31.0 LPSC XVIII

SURFACE LITHOLOGIC HETEROGENEITY AND BODY SHAPE FOR (15) EUNOMIA: EVIDENCE FROM ROTATIONAL SPECTRAL VARIATIONS AND MULTI-COLOR LIGHTCURVE INVERSIONS. Michael J. Gaffey, Department of Geology, Rensselaer Polytechnic Institute, Troy, New York, 12181; and Steven J. Ostro, Jet Propulsion Laboratory, Pasadena, California, 91109.

An understanding of the nature of the S-type is fundamental to our models of the conditions and processes in earliest solar system history. If the S-type objects are composed of undifferentiated assemblages, analogous to the ordinary chondrites, then the the compositional gradient across the would represent the fossil signature of the temperature gradient within the solar nebula. If, however, as now seems probable, the S-type objects represent metal-rich internal layers of strongly heated and magmatically differentiated parent planetesimals, the compositional gradient in the asteroid belt is primarily a function of the earliest post-accretionary heating processes. These different interpretations provide profoundly different constraints on our models of the early solar system.

The asteroid (15) Eunomia, at -270km diameter, is the largest member of the S-class of minor . Eunomia was observed in December, 1981 as part of a survey of asteroidal rotational spectral variability. Dual-beam photometer (24 channels, 0.33-l.Oum, 101 obs). and CVF (120 channels, 0.6-2.6um. 66 obs.) spectra were obtained which covered the corplete rotational cycle. This data has been analyzed to detect the presence of any rotational spectral variability, to define the nature of any gross lithologic heterogeneity across the surface, and to derive the mean cross section [l] of the asteroid at each wavelength. It was hoped that the correlation of gross lithology with body shape - or the lack thereof - would shed light on the interior structure of Eunomia's parent planetesimal.

The average normalized reflectance spectrum (relative to two solar-like standard ) is shown on figure 1. This spectral curve exhibits the reddened slope (high 1-2.5um reflectance, lum band minimum reflectance > 1.0) and the 1 and 2um mafic silicate absorption features which characterize the S-type spectra. This curve indicates subequal metal and silicate abundances [2,3]. The ratio of the areas of the 1 and 2um absorptions is 0.58fl.05, indicating an average surface abundance of pyroxene in the mafic silicate fraction of 29+2% (px/ol+px) using a calibration for -orthopyroxene mixtures [&I. The average surface material is a metal-olivine assemblage with pyroxene as a lesser phase.

The spectra at different rotational phases exhibit small but systematic variations in: I. spectral slope (-6% in the 0.40/~.81)um reflectance ratio), 11. band depth (-3% in both bands), 111. band area ratios (0.33 to 0.68), and IV. Upositions.

These spectral variations exhibit the wrong sign for spectral phase effects due to a non-spherical body shape [5], and therefore must represent the effects of surface variegation. The net effect of the shape induced spectral variations on this body is to decrease the of the rotational spectral variations. At the present level of analysis, the relative variations of the band area ratios, and of the 1 and 2um band positions are consistent with the presence of an olivine-orthopyroxene mafic component, but are not generally consistent with an undifferentiated, chondritic assemblage.

Eunomia exhibits a large (-0.34 mag) lightcurve variation (fig. 2). The convex profile (fig. 3) is the estimate of Eunomia's mean cross section at 0.56um under the assumption of: a) convexity, b) geometrical scattering, and c) an equatorial illumination/viewing geometry. The profiles for the lightcurves at all the available wavelengths have been obtained and are being compared quantitatively. With very few exceptions, the shapes of the profiles obtained from our different wavelengths are very similar to each other, and to the profiles obtained from the broad bandpass lightcurves published during the last three decades. The variance in the shapes of the profiles at different wavelengths is at the same level (

Current efforts are focused on exploring the possible small systematic differences between the convex profiles at different wavelengths as a means of constraining the

O Lunar and Planetary Institute Provided by the NASA Data System (15) EUNOMIA INVESTIGATION LPSC XVIII 311 Gaffey, M.J. and Ostro, S.J. longitudinal distribution of surface units. Effort is also focusing on exploring the viable range of surface scattering (i.e. non-geometric) and albedo properties which are consistent with the observed spectral lightcurve behavior.

At the present time, the following conclusions can be reached concerning the nature of the lithologic heterogeneity across the surface of Eunomia. Both "ends" of the asteroid are redder than the "sides", with the blunt end being the reddest region of the surface. The more pointed end - associated with the deeper lightcurve minimum - shows the deepest and shortest lum feature. The form of Eunomia's lightcurve (and the derived convex profile) is nearly - but not entirely - independent of wavelength. The form of any given lightcurve is due largely to the shape of Eunomia. The small differences between the multi-spectral lightcurves argues strongly for compositional variations as a function of longitude on the asteroid.

The research (MJG) at RPI was supported by NASA grant NAGW-642. Part of this research (SJO) was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.

References: 1. Ostro, S.J., and R. Connelly (1984) Icarus 57, 443-463. 2. Gaffey, M.J. (1984) Icarus 60, 83-114; 3. Gaffey, M.J. (1986) Icarus 66, 468-486; 4. Cloutis, E.A., et -al. (1986) JGR 91, 11,641-11,653; 5. Gradie, J. and J. Veverka (1981) Proc. Lunar . Sci. Conf. 12th pp. 1769-1779;

+ + I 1 I 1 0.5 1.0 1.5 2.0 2.5 0.0 0.2 0.4 0.6 0.8 1.0 Wavelength (urn) Rotational Phase Figure 1: Average spectral reflectance Figure 2: Lightcurve variation for Eunomia curve of asteroid (15) Eunomia. at 0.56um relative to the standard . Normalized to 0.56um. Key: +=12/29/81 X=12/30/81 a=12/31/81 data

Figure 3: Mean Equatorial cross section of Eunomia derived from the 0.56um lightcurve [I].

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System