Elemental Composition of 433 Eros: New Calibration of the NEAR-Shoemaker XRS Data

We present the results of a new calibration of the elemental-abundance data of the asteroid 433~Eros taken by the X-ray Spectrometer (XRS) aboard the NEAR-Shoemaker spacecraft, which orbited Eros from 2000--2001. The XRS measured solar-induced X-ray fluorescence from the top 100~μm of the surface of Eros. The relationship of the measured fluorescence line ratios to asteroidal elemental abundance ratios is critically dependent on the incident solar X-ray spectrum, which varies rapidly in time. The data from NEAR's surviving onboard solar monitor proved difficult to calibrate, and the previously published elemental results [1] relied on an imperfect preliminary calibration that resulted in significant and incompletely characterized systematic uncertainties. The solar monitor response and associated uncertainties have now been characterized by cross-calibration of a large sample of NEAR solar flight data against contemporary broadband solar X-ray data from the GOES-8 satellite. The results have been used to produce new X-ray spectra and errors for eight major solar flares (five of which were covered in [1]) during which the NEAR XRS collected data from 433~Eros. The revised surface elemental ratios for 433~Eros include new error estimates that accurately reflect the remaining uncertainties in the solar flare spectra. The new elemental ratios fall within the cited uncertainties of the previous results [1]. and are therefore consistent with the conclusions [1,2] that 433~Eros has an ordinary chondritic composition altered in S/Si and possibly Fe/Si at the surface by space weathering. The 1--10 keV NEAR solar X-ray spectra were successfully matched by a two-temperature model using isothermal components generated by the CHIANTI code [3]. This approach may be applicable to the analysis of data from the Hayabusa XRS, particularly in the analysis of the S/Si ratio, since Hayabusa's onboard standard lacked sulfur [4]. This will enable comparison of Itokawa's S/Si value with Eros' highly depleted S/Si. This approach may also be helpful in analysis of data from the MESSENGER XRS. [1] Nittler et al.~(2001) M&PS 36, 1673. [2] Foley et al.~(2006) Icarus 184, 338. [3] Landi et al.~(2006) ApJS 162, 261. [4] Okada et al.~(2006) Science 312, 1338.