共查询到20条相似文献,搜索用时 15 毫秒
1.
832 Karin is the largest member of the young Karin cluster that formed 5.75±0.05 Myr ago in the outer main belt. Surprisingly, recent near-IR spectroscopy measurements [Sasaki, T., Sasaki, S., Watanabe, J., Sekiguchi, T., Yoshida, F., Kawakita, H., Fuse, T., Takato, N., Dermawan, B., Ito, T., 2004. Astrophys. J. 615 (2), L161-L164] revealed that Karin's surface shows different colors as a function of rotational phase. It was interpreted that 832 Karin shows us the reddish space-weathered exterior surface of the parent body as well as an interior face, which has not had time to become space-weathered. This result is at odds with recent results including seismic and geomorphic modeling, modeling of the Karin cluster formation and measurements of the space weathering rate. Consequently, we aimed to confirm/infirm this surprising result by sampling Karin's spectrum well throughout its rotation. Here, we present new visible (0.45-0.95 μm) and near-infrared (0.7-2.5 μm) spectroscopic observations of 832 Karin obtained in January and April 2006, covering most of Karin's longitudes. In the visible range, we find that Karin shows no rotational spectral variations. Similarly, we find that Karin exhibits very little (to none) spectral variations with rotation in the near-IR range. Our results imply that 832 Karin has a homogeneous surface, in terms of composition and surface age. Our results also imply that the impact that generated the family refreshed entirely Karin's surface, and probably the surfaces of all members. 相似文献
2.
Michael K. Shepard Beth Ellen Clark Lance A.M. Benner Jon D. Giorgini Kandy Jarvis Lucy F. Lim James Bell Andrew Rivkin Daniel Scheeres 《Icarus》2008,193(1):20-38
We observed near-Earth asteroid (NEA) 2100 Ra-Shalom over a six-year period, obtaining rotationally resolved spectra in the visible, near-infrared, thermal-infrared, and radar wavelengths. We find that Ra-Shalom has an effective diameter of Deff=2.3±0.2 km, rotation period P=19.793±0.001 h, visual albedo pv=0.13±0.03, radar albedo , and polarization ratio μc=0.25±0.04. We used our radar observations to generate a three-dimensional shape model which shows several structural features of interest. Based on our thermal observations, Ra-Shalom has a high thermal inertia of ∼103 J m−2 s−0.5 K−1, consistent with a coarse or rocky surface and the inferences of others [Harris, A.W., Davies, J.K., Green, S.F., 1998. Icarus 135, 441-450; Delbo, M., Harris, A.W., Binzel, R.P., Pravec, P., Davies, J.K., 2003. Icarus 166, 116-130]. Our spectral data indicate that Ra-Shalom is a K-class asteroid and we find excellent agreement between our spectra and laboratory spectra of the CV3 meteorite Grosnaja. Our spectra show rotation-dependent variations consistent with global variations in grain size. Our radar observations show rotation-dependent variations in radar albedo consistent with global variations in the thickness of a relatively thin regolith. 相似文献
3.
Michael K. Shepard Jean-Luc Margot Michael C. Nolan Benjamin Estes Eric L. Volquardsen Lance A.M. Benner Steven J. Ostro 《Icarus》2006,184(1):198-210
We observed near-Earth Asteroid (NEA) 2002 CE26 in August and September 2004 using the Arecibo S-band (2380-MHz, 12.6-cm) radar and NASA's Infrared Telescope Facility (IRTF). Shape models obtained based on inversion of our delay-Doppler images show the asteroid to be 3.5±0.4 km in diameter and spheroidal; our corresponding nominal estimates of its visual and radar albedos are 0.07 and 0.24, respectively. Our IRTF spectrum shows the asteroid to be C-class with no evidence of hydration. Thermal models from the IRTF data provide a size and visual albedo consistent with the radar-derived estimate. We estimate the spin-pole to be within a few tens of degrees of λ=317°, β=−20°. Our radar observations reveal a secondary approximately 0.3 km in diameter, giving this binary one of the largest size differentials of any known NEA. The secondary is in a near-circular orbit with period 15.6±0.1 h and a semi-major axis of 4.7±0.2 km. Estimates of the binary orbital pole and secondary rotation rate are consistent with the secondary being in a spin-locked equatorial orbit. The orbit corresponds to a primary mass of M=1.95±0.25×1013 kg, leading to a primary bulk density of , one of the lowest values yet measured for a main-belt or near-Earth asteroid. 相似文献
4.
The Karin cluster is one of the youngest known families of main-belt asteroids, dating back to a collisional event only 5.8±0.2 Myr ago. Using the Spitzer Space Telescope we have photometrically sampled the thermal continua (3.5-22 μm) of 17 Karin cluster asteroids of different sizes, down to the smallest members discovered so far, in order to make the first direct measurements of their sizes and albedos and study the physical properties of their surfaces. Our targets are also amongst the smallest main-belt asteroids observed to date in the mid-infrared. The derived diameters range from 17.3 km for 832 Karin to 1.5 km for 75176, with typical uncertainties of 10%. The mean albedo is pv=0.215±0.015, compared to 0.20±0.07 for 832 Karin itself (for H=11.2±0.3), consistent with the view that the Karin asteroids are closely related physically as well as dynamically. The albedo distribution (0.12?pv?0.32) is consistent with the range associated with S-type asteroids but the variation from one object to another appears to be significant. Contrary to the case for near-Earth asteroids, our data show no evidence of an albedo dependence on size. However, the mean albedo is lower than expected for young, fresh “S-type” surfaces, suggesting that space weathering can darken main-belt asteroid surfaces on very short timescales. Our data are also suggestive of a connection between surface roughness and albedo, which may reflect rejuvenation of weathered surfaces by impact gardening. While the available data allow only estimates of lower limits for thermal inertia, we find no evidence for the relatively high values of thermal inertia reported for some similarly sized near-Earth asteroids. Our results constitute the first observational confirmation of the legitimacy of assumptions made in recent modeling of the formation of the Karin cluster via a single catastrophic collision 5.8±0.2 Myr ago. 相似文献
5.
Detailed near-infrared spectral observations of Asteroid 1459 Magnya reveal an asteroid that is primarily composed of pyroxene and plagioclase feldspar, confirming earlier suggestions that Magnya has a basaltic composition. The average Magnya spectrum for March 23, 2002 has a Band I center of 0.926 μm and a Band II center of 1.938 μm. Observations over hours show little variation in band center positions. The feldspar-to-pyroxene ratio is ∼0.6 on Magnya's surface. Comparing Magnya with the spectral parameters from 4 Vesta shows discordant pyroxene chemistries; Magnya's pyroxenes contain ∼10 mol% less Fs than Vesta's pyroxenes. This suggests that Magnya originated from a parent body other than 4 Vesta and that its progenitor formed in a more chemically reduced region of the solar nebula within the asteroid belt. 相似文献
6.
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. 相似文献
7.
The potentially hazardous Asteroid (33342) 1998 WT24 approached the Earth within 0.0125 AU on 2001 December 16 and was the target of a number of optical, infrared, and radar observing campaigns. Interest in 1998 WT24 stems from its having an orbit with an unusually low perihelion distance, which causes it to cross the orbits of the Earth, Venus, and Mercury, and its possibly being a member of the E spectral class, which is rare amongst near-Earth asteroids (NEAs). We present the results of extensive thermal-infrared observations of 1998 WT24 obtained in December 2001 with the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii and the ESO 3.6-m telescope in Chile. A number of thermal models have been applied to the data, including thermophysical models that give best-fit values of 0.35±0.04 km for the effective diameter, 0.56±0.2 for the geometric albedo, pv, and 100-300 J m−2 s−0.5 K−1 for the thermal inertia. Our values for the diameter and albedo are consistent with results derived from radar and polarimetric observations. The albedo is one of the highest values obtained for any asteroid and, since no other taxonomic type is associated with albedos above 0.5, supports the suggested rare E-type classification for 1998 WT24. The thermal inertia is an order of magnitude higher than values derived for large main-belt asteroids but consistent with the relatively high values found for other near-Earth asteroids. A crude pole solution inferred from a combination of our observations and published radar results is β=−52°, λ=355° (J2000), but we caution that this is uncertain by several tens of degrees. 相似文献
8.
The Amor-type near-Earth Asteroid (10302) 1989 ML has an “Earth-like” orbit (period 1.44 yr, eccentricity 0.14, inclination 4.4°), therefore the energy required to reach it from the Earth is relatively small making it a very attractive target for rendezvous missions. We have observed 1989 ML in the thermal-infrared using the Spitzer Space Telescope, and compared these data with optical and near-infrared observations. The Spitzer results imply a diameter of 0.28±0.05 km and a geometric albedo of 0.37±0.15; together with the reflectance spectrum they are consistent with the relatively rare E classification. 相似文献
9.
We present a mineralogical assessment of near-Earth Asteroid, (1036) Ganymed, using data obtained May 18, 2006 UT combined with 24 Color Asteroid Survey data to cover the spectral interval of 0.3-2.45 μm. Results of the analysis indicate (1036) Ganymed is an S (VI) asteroid with a surface silicate assemblage consisting primarily of orthopyroxene, (Fs23(±5)Wo3(±3)), consistent with calculated band centers and band area ratios (BAR). (1036) Ganymed appears to be once part of a large mesosiderite containing howardite, eucrite, and diogenite (HED) pyroxenes mixed with metal that was broken apart and dispersed. The calculated composition of the average pyroxenes in the surface material of (1036) Ganymed is consistent with mesosiderite pyroxenes, in particular the diogenites. A second possibility could be (1036) Ganymed is not yet represented in the meteorite collection. Our investigation has confirmed Ganymed is not a parent body of the ordinary chondrites and is not genetically related to (433) Eros. 相似文献
10.
Michael Mueller Franck Marchis Joshua P. Emery Stefano Mottola Jérome Berthier 《Icarus》2010,205(2):505-515
We present mid-infrared observations of the binary L5-Trojan system (617) Patroclus-Menoetius before, during, and after two shadowing events, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. For the first time, we effectively observe changes in asteroid surface temperature in real time, allowing the thermal inertia to be determined very directly. A new detailed binary thermophysical model is presented which accounts for the system’s known mutual orbit, arbitrary component shapes, and thermal conduction in the presence of eclipses.We obtain two local thermal-inertia values, representative of the respective shadowed areas: and . The average thermal inertia is estimated to be , potentially with significant surface heterogeneity. This first thermal-inertia measurement for a Trojan asteroid indicates a surface covered in fine regolith. Independently, we establish the presence of fine-grained (<a few μm) silicates on the surface, based on emissivity features near 10 and similar to those previously found on other Trojans.We also report V-band observations and report a lightcurve with complete rotational coverage. The lightcurve has a low amplitude of peak-to-peak, implying a roughly spherical shape for both components, and is single-periodic with a period equal to the period of the mutual orbit, indicating that the system is fully synchronized.The diameters of Patroclus and Menoetius are 106±11 and , respectively, in agreement with previous findings. Taken together with the system’s known total mass, this implies a bulk mass density of , significantly below the mass density of L4-Trojan asteroid (624) Hektor and suggesting a bulk composition dominated by water ice.All known physical properties of Patroclus, arguably the best studied Trojan asteroid, are consistent with those expected in icy objects with devolatilized surface (extinct comets), consistent with what might be implied by recent dynamical modeling in the framework of the Nice Model. 相似文献
11.
The Keck Observatory's adaptive optics (AO) system has been used to observe Asteroid 4 Vesta during its 2003 closest approach to Earth. Broadband K′- and L′-band images, centered at 2.1 and 3.6 μm, respectively, are presented here. The sharpness of the images was improved by applying a deconvolution algorithm, MISTRAL, to the images. The K′- and L′-band images at spatial resolutions of 53 km (0.055″) and 88 km (0.085″), respectively, display albedo features on the surface of the asteroid that can also be seen in the HST images (673 nm) presented by Thomas et al. [1997. Impact excavation on Asteroid 4 Vesta: Hubble Space Telescope results. Science 277, 1492-1495] and Binzel et al. [1997. Geologic mapping of Vesta from 1994 Hubble Space Telescope images. Icarus 128, 95-103] at the same latitudes and longitudes. While we cannot determine the morphology of these features, we can speculate that some of the albedo features may be impact craters filled with dark material. Spectra, centered at 1.65 and 2.1 μm, were also obtained. Spectra were corrected for the solar flux and are similar to those published by Gaffey [1997. Surface lithologic heterogeneity of Asteroid 4 Vesta. Icarus 127, 130-157], along the same wavelength range. 相似文献
12.
Faint absorption bands in the visible range of the smoothed vestoid spectra have been found. The bands centered near 505, 530, and 550 nm are attributed to ferrous iron in low-calcium pyroxene and are typical for pyroxene-bearing vestoid surfaces. In accordance with characteristics of the faint absorption bands around 600 and 650 nm the studied vestoid spectra can be sorted into five types. Since the same absorptions are also seen in the laboratory spectra of the minerals and meteorites, which appear to be similar to vestoid material, spectral types of the vestoids can be related to their surface compositions. Regolith of the Type-I vestoids consists of pure low-calcium pyroxenes. Minor amount of olivine along with pyroxene appear to be on the Type-II vestoids whereas the mixtures of low-calcium pyroxene with minor chromite define the Type-III and -IV. The causes for the fifth spectral type in terms of minor mineral phases are unclear now. Simulation of the spectra of vestoids was employed to estimate content of olivine (∼6-12 vol%) and chromite (∼12-30 vol%) on their surfaces. 相似文献
13.
Michael W. Busch Jon D. Giorgini Lance A.M. Benner Randy Rose Petr Pravec Michael J. Ireland Stephen B. Broschart Michael C. Nolan Jean-Luc Margot 《Icarus》2007,190(2):608-621
Near-Earth Asteroid (29075) 1950 DA may closely encounter Earth in 2880. The probability of Earth impact may be as high as 1/300, but the outcome of the encounter depends critically on the physical properties of the asteroid [Giorgini et al., 2002. Science 196, 132-136]. We have used Arecibo and Goldstone radar data and optical lightcurves to estimate the shape, spin state, and surface structure of 1950 DA. The data allow two distinct models. One rotates prograde and is roughly spheroidal with mean diameter 1.16±0.12 km. The other rotates retrograde and is oblate and about 30% larger. Both models suggest a nickel-iron or enstatite chondritic composition. Ground-based observations should be able to determine which model is correct within the next several decades. 相似文献
14.
An optimization method of smoothing noisy spectra was developed to investigate faint absorption bands in the visual spectral region of reflectance spectra of asteroids and the compositional information derived from their analysis. The smoothing algorithm is called “optimal” because the algorithm determines the best running box size to separate weak absorption bands from the noise. The method is tested for its sensitivity to identifying false features in the smoothed spectrum, and its correctness of forecasting real absorption bands was tested with artificial spectra simulating asteroid reflectance spectra. After validating the method we optimally smoothed 22 vestoid spectra from SMASS1 [Xu, Sh., Binzel, R.P., Burbine, T.H., Bus, S.J., 1995. Icarus 115, 1-35]. We show that the resulting bands are not telluric features. Interpretation of the absorption bands in the asteroid spectra was based on the spectral properties of both terrestrial and meteorite pyroxenes. The bands located near 480, 505, 530, and 550 nm we assigned to spin-forbidden crystal field bands of ferrous iron, whereas the bands near 570, 600, and 650 nm are attributed to the crystal field bands of trivalent chromium and/or ferric iron in low-calcium pyroxenes on the asteroids' surface. While not measured by microprobe analysis, Fe3+ site occupancy can be measured with Mössbauer spectroscopy, and is seen in trace amounts in pyroxenes. We believe that trace amounts of Fe3+ on vestoid surfaces may be due to oxidation from impacts by icy bodies. If that is the case, they should be ubiquitous in the asteroid belt wherever pyroxene absorptions are found. Pyroxene composition of four asteroids of our set is determined from the band position of absorptions at 505 and 1000 nm, implying that there can be orthopyroxenes in all range of ferruginosity on the vestoid surfaces. For the present we cannot unambiguously interpret of the faint absorption bands that are seen in the spectra of 4005 Dyagilev, 4038 Kristina, 4147 Lennon, and 5143 Heracles. Probably there are other spectrally active materials along with pyroxenes on the surfaces of these asteroids. 相似文献
15.
Michael K. Shepard Karelyn M. Kressler Maureen E. Ockert-Bell Ellen S. Howell Jon D. Giorgini Steven J. Ostro 《Icarus》2008,195(1):220-225
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
16.
Jian-Yang Li Lucille Le Corre Stefan E. Schröder Vishnu Reddy Brett W. Denevi Bonnie J. Buratti Stefano Mottola Martin Hoffmann Pablo Gutierrez-Marques Andreas Nathues Christopher T. Russell Carol A. Raymond 《Icarus》2013
Dawn spacecraft orbited Vesta for more than one year and collected a huge volume of multispectral, high-resolution data in the visible wavelengths with the Framing Camera. We present a detailed disk-integrated and disk-resolved photometric analysis using the Framing Camera images with the Minnaert model and the Hapke model, and report our results about the global photometric properties of Vesta. The photometric properties of Vesta show weak or no dependence on wavelengths, except for the albedo. At 554 nm, the global average geometric albedo of Vesta is 0.38 ± 0.04, and the Bond albedo range is 0.20 ± 0.02. The bolometric Bond albedo is 0.18 ± 0.01. The phase function of Vesta is similar to those of S-type asteroids. Vesta’s surface shows a single-peaked albedo distribution with a full-width-half-max ∼17% relative to the global average. This width is much smaller than the full range of albedos (from ∼0.55× to >2× global average) in localized bright and dark areas of a few tens of km in sizes, and is probably a consequence of significant regolith mixing on the global scale. Rheasilvia basin is ∼10% brighter than the global average. The phase reddening of Vesta measured from Dawn Framing Camera images is comparable or slightly stronger than that of Eros as measured by the Near Earth Asteroid Rendezvous mission, but weaker than previous measurements based on ground-based observations of Vesta and laboratory measurements of HED meteorites. The photometric behaviors of Vesta are best described by the Hapke model and the Akimov disk-function, when compared with the Minnaert model, Lommel–Seeliger model, and Lommel–Seeliger–Lambertian model. The traditional approach for photometric correction is validated for Vesta for >99% of its surface where reflectance is within ±30% of global average. 相似文献
17.
We describe interferometric observations of the Asteroid (41) Daphne in the thermal infrared obtained with the Mid-Infrared Interferometric Instrument (MIDI) and the Auxiliary Telescopes (ATs) of the European Southern Observatory (ESO) Very Large Telescope Interferometer (VLTI). We derived the size and the surface thermal properties of (41) Daphne by means of a thermophysical model (TPM), which is used for the interpretation of interferometric data for the first time. From our TPM analysis, we derived a volume equivalent diameter for (41) Daphne of 189 km, using a non-convex 3-D shape model derived from optical lightcurves and adaptive optics images (B. Carry, private communication). On the other hand, when using the convex shape of Kaasalainen et al. (Kaasalainen, M., Mottola, S., Fulchignoni, M. [2002]. Icarus 159, 369-395) in our TPM analysis, the resulting volume equivalent diameter of (41) Daphne is between 194 and 209 km, depending on the surface roughness. The shape of the asteroid is used as an a priori information in our TPM analysis. No attempt is made to adjust the shape to the data. Only the size of the asteroid and its thermal parameters such as, albedo, thermal inertia and roughness are adjusted to the data. We estimated our model systematic uncertainty to be of 4% and of 7% on the determination of the asteroid volume equivalent diameter depending on whether the non-convex or the convex shape is used, respectively. In terms of thermal properties, we derived a value of the surface thermal inertia smaller than 50 J m−2 s−0.5 K−1 and preferably in the range between 0 and ∼30 J m−2 s−0.5 K−1. Our TPM analysis also shows that Daphne has a moderate macroscopic surface roughness. 相似文献
18.
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
19.
Low-albedo near-Earth objects (NEOs) are warm enough to emit detectable thermal flux at 2.5 μm when near perihelion. Thermal radiation can account for 33% or more of the total flux for an object with an albedo ?0.04 at 1.0 AU. This is measurable using near-infrared spectroscopic instruments enabling albedos to be constrained for a larger sample of NEOs. 相似文献
20.
Lance A.M. Benner Steven J. Ostro Michael C. Nolan Jon D. Giorgini Jean-Luc Margot Michael W. Busch 《Icarus》2008,198(2):294-304
Radar observations of 214 near-Earth asteroids (NEAs) reveal a very strong correlation of circular polarization ratio with visible-infrared taxonomic class, establishing distinct differences in the centimeter-to-several-decimeter structural complexity of objects in different spectral classes. The correlation may be due to the intrinsic mechanical properties of different mineralogical assemblages but also may reflect very different formation ages and collisional histories. The highest ratios are measured for groups associated with achondritic igneous rocky meteorites: the E class, whose parent body may be 3103 Eger, and the V class, derived from the mainbelt asteroid (and Dawn mission target) 4 Vesta. 相似文献