Abstract-We report major element ratios determined for the S-class asteroid 433 Eros using remotesensing x-ray fluorescence spectroscopy with the near-Earth asteroid rendezvous Shoemaker x-ray spectrometer (XRS). Data analysis techniques and systematic errors are described in detail. Data acquired during five solar flares and during two extended "quiet Sun" periods are presented; these results sample a representative portion of the asteroid's surface. Although systematic uncertainties are potentially large, the most internally consistent and plausible interpretation of the data is that Eros has primitive Mg/Si, AI/Si, Ca/Si and Fe/Si ratios, closely similar to H or R chondrites. Global differentiation of the asteroid is ruled out. The S/Si ratio is much lower than that of chondrites, probably reflecting impact-induced volatilization and/or photo-or ion-induced sputtering of sulfur at the surface of the asteroid. An alternative explanation for the low S/Si ratio is that it reflects a limited degree ofmelting with loss of an FeS-rich partial melt. Size-sorting processes could lead to segregation of Fe-Ni metal from silicates within the regolith of Eros; this could indicate that the Fe/Si ratios determined by the x-ray spectrometer are not representative of the bulk Eros composition.

We report major element composition ratios for regions of the asteroid 433 Eros imaged during two solar flares and quiet sun conditions during the period of May to July 2000. Low aluminum abundances for all regions argue against global differentiation of Eros. Magnesium/silicon, aluminum/silicon, calcium/silicon, and iron/silicon ratios are best interpreted as a relatively primitive, chondritic composition. Marked depletions in sulfur and possible aluminum and calcium depletions, relative to ordinary chondrites, may represent signatures of limited partial melting or impact volatilization.

The Apollo orbital geochemistry, photogeologic, and other remote sensing data sets were used to identify and characterize geochemical anomalies on the eastern limb and farside of the Moon and to investigate the processes responsible for their formation. The anomalies are located in the following regions: (I) Balmer basin, (2) terrain northeast of Mare Smythii, (3) near Langemak crater, (4) Pasteur crater, (5) terrain northwest af MiIne basin, (6) northeast of Hendefeev basin, (7) north and northeast of Koroiev basin, (8) terrain north of Taruntius crater, and (9) terrain north of Orientale basin. The anomalies are commonly associated with Imbrian-or ~ec~a~~an-aged light plains units which exhibit d~k-Hanoi impact craters. The resuffs of recent spectral reflectance studies of dark~a~ued impact craters plus consideration of the surface chemistry of the anomalies strongly indicate that those geochemical anomaties associated with light plains deposits which display dark-haloed impact craters result from the presence of basaltic units that arc either covered by varying thickness of highland debris or have a surface contaminated with significant amounts of highlands material. The burial or contamination of ancient volcanic surfaces by varying amounts of highland material appears to have been an important (though not the dominant) process in the formation of lunar light plains. Basaltic volcanism on the eastern limb and farside of the Moon was more extensive in both space and time than has been accepted. MethodDigital versions of the various orbital chemistry data sets were obtained and utilized in this study. The newly revised Al]% and -Mg/Si data derived using the technique described by Clark and Hawke (1981) were utilized. The digital irtrn and titanium data used were those presented' by Davis fI%U). The tho~~m abundances are those presented by Metzger ef raf. (1977). In 3ddjtjo~, images s~aw~~~ Fe and Ti abundances as determined by Metzger and coworkers (Metzger and Parker, 1979; Haines and Metzger, X950) were provided by I% A. E. Metzger. Selected Geochemical ArmnaliesORIGIN OF SELECTED LUNAR GEOCHE~ICAL ANOMALIES 259 LUNAR GEOCHEMfCAL ANOMALIES 271