Validation of methodology to derive elemental abundances from X-ray observations on Chandrayaan-1

The Chandrayaan-1 X-ray Spectrometer (C1XS) observed characteristic X-ray fluorescence (XRF) lines of major rock forming elements from the lunar surface during different solar flare conditions. As the Sun was undergoing an unusually low activity cycle during the observing period of C1XS (28th Nov. 2008-29th Aug. 2009), most observations were made during weak flares. To derive the lunar surface chemistry, observed XRF line fluxes were converted to corresponding elemental abundances. Line flux dependencies like incident solar spectra (I _o), geometry of observation and matrix effects are taken into consideration while converting line fluxes to abundances. We describe our approach which uses the concept of flux fraction to derive abundances of major elements (weight percentages >1%). The standard Fundamental Parameter (FP) method is employed. The paper also discusses the validation of our method using laboratory experiments with samples of known composition. We demonstrate our ability to derive elemental abundances with only a few wt% errors from remote sensing data.