Circular Polarization Ratio Values Research Articles

Mini-RF S-band observation of Ohm and Stevinus craters using circular polarization ratio and m-chi decomposition techniques

The aim of the study was to observe two lunar impact craters of Copernican age, Ohm (18.4°N, 113.5°W) and Stevinus (32.5°S, 54.2°E) under microwave radar data of Miniature Radio Frequency (Mini-RF), an instrument onboard Lunar Reconnaissance Orbiter (LRO) of NASA, where Ohm is located on the far side of the Moon and Stevinus is situated on the near side of the Moon. We have analyzed the characters of impact ejecta melt of both the craters in radar data which are not evidently distinguished in the high resolution optical data of narrow angle camera (NAC) and wide angle camera (WAC) of LRO mission. Circular polarization ratio (CPR) and m-chi decomposition images were developed using ENVI and ArcGIS software which were used to understand surface roughness and backscattering properties of Ohm and Stevinus craters. Both the craters evidently have high CPR values indicating either exposure of fresh material or elevated surface roughness due to surface geometry. The m-chi decomposition of Ohm and Stevinus craters shows dominant yellowish hue suggesting a backscatter combination of double-bounce (db) scattering and volume scattering (vs) in contrast to the surrounding terrain which shows Bragg scattering (bs) according to the 7-fold classification colour-wheel. Using available optical and Mini-RF data geological maps of both the craters were generated including features such as boundary of ejecta blanket, ejecta boulders and mass wasting in the crater.

Unravelling the Mystery of Lunar Anomalous Craters Using Radar and Infrared Observations

In Miniature Radio Frequency (Mini‐RF) radar images, anomalous craters are those having a high circular polarization ratio (CPR) in their interior but not exterior to their rims. Previous studies found that most CPR‐anomalous craters contain permanently shadowed regions and that their population is overabundant in the polar regions. However, there is considerable controversy in the interpretation of these signals: Both water ice deposits and rocks/surface roughness have been proposed as the source of the elevated CPR values. To resolve this controversy, we have systematically analyzed >4,000 impact craters with diameters between 2.5 and 24 km in the Mini‐RF radar image and Diviner rock abundance (RA) map. We first constructed two controlled orthorectified global mosaics using 6,818 tracks of Mini‐RF raw data and then analyzed the correlations between radar CPR and surface slope, RA, and depth/diameter ratios of impact craters. Our results show that CPR‐anomalous craters are distributed relatively uniformly across the lunar surface, with no apparent difference in CPR between the polar, potentially icy, and nonpolar, not icy, craters. Most CPR‐anomalous craters are relatively young with a large depth/diameter ratio, and they actually represent an intermediate stage of crater evolution. Comparison with a two‐component radar scattering model suggests that rocks and surface roughness are major contributors to the observed CPR values. Using craters of 4.7–22 km in diameter with known ages, we find that craters spend up to 120 Ma with a high exterior RA, and ∼3 Ga in the CPR‐anomalous phase.

Chandrayaan-2 dual-frequency SAR: Further investigation into lunar water and regolith

The Space Applications Centre (SAC), one of the major centers of the Indian Space Research Organization (ISRO), is developing a high resolution, dual-frequency Synthetic Aperture Radar as a science payload on Chandrayaan-2, ISRO’s second moon mission. With this instrument, ISRO aims to further the ongoing studies of the data from S-band MiniSAR onboard Chandrayaan-1 (India) and the MiniRF of Lunar Reconnaissance Orbiter (USA). The SAR instrument has been configured to operate with both L- and S-bands, sharing a common antenna. The S-band SAR will provide continuity to the MiniSAR data, whereas L-band is expected to provide deeper penetration of the lunar regolith. The system will have a selectable slant-range resolution from 2m to 75m, along with standalone (L or S) and simultaneous (L and S) modes of imaging. Various features of the instrument like hybrid and full-polarimetry, a wide range of imaging incidence angles (∼10° to ∼35°) and the high spatial resolution will greatly enhance our understanding of surface properties especially in the polar regions of the Moon. The system will also help in resolving some of the ambiguities in interpreting high values of Circular Polarization Ratio (CPR) observed in MiniSAR data. The added information from full-polarimetric data will allow greater confidence in the results derived particularly in detecting the presence (and estimating the quantity) of water–ice in the polar craters.Being a planetary mission, the L&S-band SAR for Chandrayaan-2 faced stringent limits on mass, power and data rate (15kg, 100W and 160Mbps respectively), irrespective of any of the planned modes of operation. This necessitated large-scale miniaturization, extensive use of on-board processing, and devices and techniques to conserve power. This paper discusses the scientific objectives which drive the requirement of a lunar SAR mission and presents the configuration of the instrument, along with a description of a number of features of the system, designed to meet the science goals with optimum resources.

Investigation of tectonic processes in the lunar South Polar Region using Mini-SAR and other data

Studies focusing on the geological processes on the moon have found various tectonic features suggesting similar geological activity occurring on the Earth (Shwarz, 1928). Also, it can be inferred that meteoritic impact, cosmic and solar wind flux variability coupled with the interior changes of the Moon may have manifested in the form of tectonic activity on the moon. Lobate scarps which are generated from thrust faulting on the moon and are indicative of recent tectonic activity have also been identified in the sub-polar and equatorial regions (Watters et al., 2010). Alternatively, “grabens” which are indicative of extensional tectonics have also been identified at various locations in the South Polar Region (Kramer et al., 2012). Previous studies investigating tectonic activity have used optical data primarily to study lunar tectonics. But areas such as partially and completely shadowed crater interiors present in the polar regions can also contain unique features indicative of tectonic activity. The study therefore focuses on the identification of features indicating the presence of faults/lineaments/ fractures on the lunar topography using mini-SAR data from CHANDRAYAAN-1 mission as well as optical data from Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) from LROC mission. Mini-SAR imaging radar onboard CHANDRAYAAN-1 mission was the first mono-static lunar orbiting synthetic aperture radar (Spudis et al., 2009). MiniSAR data is primarily used to identify signatures suggesting the possibility of the presence of water ice within the completely shadowed regions of the lunar poles (Fa and Cai, 2013). Further, this data has been used to view various other geological features such as melt flows, crater ejecta blankets, secondary craters etc. (Saran et al., in press). Therefore, identification of distinct morphological features in shadowed as well as illuminated regions of lunar surfaces can be easily done using mini-SAR data (Spudis et al., 2009). The SAR technology can also penetrate the surface and therefore is an effective tool to look for hidden fault lines in the shadowed portions on the lunar regolith. Alternatively, other distinct surface and subsurface geological features have also been identified which can further help to study the process of impact cratering as well as understand the formation of secondary features developed post impact. Digital image processing of mini-SAR data using m-χ decomposition technique was done to achieve single, double, and multiple/diffuse backscattering contributions to view tectonically derived morphological features in detail (Saran et al., 2012). Fault lines overlying possible subsurface faults depicting presence of past or present seismic activity can be seen well after using m-χ decomposition technique. Other features such as rock/ debris avalanches within the inner crater walls were also identified lying perpendicular to the inferred fault lines on m-χ decomposition images within the South Polar Region. Slumping activity on the shadowed inner crater walls forming terraced structures can also be seen using m-χ decomposition images. These could have formed due to nearby impact shaking or seismic activity from underlying dislocation along fault planes making the material to slide down in single or multiple blocks. Circular polarization ratio (CPR) was another parameter calculated to infer surface roughness in the regions having the tectonically derived features. Circular polarization ratio gives the ratio of the same sense circular polarization backscatter power received as emitted over the opposite sense backscatter power received as emitted (Calla et al., 2013). The inferred rock/ debris avalanche associated with tectonic activity have high values of CPR ranging between 0.1–2.7, which indicates the presence of highly rough surfaces. Such type of mass wasting activity can also be seen on terrestrial hilly terrains being formed after seismic shaking of unconsolidated material. Microwave remote sensing therefore helps to reveal a number of geological features on varying terrains otherwise not visible on optical data. Mini-SAR data makes it easy to view several of such unique features over the lunar surface both in the illuminated and the shadowed regions. The identification of such features can help to further understand the past and present geological activity taking place resulting in the formation of such features. It can also explain the physics behind the process of impact catering and the subsequent formation of other secondary structures. Further the results obtained can be correlated and compared with similar features seen on terrestrial surfaces. High CPR values derived from Stokes vectors and degree of polarization can primarily be attributed to the rough and rugged surfaces mainly found on the inner walls of impact craters and on the outer

Synergetic use of SAR and Thermal Infrared data to study the physical properties of the lunar surface

The surface layer of the Moon preserves vital evidences of lunar impact and cratering processes due to the absence of any Aeolian and fluvial erosion processes acting on it. By examining these evidences, which are recorded throughout the evolutionary history of the Moon, several basic aspects of lunar science can be understood, and this has direct relevance to the surfaces of other airless bodies within the solar system. In this study, rock abundance data obtained from Thermal Infrared (TIR) observations and radar Circular Polarization Ratio (CPR) data sets obtained from polarimetric SAR observations were correlated at some sample sites on the lunar surface. Preliminary results yielded qualitative and quantitative estimates for surface rock abundances. Except at distal ejecta deposits of young, bright craters a general correlation was observed between the two datasets. Mixed results were observed from the impact melt flows where the situation is complex due to the possible subsurface-volume and volume-subsurface interactions of the radar waves. But the flow features were clearly separated from the interior and ejecta regions of their parent craters in terms of CPR and rock abundances. The extent and distributions of pyroclastic deposits and dark haloed regions could not be distinctly identified at the resolution of datasets utilized. Near Gerasimovich D crater, the Diviner Radiometer has provided the first TIR observations of a newly discovered impact melt flow which was not visible in the optical imagery. This facilitated the first ever quantitative comparisons of the radar CPR and rock abundance values near such a region. Also, significant differences in spatial patterns between the radar and rock concentration data sets were observed, owing to the differences in the sensitivity of the two observations.

Evidence for water ice on the Moon: Results for anomalous polar craters from the LRO Mini‐RF imaging radar

The Mini‐RF radar instrument on the Lunar Reconnaissance Orbiter spacecraft mapped both lunar poles in two different RF wavelengths (complete mapping at 12.6 cm S‐band and partial mapping at 4.2 cm X‐band) in two look directions, removing much of the ambiguity of previous Earth‐ and spacecraft‐based radar mapping of the Moon's polar regions. The poles are typical highland terrain, showing expected values of radar cross section (albedo) and circular polarization ratio (CPR). Most fresh craters display high values of CPR in and outside the crater rim; the pattern of these CPR distributions is consistent with high levels of wavelength‐scale surface roughness associated with the presence of block fields, impact melt flows, and fallback breccia. A different class of polar crater exhibits high CPR only in their interiors, interiors that are both permanently dark and very cold (less than 100 K). Application of scattering models developed previously suggests that these anomalously high‐CPR deposits exhibit behavior consistent with the presence of water ice. If this interpretation is correct, then both poles may contain several hundred million tons of water in the form of relatively “clean” ice, all within the upper couple of meters of the lunar surface. The existence of significant water ice deposits enables both long‐term human habitation of the Moon and the creation of a permanent cislunar space transportation system based upon the harvest and use of lunar propellant.

Study of scattering characteristics of lunar equatorial region using Chandrayaan-1 Mini-SAR polarimetric data

The miniaturized synthetic aperture radar Mini-SAR onboard Indian Chandrayaan-1 mission was the first ever lunar orbiting SAR that acquired several data strips covering a wide variety of geological units over lunar equatorial and low latitude regions, some of which were not studied earlier at radar wavelengths. The Mini-SAR observations, complemented by high resolution optical imagery and higher incidence angle radar datasets, were effectively used to create a catalog of SAR backscatter properties of various lunar geological features. The radar backscatter along with the parameters circular polarization ratio (CPR), relative phase (δ) and m (the degree of polarization)–chi (the Poincare ellipticity) decomposition technique were used to study the scattering mechanisms, surface/sub-surface roughness and regional topography of some of the craters and their ejecta fields. The study revealed that the Taylor and Descartes craters in the lunar highlands region were characterized by high backscatter and low CPR values, while the Maunder and Kopff craters in the Mare Orientale basin were characterized by relatively low backscatter and elevated CPR values. The fresh crater Jackson and its ejecta blankets on the lunar far-side showed very high backscatter along with elevated CPR values due to the presence of abundant wavelength-scale scatterers. The radar dark Pyroclastic Orientale ring deposits showed both low backscatter and low CPR values, as reported by previous radar observations of pyroclastic deposits. The elevated CPR values corresponding to the interior and exterior of Santos Dumont crater could be attributed to rough crater walls and regional topography, respectively. Finally, attempt was made to bring out the differences in the origin of two similar-sized craters Taylor and Kopff, with the help of SAR polarimetric parameters. Differences in the CPR values of Mare and highland craters were attributed to the sensitivity of CPR to the ilmenite content and presence of surface/sub-surface rocks with diameter of about one-tenth the radar wavelength and larger. This could act as a potential method to distinguish craters of different origin and (or) composition, where the information regarding relative age and regolith composition are not available.

The nature of lunar volatiles as revealed by Mini-RF observations of the LCROSS impact site

[1] On 9 October 2009 the Lunar Crater Observation and Sensing Satellite (LCROSS) impacted Cabeus crater, located near the south pole of the Moon. Prior to that impact, the Mini-RF instruments on ISRO's Chandrayaan-1 and NASA's Lunar Reconnaissance Orbiter (LRO) obtained S band (12.6 cm) synthetic aperture radar images of the impact site at 150 and 30 m resolution, respectively. These observations show that the floor of Cabeus has a circular polarization ratio (CPR) comparable to or less than the average of nearby terrain in the southern lunar highlands. Furthermore, <2% of the pixels in Cabeus crater have CPR values greater than unity. This observation is not consistent with the presence of thick deposits of nearly pure water ice within a few meters of the lunar surface, but it does not rule out the presence of small (<∼10 cm), discrete pieces of ice mixed in with the regolith. In addition, Mini-RF on LRO acquired a postimpact S band image of the region surrounding the LCROSS impact site, providing important geologic context for the site. Registering the LRO image to a near-infrared (NIR) image taken by the LCROSS shepherding spacecraft, we find that the impactor landed in the ray of a fresh, radar-bright, 1 km crater. However, the difference between preimpact and postimpact images is not above the speckle noise. This implies that the size of the LCROSS impact crater is less than Mini-RF's resolution (30 m), and/or that the impact did not excavate more decimeter-size blocks than were already present at the impact site.

Earth-based 12.6-cm wavelength radar mapping of the Moon: New views of impact melt distribution and mare physical properties

We present results of a campaign to map much of the Moon’s near side using the 12.6-cm radar transmitter at Arecibo Observatory and receivers at the Green Bank Telescope. These data have a single-look spatial resolution of about 40 m, with final maps averaged to an 80-m, four-look product to reduce image speckle. Focused processing is used to obtain this high spatial resolution over the entire region illuminated by the Arecibo beam. The transmitted signal is circularly polarized, and we receive reflections in both senses of circular polarization; measurements of receiver thermal noise during periods with no lunar echoes allow well-calibrated estimates of the circular polarization ratio (CPR) and the four-element Stokes vector. Radiometric calibration to values of the backscatter coefficient is ongoing. Radar backscatter data for the Moon provide information on regolith dielectric and physical properties, with particular sensitivity to ilmenite content and surface or buried rocks with diameter of about one-tenth the radar wavelength and larger. Average 12.6-cm circular polarization ratio (CPR) values for low- to moderate-TiO 2 mare basalt deposits are similar to those of rough terrestrial lava flows. We attribute these high values to abundant few-centimeter diameter rocks from small impacts and a significant component of subsurface volume scattering. An outflow deposit, inferred to be impact melt, from Glushko crater has CPR values near unity at 12.6-cm and 70-cm wavelengths and thus a very rugged near-surface structure at the decimeter to meter scale. This deposit does not show radar-brightness variations consistent with levees or channels, and appears to nearly overtop a massif, suggesting very rapid emplacement. Deposits of similar morphology and/or radar brightness are noted for craters such as Pythagoras, Rutherfurd, Theophilus, and Aristillus. Images of the north pole show that, despite recording the deposition of Orientale material, Byrd and Peary craters do not have dense patterns of radar-bright ejecta from small craters on their floors. Such patterns in Amundsen crater, near the south pole, were interpreted as diagnostic of abundant impact melt, so the fraction of Orientale-derived melt in the north polar smooth plains, 1000 km farther from the basin center, is inferred to be much lower.

Initial results for the north pole of the Moon from Mini‐SAR, Chandrayaan‐1 mission

We present new polarimetric radar data for the surface of the north pole of the Moon acquired with the Mini‐SAR experiment onboard India's Chandrayaan‐1 spacecraft. Between mid‐February and mid‐April, 2009, Mini‐SAR mapped more than 95% of the areas polewards of 80° latitude at a resolution of 150 meters. The north polar region displays backscatter properties typical for the Moon, with circular polarization ratio (CPR) values in the range of 0.1–0.3, increasing to over 1.0 for young primary impact craters. These higher CPR values likely reflect surface roughness associated with these fresh features. In contrast, some craters in this region show elevated CPR in their interiors, but not exterior to their rims. Almost all of these features are in permanent sun shadow and correlate with proposed locations of polar ice modeled on the basis of Lunar Prospector neutron data. These relations are consistent with deposits of water ice in these craters.

Radar remote sensing of pyroclastic deposits in the southern Mare Serenitatis and Mare Vaporum regions of the Moon

We use polarimetric radar observations to study the distribution, depth, and embedded rock abundance of nearside lunar pyroclastic deposits. Radar images were obtained for Mare Vaporum and the southern half of Mare Serenitatis; the imaged areas contain the large Rima Bode, Mare Vaporum, Sulpicius Gallus, and Taurus‐Littrow pyroclastic deposits. Potential pyroclastic deposits at Rima Hyginus crater, the Tacquet Formation, and a dome in Mare Vaporum are also included. Data were acquired at S band (12.6 cm wavelength) using Arecibo Observatory and the Green Bank Telescope in a bistatic configuration. The S band images have resolutions between 20 and 100 m/pixel. The pyroclastic deposits appear dark to the radar and have low circular polarization ratios at S band wavelengths because they are smooth, easily penetrable by radar waves, and generally contain few embedded blocks. Changes in circular polarization ratio (CPR) across some of the pyroclastic deposits show areas with increased rock abundance as well as deposits that are shallower. Radar backscatter and CPR maps are used to identify fine‐grained mantling deposits in cases where optical and near‐infrared data are ambiguous about the presence of pyroclastics. The Tacquet Formation in southern Serenitatis, areas near Hyginus crater, and a dome in Mare Vaporum have lower‐backscatter cross sections than would be expected for mare basalts of similar estimated titanium content. Combined with very low CPR values, this is strong evidence that these areas are covered in fine‐grained pyroclastic mantling material.