Asteroid (354) Eleonora: Plucking an Odd Duck

Abstract During a survey of the S-type asteroids, Gaffey et al. (Gaffey, M.J., Bell, J.F., Brown, R.H., Burbine, T.H., Piatek, J., Reed, K.L., Chaky, D.A. [1993]. Icarus 106, 573–602) identified Asteroid (354) Eleonora as anomalous with a 1 μm absorption feature ∼2.5 times stronger than any S-asteroid of comparable size. Subsequent investigation revealed significant differences in the 1 μm absorption feature between the visible & very near-infrared CCD spectra ( λ 3 OS 2 ) of Eleonora. These differences could potentially arise from spectral variations across the asteroid surface, from observational phase angle differences, from surface temperature differences, from viewing geometry for a nonspherical body, or from the use of standard stars with deviated to different degrees from a true solar standard. In June 2011 Asteroid (354) Eleonora was observed over two nights using the NASA Infrared Telescope Facility (IRTF) at Mauna Kea Observatory in order to test these possible scenarios and to better understand the nature and history of Eleonora and its relationships to other asteroids and to the meteorites. Analysis of this data set has eliminated the following options as the cause of the differences in the 1 μm absorption feature within the CCD data sets and between the CCD data sets and the other spectral data: (1) rotational spectral variations; (2) variation in surface composition with latitude; (3) observation phase; (4) surface temperature variations with differing heliocentric distance in the asteroid’s elliptical orbit; (5) spectral effects of viewing geometry for a nonspherical body; and (6) differences in spectral standard stars. We conclude that none of the CCD spectra of (354) Eleonora are reliable, and that within the limits of their spectral coverage, analyses of the three CCD spectra would produce significantly different – and generally unreliable – indications of surface mineralogy. An effort needs to be made to determine whether “bad” CCD spectra are rare with the case of (354) Eleonora being an uncommon occurrence or whether there is a broader problem with the CCD asteroid survey data sets, and if so, how to identify the “bad” spectra. While CCD survey spectra show apparently irreconcilable differences, the near-infrared spectra of (354) Eleonora from various observers show only minor differences, primarily in the overall spectral slope, most of which can be attributed to slight differences in the standard stars used to calibrate the data. In June 2011, 226 near-infrared (∼0.76–2.5 μm) spectra of (354) Eleonora were obtained using the SpeX instrument on the NASA Infrared Telescope Facility at Mauna Kea Observatory. These spectra were consistent with the six sets of NIR spectra obtained for Eleonora by previous observers. The primary variation observed in this new data set was an approximately 10% variation in spectral slope between ∼0.8 μm and ∼1.6 μm during the rotation period of the asteroid. Mineralogically diagnostic spectral parameters extracted from this new data are most consistent with a surface assemblage of fine-grained intimately mixed olivine (∼60–70%, ∼Fo 61–71 ) and low nickel ( 66±5 ) by Sanchez et al. (Sanchez, J.A., Reddy, V., Kelley, M.S., Cloutis, E.A., Bottke, W.F., Nesvorný, D., Lucas, M.P., Hardersen, P.S., Gaffey, M.J., Abell, P.A., Le Corre, L. [2014]. Icarus 228, 288–300), but not with the estimate (∼Fo 90 ) of Sunshine et al. (Sunshine, J.M., Bus, S.J., Corrigan, C.M., McCoy, T.J., Burbine, T.H. [2007]. Meteorit. Planet. Sci. 42, 155–170). The surface assemblage appears to contain a small component (∼8–10%) of igneous pyroxene (weakly constrained at ∼Fs 50 Wo 10 ). The parent lithology of the surface regolith may be similar to a pallasite assemblage, although none of the three known types of pallasites are good mineralogical matches.

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