Study of lunar surface chemistry using swept charge devices

Abstract

The study of origin and evolution of the Moon have generated scientific curiosity for a long time. With the improved technology, with larger telescopes and better scientific instruments the lunar surface got revealed and created more research interests. The surface exploration of Moon gained momentum soon after the samples brought back from the Apollo and Luna missions. Since then, various orbiting remote-sensing experiments explored the lunar surface at multi-wavelengths. This thesis deals with one such study of understanding the chemical composition of the lunar surface using x-rays. This provides a unique opportunity to sample the top few micron thick layer of the lunar surface which undergoes severe modifications and mixing due to different physical/chemical processes. Context : Since the Apollo era, there have been very few successful observations of the x-ray studies of the lunar surface. Spectral resolution of some of the instruments was badly affected due to charged particles. Chandrayaan-1 X-ray Spectrometer (C1XS) onboard Chandrayaan-1 was launched on October 2008 to study surface elemental abundances with a spatial resolution of 25 km on the lunar surface. C1XS was designed to map the abundances of major rock-forming elements viz., Mg, Al, Si, Ca, Ti and Fe on the lunar surface using the X-ray fluorescence (XRF) technique. C1XS was the first remote sensing x-ray experiment to spectrally resolve the x-ray lines of all major rockforming elements from the Moon simultaneously under different solar flare conditions. It used ‘swept charge devices (SCD)’, a variant of x-ray CCDs as detectors. Description : In this thesis, we present a summary of entire C1XS observations. Steps involved in data selection, spectral analysis procedures viz., x-ray background estimation and modeling incident solar x-ray spectrum for certain flare observations are described in detail. We describe the spectral observations of all major rock-forming elements as seen by C1XS. The first spectral evidence of Sodium (Na) from the Moon as observed by C1XS is demonstrated in the thesis. Elemental abundances are derived from the observed x-ray line intensities. vii My thesis work addresses the development of XRF inversion algorithm x2abundance for remote-sensing measurements to convert XRF line intensities to abundances with uncertainties. Validation of x2abundance using laboratory XRF experiments with metal alloys and lunar analogue rocks are discussed in detail in Chapter 2. In Chapter 3, we show the spectral analysis of C1XS data, where we extract the XRF line fluxes of major-rock forming elements. We demonstrate the first direct evidence of x-ray signature of Na from the Moon. We determine the abundances of lunar surface elements including Na using x2abundance. Results from C1XS abundance analysis exploring the lunar surface geochemistry are discussed in detail in this chapter. However, global geochemistry of the Moon could not be completely studied by C1XS due to lack of solar x-ray activity and limited mission life-time. Chandrayaan-2 Large Area Soft x-ray Spectrometer (CLASS) is being developed for Chandrayaan-2 to answer the questions raised by C1XS results and complete global mapping with enhanced sensitivity using new generation SCDs (CCD-236). In fourth chapter, we describe a detailed physical model developed to simulate x-ray photon interaction in SCDs. This Monte Carlo simulation aims at modeling device level interactions to better understand the spectral redistribution function of SCDs. Algorithm of the model, implementation and comparison with C1XS ground calibration data are presented here. This model will be used during the ground calibration of CLASS experiment. In the last chapter, we summarize the major findings from this thesis work. We discuss the scientific importance of the discovery of Na on the lunar surface. A description on science cases for future lunar surface studies is presented here. Further, to carry out better scientific investigations in future, we propose a few design aspects which can potentially improve the surface exploration of the Moon/airless planetary bodies in xrays.