66 research outputs found
Compositional structure in the asteroid belt : results of a spectroscopic survey
No full textThesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1999.Includes bibliographical references (p. 268-280).Visible wavelength spectra have been obtained for 1189 main-belt asteroids during the second phase of the Small Main-belt Asteroid Spectroscopic Survey. These telescopic measurements were made using charge-coupled device (CCD) detectors, allowing for the targeting of smaller asteroids. A majority of the asteroids sampled have estimated diameters of 30 km or less. The SMASSII survey provides the largest internally-consistent sample of asteroid spectra ever obtained, and reveals a greater range of spectral diversity among asteroids than has been previously shown. This diversity may arise from a broad range of surface ages, where smaller (collisionally younger) asteroids may have less evolved or processed surfaces than do larger asteroids. Processes that affect the development of a regalia, and the distribution of regalia particle sizes may produce the observed variations in spectral features. The larger sample size, greater spectral resolution, and greater photometric precision of the SMASSII survey, compared with the most extensive previous survey, provide a basis for developing a new "feature-based" taxonomic classification system for asteroids. This new taxonomy builds on the robust, large-scale structure of existing taxonomies, and relies on the presence or absence of specific spectral features to define new class boundaries. The SMASSII measurements reveal that many of the previous taxonomic classes that appeared to be distinctly separate, are instead spanned by a nearly continuous transition of spectral properties. This continuum may be evidence of sampling a more complete range of mineralogies. This newly found continuum makes it difficult to define a unique taxonomic structure using classical multivariate techniques, requiring that subjective boundaries be defined in some cases. The scale-length in spectral variance observed in many dynamical asteroid families was used as a metric for constraining the class sizes in this new taxonomy. The resulting new taxonomy consists of twenty-six spectral classes. Members of the previously defined X-class (which could be subdivided only using albedo information) display sufficient variation in spectral features to allow subgroups to be distinguished without the use of albedo. A subset of 465 SMASSII asteroids were specifically targeted to test the genetic reality of small dynamical "families." These families likely result from the collisional disruption of larger parent asteroids, and are identified as clusters of objects having similar orbital parameters. The targeted region, encompassing the heliocentric distance range of 2.690 to 2.815 AU, contains 14 families that had been previously identified, based on numerous analyses of orbital distributions in this region of the;1 main belt. A newly developed multivariate technique that analyzes the combination of spectral characteristics and orbital parameters reveals that all 14 of these previously proposed families are distinct from the "background" population mt asteroids. This result implies that each of these families is likely to have been truly formed by a collisional event, providing strong evidence for an extensive collisional history within the asteroid belt. Cand S-type 2 asteroids appear equally capable of forming families. Each family is found to be relatively homogeneous in its spectral characteristics, allowing the boundaries of tile families, and potential interlopers, to be more precisely identified than was possible in previous dynamical analyses. The relative spectral homogeneity within each of these families provides little evidence for any to have been formed from a differentiated parent body. It remains a mystery as to why there is no clear evidence of a major family containing members from· different geologic units, derived from a differentiated parent asteroid. In addition to the 14 previously identified dynamical families, our analysis has revealed five additional associations of spectrally similar asteroids in this same small heliocentric range. These spectral clusters may represent the dispersed fragments of older collisionally derived families.by Schelte John Bus.Ph.D
Distribution of olivine and pyroxene in S-type asteroids throughout the inner main belt
No full textThesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2008.Cataloged from PDF version of thesis. "May 5, 2008." "This thesis was submitted to the Institute Archives without all the required signatures"--Disclaimer Notice page.Includes bibliographical references (page 33).The mineralogical composition of asteroids can be constrained using visible and near-infrared (VNIR) spectroscopy. The most prominent spectral features observed over this wavelength range are due to olivine and pyroxene, the two most abundant minerals in both chondritic and achondritic meteorites. The observed ratio of these two minerals is highly dependent on the amount of heating that an asteroid has undergone. The 1-micron band center wavelength and the band area ratio (BAR) between the 2- and 1-micron bands reveal relative abundances of olivine and/or pyroxene on an asteroid surface (Gaffey, 1993). A large sample of S-, A-, V-, and R-type asteroid spectra was collected over the visible and near-IR wavelengths during the second phase of the Small Main-belt Asteroid Spectroscopic Survey (Bus and Binzel, 2002) and using the low-resolution SpeX spectrograph (Rayner, 2003) at NASA's Infrared Telescope Facility (IRTF). Here we present a methodology for calculating the location of the 1-micron band center wavelength and BAR with appropriate 1- sigma uncertainties. This method was used to characterize 188 S-type asteroids throughout the inner main belt. We will also present the distribution of olivine / pyroxene throughout the main belt by measuring how the S-type mineralogy varies with heliocentric distance. This will provide a better understanding of both the thermal processing across the main belt and subsequent mixing of asteroids through collisional and dynamical processes.by Shaye Perry Storm.S.B
Physical characterization of the potentially-hazardous high-albedo Asteroid (33342) 1998 WT<sub>24</sub> from thermal-infrared observations
No full textThe potentially-hazardous asteroid (33342) 1998 WT24 approached the Earth within 0.0125 AU on 2001 Dec. 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
The 2060 Chiron: CCD photometry
No full textR-band CCD photometry of 2060 was carried out on nine nights in Nov. and Dec. 1986. The rotation period is 5.9181 + or - 0.0003 hr and the peak to peak lightcurve amplitude is 0.088 + or - 0.0003 mag. Photometric parameters are H sub R = 6.24 + or - 0.02 mag and G sub R = + or - 0.15, though formal errors may not be realistic. The lightcurve has two pairs of extrema, but its asymmetry, as evidenced by the presence of significant odd Fourier harmonics, suggests macroscopic surface irregularities and/or the presence of some large scale albedo variegation. The observational rms residual is + or - 0.015 mag. On time scales from minutes to days there is no evidence for nonperiodic (cometary) brightness changes at the level of a few millimagnitudes
Meteorite Source Regions as Revealed by the Near-Earth Object Population
No full textInternational audienceSpectroscopic and taxonomic information is now available for 1000 near-Earth objects, having been obtained through both targeted surveys (e.g. [1], [2], [3]) or resulting from all-sky surveys (e.g. [4]). We determine their taxonomic types in the Bus-DeMeo system [5] [6] and subsequently examine meteorite correlations based on spectral analysis (e.g. [7],[8]). We correlate our spectral findings with the source region probabilities calculated using the methods of Bottke et al. [9]. In terms of taxonomy, very clear sources are indicated: Q-, Sq-, and S-types most strongly associated with ordinary chondrite meteorites show clear source signatures through the inner main-belt. V-types are relatively equally balanced between nu6 and 3:1 resonance sources, consistent with the orbital dispersion of the Vesta family. B- and C-types show distinct source region preferences for the outer belt and for Jupiter family comets. A Jupiter family comet source predominates for the D-type near-Earth objects, implying these "asteroidal" bodies may be extinct or dormant comets [10]. Similarly, near-Earth objects falling in the spectrally featureless "X-type" category also show a strong outer belt and Jupiter family comet source region preference. Finally the Xe-class near-Earth objects, which most closely match the spectral properties of enstatite achondrite (aubrite) meteorites seen in the Hungaria region[11], show a source region preference consistent with a Hungaria origin by entering near-Earth space through the Mars crossing and nu6 resonance pathways. This work supported by the National Science Foundation Grant 0907766 and NASA Grant NNX10AG27G.[1] Lazzarin, M. et al. (2004), Mem. S. A. It. Suppl. 5, 21. [2] Thomas, C. A. et al. (2014), Icarus 228, 217. [3] Tokunaga, A. et al. (2006) BAAS 38, 59.07. [4] Hasselmann, P. H., Carvano, J. M., Lazzaro, D. (2011) NASA PDS, EAR-A-I0035-5-SDSSTAX-V1.0. [5] Bus, S.J., Binzel, R.P. (2002). Icarus 158, 146. [6] DeMeo, F.E. et al. (2009), Icarus 202, 160. [7] Dunn et al. (2010) Icarus 208, 789. [8] Dunn et al. (2013) Icarus 222, 273. [9] Bottke, W.F. et al. (2002), Icarus 156, 399. [10] DeMeo, F., Binzel, R. P. (2007) Icarus 194, 436. [11] Gaffey, M. J. et al. (1992) Icarus 100, 95
The surface properties of small asteroids: Peculiar Betulia - a case study
No full textWe present the results of extensive thermal–infrared observations of the C-type near-Earth Asteroid (1580) Betulia obtained in June 2002 with the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Betulia is a highly unusual object for which earlier radiometric observations, interpreted on the basis of simple thermal models, indicated a surface of high thermal inertia. A high thermal inertia implies a lack of thermally insulating regolith. Radiometric observations of other asteroids of comparable size indicate that regolith is present in nearly all cases. Knowledge of the surface thermal properties of small near-Earth asteroids is crucial for meaningful calculations of the Yarkovsky effect, which is invoked to explain the delivery of collisional fragments from the main belt into near-Earth orbits, and apparently has a significant influence on the orbital evolution of potentially hazardous near-Earth asteroids. Furthermore, apart from being an indicator of the presence of thermally insulating regolith on the surface of an asteroid, the thermal inertia determines the magnitude of the diurnal temperature variation and is therefore of great importance in the design of instrumentation for lander missions to small asteroids. In the case of Betulia our database is sufficiently broad to allow the use of more sophisticated thermal models than were available for earlier radiometric observations. The measured fluxes have been fitted with thermal-model emission continua to determine the asteroid's size and geometric albedo, pv. Fits obtained with a new thermophysical model imply an effective diameter of 4.57±0.46 km and an albedo of 0.077±0.015 and indicate a moderate surface thermal inertia of around 180 J m−2 s−0.5 K−1. It is difficult to reconcile our results with earlier work, which indicate a larger diameter for Betulia and a high-thermal-inertia surface of bare rock
Mineralogical analysis of the Eos family from near-infrared spectra
No full textInternational audienceThe aim of this work is to analyze the mineralogy of the Eos family, which exhibits considerable taxonomic diversity. Its biggest fragment, (221) Eos has previously been associated, through direct spectral comparisons, with such diverse mineralogies as CV/CO and achondrite meteorites [Burbine, T.H., Binzel, R.P., Bus, S.J., Clark, B.E., 2001. Meteorit. Planet. Sci. 36, 245-253; Mothé-Diniz, T., Carvano, J.M., 2005. Astron. Astrophys. 174, 54-80]. In order to perform such analysis we obtained spectra of 30 family members in the 0.8-2.5 mum range, and used three different methods of mineralogical inference: direct spectral comparison with meteorites, intimate mixing using Hapke's theory, and fitting absorption features with the MGM. Although the direct comparison failed to yield good matches---the best candidates being R-chondrites---both mixing model and MGM analysis suggest that the bulk of the family is dominated by forsteritic ( Fa) olivine, with a minor component of orthopyroxene. This composition can be compatible with what would be expected from the partial differentiation of a parent-body with an original composition similar to ordinary chondrites, which probably formed and differentiated closer to the Sun than the present location of the family. A CK-like composition is also possible, from the inferred mineralogy, as well as from the similarities of the spectra in the NIR
Spectroscopy of K-complex asteroids: Parent bodies of carbonaceous meteorites?
No full textThis 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. © 2009 Elsevier Inc. All rights reserved
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