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O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System GENETIC LINK BITWEEN the NYSA and HERTHA ASTEROID FAMILIES: Kelley, M

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O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System GENETIC LINK BITWEEN the NYSA and HERTHA ASTEROID FAMILIES: Kelley, M COMPOSITIONAL EVIDENCE IN FAVOR OF A GENETIC LINK BETWEEN THE NYSA AND HERTHA ASTEROID FAMILIES; M. S. Kelley and M. J. Gaffey, Dept. of Earth & Env. Sci., Rensselaer Polytechnic Institute, Troy, NY 12180-3590; J.G. Williams, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91 109 High resolution spectral data has been obtained for 135 Hertha using the 52-channel double CVF (0.8-2.5pm) system at the Infrared Telescope Facility on Mauna Kea. The spectrum of Hertha exhibits a narrow, weak feature centered near 0.9pm indicating the presence of at least a small amount of silicate material on its surface. To date, this spectral feature has not been seen in published spectra of other M-types. 44 Nysa also exhibits a similar narrow absorption feature near 0.9pm [I] which is not seen in the high resolution spectra of other E-type asteroids (e.g. 3103 [2]). This distinct spectral feature, indicating a unique pyroxene composition, makes both these asteroids anomalous within their taxonomic classes. This distinctive feature also provides the genetic link in their mineralogy. Probable compositions of Nysa and Hertha can be reconciled with plausible compositions of F-types in this region and with extensive igneous processing of a parent body with E-chondrite starting composition. Previously, several investigators have suggested a possible connection between the Nysa and Hertha families. Zellner, et al. [3] suggested that 44 Nysa is the largest surviving fragment of a silicate crust, broken away from the iron-core body 135 Hertha. However, they were unable to volumetrically reconcile these two objects with an enstatite-chondrite starting composition for the parent body. Gradie, et al. [4] also favored a connection and suggested ways to resolve the volumetric considerations. Williams [5] considered these to be separate families, but admitted that the close location of Nysa and Hertha suggests a connection. Other investigators have argued against a common origin for these families. Bell [6] considered Nysa to be an interloper in its own family and Hertha and its family to be completely unrelated. These conclusions were primarily based on taxonomic classifications of family members. Chapman, et al. [7] and Granahan and Bell [8] reached the same conclusions again based almost solely on taxonomic classifications. Recent additions of higher numbered objects to the previously identified [9] Nysa and Hertha families show a definite structure in the distribution of objects in this region. With a diameter of 57 kilometers, 142 Polana is the largest known F-type asteroid in this region, and it is second in size to 44 Nysa (73 kilometers) in the Nysa family. At least two distinct cratering events off of 142 Polana are identifiable. One of these cratering events has yielded a string of objects extending from Polana to the 3:l Jupiter resonance. All of the objects in this structure that have been taxonomically identified are F-types. Spectral data (primarily ECAS) available for these objects show no obvious inconsistencies. However, the similarity between some asteroids (e.g. Nysa and Hertha) only becomes apparent in the subtle features of high resolution data. Taxonomy-based studies [3-8,101 are an important first step in making general statements about asteroid families and they often point out intriguing directions for one to explore. For instance, part of the interest in the Nysa-Hertha region is the high concentration of F-asteroids relative to the background population, and their apparently odd association with M- and E- types. However, considering the mineralogical variability that exists within just one class of asteroids, the S-types [ll], use of taxonomy is generally inadequate for testing genetic relationships between potential members of asteroid families. Therefore, arguments against the genetic reality of asteroid families based on their apparent inconsistencies between taxonomy of members may be invalid. The only rigorous test of the genetic reality of an asteroid family is one based on mineralogical characterization of the proposed family members and development of plausible thermal history and differentiation models to support or rule out membership. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System GENETIC LINK BITWEEN THE NYSA AND HERTHA ASTEROID FAMILIES: Kelley, M. S., et al. If the Nysa and Hertha families originated from a common parent body, that body must have been heated sufficiently to allow a core (Hertha) to separate from silicate mantle (Nysa) material. During the differentiation process, it is reasonable to expect that partial melts (basalts) would be extruded onto the surface of the body. To date no basaltic objects have been identified in the Nysa-Hertha region. Therefore, this material has either been lost, or it is somehow going undetected. It has been suggested that this material could have been lost to space by energetic eruptions during differentiation of the parent body [12,13]. At the high temperature required to melt a body of E-chondrite composition abundant CO would have been produced. CO produced in magma at depth in the parent body would have been converted to C02 at lower pressure as the gas migrated toward the surface. This may have resulted in the deposition of carbon in the cooler, primitive material near the surface. Subsequent disruption of the parent body would have yielded M- (core), E- (mantle), and F-type (carbon-darkened, E- or F-chondrite) objects. In order to resolve this issue, rotational spectral coverage and visible and thermal infrared lightcurves need to be obtained for 44 Nysa, 135 Hertha and 142 Polana. This level of detail is required to distinguish spectral variations due to body shape from those due to lithologic variations on the asteroid surfaces. Understanding the arrangement of family member surface lithologies allows one to constrain the internal compositional structure of the parent body. In addition, high resolution spectral data in the visible to near infrared are needed to characterize several F-type objects inside and outside the Nysa-Hertha region. ACKNOWLEDGMENTS: Portions of this work were supported by NSF Solar System Astronomy grant AST-9012180, NASA Planetary Geology and Geophysics grant NAGW-642, and an Ernest F. Fullam Award from The Dudley Observatory. M. J. Gaffey and M. S. Kelley are Visiting Astronomers at the Infrared Telescope Facility which is operated by the University of Hawaii under contract to the National Aeronautics and Space Administration. Thanks to Pranoti M. Asher for the use of a portable computer. REFERENCES: [I] Gaffey M.J., Bell J.F. and Keil K. (in preparation). [2] Gaffey M.J., Reed K.L. and Kelley M.S. (1992) Icarus, 100, 95-109. [3] Zellner B., Leake M., Morrison D. and Williams J.G. (1977) Geochim. Cosmochem. Acta, 41, 1759-1767. [4] Gradie J.C., Chapman C.R. and Williams J.G. (1979) in Asteroids, U. Arizona Press, pp. 359-390. [5] Williams J.G. (1992) Icarus, 96, 251-280. [6] Bell J.F. (1989) Icarus, 78, 426-440. [7] Chapman C.R., Paolicchi P., Zappala V., Binzel R.P. and Bell J.F. (1989) in Asteroids 11, pp. 386-415. [8] Granahan J.C. and Bell J.F. (1993) lcarus (submitted). [9] Williams J.G. (1989) in Asteroids 11, pp. 1034-1072. [lo] Gradie J.C. (1978) Ph.D. dissertation, Univ. of Arizona [I11 Gaffey M.J., Bell J.F., Brown R.H., Burbine T.H., Piatek J.L., Reed K.L. and Chaky D.A. (1993) lcarus (in press). [12] Wilson L. and Keil K. (1991) Earth and Planet. Sci. Let., 104, 505-512. [13] Wilson L. and Keil K. (1991) Lunar and Planet. Sci. XXII, 1515-1 51 6. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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