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DIOGENITE-LIKE FEATURES in the SPITZER IRS (535Pm) SPECTRUM of 956 ELISA

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DIOGENITE-LIKE FEATURES in the SPITZER IRS (535Pm) SPECTRUM of 956 ELISA DIOGENITE-LIKE FEATURES IN THE SPITZER IRS (535pm) SPECTRUM OF 956 ELISA. Lucy F. Lin~', Joshua P. Emery2,Nicholas A. b'lo~kovitz,~]Code 691. NASAIGoddard Spice Flight Center([email protected]), ?~arthand Plandarj Sciences Dcpt & Planetary Geoscience Inst., University of Tennessee Knoxville, 'University of Hawaii IfA We report preliminary results from the Spitzer IRS (,'InfraredSpectrograph) observations of the V-type aster- oid 956 Elisa. Elisa was observed as part of a campaign to measure the 5.2-38 micron spectra of smdl basaltic asteroids with the Spitzer IRS. Targets include members of the dynamical family of the unique large differcilti- ated asteroid 4 Vesta i"Vesroids"), several outer-main- helt basaltic asteroids whose orbits exclude them fiwm originating on 4 Vesta, and the basaltic near-Earth aster- oid 4055 Magellan. 4 Vesta's dynamical hmily of small basaltic asteroids is believed to be the proximate stturce of the HF,I) mete- orites, the majority of which are thought to have their ultimate origin on Vesta itself [I, 21 . Hubhle imaging of Vesta [3] has reveiilrd nn impact hasin large enough ID zz 460 km) to be the source of all known Vestoids. Figure 1: Spitzer 1RS spectnim of 956 Elisa with STM Spectral studies of Vestoids in the visible and near-IR fit ovcrplotted. have characterized their olivine and pyroxene composi- tions and revealed significant differences between HED and Vestoid populations. For example, about 25% of HED meteorites are diogenites, although to date, obser- Preliminary Assessment of Spectral Features: The vations of diogenite-like near-IR spectra among Vestoids spectral emissivity variation of the asteroid can then be have been very rare: whereas a number of vestoids [e.g. found by dividing the asteroid's spcctrum by the thermal 2579 Spartacus: 41 appear to have olivine-rich compo- continuum. The IRS emissivity spectrum of 956 Elisa is sitions not represented among meteorites. Similar NIR illustrated in Fig. 2. The waveiengrh of the Christiansen features suggest regional olivine exposures on Vesta it- feature cemissivity mirximum just shortward of 9 pm), self l5.61.., . the positions and shapes of the narrow maxima I10 pm, 1 1 pm) within the broad 9-14 pm silicate band, and the Thermal Model Fitting: To model the thermal con- 19-20 pm are similar to features ibund in the tinuum of 956 Elisa (Fig. 1). we apply a variant of the laboratory of diogenites [8] and of orthopyrox. Standard Thennal Model 1 "STM. Lebofsky et al. 1986) enes similar in composition to diogenitic orthopyroxene to the 1RS emission spectra of the asteroids. In each of iY1 Howardites, which diogenitic clasts, also our fits, just two bee parameters are allowed to float: the display features, ;ilkit at substantidly lower spec. radius of the asteroid and its maximum (subsolar-point) 3), temperature, T,, = i (1 l -albedo)(insolation))I ( qco ))'"" Several features of the IRS spectrum have yet to where o is the Srefm-Boltzmann constant, r is the rffec- It remains uncertain whether the tivc emissivity, and 11 is the "beaming parameter'., rep- 14.5 pm peak is red or an instance of the IRS "teardrop" resenting a combination of thrrmophysical propmies of ifa act known to occur sporadicall). at this wavelength, the surface 1e.p.- thermal inertia) and surface roughness.. The preliminary STM fit to the 5-35 krm spectnlm of Comparison with Near-IR Spectra: The near-IR spec- 956 Elisa gives a radius of 5.4 -i- 0.3 km and a suhsoiar- tum of 956 Elisa is not consistent with a predominantly point temperature of 182.2 i 0.5 K. This temperature diogcnitic global siirljce composition. The temperature- corresonds to I) Z 1.06 + 0.02, which is substantially corrected band centers at 0.9252 and 1.9592 pm corre- higher than the 7 1. 0.756 characterisic of large main- spond to Fs and Uh molar contents of 32 -t 3 and 7 f helt itsteroids [:]. Unlike 1 Vesra and other large asler- 1 respectively. placing the NIR spectrum rnerall within oids, therefore, 956 Elisa has significant thermal inertia the howardite range. However, because of the very great in its surface layer. variation in rriid.fR spectral contrast !Fig. 31 with corn- Figure 2: Spitzer IRS specttum of 956 Elisa in emissiv- Figure 3: Laboratory spectra of HE11 meteorites and a ity, with laboratory spectra of HED meteorites and rele- hypersthene similar to the orthopyroxenes that dominate vant minerals [8,91. In this plot, the spectral contrast of the composition of diogenites 18. 91. This plot illus- the laboratory spectra has been varied in order to make trates the significant differences in the spectral contrast the w;~velengths of features apparent. (See Fig. 3 for of the 8-14 and 16-24 micron features among samples. the relative spectral contrasts of the lalnlratory spectra. I Spectral contrast is much greater in hne-grlned diogen- "Coarse" indicates lab samples of 75-250 pm particle ires than in fine-grained howardites or eucrites. Coarse- size: "Fine" describes samples of 0-75 pm particle size. grained orihopyroxenes, represented here by a terrestrial sample of diogenite-like composition [the "hypzrsthene" !?6.l% .MgO, 1.3% CaO) of 91, hiwe still stronger spec- tral fealures. position and particle size, the mid-IR spectrum may still be dominated by diogenitic features even if the overall composition is not. The Opx-rich material may be coarse in grain size compared with the majority of the asteroid. 4 Vesta: Hubhle Space Telescope results. Science. 277: 1492-1495, 1997. Alternatively, it may be concentrated close to the subso- 141 T H. Burhine. P. C. Buchanan. R. P Binzel, S. J. Bus. lar point at the time of observation, as the warmest parts T. tliroi. I. I.. Hinrichs. A. Meibom. and T. I. Mc- of the asteroid will he overrepresented in any thermai IR Coy. Vesta. Vestoids. and the howardite. eucrite. diogenite spectrum. group: Relationships and the origin of spectml differences. ,Meteoritics & Planer07 Science. 36:761-781. June 2Wl. 15) M. J. Gaffey. SurfaceLithologicHeterogeneity uf Asteroid This work is based [in part] on observations made J Vesta. Icor~is.127:130-157. May 1997. with the Spitzer Space Telescope, which is operated /6] R. P Binzel. M. J. Gaffey. P. C. Thomas. B. H. Zellner. by the Jet Propulsion Laboratoty, California Institute of A. D. Storrs. and E. N. WeIIs. Geologic Mapping uf Vesm Technology under a contract with NASA. Support for from 1994 Hubble Space Telescope Images. Icn,~rs. 128: this work was provided by NASA through an award is- 95-103. July 1997. dci: 10.10Oh/icai.l0975714, sued by JPUCaItech. [7j I. R. Spencer. A rough-surface thermophysical model for References airless planets. Irarur. 832-38. January 1990, doi: 10. 101610019-1035l9~~90004-S. [lj T. B. McCord. J. B.Adams. and T,V Johnson. Asteroid [8] J, W Salisbury. D. M. Dr\ria. snd E, Jarosewich. Niidin- Vesta: Spectral reflectivity and compositional implications. frmed 12.5-13.5 microns! reflectance specea of powdered Scicnrr. 1?8:745--747. 1970. stony meteorites. Icants. 92:280-297. August 1991 121 R. P Blnzel and S. Xu. Chips off of asteroid J Vesta - Evi- [9] J. W. Salisbury. D. M. U'ha. 1.. S. Walter. and N. Vergo. dence for the parent body of basaltic achondritc meteorites. Infrared (2.1- 25 micromcrerr) Spectra "f'Mineruk. Johns Sciencr. 2hO:lXh-191. April 1993. fiupkins University Press. 1991 [?I P <:.Thomas. R. I? Xinrel. M, J. Gaffey,A. 11. Stons, E. N. Wells. and 8. if. 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