Mini-RF S-Band Observation of Ohm and Stevinus Craters Using Circular Polarization Ratio and M-Chi Decomposition Techniques

Home , Bragg (crater), Ohm (crater), Shternberg (crater)

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 Ã S3 ¼ 2Re\EH EV [ ; Ã 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 Ã 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 Ã 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 Ã ðÞ1Àm ; ð5Þ to thousands of crater radii and is quite patchy and 1=2 thin (Melosh 1989). The Mini-RF data of Ohm crater B ¼½S1 Ã m Ãð1À Sin 2vÞ=2 : illustrates much higher CPR values than the sur- In this order, R (red) exhibits double bounce, rounding terrain (Bgure 2C), suggesting rougher sur- G (green) exhibits volume scattering and B (blues) face and sub-surface in scale of centimetre to indicates single bounce backscattering. decimetre. The CPR values range from 0.9 to 1.5. The The calculations of CPR and m-chi decomposition ejecta material of the ray system surrounding the were made using the ENvironment for Visualising crater which is not visible in the optical data Images (ENVI) software. Initially, the typically thin (Bgure 2A), can be seen distinctly in the SAR data strips of S1, S2, S3 and S4 images were adjusted and (Bgure 2B–D). mosaicked. Later, using the basic tool Band Math, Some regions of the crater appear smooth in NAC CPR and m-chi decomposition were generated. and WAC data but exhibit higher CPR values which Afterwards, S1, CPR and m-chi images for both mightbeduetotheradardetectingpresenceoffea- Ohm and Stevinus craters were generated using tures like boulders, blocky regolith, secondary craters 105 Page 4 of 11 J. Earth Syst. Sci. (2020) 129:105

Figure 2. (A) WAC image of Ohm crater, (B) total power radar backscatter (S1), (C) colour-stretched CPR image overlain on S1 image, and (D) m-chi decomposition overlain on S1 image of Ohm crater. The colour wheel highlights each m-chi scattering (red: double bounce (db); blue: single bounce (bs); green: volume scattering (vs)) and combinations of these scatterings that may appear visually.

Figure 3. (A) m-chi decomposition image of Ohm, (B) geological map of Ohm crater featuring continuous ejecta blanket, ejecta boulders and mass wasting with white arrows pointing at the discontinuous ejecta melt, (C) NAC image M107622918RE, and (D) NAC image M184268782LE showing ejecta boulders from the N and NE block rich ejecta patches. or cooling cracks (Carter et al. 2012). The circular suggests that Ohm is a ‘fresh crater’ and the high CPR polarization ratio of Ohm appears even, overall inside values are due to rough surface primarily (Spudis et al. the crater as well as in the outer ejecta blanket, which 2010). J. Earth Syst. Sci. (2020) 129:105 Page 5 of 11 105

Figure 4. (A) NAC images M143018370LE and M143018370LE showing small primary and secondary craters, (B) colour stretched CPR image showing high CPR values for small craters, (C) marked locations of craters on the left and boulders on the right, (D) NAC image M10526384LE showing ejecta boulders, and (E) colour stretched CPR image showing high CPR values for ejecta boulders.

Figure 5. (A) Granular Cow highlighted on WAC image, (B) NAC images M105270662RE, M102908871LE and M102908871RE, (C) total radar backscatter (S1), (D) colour stretched CPR image, and (E) m-chi decomposition image overlain on LRO WAC global morphology map highlighting granular melt Cow on the outer side of north-western crater wall. 105 Page 6 of 11 J. Earth Syst. Sci. (2020) 129:105

However, few regions of Ohm including raised Apart from these boulder rich patches, another crater rim, walls, crater Coor and extended ejecta factor responsible for the higher CPR values is the melt show higher CPR values indicated by the red presence of numerous primary and secondary cra- coloured sprinkle in Bgure 2(C). These higher CPR ters. They are present almost everywhere in the values are the result of decimetre scale surface ejecta blanket, but are especially abundant in the roughness and its geometry. Upon closer inspection W and SW directions (Bgure 4A). of these red tinged regions, we observed several The m-chi decomposition of Ohm crater shows ejecta boulder Belds, small primary and secondary an overall yellowish tinge (Bgure 2D) suggesting craters. The ejecta boulder distribution around the the crater is dominated by a mixture of double Ohm crater has been mapped using high resolution bounce (db) backscattering and volume backscat- optical images (Bgure 3). Similar to the ejecta tering (vs), as per the seven-fold classiBcation distribution, the block rich deposits are also non- (Raney et al. 2012). The surrounding terrain shows uniform in nature. Most extensive boulder-rich an abundant amount of Bragg scattering suggested ejecta is observed in NE, N and SE directions by the blue tinge of the area. The db scattering (Bgure 4C, D). In these regions, apart from their suggests a natural dihedral formed by a combina- abundance, the size of these boulders is also bigger tion of the crater Coor and the far wall from the in comparison to others. The boulder size ranges perspective of the radar, whereas the vs suggests from around *5 m diameter to as large as *25 m randomly polarized multiple internal reCections diameter. In the SW and S directions, the boulders (Raney et al. 2012). Presence of rough surface are comparatively much less. Their size compared geometry due to ejecta boulders in N and NE to those in northern part is also relatively small. patches and due to small craters in W and SW

Figure 6. (A) Ropey structure highlighted with a box on S1 image, (B) zoomed-in total radar backscatter image with arrows pointing at the rope-like structure, (C) structure visible in the optical data (NAC image M102894594LE), (D) CPR image, and (E) m-chi decomposition image overlain on LRO WAC global morphology map highlighting the ropey structure on the crater Coor near the central mounds. J. Earth Syst. Sci. (2020) 129:105 Page 7 of 11 105

Figure 7. (A) LROC WAC image of Stevinus crater and the surrounding area. The highlighted impact melt Cow on the east side is covered by regolith, Cow outlines highlighted on (B) total radar backscatter image (S1), (C) CPR image, and (D) m-chi decomposition image. The impact Cow has uniform radar brightness. patches further suggest dominance of double strongest db signatures of wall-terraced craters by bounce scattering over volume scattering in the Raney et al. (2012). Near the central mounds as yellowish tinged mixture in the m-chi decomposi- well as the crater rim, a mixture of Bragg scatter- tion of Ohm. ing (single bounce) and volume scattering (vs) is in Conversely, the southern ejecta blanket in case abundance which might be due to the presence of of Ohm crater lacks significant boulders or small debris or small scale boulder Belds present in the craters and appears relatively smooth. Even so, the region. m-chi decomposition shows a dominance of yel- In Ohm crater, on the outer part of the north- lowish tinge suggesting a mix of double-bounce and western wall, granular melt Cow can be distin- volume scatter. This phenomenon points towards guished in the SAR data, contrary to the optical dominating volume scatter as a result of subsurface data (Bgure 5B, C). Such granular melt Cow might level roughness caused by suspended rock debris have formed by slope movements of Bne granular (Campbell 2016). material either by erosion or transportation Morphologically, Ohm has several tires of wall (Shevchenko et al. 2012). The absence of the terraces which further support the theory of structure in optical data might be caused by 105 Page 8 of 11 J. Earth Syst. Sci. (2020) 129:105 gradual space weathering of the lunar surface or On the crater Coor, nearer to the central due to formation of a regolith material over the mounds a ropey structure is formed which might ejecta Cow (Carter et al. 2012). Contrary to optical suggest rapid cooling of the impact melt and data, the radar penetrates the subsurface up to churning of the dissolved materials to shape a some extent suggesting the presence of such rope-like structure (Bgure 6). Previously such a granular Cow on a subsurface level. structure has been associated with plausible From Bgure 5(D), it is clearly seen that the CPR volcanic origin (https://www.asprs.org/wp-con of the granular ejecta is much higher than the tent/uploads/pers/1969journal/mar/1969˙mar˙ surrounding region. This might be because of its 239-245.pdf), but similar structure might also form grainy nature, which forms a rugged surface. The because of ejecta melt. Thus, it is difBcult to make m-chi decomposition of the Cow region shows a statement on its origin without further studying dominant mixture of double bounce and volume its composition using different datasets. The scattering (Bgure 5E), which is consistent with the structure can be clearly seen in both the datasets, rest of the crater. The grainy texture suggests optical and SAR (Bgure 6B–E). The CPR image dominance of double bounce scatter over volume and the m-chi decomposition do not show any scattering in the mixture. The surrounding terrain significant variation in the CPR values or shows bluish hue in the image suggesting Bragg backscattering regime (double bounce + volume) scattering along with a minor amount of a mixture of the ropey structure than the surrounding crater of Bragg and volume scattering. Coor (Bgure 6D).

Figure 8. (A) NAC image showing multiple secondary craters, (B) colour stretched CPR image showing high CPR values for secondary craters, (C) locations of secondary craters (left) and impact Cow (right) highlighted, (D) NAC image showing thick ejecta Cow, and (E) colour stretched CPR image showing high CPR values for the melt Cow. J. Earth Syst. Sci. (2020) 129:105 Page 9 of 11 105

In case of Stevinus crater, the available Mini-RF underneath the regolith, the impact melt Cow data shows high values of CPR for the visible still might be present. The total radar backscatter ejecta blanket of the crater on the east side, com- (S1), CPR and m-chi decomposition images pared to the surrounding terrain. In contrast to (Bgure 7B–D) reveal the Cow outlines as the radar Ohm crater, the ejecta material of Stevinus is far wave penetrates through the regolith cover. less continuous. The high CPR value is similar to The m-chi decomposition image of Stevinus melt Ohm ranging from 0 to 1.5, but is ambiguous, since Cow, instead of pure blue-green-red colouration, it might have resulted from the exposure of fresh shows combination of backscattering regimes – material as Stevinus is a relatively young crater or mostly a dominant mixture of double bounce and it might be due to a rough lunar surface. The radar volume scattering showing yellowish hue, where brightness is distributed uniformly in the region the double bounce backscatter further indicates (Bgure 7B). large scale surface roughness to be the reason The impact melt Cow on the east of Stevinus can behind high CPR (Raney et al. 2012). The sur- be distinguished clearly in the radar dataset com- rounding terrain including secondary craters show pared to optical NAC or WAC data. The undis- Bragg backscatter having blue tinge, indicative of tinguished impact Cow in the optical data might be moderately rough soil (Raney et al. 2012). because a thin (less than a meter thick), smooth Compared to Ohm, the ejecta blanket of the layer of lunar regolith was formed covering the Stevinus crater has far less block rich deposits and ejecta Cow, but the high CPR values indicate that most of the rugged appearance under optical and

Figure 9. (A) Geological map of Stevinus crater using (B) m-chi decomposition image as a basemap and featuring continuous ejecta blanket and white arrows showing discontinuous ejecta patches. 105 Page 10 of 11 J. Earth Syst. Sci. (2020) 129:105 radar data is a result of numerous secondary by energy bouncing oA of the crater Coor and the craters formed after the main impact event as well wall, respectively, or the other way round. Another as thick ejecta melt (Bgure 8). Lack of significant explanation for the high CPR values is the surface blocks or boulders in the region might be the pro- morphology. The presence of small scale rock duct of these micro-meteoroid secondary impacts debris, melt deposits, wall terraces cause double as such impacts destroyed the boulders previously bounce and volume scattering turning the CPR present in the ejecta. For boulders [ 2m in values high. The m-chi decomposition is not always diameter, estimated survival time for 99% of rock pure but shows combination of two different population is about 150–300 Ma (Basilevsky et al. backscatters. The most dominant combination in 2013). The absence of such boulders from Stevinus case of both the craters is of double bounce and ejecta suggests the crater to be older than the volume scattering shown by yellowish tinge in the mentioned time window. Moreover, the abundance image, wherein, the double bounce scatter is the of blocks and boulders in the ejecta blanket of Ohm result of rugged surface, ejecta boulders and cool- leads us to believe that albeit both craters ing cracks. Volume scattering on the other hand is belonging to Copernican age, Ohm is younger in an indicator of rocks suspended within the Bne age compared to Stevinus. grained debris at a subsurface level of a lunar Apart from the surface morphology, presence of regolith. ejecta melt and its thickness also aAect the scattering properties. Even though quite difBcult to measure Acknowledgements direct melt thickness using optical and Mini-RF data, it is possible to make an observationthatincaseof We would like to thank the Lunar Reconnaissance both the craters, with distance as the continuous Orbiter mission of NASA for providing high reso- ejecta transforms into discontinuous melt, the lution data in public domain. The authors are thickness of the melt sheet decreases (Bgure 9). grateful to the reviewer for their constructive In case of Ohm, in the vicinity of continuous comments and suggestions for the betterment of ejecta blanket, we observed abundant volume and the manuscript. double bounce scattering further accompanied by higher CPR values. These scattering properties and CPR values are fairly uniform in the ejecta References blanket itself. However, as we move further away from the crater, the amount of db and volume Andersson L E and Whitaker E A 1982 NASA Catalogue of scatter decreases and after approximately *0.65 Lunar Nomenclature; NASA Reference Publication 1097, radii distance from the crater rim, the scattering Arizona, pp. 28–66. Basilevsky A T, Head J W and Horz F 2013 Survival times of properties blends into more general scattering of meter-sized boulders on the surface of the Moon; Planet. the lunar regolith. The decrease in the double Space Sci. 89 118–126. bounce scatter is mostly because of decreasing Cahill J T S, Thomson B J, Patterson G W, Bussey D B J, ejecta boulders with increasing distance from the Neish C D, Lopez N R, Turner F S, Aldridge T, McAdam craters. On the other hand, the decrement of vol- M, Meyer H M, Raney R K, Carter L M, Spudis P D, ume scattering indicates lesser suspended rock Hiesinger H and Pasckert J H 2014 The miniature radio frequency instruments’s (Mini-RF) global observations of debris. In case of Stevinus, the available data Earth’s Moon; Icarus 243 173–190. makes it impossible to make such observations. Campbell B A 2016 Planetary geology with imaging radar: Insights from Earth-based lunar studies, 2001–2015; Astron. Soc. Pac. 128(062001) 20. 5. Conclusion Campbell B A, Carter L M, Campbell D B, Nolan M, Chandler J, Ghent R R, Hawke B R, Anderson R F and Wells K 2010 Earth-based 12.6-cm wavelength radar mapping of the The radar datasets provide details on physical Moon: New views of impact melt distribution and mare properties of ejecta material not provided by the physical properties; Icarus 208(2) 565–573. optical data which might be due to regolith coating Carter L M, Neish C D, Bussey D B J, Spudis P D, Patterson over the region. Both the craters, Ohm and Stevi- G W, Cahill J T and Raney R K 2012 Initial observations of nus are of Copernican age and thus depict fresh lunar impact melts and ejecta Cows with the Mini-RF radar; J. Geophys. Res. 117(E00H09)1–13. material. High CPR values might have resulted Lucey P G, Blewett D T, Taylor G J and Hawke B R 2000 due to the crater geometry scattering large amount Imaging of lunar surface maturity; J. Geophys. Res. Planets of energy back to the receiver, which might occur 105(E8) 20,377–20,386. J. Earth Syst. Sci. (2020) 129:105 Page 11 of 11 105

Melosh H J 1989 Impact Cratering: A Geologic Process; Senft L E and Stewart S T 2007 Modeling impact cratering in Oxford Monographs on Geology and Geophysics, 11, Layered Surfaces; J. Geophys. Res.: Planets. 112(E11002) Oxford University Press, Oxford. 1–18. Neish C D, Madden J, Carter L M, Hawke B R, Giguere T, Shevchenko V V, Pinet P C, Shevrel S D, Dadu Y, Lu Y, Skobeleva Bray V J, Osinski G R and Cahill J T S 2014 Global T P, Kvaratskhelia O I and Rosemberg K 2012 Modern slope distribution of lunar impact melt Cows; Icarus 239 105–117. processes on the Moon; Sol. Sys. Res. 46(1)1–17. Raney R K 2006 Dual-polarized SAR and Stokes parameters; Spudis P D, Bussey D B J, Baloga S M, Butler B J, Carl D, IEEE Geosci. Remote Sens. Lett. 3(3) 317–319. Carter L M, Chakraborty M, Elphic R C, Gillis-Davis J J, Raney R K 2007 Hybrid-polarity SAR architecture; IEEE Goswami J N, Heggy E, Hillyard M, Jensen R, Kirk R L, Trans. Geosci. Remote Sens. 45(11) 3397–3404. LaVallee D, McKerracher P, Neish C D, Nozette S, Nylund S, Raney R K, Cahill J T S, Patterson G W and Bussey D B J Palsetia M, Patterson W, Robinson M S, Raney R K, Schulze 2012 The m-chi decomposition of hybrid dual-polarimetric R C, Sequeira H, Skura J, Thompson T W, Thomson B J, radar data with application to lunar craters; J. Geophys. Ustinov E A and Winters H L 2010 Initial results for the north Res. 117(E00H21)1–8. pole of the Moon from Mini-SAR, Chandrayaan-1 mission; Saran S, Das A, Mohan S and Chakraborty M 2014 Synergetic Geophys. Res. Lett. 37(L06204)1–6. use of SAR and thermal infrared data to study the physical Wilhelms D E with sections by McCauley J F and Trask N J properties of the lunar surface; Adv. Space Res. 54(10) 1987 The Geologic History of the Moon; U.S. Geological 2101–2113. Survey Prof. Paper 1348 265p.

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