kirkwood Gap can be used as a tool to “map” conditions and processes in the early Solar System. The chronological studies of the meteorite types provide a “clock” for the relative timing of those events and processes. By identifying the source asteroids of particular meteorite types, the “map” and “clock” can be combined to provide a much more sophisticated understanding of the history and evolution of the late solar nebula and the early Solar System.A mineralogical assessment of seven 3:1 Kirkwood Gap asteroids has been carried out using near-infrared spectral data obtained over the years 2006–2009 combined with visible spectral data (when available) to cover the spectral interval of 0.4–2.5 μm. We explore the diversity, uniqueness, and possible links between the asteroids (198) Ampella, (329) Svea, (495) Eulalia, (556) Phyllis, (623) Chimaera, (908) Buda, and (1772) Gagarin, which are located adjacent to the 3:1 resonance, and the meteorite types in the terrestrial collections.     

Thermal metamorphism of the C,G, B,and F asteroids seen from the 0.7 μm, 3 μm,and UV absorption strengths in comparison with carbonaceous chondrites

Abstract Thermal metamorphism study of the C, G, B, and F asteroids has been revisited using their UV, visible, NIR, and 3 μm reflectance spectra. High-quality reflectance spectra of seven selected C, G, B, and F asteroids have been compared with spectra for 29 carbonaceous chondrites, including thermally-metamorphosed CI/CM meteorites. There are three sets of spectral counterparts, among which 511 Davida and B-7904 are the most similar to each other in terms of both spectral shape and brightness. By comparing the 0.7 μm and 3 μm absorption strengths of 21 C, G, B, and F asteroids and heated Murchison samples, these asteroids have been grouped into three heating-temperature ranges. These correspond to (1) <400 °C: phyllosilicate-rich; (2) 400–600 °C: phyllosilicates transformed to anhydrous silicates; and (3) >600 °C: fully anhydrous. A good correlation between the UV and 3 μm absorption strengths has been confirmed for the C, G, B, and F asteroids and the CI, CM, and CR meteorites. A plot of the UV absorption strength vs. the IRAS diameter for 142 C, G, B, and F asteroids shows that the maximum UV absorption strength decreases as the diameter increases for the asteroids >60 km, with a notable exception, Ceres. These relationships suggest that some of the larger asteroids may be the heated inner portions of once larger bodies and that common CI/CM meteorites may have come from the lost outer portions, which escaped extensive late-stage heating events.     

S-type asteroids,ordinary chondrites,and space weathering: The evidence from Galileo's fly-bys of Gaspra and Ida

Abstract— New observations of the S-type asteroids Gaspra and Ida, especially, by the Galileo spacecraft demonstrate that a “space weathering” process operates, which modifies the reflectance spectra of fresh material to be redder, straighter, and have shallower absorption bands. The weathering process appears similar to but less potent than that observed on the Moon. It operates in the sense that it would tend to convert spectra of ordinary chondrites (OC) to having the spectral traits of S-type asteroids. These results appear to resolve the major obstacles of the long standing “S-type conundrum” about the provenance of ordinary chondrite meteorites. A wide body of recent research, which is reviewed here, builds on previous meteoritical evidence to support a new, developing consensus that the larger S-type asteroids are a diverse assortment of silicaceous assemblages, which includes the ordinary chrondite parent bodies. Recent fairly realistic laboratory simulations of space weathering processes have changed OC spectra to resemble S-type spectra. J. F. Bell's highly regarded earlier paradigm that OC parent bodies would be found only among sub-10 km main-belt asteroids has been tested by an extensive observational survey by R. Binzel and his proteges; the result is that no small, main-belt analog for the OCs has been found, not even the somewhat OC-like object Boznemcová. This article reviews the history of the S-type conundrum, which set the stage for Galileo's historic flybys. I review the findings about Gaspra and Ida, including results about their geology, their geophysical structure and probable origins, and about Ida's small moonlet, Dactyl. Density constraints on Ida set by Dactyl's orbit rule out (for Ida, at least) the classic view of S-types as metallic, stripped cores of differentiated precursor asteroids. New analysis of the Galileo spectral images of Ida is presented that provides strong evidence that space weathering occurs on Ida, and that Ida, and the Koronis family asteroids in general, are plausibly OC-like in composition. After reviewing 1990s developments on the S-type conundrum, I advocate a new perspective that the ordinary chondrite parent bodies are among the S-type asteroids, a diverse grouping that also contains other silicaceous and silicate/metal assemblages, presumably including various stony-irons and primitive achondrites represented in meteorite collections; Gaspra may be such a metal-rich assemblage.     

Could G-class asteroids be the parent bodies of the CM chondrites?

Abstract— I review the dynamical and compositional evidence for possibly linking CM chondrites and asteroids having G-class taxonomic designations. Three G asteroids have been identified through previous theoretical studies as being likely meteorite source bodies due to their locations near resonances. Two of these objects, 19 Fortuna and 13 Egeria, have spectral properties that are consistent with such a linkage with CM chondrites. Fortuna has a similar strength 0.7 μm absorption feature and near-infrared spectral slope to CM chondrites but a weaker ultraviolet feature. Egeria also has the characteristic 0.7 μm feature of CM chondrite spectra but does not match as well in the near-infrared. However, since the 0.7 μm feature is apparent in the spectra of approximately one-half of measured C-type asteroids, no definitive statement about any linkages can be made. Ceres is spectrally different from known meteorites in the 3 μm wavelength region and cannot be convincingly linked with any meteorite group.     

Polarization and brightness opposition effects for the E-type Asteroid 44 Nysa

We present new polarimetric and photometric observations of high-albedo E-type Asteroid 44 Nysa in the BVRI wavebands at phase angles ranging from 0.41° to 7.49° during the 2005 opposition. A bimodal phase-angle dependence of polarization was found for Nysa in the V band. The polarization opposition effect was revealed in the form of a secondary minimum of negative polarization with amplitude ∼0.3% centered at a phase angle ∼0.8°. It is superimposed on the regular negative polarization branch with minimal polarization −0.30% at a phase angle 5.8°. We analyzed all available polarimetric data for E-type Asteroids 44 Nysa, 64 Angelina, and 214 Ashera and confirmed the presence of the polarization opposition effect for high-albedo asteroids at phase angle ∼1° with an amplitude ∼0.35%. The magnitude-phase curves reveal the presence of spike-like opposition effect of brightness for 44 Nysa in the BVRI spectral bands. 44 Nysa is the second high-albedo asteroid after 64 Angelina for which both the polarization opposition effect and the brightness opposition effect are detected. The differences between the parameters of the opposition effects for silicate surfaces (44 Nysa, 64 Angelina, Io) and icy surfaces (Europa, Ganymede, Iapetus, Saturn's rings) are discussed. The specific morphological parameters of opposition effects, in particular the angular width of the polarization opposition effect is comparable to that of the brightness opposition effect, provide almost unequivocal evidence that they are caused by coherent backscattering. One of unexpected results of our investigation is that 44 Nysa becomes bluer with increasing phase angle, while 64 Angelina shows phase reddening.     

Polarization and brightness opposition effects for the E-type Asteroid 64 Angelina

We present new polarimetric and photometric observations of the high-albedo Asteroid 64 Angelina in the UBVRI wavebands at phase angles ranging from 0.43° to 13.02° during oppositions in 1995, 1999, and 2000/2001. The polarization opposition effect has been observed in the form of a sharp peak of negative polarization with amplitude of about −0.4% centered at αmin≈1.8°, which is superimposed on the regular negative polarization branch. The amplitude of the polarization opposition effect appears to be apparition-dependent. Our photometric data confirm the early detected by Harris et al. [1989. Phase relations of high-albedo asteroids: The unusual opposition brightening of 44 Nysa and 64 Angelina. Icarus 81, 365-374] of a very strong and unusually narrow opposition spike, i.e., brightness opposition effect, for Angelina. Thus, 64 Angelina is the first asteroid for which both the polarization opposition effect and the brightness opposition effect have been detected. We observed that the polarization opposition effect as well as the regular negative polarization branch depends on the wavelength of scattered light, but in different manners. In addition, the colors B-V and V-R show little phase-angle dependence, while the color U-B increases with increasing phase angle, thus indicating that the amplitude of the brightness opposition effect is larger in the U band and almost the same in the B, V, and R bands. It appears that all colors indices begin to increase with decreasing phase angle to zero. The composite lightcurve computed with a period of 8.752 h has amplitude of 0.13 magnitude.     

Spectroscopy of K-complex asteroids: Parent bodies of carbonaceous meteorites?

This is the first focused study of non-Eos K asteroids. We have observed a total of 30 K-complex objects (12 K-2 Sk- and 13 Xk-type asteroids (from the Bus taxonomy), plus 3 K-candidates from previous work) and we present an analysis of their spectral properties from 0.4 to 2.5 μm. We targeted these asteroids because their previous observations are spectrally similar enough to suggest a possible compositional relationship. All objects have exhibited spectral redness in the visible wavelengths and minor absorptions near 1 micron. If, as suggested, K-complex asteroids (including K, Xk, and Sk) are the parent bodies of carbonaceous meteorites, knowledge of K-asteroid properties and distribution is essential to our understanding of the cosmochemical importance of some of the most primitive meteorite materials in our collection. This paper presents initial results of our analysis of telescopic data, with supporting analysis of laboratory measurements of meteorite analogs. Our results indicate that K-complex asteroids are distinct from other main belt asteroid types (S, B, C, F, and G). They do not appear to be a subset of these other types. K asteroids nearly span the range of band center positions and geometric albedos exhibited by the carbonaceous chondrites (CO, CM, CV, CH, CK, CR, and CI). We find that B-, C-, F- and G-type asteroids tend to be darker than meteorites, and can have band centers longer than any of the chondrites measured here. This could indicate that K-complex asteroids are better spectral analogues for the majority of our carbonaceous meteorites than the traditional B-, C-, F- and G-matches suggested in the literature. This paper present first results of our ongoing survey to determine K-type mineralogy, meteorite linkages, and significance to the geology of the asteroid regions.     

Spectroscopic survey of X-type asteroids

We present reflected light spectral observations from 0.4 to 2.5 μm of 24 asteroids chosen from the population of asteroids initially classified as Tholen X-type objects (Tholen, 1984). The X complex in the Tholen taxonomy comprises the E, M and P classes which have very different inferred mineralogies but which are spectrally similar to each other, with featureless spectra in visible wavelengths.The data were obtained during several observing runs in the 2004-2007 years at the NTT, TNG and IRTF telescopes. Sixteen asteroids were observed in the visible and near-infrared wavelength range, seven objects in the visible wavelength range only, and one object in the near-infrared wavelength range only. We find a large variety of near-infrared spectral behaviors within the X class, and we identify weak absorption bands in spectra of 11 asteroids. Our spectra, together with albedos published by Tedesco et al. (2002), can be used to suggest new Tholen classifications for these objects. We describe 1 A-type (1122), 1 D-type (1328), 1 E-type (possibly, 3447 Burckhalter), 10 M-types (77, 92, 184, 337, 417, 741, 758, 1124, 1146 and 1355), 5 P-types (275, 463, 522, 909, 1902), and 6 C-types (50, 220, 223, 283, 517, and 536). In order to constrain the possible composition of these asteroids, we perform a least-squares search through the RELAB spectral database. Many of the best fits are consistent with meteorite analogue materials suggested in the published literature. In fact, we find that seven of the new M-types can be fit with metallic iron (or pallasite) materials, and that the low albedo C/P-type asteroids are best fitted with CM meteorites, some of which have been subjected to heating episodes or laser irradiation. Our method of searching for meteorite analogues emphasizes the spectral characteristics of brightness and shape, and de-emphasizes minor absorption bands. Indeed, faint absorption features like the 0.9 μm band seen on four newly classified M-type asteroids are not reproduced by the iron meteorites. In these cases, we have searched for geographical mixture models that can fit the asteroid spectrum, minor bands, and albedo. We find that a few percent (less than 3%) of orthopyroxene added to iron or pallasite meteorite, results in good spectral matches, reproducing the weak spectral feature around 0.9 μm seen on 92 Undina, 417 Suevia, and 1124 Stroobantia. For 337 Devosa, a mixture model that better reproduces its spectral behavior and the 0.9 μm feature is made with Esquel pallasite enriched with goethite (2%).Finally, we consider the sample of the X-type asteroids we have when we combine the present observations with previously published observations for a total of 72 bodies. This sample includes M and E-type asteroid data presented in [Fornasier et al., 2008] and [Fornasier et al., 2010]. We find that the mean visible spectral slopes for the different E, M and P Tholen classes are very similar, as expected. An analysis of the X-type asteroid distribution in the main belt is also reported, following both the Tholen and the Bus-DeMeo taxonomies (DeMeo et al., 2009).     

Thermal infrared (8-13 μm) spectra of 29 asteroids: the Cornell Mid-Infrared Asteroid Spectroscopy (MIDAS) Survey

We report the results of the Cornell Mid-IR Asteroid Spectroscopy (MIDAS) survey, a program of ground-based observations designed to characterize the 8-13 μm spectral properties of a statistically significant sample of asteroids from a wide variety of visible to near-IR spectral classes. MIDAS is conducted at Palomar Observatory using the Spectrocam-10 (SC-10) spectrograph on the 200-in Hale telescope. We have measured the mid-infrared spectra of twenty-nine asteroids and have derived temperature estimates from our data that are largely consistent with the predictions of the standard thermal model. We have also generated relative emissivity spectra for the target asteroids. On only one asteroid, 1 Ceres, have we found emissivity features with spectral contrast greater than 5%. Our spectrum of 4 Vesta suggests emissivity variation at the 2-3% level. Published spectra of several of the small number of asteroids observed with ISO (six of which are also included in our survey), which appeared to exhibit much stronger emissivity features, are difficult to reconcile with our measurements. Laboratory work on mineral and meteorite samples has shown that the contrast of mid-IR spectral features is greatly reduced at fine grain sizes. Moreover, the NEAR mission found that 433 Eros is covered by a relatively thick fine-grained regolith. If small bodies in general possess such regoliths, their mid-IR spectral features may be quite subtle. This may explain the evident absence of strong emissivity variation in the majority of the MIDAS spectra.