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Science  12 Sep 2014:
Vol. 345, Issue 6202, pp. 1254
DOI: 10.1126/science.345.6202.1254-a

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 55Fe 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.


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