SAR-based Investigation of Water Ice in Lunar Polar Permanently Shadowed Regions (PSRs)

THE nature and distribution of deposits in the lunar PSRs have long been subjects of scientific debate. Although the Lunar Crater Observation and Sensing Spacecraft (LCROSS) impact experiment provided evidence for the existence of cold-trapped volatiles, Arecibo monostatic radar and the Clementine bistatic radar experiment for ice in polar craters using radar backscatter turned up no evidence for macroscopic water ice within 1 m of the surface^[1].

Since 2008, orbiters equipped with synthetic Aperture Radar (SAR) instruments have advanced our understanding of the PSRs in the lunar polar regions through microwave remote sensing, these include Chandrayaan-1 Mini-SAR, Lunar Reconnaissance Orbiter (LRO) Mini-RF and Chandrayaan-2 Dual Frequency Synthetic Aperture Radar (DFSAR) ^[2]. The Mini-SAR is a S-band single-frequency, hybrid polarity imaging radar designed to collect information about the scattering properties of the permanently dark areas near the lunar poles^[3]. The LRO Mini-RF combines SAR at two wavelengths (S-band and X-band) and utilizes hybrid polarization architecture to measure the Stokes parameters of the reflected signal^[4]. DFSAR sensor is the first to operate at L-band and S-band in fully and hybrid polarimetric modes^[5].

Over the past decade, the exploration of water ice in the lunar polar regions using SAR data has been steadily advancing^[6-17]. The possibility of surface ice clusters has been investigated by including CPR and PolSAR-based scattering properties in the PSRs. The important information related to roughness patterns, dielectric constant, subsurface rock abundance and composition have also been analyzed while identifying the regions containing water-ice deposits. Radar backscatter was analyzed for the effect of surface roughness, whereas polarimetric parameters were used for identifying the scattering mechanism. By decoupling the effects of surface roughness, the dielectric constant of the lunar surface is inverted.

In the research, we will employ the Chandrayaan-2 L-band SAR data to establish a multi-polarimetric radar echo model for lunar surface/subsurface characterization, with surface roughness effects decoupled to retrieve subsurface dielectric constants. Model validation will be conducted using Apollo and Chang'E-5/6 sample constraints. Subsequently, the validated approach will be implemented on Chang'E-7's L-band polarimetric SAR to analyze the polarimetric signatures and dielectric properties in PSRs. Additionally, a multi-instrument synthesis (spectroscopy, neutron spectrometry, and thermophysical data) will enable comprehensive assessment of water ice presence with quantitative estimates of its abundance, composition, and distribution.

 

References:

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