XHE S-TYPZ AND 'XEE ,ORDINARY UfIIONDBITES : 'LHE (8)FLORA CASE. Michael I. Gaf fey, Planetary Geosciences, Hawaii Institute of Geophysics, University of Hawaii, 2525 Correa Road, Eonolulu, Hawaii 96822. The S-type asteroids are a class defined on albedo, polarization, color, and spectral ariteria [lj, which constitcte a large fraction.of the members of the inner portion of the belt. The S-type objects exhibit spectral reflectance curves generally interpreted as indicative of mixtures of , pyroxene, and nickel- metal. The history and meteoritic analogues of these assemblages has been the subject of considerable controversy, sae investigators suggesting that the surface materials of these asteroids are analogous to differentiated. stony-iron meteorite assemblages [2-53, and others suggesting that these asteroids are analogon* to undifferentiated, ordinary chondrite-like assemblrgos [6-81. These two opposed interpretations have important and profoundly different implications for: (a) the sources of the noteorites, (b) the evolutionary history of inner belt aateroids, and (c) the selectivity and intensity of post-accretzonary heating events in tho early solar lystsm. Flora is an S-type asteroid which has classification parametera near the median for the S-type. It is 156 kilanetcrs in diameter, andsir the largest body in the dynmical nor8 fmily located in the inner belt. A recent investigation of 11 S-type asteroids aoncluded that they were probably undifferentiated objects, and that Flora was the most ordinary chondrite-like of that sample [81. Observations obtained with the W 2.2 meter telescope atop huna Lea in December, 1981 ahw that the spectral reflectance curve of Flora varies with rotation. Although rotational perioda of ~8.834and ~9.011hours are not grossly inconaistont with our lightcurve data, a period of z13.653 hsurs best matcher I.II,..,~~.,...I~~ tho data. The five spectral curves I shorn as the points with error bars Figure 1 (21~)on Figures 1 and 2, represent five different rotational aspects frm 1.10 - li htcurve maximum (A) through minimum (D! and during the rapid rise from minimum to maximum (El. [In a11 cares the overall average is plotted as the 1.05 - solid line for reference.] The depth of the lp band varies from 9-84, at maximum light (Pig. 1A) to =4% midway 'A between maximum and minimum light (Fig. 1.00 - 1B). The 2pm band is most pronoun~ed and centered at longeat wavelength near li8htcurve maximum. The validity of the observed rotationrl spectral variations o.rs 1 is supported by duplicate observations in the lpm region, and by similar variations (in sense and magnitude) for 1.00 the J and K filters at similar lifitcurve aspects from independent observations [5 1. These spectral variations are much larger and in the incorrect sense with respect to the lightcurve variations than can be x explained by body shape sf fects [91. 2 C Reoently derived cal ibrations from , mineral abundance in 01ivine-py roxene 6 mixtorea [lo] and for the nature of the metal phase [Ill have been utilized to interprrt tha Flora spectral curve and variations. Even when an Z0.025pm 0.1 ibration error between the instrnmenta used in the old laboratory measurements md in these observationa is taken into account, the pyroxene phaso(s) present in the surface layer of Flora either are quite iron-rich or inolude an abundant cl inopyroxene phasr. The strong metallic NiFe E 8i~aaturein the Flora spectrum (and oharacteristia of the S-type) cannot be prodocod f roa tho (spectrally distinct) wtal phase in ordinary chondritstype aasomblaces without invokina ver~ iipiauriihe regolith proces~rs. - 0.90' , I B I n. widence of which should be present (but is not) in aae of the chondritic 0.8 1.0 broocias. Iatolon~lh(pm) .

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System THE (8) FLORA CASE

Gaffey, M.J.

Five mineralogic and petrologic parameters of the surface material of the typical S- type asteroid, (8) Flora, have been derived from interpretatio~of the spectral reflectance aad rotational spectral variations of this object. These include: bulk mineralogy INiFe metal, olivine, pyroxene I, mafic mineral abundance [ol/py :: 2.81, metal natnre [abondant(=505) and coarse-grained in the substratelr;'pyroxene composition [opx ) Fs and/or abandant cpxl. and spatial mineralogic variations C olivine/pyroxene ratio ango pyroxene composition that do not covary in a chondritic manner]. All of the relevant and diagnostic mineralogic properties of the Flora surface material indicate that this is a differentiated body. None of the derived surface properties unambiguously support a chondritic-type material and several contradict any type of.undifferentiated assemblage. Flora is most probably the residual core of a strongly heated, thermally evolved, and magmatically differentiated (or extensively partially melted) planetesimal which was robsequently disrupted. The present surface samples layers formed at and near the core- .amtle boundary in the parent body. Several strong lines of evidence suggest that the S-type asteroids as a class are .predominantly the core fragments of disrupted, differentiated planetesimals. It seams most probable that an efficient, radially dependent heating neckanism was active in the early solar systm producing the magmatically differentiated parent bodies from which the S, E mad Ptype asteroids, which dominate the innermost belt [12l, are derived. Vests, with its brraltic surface, is unique not in its thermal history bat in being intact. A main belt source of the ordinary chondrites rsmains elusive. It is plausible that ordinary chondritic assemblages are present among the smallest, and most poorly charaoterized, members of the S group, which would be distinguishable from the real !+type by nearly flat VJHK colors neglecting the silicate absorption bands. It is also plausible that ordinary chondritio assemblages actually are absent or extremely rare in the main bmlt . L~~li~~..~~~Qll.l*~~~i Various aspects of this work were rapported by NASA grants NSG 7462, : 1.4 - 7312, and 7323. Figure 2 C WEBENCES: [I1 Bowel1 E. .978, Jcarus 35, 315-335. WCord B. and Gaffey H.J. (1974) Science @, 352-355. [3l Chapman C.R. (1976) ~oohim. Cosmochim. Acta 40, 701-719. II Gaffey M.J. and McCord T.B. (1978) lace Sci. Rev, 21, 555-628. 151 (lv8) As r n J 83, I;%39?&Zellner et :lo '(lh7) ;eo. Junar Sci. Conf. &&: pp. 1091- .lo. [71 Anders E. (1978) in Beroids: An Ex~lorationAssessment 1. Morrison and W.C. Wells, Eds.) pp. '-78. NASA CP-2053. [81 Feierberg M.A. (1982) Astrouhvs. J, 257, il-372. [9] Gradie J. and Veverka J. 1981) u.Lunar P . u. Conf. u,pp. ~76rn~Wcloutis E. (1984) Ic rus, to be submitted. +Gaffey M.J. ?19114) Icaru~.to be tbaitted. [I21 Gradie J. and Tedesco , (1982) Science 216, 1405-1407.

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