Chandrayaan2: latest updates

November 13, 2019

Terrain Mapping Camera-2 (TMC-2) is a follow-on of the TMC on-board Chandrayaan-1. TMC-2 provides images (0.4μm to 0.85μm) at 5m spatial resolution & stereo triplets (fore, nadir and aft views) from a 100 km orbit for preparing Digital Elevation model (DEM) of the complete lunar surface.

The triplet images from TMC-2 when processed into Digital Elevation Models, enable mapping of surface landform morphologies. These include

• Craters (formed by impactors)
• Lava tubes (potential sites for future habitability)
• Rilles (furrows formed by  lava channels or collapsed lava tubes)
• Dorsa or wrinkle ridges (formed mostly in Mare regions depicting cooling of and contraction of basaltic lava)
• Graben structures (depicts the structural dislocations on the lunar surface )
• Lunar Domes/ Cones (denoting localized vents of past volcanism on the Moon).

The derived information facilitates estimation of dimensions of above features and its comparison for reconstructing the morpho-structural framework, crater characterization to derive impact geometries, surface age determination through Crater Size –Frequency Distribution (CSFD) methods, Rheological analysis based on the derived morphometric parameters, Lunar reflectance estimation etc.

October 31, 2019

Detection of Argon-40 in the lunar exosphere

Planetary scientists prefer to call the thin gaseous envelope around the Moon as the ‘Lunar exosphere’ since it is so tenuous that the gas atoms very rarely collide with each other. While the Earth’s atmosphere near the mean sea level contains ~1019 atoms in a cubic centimetre of volume, the lunar exosphere contains ~ 104 to 106atoms in a cubic centimetre.

Argon-40 (40Ar), which is one of the isotopes of the noble gas Argon, is an important constituent of the lunar exosphere. It originates from the radioactive disintegration of Potassium-40 (40K), which has a half-life of ~1.2 X 109 years. The radioactive 40K nuclide, which is present deep below the lunar surface, disintegrates to 40Ar, which, in turn, diffuses through the intergranular space and makes way up to the lunar exosphere through seepages and faults.

Schematic of the origin and dynamics of 40Ar in lunar exosphere

October 22, 2019

Initial imaging and observations by Chandrayaan-2 Dual-Frequency Synthetic Aperture Radar (DF-SAR)

Moon has been continuously bombarded by meteorites, asteroids and comets since its formation. This has resulted in the formation of innumerable impact craters that form the most distinct geographic features on its surface. Impact craters are approximately circular depressions on the surface of the moon, ranging from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. The study of the nature, size, distribution and composition of impact craters and associated ejecta features reveal valuable information about the origin and evolution of craters. Weathering processes result in many of the crater physical features and ejecta material get covered by layers of regolith, making some of them undetectable using optical cameras. Synthetic Aperture Radar (SAR) is a powerful remote sensing instrument for studying planetary surfaces and subsurface due to the ability of the radar signal to penetrate the surface. It is also sensitive to the roughness, structure and composition of the surface material and the buried terrain.

Previous lunar-orbiting SAR systems such as the S-band hybrid-polarimetric SAR on ISRO’s Chandrayaan-1 and the S & X-band hybrid-polarimetric SAR on NASA’s LRO, provided valuable data on the scattering characterisation of ejecta materials of lunar impact craters. However, L & S band SAR on Chandraayan-2 is designed to produce greater details about the morphology and ejecta materials of impact craters due to its ability of imaging with higher resolution (2 - 75m slant range) and full-polarimetric modes in standalone as well as joint modes in S and L-band with wide range of incidence angle coverage (9.5° - 35°). In addition, the greater depth of penetration of L-band (3-5 meters) enables probing the buried terrain at greater depths. The L & S band SAR payload helps in unambiguously identifying and quantitatively estimating the lunar polar water-ice in permanently shadowed regions.

A convenient approach towards discerning the radar information is to prepare images using two derived parameters, ‘m’ the degree of polarization and ‘δ’ the relative phase between the transmit-receive polarized signals. These parameters are used to generate colour composite images with ‘even-bounce’, ‘volume or diffused’ and ‘odd-bounce’ scatterings of a pixel represented in red (R), green(G), and blue (B) image planes, respectively. The genesis of the scattering mechanism is as illustrated in Figure 1.

Figure 1: Conceptual diagram explaining different types of Radar scattering mechanisms on lunar surface and sub-surface

Figure 2 is one of the m- δ decomposition images from the first datasets acquired over lunar south polar regions in L-band high-resolution (2mslant-range resolution) hybrid polarimetric mode. It is important to note that the obtained resolution is one-order better than the earlier best by a lunar-radar. This image presents many interesting facts about the secondary craters of different ages and origins in the lunar south polar region. The yellowish tone around crater rims in the image shows ejecta fields. The distribution of ejecta fields, whether uniformly distributed in all directions or oriented towards a particular side of a crater, indicates the nature of the impact.  The image shows craters of vertical impact and oblique impact on the top-right and bottom-right, respectively. Similarly, the roughness of the ejecta materials associated with the impact craters indicates the degree of weathering a crater has undergone. Three similar sized craters along a row on the bottom-right of the image show examples of young crater, moderately weathered crater and an old degraded crater. Many of the ejecta fields seen in the image are not visible in high-resolution optical image over the same region, indicating the ejecta fields are buried beneath regolith layers.

Figure 2

Chandrayaan-2 Orbiter’s DF-SAR has been operated in full-polarimetry mode- a gold standard in SAR polarimetry, and is the first-ever by any planetary SAR instrument. Figure 3 shows an L-band fully-polarimetric, 20m slant-range resolution image of Pitiscus-T crater. The image is a colour composite of different transmit-receive polarization responses of the imaged region.