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1.
We present the results of a visible spectroscopic and photometric survey of Jupiter Trojans belonging to different dynamical families. The survey was carried out at the 3.5 m New Technology Telescope (NTT) of the European Southern Observatory (La Silla, Chile) in April 2003, May 2004 and January 2005. We obtained data on 47 objects, 23 belonging to the L5 swarm and 24 to the L4 one. These data together with those already published by Fornasier et al. [Fornasier, S., Dotto, E., Marzari, F., Barucci, M.A., Boehnhardt, H., Hainaut, O., de Bergh, C., 2004a. Icarus 172, 221-232] and Dotto et al. [Dotto, E., Fornasier, S., Barucci, M.A., Licandro, J., Boehnhardt, H., Hainaut, O., Marzari, F., de Bergh, C., De Luise, F., 2006. Icarus 183, 420-434], acquired since November 2002, constitute a total sample of visible spectra for 80 objects. The survey allows us to investigate six families (Aneas, Anchises, Misenus, Phereclos, Sarpedon, Panthoos) in the L5 cloud and four L4 families (Eurybates, Menelaus, 1986 WD and 1986 TS6). The sample that we measured is dominated by D-type asteroids, with the exception of the Eurybates family in the L4 swarm, where there is a dominance of C- and P-type asteroids. All the spectra that we obtained are featureless with the exception of some Eurybates members, where a drop-off of the reflectance is detected shortward of 5200 Å. Similar features are seen in main belt C-type asteroids and commonly attributed to the intervalence charge transfer transition in oxidized iron. Our sample comprises fainter and smaller Trojans as compared to the literature's data and allows us to investigate the properties of objects with estimated diameter smaller than 40-50 km. The analysis of the spectral slopes and colors versus the estimated diameters shows that the blue and red objects have indistinguishable size distribution, so any relationship between size and spectral slopes has been found. To fully investigate the Trojans population, we include in our analysis 62 spectra of Trojans available in literature, resulting in a total sample of 142 objects. Although the mean spectral behavior of L4 and L5 Trojans is indistinguishable within the uncertainties, we find that the L4 population is more heterogeneous and that it has a higher abundance of bluish objects as compared to the L5 swarm. Finally, we perform a statistical investigation of the Trojans's spectra property distributions as a function of their orbital and physical parameters, and in comparison with other classes of minor bodies in the outer Solar System. Trojans at lower inclination appear significantly bluer than those at higher inclination, but this effect is strongly driven by the Eurybates family. The mean colors of the Trojans are similar to those of short period comets and neutral Centaurs, but their color distributions are different.  相似文献   

2.
The Eurybates family is a compact core inside the Menelaus clan, located in the L4 swarm of Jupiter Trojans. Fornasier et al. (Fornasier, S., Dotto, E., Hainaut, O., Marzari, F., Boehnhardt, H., De Luise, F., Barucci, M.A. [2007]. Icarus 190, 622-642) found that this family exhibits a peculiar abundance of spectrally flat objects, similar to Chiron-like Centaurs and C-type main belt asteroids. On the basis of the visible spectra available in literature, Eurybates family’s members seemed to be good candidates for having on their surfaces water/water ice or aqueous altered materials.To improve our knowledge of the surface composition of this peculiar family, we carried out an observational campaign at the Telescopio Nazionale Galileo (TNG), obtaining near-infrared spectra of 7 members. Our data show a surprisingly absence of any spectral feature referable to the presence of water, ices or aqueous altered materials on the surface of the observed objects. Models of the surface composition are attempted, evidencing that amorphous carbon seems to dominate the surface composition of the observed bodies and some amount of silicates (olivine) could be present.  相似文献   

3.
We have conducted a radar-driven observational campaign of 22 main-belt asteroids (MBAs) focused on Bus–DeMeo Xc- and Xk-type objects (Tholen X and M class asteroids) using the Arecibo radar and NASA Infrared Telescope Facilities (IRTF). Sixteen of our targets were near-simultaneously observed with radar and those observations are described in a companion paper (Shepard, M.K., and 19 colleagues [2010]. Icarus, in press). We find that most of the highest metal-content asteroids, as suggested by radar, tend to exhibit silicate absorption features at both 0.9 and 1.9 μm, and the lowest metal-content asteroids tend to exhibit either no bands or only the 0.9 μm band. Eleven of the asteroids were observed at several rotational longitudes in the near-infrared and significant variations in continuum slope were found for nine in the spectral regions 1.1–1.45 μm and 1.6–2.3 μm. We utilized visible wavelength data (Bus, S.J., Binzel, R.P. [2002b]. Icarus 158, 146–177; Fornasier, S., Clark, B.E., Dotto, E., Migliorini, A., Ockert-Bell, M., Barucci, M.A. [2010]. Icarus 210, 655–673.) for a more complete compositional analysis of our targets. Compositional evidence is derived from our target asteroid spectra using two different methods: (1) a χ2 search for spectral matches in the RELAB database, and (2) parametric comparisons with meteorites. This paper synthesizes the results of the RELAB search and the parametric comparisons with compositional suggestions based on radar observations. We find that for six of the seven asteroids with the highest iron abundances, our spectral results are consistent with the radar evidence (16 Psyche, 216 Kleopatra, 347 Pariana, 758 Mancunia, 779 Nina, and 785 Zwetana). Three of the seven asteroids with the lowest metal abundances, our spectral results are consistent with the radar evidence (21 Lutetia, 135 Hertha, 497 Iva). The remaining seven asteroids (22 Kalliope, 97 Klotho, 110 Lydia, 129 Antigone, 224 Oceana, 678 Fredegundis, and 771 Libera) have ambiguous compositional interpretations when comparing the spectral analogs to the radar analogs. The number of objects with ambiguous results from this multi-wavelength survey using visible, near-infrared, and radar wavelengths indicates that perhaps a third diagnostic wavelength region (such as the mid-infrared around 2–4 μm, the mid-infrared around 10–15 μm, and/or the ultraviolet around 0.2–0.4 μm) should be explored to resolve the discrepancies.  相似文献   

4.
S. Fornasier  B.E. Clark 《Icarus》2011,214(1):131-146
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).  相似文献   

5.
We observed the E-class main-belt Asteroids (MBAs) 44 Nysa and 434 Hungaria with Arecibo Observatory's S-band (12.6 cm) radar. Both asteroids exhibit polarization ratios higher than those measured for any other MBA: Nysa, μc=0.50±0.02 and Hungaria, μc=0.8±0.1. This is consistent with the high polarization ratios measured for every E-class near-Earth asteroid (NEA) observed by Benner et al. [Benner, L.A.M., and 10 collegues, 2008. Icarus, submitted for publication] and suggests a common cause. Our estimates of radar albedo are 0.19±0.06 for Nysa and 0.22±0.06 for Hungaria. These values are higher than those of most MBAs and, when combined with their high polarization ratios, suggest that the surface bulk density of both asteroids is high. We model Nysa as an ellipsoid of dimension 113×67×65 km (±15%) giving an effective diameter Deff=79±10 km, consistent with previous estimates. The echo waveforms are not consistent with a contact binary as suggested by Kaasalainen et al. [Kaasalainen, M., Torppa, J., Piironen, J., 2002. Astron. Astrophys. 383, L19-L22]. We place a constraint on Hungaria's maximum diameter, Dmax?11 km consistent with previous size estimates.  相似文献   

6.
Abstract— ESA's Rosetta spacecraft will fly by main‐belt asteroid 2867 Steins on September 5, 2008. We obtained new visible wavelength spectra of 2867 Steins on December 19, 2006 (UT), using the Palomar 5 m telescope and the facility Double Spectrograph. Two sets of spectra, taken ～3 h apart, one half of the rotation period for 2867 Steins, show it to be an E‐type asteroid. The asteroid displays a 0.50 μm feature that is considered diagnostic of the E(II) sub‐class, but is deeper than any previously observed E‐type. This feature is most likely due to the presence of oldhamite (CaS) on the asteroid's surface. Also, the observed Steins spectra are far redder than any other known E‐types. There is potential evidence for heterogeneity on hemispheric scales, one side of the asteroid appearing to be significantly redder than the other. No known recovered meteorite sample matches the unusual spectra of 2867 Steins, but the closest analog would be similar to an enstatite achondrite (aubrite).  相似文献   

7.
Using the S-band radar at Arecibo Observatory, we observed six new M-class main-belt asteroids (MBAs), and re-observed one, bringing the total number of Tholen M-class asteroids observed with radar to 19. The mean radar albedo for all our targets is , significantly higher than the mean radar albedo of every other class (Magri, C., Nolan, M.C., Ostro, S.J., Giorgini, J.D. [2007]. Icarus 186, 126-151). Seven of these objects (Asteroids 16 Psyche, 129 Antigone, 216 Kleopatra, 347 Pariana, 758 Mancunia, 779 Nina, 785 Zwetana) have radar albedos indicative of a very high metal content , and consistent with a remnant iron/nickel core interpretation (irons) or exotic high metal meteorite types such as CB. We propose designating these high radar albedo objects as Mm. Two asteroids, 110 Lydia and 678 Fredegundis, have more moderate radar albedos , but exhibit high values at some rotation phases suggesting a significant metal content. The remaining 10 objects have moderate radar albedos at all rotation phases. Most of our targets have visible/near-infrared spectra (Hardersen, P.S., Gaffey, M.J., Abell, P.A. [2005]. Icarus 175, 141-158; Fornasier, S., Clark, B.E., Dotto, E., Migliorini, A., Ockert-Bell, M., Barucci, M.A. [2009]. Icarus, submitted for publication) that indicate the presence of at least some silicate phases. All of the non-Mm asteroids show a positive correlation between visual and radar albedo but the reasons for this are not clear. All of the higher radar albedo targets (the 7 Mm asteroids, Lydia, and Fredegundis) show moderate to large variations in radar albedo with rotation phase. We suggest that their high radar reflectivity exaggerates irregularities in the asteroid shape to cause this behavior. One-third of our targets show evidence for asteroid-scale concavities or bifurcation. Based on all the evidence available, we suggest that most Tholen M-class asteroids are not remnant iron cores or enstatite chondrites, but rather collisional composites of silicates and irons with compositions more analogous to stony-iron meteorites and high-iron carbonaceous chondrites.  相似文献   

8.
We report new radar observations of E-class Asteroid 64 Angelina and M-class Asteroid 69 Hesperia obtained with the Arecibo Observatory S-band radar (2480 MHz, 12.6 cm). Our measurements of Angelina’s radar bandwidth are consistent with reported diameters and poles. We find Angelina’s circular polarization ratio to be 0.8 ± 0.1, tied with 434 Hungaria for the highest value observed for main-belt asteroids and consistent with the high values observed for all E-class asteroids (Benner, L.A.M., Ostro, S.J., Magri, C., Nolan, M.C., Howell, E.S., Giorgini, J.D., Jurgens, R.F., Margot, J.L., Taylor, P.A., Busch, M.W., Shepard, M.K. [2008]. Icarus 198, 294-304; Shepard, M.K., Kressler, K.M., Clark, B.E., Ockert-Bell, M.E., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M., Ostro, S.J. [2008b]. Icarus 195, 220-225). Our radar observations of 69 Hesperia, combined with lightcurve-based shape models, lead to a diameter estimate, Deff = 110 ± 15 km, approximately 20% smaller than the reported IRAS value. We estimate Hesperia to have a radar albedo of , consistent with a high-metal content. We therefore add 69 Hesperia to the Mm-class (high metal M) (Shepard, M.K., Clark, B.E., Ockert-Bell, M., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M., Ostro, S.J., Harris, A.W., Warner, B.D., Stephens, R.D., Mueller, M. [2010]. Icarus 208, 221-237), bringing the total number of Mm-class objects to eight; this is 40% of all M-class asteroids observed by radar to date.  相似文献   

9.
We have conducted a radar-driven observational campaign of main-belt asteroids (MBAs) focused on X/M class asteroids using the Arecibo radar and NASA Infrared Telescope Facilities (IRTF). M-type asteroids have been identified as metallic, enstatite chondrites and/or heavily altered carbonaceous chondrites [Bell, J.F., Davis, D., Hartmann, W.K., Gaffey, M.J., 1989. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press, Tucson, pp. 921-948; Gaffey, M.J., McCord, T.B., 1979. In: Gehrels, T., Matthews, M.S. (Eds.), Asteroids. Univ. of Arizona Press, Tucson, pp. 688-723; Vilas, F., 1994. Icarus 111, 456-467]. Radar wavelength observations can determine whether an asteroid is metallic and provide information about the porosity and regolith depth. Near-infrared observations can help determine the grain size, porosity and composition of an object. Concurrent observations with these tools can give us a wealth of information about an object. Our objectives for this observation program were to (a) determine if there are any consistent relationships between spectra in the near-infrared wavelengths and radar signatures and (b) look for rotationally resolved relationships between asteroid radar properties and near-infrared spectral properties. This paper describes preliminary results of an ongoing survey of near-infrared observations of M-type asteroids and is a companion paper to radar observations reported by Shepard [Shepard, M.K., and 19 colleagues, 2008a. Icarus 195, 184-205]. In the analysis of 16 asteroid near-infrared spectra and nine radar measurements, we find a trend indicating a correlation between continuum slope from 1.7 to 2.45 μm and radar albedo—an asteroid with a steep continuum slope also has a bright radar albedo, which suggests a significant metal content. This may provide a means to use near-IR observations to predict the most likely metallic candidates for radar studies.  相似文献   

10.
Bottke et al. [Bottke, W.F., Vokrouhlicky, D., Nesvorný, D., 2007. Nature 449, 48–53] linked the catastrophic formation of Baptistina Asteroid Family (BAF) to the K/T impact event. This linkage was based on dynamical and compositional evidence, which suggested the impactor had a composition similar to CM2 carbonaceous chondrites. However, our recent study [Reddy, V., Emery, J.P., Gaffey, M.J., Bottke, W.F., Cramer, A., Kelley, M.S., 2009. Meteorit. Planet. Sci. 44, 1917–1927] suggests that the composition of (298) Baptistina is similar to LL-type ordinary chondrites rather than CM2 carbonaceous chondrites. This rules out any possibility of it being related to the source of the K/T impactor, if the impactor was of CM-type composition. Mineralogical study of asteroids in the vicinity of BAF has revealed a plethora of compositional types suggesting a complex formation and evolution environment. A detailed compositional analysis of 16 asteroids suggests several distinct surface assemblages including ordinary chondrites (Gaffey SIV subtype), primitive achondrites (Gaffey SIII subtype), basaltic achondrites (Gaffey SVII subtype and V-type), and a carbonaceous chondrite. Based on our mineralogical analysis we conclude that (298) Baptistina is similar to ordinary chondrites (LL-type) based on olivine and pyroxene mineralogy and moderate albedo. S-type and V-type in and around the vicinity of BAF we characterized show mineralogical affinity to (8) Flora and (4) Vesta and could be part of their families. Smaller BAF asteroids with lower SNR spectra showing only a ‘single’ band are compositionally similar to (298) Baptistina and L/LL chondrites. It is unclear at this point why the silicate absorption bands in spectra of asteroids with formal family definition seem suppressed relative to background population, despite having similar mineralogy.  相似文献   

11.
Mark Willman  Robert Jedicke 《Icarus》2008,195(2):663-673
We have obtained moderate S/N (∼85) spectra at a realized resolution of R∼100 for 11 members of the Iannini family, until recently the youngest known family at under 5 million years of age [Nesvorný, D., Bottke, W.F., Levison, H.F., Dones, L., 2003. Astrophys. J. 591, 486-497, 720-771]. The spectra were acquired using the Echellette Spectrograph and Imager in its low-resolution prism mode on the Keck II telescope. The family members belong to the S-complex of asteroids with perhaps some K class members. The Iannini family members' average spectral slope, defined as the slope of the best-fit line constrained to pivot about 1 at 550 nm, is (0.30±0.04)/μm, matching the (0.26±0.03)/μm reported by Jedicke et al. [Jedicke, R., Nesvorný, D., Whiteley, R.J., Ivezi?, ?., Juri?, M., 2004. Nature 429, 275-277] using SDSS [Ivezi?, ?., Juri?, M., Lupton, R.H., Tabachnik, S., Quinn, T., 2002. In: Tyson, J.A., Wolff, S. (Eds.), Survey and Other Telescope Technologies and Discoveries. In: Proc. SPIE, vol. 4836. SPIE, Bellingham, pp. 98-103] color photometry. Using our spectra for this family as well as new observations of Karin family members [Vernazza, P., Birlan, M., Rossi, A., Dotto, E., Nesvorný, D., Brunetto, R., Fornasier, S., Fulchignoni, M., Renner, S., 2006. Astron. Astrophys. 460, 945-951] and new classifications of some older families we have revised the space weathering rate of S-complex asteroids originally determined by Jedicke et al. [Jedicke, R., Nesvorný, D., Whiteley, R.J., Ivezi?, ?., Juri?, M., 2004. Nature 429, 275-277]. Following Jedicke et al. [Jedicke, R., Nesvorný, D., Whiteley, R.J., Ivezi?, ?., Juri?, M., 2004. Nature 429, 275-277] we parameterize the space weathering rate of the principal component color of the spectrum (PC1), which is correlated with the spectral slope, as PC1(t)=PC1(0)+ΔPC1[1−exp−α(t/τ)]. Our revised rate suggests that the characteristic time scale for space weathering is τ=570±220 Myr and that new S-complex clusters will have an initial color of PC1(0)=0.31±0.04. The revised time scale is in better agreement with lab measurements and our measurements support the use of space weathering as a dating method. Under the assumption that all the spectra should be identical, since the members all derive from the same parent body and are presumably covered with similar regolith, we combined them to obtain a high-S/N composite spectrum for the family. The combined spectrum is within the S-complex.  相似文献   

12.
Near-infrared (∼0.7 to ∼2.5 μm) spectra of S-asteroids 138 Tolosa, 306 Unitas, 346 Hermentaria, and 480 Hansa suggest the presence of variable amounts of orthopyroxene ± clinopyroxene ± olivine ± plagioclase feldspar on the surfaces of these asteroids. The spectra of these asteroids were compared to laboratory mineral mixtures of orthopyroxene, clinopyroxene, and olivine [Singer, R.B., 1981. J. Geophys. Res. 86 (B9), 7967-7982; Cloutis, E.A., 1985. Master's thesis]. The band parameters (band centers, band areas) were quantified and temperature-corrected [Moroz et al., 2000. Icarus 147, 79-93; Gaffey et al., 2002. In: Bottke Jr., W.F., Cellino, A., Paolicchi, P., Binzel, R.P. (Eds.), Asteroids III. The University of Arizona Press, Tucson, pp. 183-204]. Each S-asteroid in this paper exhibits an overall spectral shape with band parameters that are inconsistent with ordinary chondrite near-infrared spectra and their inferred mineral abundances and/or pyroxene chemistries. 138 Tolosa displays a complex spectrum with a broad ∼1 μm absorption feature that displays a double Band I minimum, a well-defined absorption at ∼1.3 μm, and a broad, but weak absorption in the ∼2 μm region. Although different interpretations exist, the Tolosa spectrum is most consistent with a ∼60/40 mixture of Type B clinopyroxene and orthopyroxene. Spectra of 306 Unitas suggest a surface with variable amounts of low-Ca pyroxene and olivine. Unitas is located in the S-(IV) and S-(VI) subtype regions in Gaffey et al. [1993. Icarus 106, 573-602]. 346 Hermentaria exhibits a complex, broad Band I absorption feature and a weak Band II feature, which suggests a ∼50/50 mixture of clinopyroxene and orthopyroxene. Hermentaria is classified as an S-(III). Spectra of 480 Hansa suggest a dominant low-Ca pyroxene component with lesser amounts of olivine. Based on these characterizations, these four S-asteroids should not be considered as potential ordinary chondrite parent bodies. Furthermore, these results suggest that these S-asteroids experienced at least partial melting temperatures [T?∼950 °C: Gaffey et al., 1993. Icarus 106, 573-602; Keil, K., 2000. Planet. Space Sci. 48, 887-903] during the formation epoch in the early Solar System. Continuing spectroscopic investigations will discern the relative abundance of chondritic and thermally-evolved objects among the S-type asteroids that have survived since the formation epoch ∼4.56 billion years ago.  相似文献   

13.
The space weathering process and its implications for the relationships between S- and Q-type asteroids and ordinary chondrite meteorites is an often debated topic in asteroid science. Q-type asteroids have been shown to display the best spectral match to ordinary chondrites (McFadden, L.A., Gaffey, M.J., McCord, T.B. [1985]. Science 229, 160–163). While the Q-types and ordinary chondrites share some spectral features with S-type asteroids, the S-types have significantly redder spectral slopes than the Q-types in visible and near-infrared wavelengths. This reddening of spectral slope is attributed to the effects of space weathering on the observed surface composition. The analysis by Binzel et al. (Binzel, R.P., Rivkin, A.S., Stuart, J.S., Harris, A.W., Bus, S.J., Burbine, T.H. [2004]. Icarus 170, 259–294) provided a missing link between the Q- and S-type bodies in near-Earth space by showing a reddening of spectral slope in objects from 0.1 to 5 km that corresponded to a transition from Q-type to S-type asteroid spectra, implying that size, and therefore surface age, is related to the relationship between S- and Q-types. The existence of Q-type asteroids in the main-belt was not confirmed until Mothé-Diniz and Nesvorny (Mothé-Diniz, T., Nesvorny, D. [2008]. Astron. Astrophys. 486, L9–L12) found them in young S-type clusters. The young age of these families suggest that the unweathered surface could date to the formation of the family. This leads to the question of whether older S-type main-belt families can contain Q-type objects and display evidence of a transition from Q- to S-type. To answer this question we have carried out a photometric survey of the Koronis family using the Kitt Peak 2.1 m telescope. This provides a unique opportunity to compare the effects of the space weathering process on potentially ordinary chondrite-like bodies within a population of identical initial conditions. We find a trend in spectral slope for objects 1–5 km that shows the transition from Q- to S-type in the main-belt. This data set will prove crucial to our understanding of the space weathering process and its relevant timescales.  相似文献   

14.
In this paper, we present arguments for the presence of optically active Ti3+ in the structure of low iron pyroxenes to explain the strong 0.49-μm absorption band in spectra of E[II] asteroids. Such an interpretation supports the idea that E asteroid material is formed at high temperature in a strongly reducing medium. We also discuss the possibility of high temperature and hydrated rocks coexisting on some E asteroids. Our discussion is based on the idea of collisional evolution of aubrite parent bodies that was presented by Keil (1989).  相似文献   

15.
Absorption bands near 0.43 and 0.60–0.80 m, with a relative intensity of approximately 5–10%, were discovered in the reflectance spectra of five M asteroids (21, 75, 161, 201, and 497) and two S asteroids (11 and 198). It is highly probable that these absorption bands are related to oxidized and/or hydrated silicates that incorporate OH structural groups. Absorbed water in the surface material of about 24% of the known M asteroids and three E asteroids has been identified by a group of astronomers at the University of Arizona (Rivkin et al., 1995; Rivkin, 1997; Rivkin et al., 2000) on the basis of the characteristic 3-m absorption band. In addition to this, four of the six E-type asteroids observed spectrally in the visible region quite recently were found to have an absorption band at 0.5 m (Burbine et al., 1998; Fornasier and Lazzarin, 2001), which could likewise be associated with the presence of hydrated silicates in their material. The above data contradict the generally accepted viewpoint on the nature of M, S, and E asteroids, which is based on their common observed characteristics as bodies that arose at high temperatures (in the 1000–2000°C range). This contradiction, however, can be eliminated if use is made of the cosmogonic model developed by V.S. Safronov and co- workers. According to this model, the initial evolution of the main-belt asteroids was directly influenced by the process of the growth of Jupiter and was later controlled by its gravitational field.  相似文献   

16.
In this paper we present the observational campaign carried out at ESO NTT and VLT in April and May 2006 to investigate the nature and the structure of the near-Earth object (144898) 2004 VD17. In spite of a great quantity of dynamical information, according to which it will have a close approach with the Earth in the next century, the physical properties of this asteroid are largely unknown. We performed visible and near-infrared photometry and spectroscopy, as well as polarimetric observations. Polarimetric and spectroscopic data allowed us to classify 2004 VD17 as an E-type asteroid. A good agreement was also found with the spectrum of the aubrite meteorite Mayo Belwa. On the basis of the polarimetric albedo (pv=0.45) and of photometric data, we estimated a diameter of about 320 m and a rotational period of about 2 h. The analysis of the results obtained by our complete survey have shown that (144898) 2004 VD17 is a peculiar NEO, since it is close to the breakup limits for fast rotator asteroids, as defined by Pravec and Harris [Pravec, P., Harris, A.W., 2000. Icarus 148, 12-20]. These results suggest that a more robust structure must be expected, as a fractured monolith or a rubble pile in a “strength regime” [Holsapple, K.A., 2002. Speed limits of rubble pile asteroids: Even fast rotators can be rubble piles. In: Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids, Washington, September, 2002].  相似文献   

17.
The NEAR mission to 433 Eros provided detailed data on the geology, mineralogy, and chemistry of this S-class asteroid [McCoy, T.J., Robinson, M.S., Nittler, L.R., Burbine, T.H., 2002. Chem. Erde 62, 89-121; Cheng, A.F., 1997. Space Sci. Rev. 82, 3-29] with a key science goal of understanding the relationship between asteroids and meteorites [Cheng, A.F., 1997. Space Sci. Rev. 82, 3-29; Gaffey, M.J., Burbine, T.H., Piatek, J.L., Reed, K.L., Chaky, D.A., Bell, J.F., Brown, R.H., 1993a. Icarus 106, 573-602]. Previously reported major element data revealed a bulk surface similar to that of ordinary chondrites, with the notable exception of sulfur, which was highly depleted [Trombka, J.I., and 23 colleagues, 2000. Science 289, 2101-2105; Nittler, L.R., and 14 colleagues, 2001. Meteorit. Planet. Sci. 36, 1673-1695]. The origin of this sulfur deficiency, and hence the fundamental nature of the asteroid's surface, has remained controversial. We report a new analysis of NEAR X-ray spectrometer data, indicating that Eros has Cr/Fe, Mn/Fe, and Ni/Fe ratios similar to ordinary chondrite meteorites of type LL or L. Chondritic levels of Cr, Mn, and Ni argue strongly against a partial melting explanation for the sulfur depletion. Instead, our results provide definitive evidence that Eros is a primitive body with composition and mineralogy similar to ordinary chondrites, but with a surface heavily modified by interactions with the solar wind and micrometeorites, processes collectively termed space weathering.  相似文献   

18.
A.S. Rivkin  R.P. Binzel  S.J. Bus  A. Saxena 《Icarus》2004,172(2):408-414
We have obtained near-infrared (0.8-2.5 μm) spectra of Hermes (1937 UB) using the NASA Infrared Telescope Facility on Mauna Kea. We find Hermes to have spectral properties consistent with L/LL chondrites, with a strong visual similarity to Gaffey (1976, J. Geophys. Res. 81, 905) average L6 chondrites. We define a ratio of band areas (RBA) using the Modified Gaussian Method (MGM: Sunshine and Pieters, 1990, in: Lunar and Planetary Institute Conference Abstracts, p. 1223, 1993, J. Geophys. Res. 98, 9075) to quantify near-infrared asteroidal data lacking a visible component. Hermes has a spectrum nearly indistinguishable from (19356) 1997 GH3. Together, these asteroids represent new endmembers on the continuum of spectra from ordinary chondrite meteorites to large main-belt S-class asteroids. We discuss regolith effects that may be occurring on Hermes and other possible ordinary chondrite parent bodies, and constrain the albedo of Hermes to 0.4 or higher (effective diameter 650 m or smaller) if it has a regolith. This value for albedo/diameter is consistent with radar results.  相似文献   

19.
High signal-to-noise, rotationally-resolved spectra of Asteroid 4 Vesta’s southern hemisphere from the 2007 opposition were used to constrain its compositional and mineralogical variations. The spectra were rotationally-phased using closely timed HST observations of Vesta by Li et al. (Li, J.-Y., McFadden, L.A., Thomas, P.C., Mutchler, M.J., Parker, J.Wm., Young, E.F., Russell, C.T., Sykes, M.V., Schmidt, B.E. [2010]. Icarus 208, 238–251). The average surface of Vesta’s southern hemisphere is analogous to a howardite or polymict eucrite assemblage similar to the northern hemisphere, although the band parameters are distinctly shifted towards the diogenite zone on the Band–Band plot. A few distinct compositional units were detected and they might be related to albedo features detected by Hubble Space Telescope (Li et al., 2010). We have identified two compositionally distinct regions overlaying the background surface. The first unit is a polymict eucrite and/or low-Ca eucrite compositional unit at 143° longitude that border the eucrite zone on the Band–Band plot and the second is a diogenite unit at 159°. While we did not detect any distinct olivine units as suggested by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157), we cannot rule out the possibility of smaller olivine-rich units that are below the detection limit of the instrumentation we used. Based on the analysis and the limitations of the data, we do not suggest that Vesta’s surface is olivine-free. Mean pyroxene chemistry estimates for both hemispheres broadly agree with one another (to within one-sigma) with the northern hemisphere ferrosilite (Fs) and wollastonite (Wo) values being slightly higher than southern hemisphere.  相似文献   

20.
Following the work of Rivkin et al. (Rivkin, A.S., Thomas, C.A., Trilling, D.E., Enga, M., Grier, J.A. [2011]. Icarus 211, 1294–1297) and Thomas et al. (Thomas, C.A., Rivkin, A.S, Trilling, D.E., Enga, M., Grier, J.A. [2011a]. Icarus 212, 158–166), we investigate space weathering trends in the Koronis family using the larger sample size of the Sloan Digital Sky Survey Moving Object Catalog. We confirm the trend in spectral slope seen in our earlier work and extend our results by investigating the trend in band depth (i ? z color index) to show that Koronis family asteroids smaller than 4 km show the transition from fresh Q-type to weathered S-type surfaces.  相似文献   

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