Takehiko Arai

X-ray Fluorescence Spectrometry of Asteroid Itokawa by Hayabusa

X-ray fluorescence spectrometry of asteroid 25143 Itokawa was performed by the x-ray spectrometer onboard Hayabusa during the first touchdown on 19 November 2005. We selected those data observed during relatively enhanced solar activity and determined average elemental mass ratios of Mg/Si = 0.78 ± 0.09 and Al/Si = 0.07 ± 0.03. Our preliminary results suggest that Itokawa has a composition consistent with that of ordinary chondrites, but primitive achondrites cannot be ruled out. Among ordinary chondrites, LL- or L-chondrites appear to be more likely than H-chondrites. No substantial regional difference was found on the asteroid surface, indicating its homogeneity in composition.

Instrumentation and performance evaluation of the XRS on SELENE orbiter

The x-ray fluorescence spectrometer (XRS) on board Japanese lunar polar orbiter SELENE (Kaguya) will provide global distribution of major elemental composition on the lunar surface in energy range of characteristic K-α x-ray line emission for Mg, Al, Si, Ca, Ti, and Fe. These measurements will contribute to research of lunar origin and its evolution. The XRS shows a good energy resolution within 200 eV at 5.9 keV relying on charge coupled device (CCD) as photon energy dispersive detector. Total collective area of 100 cm2 for main detector facing the lunar surface is composed of 16 CCD chips. Instrumentation of the XRS and its performance evaluated in laboratory are presented.

X-ray fluorescence/diffraction analyzer for the SELENE-B lander/rover mission

Abstract A miniaturized X-ray fluorescence and diffraction analyzer is being developed for the SELENE-B, a future Japanese lunar lander and rover mission, to analyze major elemental composition and mineralogy. An onboard micro X-ray tube with a fine focus collimator is used to generate primary X-rays that excite fluorescence X-rays characteristic of major elements in collected samples; the primary X-rays are also scattered into X-ray diffraction pattern reflecting lattice structures of the component minerals. By using a two-dimensional charge-coupled device, pulse height analysis for XRF and diffraction pattern extraction for XRD will be simultaneously carried out. The instrument covers an energy detection range from 1 to 10 KeV and measures diffraction angles from 20 to 60 degrees for elemental and mineral analysis, respectively. We show the results of laboratory experiments conducted with alumina powder.

Sulfur abundance of asteroid 25143 Itokawa observed by X-ray fluorescence spectrometer onboard Hayabusa

The Japanese Hayabusa spacecraft successfully carried out in situ observations of S-class asteroid 25143 Itokawa, including the surface major elemental analysis with the X-ray fluorescence spectrometer (XRSHayabusa). Our previous results for the X-ray experiments (Okada et al., 2006a) indicated that major elemental ratios of Mg/Si and Al/Si on the surface of Itokawa resemble ordinary LL- or L-chondrites more than any other meteorite analogues. In the NEAR Shoemaker observations of S-class asteroid 433 Eros, the results of X-ray fluorescence observations indicated the depletion of sulfur, probably reflecting impact-induced volatilization, photoor ion-induced sputtering at the surface, or the loss of FeS-rich materials due to partial melting. Here, we determined the elemental abundance of sulfur (S) on the surface of Itokawa, in addition to that of Mg, Al, and Si, and its regional variation using XRS-Hayabusa observations. In particular, we carefully corrected the fluctuation of solar X-rays, variation of surface geometry, and sensor response function in this analysis, and thus we believe that the results are more accurate than those of our previous report. In this study, the upper and lower limits for Mg/Si, Al/Si, and S/Si overlap those of meteorite analogues for ordinary chondrites or primitive achondrites. In terms of the major elemental composition, Itokawa is best classified as a ordinary chondrite or a primitive achondrite. Our models do not include the mineral mixing effects. With the effects, the abundance of sulfur is expected to be 30% lower than our results. Hence, we conclude that the abundance of sulfur on the surface of Itokawa is almost equal to or even lower than the average abundance in ordinary chondrites. Although the abundances for Mg and Si are globally homogeneous, best-fit or upper limits of mass fraction for Al and S vary in local areas. There is a negative correlation (−0.92) for Al/Si vs. S/Si in ten facets. In particular, the area with the lowest sulfur, accompanied with enriched aluminum, is found in Arcoona, close to a cratered area. Therefore, aluminum enrichment and sulfur depletion features may support events of partial melting on the parent body of Itokawa or aluminum-rich material impacts on the surface of Itokawa. In some areas, Itokawa has a brighter geometric albedo and color variation. Little altered, fresh material may be exposed in these portions of the surface. The sulfur abundance on the surface appears to vary between little and highly altered areas by space weathering. Thus, the sulfur regional variation in our result may reflect the heterogeneity of a surface altered by space weathering.

An evaluation method of reflectance spectra to be obtained by Hayabusa2 Near-Infrared Spectrometer (NIRS3) based on laboratory measurements of carbonaceous chondrites

We conducted ground-based performance evaluation tests of the Near-Infrared Spectrometer (NIRS3) onboard Hayabusa2 spacecraft in November 2013 and from April to May 2014 and established a method for evaluating its measured reflectance spectra. Reflectance spectra of nine powdered carbonaceous chondrite samples were measured by both NIRS3 and a Fourier transform infrared (FT-IR) spectrometer. We have established two methods for correcting the NIRS3 data by comparing them with the corresponding FT-IR data because raw data obtained by NIRS3 underwent spectral distortion caused by systematic offsets in sensitivity of individual pixels. The corrected NIRS3 spectra of carbonaceous chondrite samples are comparable with their FT-IR spectra. The depth of each band component Dλ is defined for each wavelength λ (μm) to characterize the absorption bands in NIRS3 spectra. It is suggested that the relationship between the D2.72/D2.79 ratio and the D2.76/D2.90 ratio would be useful for estimating the degree of heating of the asteroid surface, if contributions of terrestrial adsorbed water on D2.79 and D2.90 are properly corrected. The degrees of heating and space weathering are also comprehensively evaluated by the relationship between D2.90 and the D2.76/D2.90 ratio. Reflectance spectra of asteroid Ryugu, the target asteroid of Hayabusa2, to be recorded by the NIRS3 instrument are expected to reveal the characteristics of the surface materials by using the evaluation technique proposed in this paper. Such information will be used for choosing the touchdown points for sampling and also for investigating the distribution of the materials similar to the returned samples on Ryugu.Graphical abstract.

Retraction. X-ray fluorescence spectrometry of asteroid Itokawa by Hayabusa.

IN OUR REPORT “X-ray fluorescence spectrometry of asteroid Itokawa by Hayabusa” (1), we analyzed the major elemental ratios of asteroid 25143 Itokawa with the x-ray spectrometer (XRS) onboard the Hayabusa spacecraft. We used an improper analytical procedure, which resulted in erroneous identification of x-ray fluorescence peaks and statistically insignificant conclusions. Therefore, we retract the paper. Our XRS team confirmed that our original analysis was inaccurate in parallel with both the steering committee of the Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, and an investigation team appointed by the institute. Figure 1 of our Report shows the x-ray spectrum, which is fitted with some x-ray fluorescence peaks and continuum components. The figure shows the smoothed data. The smoother x-ray spectrum was used in the model fitting (statistical analysis). We should have used the raw data and examined it in detail before proceeding to the modeling. In our data analysis, x-ray photon energy calibration (relation of detector channel number to x-ray photon energy) was conducted using the observed data, because the instrument had no onboard calibration source, such as Fe radioactive nuclide. The observed raw data was smoothed to identify the expected x-ray fluorescence peaks effectively. In the smoothed profile, we detected two apparent intense peaks. We erroneously identified the peaks as the anticipated x-ray fluorescence of Mg and Si. The energy calibration formula we obtained differed from the formula we had obtained in our pre-flight ground testing; we attributed the difference to a drift of the XRS instrument parameters due to temperature variation of the instrument. However, recent detailed investigation of the past data shows that it is more appropriate to adopt a different formula for the energy calibration. Furthermore, the peak we identified as the x-ray fluorescence of Mg is likely to be an artificial peak from the instrument. Thus, the x-ray photon energy axis of Figure 1 was incorrect. Our reinvestigation revealed another error. In the model fitting to obtain the major elemental abundance ratio of the target asteroid Itokawa, we analyzed the smoothed data, not the raw data, which resulted in inflated statistical significance. In a properly done elemental analysis, the signal-noise ratio is too low to support our conclusion of chondritic elemental abundance. Tatsuaki Okada,* Kei Shirai, Yukio Yamamoto, Takehiko Arai, Kazunori Ogawa, Kozue Hosono, Manabu Kato