J. Earth Syst. Sci. (2020) 129:105 Ó Indian Academy of Sciences https://doi.org/10.1007/s12040-020-1370-8 (0123456789().,-volV)(0123456789().,-volV) Mini-RF S-band observation of Ohm and Stevinus craters using circular polarization ratio and m-chi decomposition techniques 1, 1,2 1 ASHKA DTHAKER *, SHREEKUMARI MPATEL and PARAS MSOLANKI 1Department of Geology, M.G. Science Institute, Ahmedabad 380 009, India. 2Research Scholar, Gujarat University, Ahmedabad 380 009, India. *Corresponding author. e-mail: [email protected] MS received 5 August 2019; revised 13 January 2020; accepted 19 January 2020 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 classiBcation 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. Keywords. Ohm; Stevinus; circular polarization ratio; m-chi decomposition. 1. Introduction camera (WAC) and narrow angle camera (NAC), ultimately the brighter or younger material will be Impact cratering is a copious geological event susceptible to space weathering masking their inCuencing the Moon and it plays a major role in original properties (Lucey et al. 2000). Further- determining material distribution on the Moon more, optical datasets are only responsive up to the surface (Melosh 1989). Determining the physical top few millimetres of the surface (Saran et al. properties of such material by observing the Bnal 2014). Contrary to this, radar provides an exclu- crater morphology can give decisive perception on sive means to analyze surface and subsurface chronological events as well as subsurface resources physical properties of the Moon by inBltrating (Senft and Stewart 2007). Albeit many things can signals of several wavelengths. The Miniature be determined by optical observation of impact Radio Frequency (Mini-RF) instrument is such craters in imaging instruments like wide angle hybrid-polarized, synthetic aperture radar carried 105 Page 2 of 11 J. Earth Syst. Sci. (2020) 129:105 by Lunar Reconnaissance Orbiter (LRO) of NASA 3. Data and methodology that penetrates up to a meter in the lunar surface. Mini-RF transmits two different wavelengths, As mentioned above, we have used S-band ‘zoom’ either S-band (12.6 cm) or X-band (4.2 cm). There mode data which was downloaded from PDS Geo- are two distinct modes, one of which is ‘baseline’ science Node of NASA. Mini-RF is accustomed of mode with 150 m resolution and other is a ‘zoom’ dual polarized radar that transmits on a single mode with 15 9 30 m resolution (Raney 2006). linear polarization (e.g., H) and receives on two In this study, we have used S-band zoom data to polarizations among which, one is corresponding to determine backscattering and surface roughness of the transmitted one (H) and another is orthogonal two lunar impact craters Ohm and Stevinus, which counterpart (V) of the linear polarization (crossed- we then compared to optical datasets of NAC and polarized) (Raney et al. 2012). This phenomenon WAC. The Mini-RF data also revealed ejecta allows calculation of all four Stokes parameters S1, materials including melt Cows which are not S2, S3 and S4 (Raney 2006). optically visible in the NAC or WAC data. 2 2 S1 ¼ \jjEH þ jjEV [ ; S ¼ \jjE 2À jjE 2 [ ; 2 H V ð1Þ 2. Regional setting and study areas à S3 ¼ 2Re\EH EV [ ; à The study area includes two lunar impact craters: S4 ¼À2Im\EH EV [ : Ohm and Stevinus. Among which, Ohm is located In these equations, E is the complex voltage in on the far side of the Moon and Stevinus is located the subscripted polarization, Re and Im stand for on the near side of the Moon. Interestingly, both the real and the imaginary value of the complex the craters are situated in highland area displaying cross-product amplitude, respectively, the ray systems extending for several kilometres. chevrons ( ) indicate spatial averaging and ‘*’ According to Wilhelms (1987), the ray systems \[ represents the conjugate (Raney et al. 2012). signify the Copernican age of both the craters. Here, the S Stokes parameter indicates mea- Ohm crater (18.4°N, 113.5°W), with a diameter 1 surement of total average power of the received of 64 km (Andersson and Whitaker 1982), lies signal. The S and S parameters calculate the linear attached to the north-eastern rim of crater Comire 2 3 polarization power and S computes whether the K and to the south of Comire crater. To its north- 4 polarized power is circularly right or left polarized. west, a large crater Shternberg and to its south- Left circularly polarized power is indicated by the west, Kamerlingh Onnes are present. In optical negative sign on the S parameter (Raney 2007). data, Ohm shows sharply deBned and mostly 4 These Stokes parameters can be used to compute symmetrical rim except for a slightly irregular several child parameters generally used for the southern end of the rim. For a complex crater, analysis of radar data. The most commonly derived Ohm lacks a prominent central peak but large product is the circular polarization ratio (CPR). sized, irregular central mounds are present. Ohm The CPR is a robust indicator of the surface exhibits diverse melt features such as cooling roughness (Campbell et al. 2010). CPR is deBned cracks on the crater Coor, melt Cows and melt as the ratio of transmitted polarization signal as ponds along the interior side as well as the exterior same-sense (SC) to the returned polarization signal side of the crater. as opposite-sense (OC) (Cahill et al. 2014). Using Stevinus (32.5°S, 54.2°E) is one of the few large Stokes vectors, CPR can be calculated as: craters situated in the south-east part of the Moon with a diameter of 75 km (Andersson and Whitaker ðÞS ÀS CPR ¼ 1 4 : ð2Þ 1982). Stevinus is a neighbour to a small crater ðÞS þ S named Stevinus A, which has a small ray system of 1 4 its own with a relatively higher albedo. In contrast to Relatively smooth surfaces lead to single-bounce Ohm, Stevinus crater has a massive but slightly oA- backscattering indicating low CPR values centred central peak. The crater walls are highly (typically \ 0.4), whereas slightly rougher lunar terraced with smooth melt ponds trapped in between. surfaces cause double-bounce backscattering The study area is highlighted on the LROC leading to high CPR values of 1 and above (Carter WAC (100 m) global morphology mosaic in et al. 2012). However, the CPR calculations are Bgure 1. simpliBed and there might be some exceptions to it. J. Earth Syst. Sci. (2020) 129:105 Page 3 of 11 105 Figure 1. LocationsofOhmandStevinuscratershighlightedwiththeredcolouredboxesonLROCWACglobalmosaic. Therefore, it is important to consider other available ArcGIS software and subsequently compared to the perceptions as well (Raney et al. 2012). optical datasets of LRO narrow angle camera Other daughter products used in this study are (0.5 m/pixel) and LRO wide angle camera global the degree of polarization (m), degree of circularity morphology mosaic (100 m/pixel) to distinguish (v) and the m-chi decomposition which we calcu- areas having high surface roughness. The NAC data lated using referenced equations (Raney et al. was downloaded from the PILOT (Planetary Image 2012). Locate Tool) imagery archive of NASA and the Degree of polarization WAC mosaic was downloaded from the Astropedia, ÀÁ lunar and planetary cartographic catalog. 2 2 2 1=2 m ¼ S2 þ S3 þ S4 =S1; ð3Þ The degree of circularity 4. Results and discussions ðvÞSin 2v ¼ÀðÞS4 = ðÞm à S1 : ð4Þ The m-chi decomposition was generated to Overall, Ohm crater exhibits asymmetrical distribu- enhance the information provided by the CPR tion of ejecta with most extensive ejecta in the SW and for detailed demarcation of types of back- direction (Neish et al. 2014). Through the available scattering such as single bounce, double bounce Mini-RF data, we observed continuous and discon- or randomly polarized backscatter (Raney et al. tinuous ejecta melt sheets in Ohm. Continuous ejecta 2012). The interpretation of backscattering is blanket of a crater roughly extends up to one crater expressed using colour-codes such as: radius from the rim (Melosh 1989) and mostly consists of broken remains of the target materials. Such con- 1=2 R ¼½m à S1 Ãð1 þ Sin 2vÞ=2 ; tinuous ejecta is thus more hummocky in nature. 1=2 Discontinuous ejecta on the other hand can extend up G ¼ ½S1 à ðÞ1Àm ; ð5Þ to thousands of crater radii and is quite patchy and 1=2 thin (Melosh 1989).