Authors– Deepak Putrevu, Tathagata Chakraborty and DFSAR team, Space Application Center, ISRO Ahmedabad.

Radar studies of the lunar surface have so far been restricted to Earth-based radio telescopes and Moon-orbiting sensors operating with limited polarimetry configuration. The Dual-frequency Synthetic Aperture Radar (DFSAR) onboard Chandrayaan-2 (Ch-2) orbiter is the first ever state-of-the-art high-resolution radar system in the lunar orbit. To this date, the DFSAR instrument operated in fully polarimetric (FP) configuration during ten imaging seasons (spanning 5 years), to acquire more than fifteen hundred datasets, imaging significant amount (~80%) of lunar polar regions (80°-90° N/S) (Figure 1) and some important targets in non-polar regions. The datasets are publicly available in ISRO portal for Planetary Science data (https://pradan.issdc.gov.in).

The DFSAR datasets are processed to achieve several key results envisaged from the mission as mentioned below.

• Characterization of radar scattering from various lunar impact craters to study crater degradation process.

• Estimation of electrical (dielectric constant) and physical (surface roughness) properties of lunar surface.
• Derivation of potential water ice bearing regions in the shadowed regions of the lunar poles.
• Radar characterization of volcanic features such as impact melts, pyroclastic deposits.
• Studying subsurface buried features like subsurface ejecta, melt flow, crypto-mare.
• Assessment of presence of volatiles and landing hazards in the lunar polar regions for defining future landing sites.

The results obtained from DFSAR helped to address some of the most fundamental questions in lunar science related to the physical properties of the lunar volcanic terrain, volatiles (especially water ice), geological evolution of impact craters and their associated melt and ejecta deposits, etc. In particular, the salient results obtained from DFSAR based studies are as follows.

Figure 1: Circular Polarization Ratio (CPR) maps derived from full-pol (FP) L-band DFSAR data for north polar region (left), and south polar region (right) from 80°-90° N/S latitude band.

Using DFSAR data, the presence of centimeter-to-decimeter scale surface roughness has been established as the major reason for anomalous nature of many of the craters in polar regions of the Moon, thereby, rectifying the previous understanding on this. The findings reveal that the crater degradation (aging) process controls the change in the physical nature of the targets present in various impact craters leading to deviation in the radar scattering behavior. Earlier radar based evidences on occurrence of water ice in the lunar poles were ambiguous because of similar radar scattering behavior of water ice and surface roughness. However, using DFSAR datasets, decoupling of the radar signatures from these two factors has been achieved. The results suggest potential presence of water ice in some of the polar anomalous craters situated in Peary and near Rozhdestvenskiy W crater in north pole, and in Shoemaker, Faustini, Haworth, Idel’son and Cabeus crater in south pole (Figure 2).

Figure 2: Map showing potential water ice bearing craters in the lunar north (left) and south pole (right). The background map is the CPR mosaic derived from DFSAR L-band FP data.

Using high-resolution DFSAR data collected over Chandrayaan-3 Vikram landing site, prior- and post-landing, fine-scale characterization of surficial changes has been demonstrated. This data revealed that the lander descent resulted in changes in regolith (upper layer) spread over an area of about 177 m² surrounding the landing site.

Lunar impact melt deposits have unique physical properties compared to other lunar terrain: they are very rough similar to those of terrestrial blocky and rubbly lava flows at the centimeter- to decimeter-scale, as interpreted from DFSAR data.

Reference:

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