The Chandrayaan-2 Dual Frequency Synthetic Aperture Radar (DFSAR)

Context: Scientists from the Physical Research Laboratory (PRL), Ahmedabad, used observations from Chandrayaan-2’s Dual Frequency Synthetic Aperture Radar (DFSAR) to discover evidence of subsurface water-ice in the Moon’s South Polar Region.

About The Chandrayaan-2 Dual Frequency Synthetic Aperture Radar (DFSAR):

What It Is?

• The Dual Frequency Synthetic Aperture Radar (DFSAR) is a state-of-the-art microwave imaging instrument operating onboard the active Chandrayaan-2 lunar orbiter. It holds the technological distinction of being the first fully polarimetric Synthetic Aperture Radar (SAR) ever deployed to study the lunar surface.

Aim:

• The primary objective of DFSAR is to map and investigate the lunar topography, surface roughness, and subsurface material composition.
• It specifically aims to explore permanently shadowed regions (PSRs) and doubly shadowed craters at the poles to identify, quantify, and map volatiles like water-ice, which are crucial for long-term human survival and fuel production on the Moon.

How It Works?

Unlike optical cameras that need visible light, DFSAR maps the Moon by emitting its own microwave signals and listening to the echoes that bounce back:

• Signal Penetration: The radar transmits structured L-band and S-band microwave radio frequencies. These long wavelengths pass directly through top-layer surface dust to interact with hidden, subsurface materials.
• Volumetric Reflection: When the waves hit pocketed subsurface ice, they bounce around internally (volumetric scattering). This changes the wave properties before reflecting back to the orbiter.
• Polarimetric Profiling: The instrument measures two critical properties of the returned echo: the Circular Polarization Ratio (CPR) and the Degree of Polarization (DOP). DOP measures how much of the reflected signal retains its original polarization state after the collision.
• Signature Separation: Rocky terrains can easily mimic ice by scattering signals wildly. DFSAR isolates genuine water-ice by filtering for regions where a high CPR (CPR > 1) matches an ultra-low DOP (DOP < 0.13), providing a highly refined blueprint for mapping subterranean volatiles.

Key Features:

• Dual-Frequency Versatility: Operates across both L-band and S-band frequencies, allowing for varying depths of subsurface penetration and high-resolution imaging capability.
• Fully Polarimetric Matrix: Collects complete polarimetric radar returns simultaneously, giving scientists a clear look at the physical geometry and orientation of subsurface targets.
• Non-Line-of-Sight Imaging: Functions entirely independently of solar illumination, enabling it to look inside dark craters that have been frozen in darkness for billions of years.
• High-Contrast Resolution Mapping: Capable of generating detailed, high-contrast radar maps to distinguish fine differences between fine lunar soil (regolith), jagged boulders, and embedded ice blocks.

Recent Discoveries Made:

• Subsurface Polar Ice Substream: Successfully confirmed the potential presence of subsurface water-ice beneath the floors of four distinct doubly shadowed craters in the lunar South Polar Region.
• The Faustini Crater Discovery: Identified strong, concrete evidence of subsurface ice inside a small, 1.1 km diameter doubly shadowed crater (F2) located within the larger Faustini crater matrix (87.39^OS, 82.31 ^OE)
• Lobate-Rim Morphological Confirmation: Discovered a distinct, flow-like lobate-rim morphology around the 1.1 km crater. This structural feature suggests that the original meteor impact cracked through a solid sheet of subsurface ice, melting and shifting the rim into a unique lobed pattern during the impact.
• Extreme Cold Preservation Proof: Mapped these ice signatures to interior crater pockets where temperatures remain pinned around an ultra-cold 25K (~ -248°C), proving that these doubly shadowed zones act as ideal deep-freezers for preserving volatile resources over long geological timescales.