Comparison of lunar red spots including the crater copernicus

Icarus 272 (2016) 125–139

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Comparison of lunar red spots including the crater copernicus Y. Shkuratov a,∗, V. Kaydash a, L. Rohacheva a, V. Korokhin a, M. Ivanov b,c, Y. Velikodsky a,d, G. Videen e,f a

Institute of Astronomy, Kharkiv V.N. Karazin National University, 35 Sumskaya St., Kharkiv 61022, Ukraine V. I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 Kosygin Str., Moscow 119991, Russia c Moscow State University of Geodesy and Cartography (MiiGAIK), 105064 Moscow, Gorokhovsky 4, Russia d National Aviation University, Cosmonaut Komarov Ave. 1, Kiev 03680, Ukraine e Space Science Institute, 4750 Walnut St. Suite 205, Boulder, CO 80301, USA f Army Research Laboratory AMSRL-CI-EM, 2800 Powder Mill Road, Adelphi, Maryland 20783, USA b

a r t i c l e

i n f o

Article history: Received 8 September 2015 Revised 11 February 2016 Accepted 23 February 2016 Available online 3 March 2016 Keywords: Moon, Surface Photometry Spectrophotometry

a b s t r a c t The lunar red spots, Helmet, Hansteen Alpha, and the NW quadrant of the crater Copernicus, were selected for a complex comparative investigation of their characteristics measured by the spacecraft Clementine, LRO, and Chandrayaan-1. For the analysis we used the following parameters: the reflectance A(750 nm), color-ratio A(750 nm)/A(415 nm), parameter of optical micro-roughness (LRO WAC), parameters deduced from LRO Diviner data, optical maturity OMAT, abundance of FeO and TiO2 (Clementine UVVIS and LRO WAC data), oxygen content determined using Lunar Prospector data, and parameters characterizing the 0.95-μm and 2.2-μm bands of Fe2+ ions (crystal field bands), and 2.8-μm band of H2 O/OH and/or Fe2+ ions. The red spots Helmet and Hansteen Alpha are considered to be extrusions of rhyolite composition, which can be attributed to the Nectarian period; we did not find contradictions of this assumption. As for the Copernicus red spot, this, perhaps, is a similar formation that has been destroyed by the impact. We demonstrate that the material of the Copernicus red spot probably has the same composition as the classical red spots Helmet and Hansteen Alpha. The distributions of the parameter of optical micro-roughness and optical maturity OMAT show that the Copernicus red anomaly was not formed during the long evolution of the lunar surface, but results from crater formation. We find several confirmations of the hypothesis that the Copernicus red spot can be a residual of a red material (possibly rhyolite) extrusion that was involved in the impact process. The red material could have been partially melted, crushed, and ejected to the crater’s north-western vicinity. The described red asymmetry of the Copernicus ejecta can be related to the eccentricity, relative to the extrusion, of the impact and/or to the inclination of the impactor trajectory. The latter also is confirmed by an analysis of the region, which is based on the geological map shown in this paper. © 2016 Elsevier Inc. All rights reserved.

1. Introduction The lunar surface reveals pronounced chemical and mineral diversity that manifests itself in the regional variations of optical characteristics, such as albedo and different color-ratios. There are, in particular, regions having intermediate albedo A, but a significant excess of the color-ratio in the visible spectral range, Ared /Ablue . Several such red spots were found many years ago with Whitaker’s color-ratio images (Whitaker, 1966, 1972). Red spots occur mostly on the western nearside of the Moon, in Oceanus Procellarum. These red formations are considered to be pre-mare

∗

Corresponding author. Tel.: +38 57719 2883; fax: +38 57700 5349. E-mail address: [email protected] (Y. Shkuratov).

http://dx.doi.org/10.1016/j.icarus.2016.02.034 0019-1035/© 2016 Elsevier Inc. All rights reserved.

materials of evolved composition (Malin, 1974; Head and McCord, 1978; Bruno et al., 1991; Raitala et al., 1999; Chevrel et al., 1999; Bondarenko and Shkuratov, 20 0 0). These unusual formations occur within the predominantly basaltic terrain of the lunar surface. The red color of the areas is due to low titanium abundance (e.g., Burns, 1993) and partially can be related to the presence of nanophase metallic iron npFe0 in regolith particles (e.g., Hapke, 2001; Tompkins and Pieters, 2010, Noble et al., 2007). Examples of red spots are the formations Helmet, Hansteen Alpha, Gruithuisen Domes, Riffaeus Montes, and others (see Fig. 1). Using Lunar Prospector data, it was found that the lunar red spots poorly correlate with Th abundances. For example, Hagerty et al. (2006) have shown that Hansteen Alpha, the Gruithuisen Domes, and the Lassell massif all have Th abundances that are consistent with the presence of lunar granites (rhyolites). On the other hand,

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of nearby highland areas (Raitala et al., 1999). Hawke et al. (2002, 2003), Wilson and Head (2003), Hagerty et al. (2006), Wagner et al. (2010), and Glotch et al. (2010) considered that both these formations, perhaps, are rhyolite extrusions. The crater Copernicus, 96 km in size, is located on the lunar near side. It is a bright, young crater, ∼779 m.y. (Heisinger et al., 2012). Fig. 2a shows a portion of the LROC WAC mosaics (Korokhin et al., 2015; 2016) at λ = 750 nm around the crater. This region is optically and, hence, compositionally heterogeneous (e.g., Pinet et al., 1993; Dhingra et al., 2013, 2015). In particular, Dhingra et al. (2015) have discussed multiple origins for olivine-bearing lithologies at Copernicus crater. Fig. 2b shows a color-ratio A(750 nm)/A(415 nm) image obtained with the LROC WAC mosaics (Korokhin et al., 2015, 2016). As one can see, there is an amazing feature in the left upper quadrant of the crater, which is very well detected in this color ratio. This color anomaly results from a spectrum characterized by strong ultraviolet absorption. A question arises: do the Copernicus red spot and the red extrusions (Helmet and Hansteen Alpha) have commonalities besides their color characteristics? We show here that the Copernicus feature can be related to the formations.

Fig. 1. The locations of several red spots on the lunar disk.

the formations Helmet and Montes Riffaeus do not have high Th content. The LRO Diviner radiometer allowed measurements of the location of the Christiansen feature (CF) in spectra near 8 μm. The feature is related to the fundamental vibrational bands of silicates. The Christiansen frequency corresponds to the case when the real part of the refractive index of a material approaches 1. The feature position depends strongly on the mineral composition and allows one to remotely determine landforms rich in silica. It has shown that some lunar red spots exhibit Christiansen frequency locations consistent with high silica content (e.g., Glotch et al., 2010; Jolliff et al., 2011). Recently, water and/or hydroxyl anomalies have been detected at several red spots (Bhattacharya et al., 2013; Pathak et al., 2015). This result is based on Chandrayaan-1 Moon Mineralogy Mapper (M3 ) measurements. This may be related to endogenic/magmatic origin of the formations. Moreover, Bhattacharya et al. (2013) also have reported the presence of the prominent spinel feature in the Hansteen Alpha formation that somewhat contradicts the hypothesis about rhyolite composition of the formation materials. Using M3 data, Pieters at al. (2014) also have found Mg-spinel at a few areas of hypothesized non-mare volcanism, in particular, in Hansteen Alpha (see also the discussion by Prissel et al., 2014). Among the red spots, there is a red feature associated with the crater Copernicus, which is centered approximately at latitude +10.2º, longitude –20.9º; it is not usually considered in the common context of the red spots. It is interesting to compare optical properties of the Copernicus feature with other red spots in order to include it or not in the class of regular lunar red spots (Shkuratov et al., 2015). We selected three areas for different comparative studies: the northwestern quadrant of the crater Copernicus, the formation Helmet that is located at the northern edge of the Humorum basin (latitude = –16.8º, longitude = –31.5º), and the polygonally shaped feature Hansteen Alpha that is centered at the latitude = –12.2º and the longitude = –50.1º. The formations Helmet and Hansteen Alpha are isolated domes 35 and 13 km in average diameter, respectively, which elevate over the surrounding mare surface. Their material appears to be brighter and more fresh-looking than that

2. The parameters studied and source data For the comparative investigation of the three areas, we used the following 12 parameters: the reflectance (albedo) A(750 nm), color-ratio A(750 nm)/A(415 nm), shaded topography images, parameter of optical micro-roughness, LRO Diviner rock abundance, parameter of optical maturity OMAT, abundance of FeO and TiO2 , oxygen content determined using Lunar Prospector data, and the parameters bend1, bend2, bend3 that characterize, respectively, the 0.95-μm and 2.2-μm bands of Fe2+ ions (crystal field bands), and 2.8-μm band of H2 O/OH and/or Fe2+ ions. We characterize the bands with the band bends calculated from Chandrayaan-1 M3 spectral data. The bend1= A750 A1109 /(A950 )2 , bend2 = A1548 A2537 /(A2218 )2 , and bend3 = A3 · A1 /(A2 )2 , where A1 = (A2657 + A2697 + A2737 )/3, A2 = (A2777 + A2817 + A2856 )/3, and A3 = (A2896 + A2936 + A2976 )/3; the albedo indices are the wavelengths given in nanometers. The first bend has been used, in particular, for the detection of lunar red spots (Raitala et al., 1999). The Clementine 100-m mosaics (McEwen and Robinson, 1997), Lunar Prospector gamma-ray spectrometer (GRS) measurements (e.g., Lawrence et al., 2002; Elphic et al., 2002), Chandrayaan-1 M3 IR spectrometer measurements (Pieters et al., 2009), and LROC WAC images (Robinson et al., 2010) were used as source data for the analysis. The 12 characteristics were calculated as described below. They are not independent of each other and the reliability of their determination is different, but it is sufficient for semiquantitative assessments. Using the LRO WAC data and Clementine mosaics we apply Lucey’s method (Lucey et al. 1995, 20 0 0) for the assessment of TiO2 , FeO abundance and the parameter of optical maturity OMAT with the following expressions:





A(950 nm )/A(750 nm ) − y FeO[%] = −17.43 arctan A(750 nm ) − x

 − 7.56, (1)





A(415 nm )/A(750 nm ) − z TiO2 [%] = 3.71 arctan A(750 nm )

5.98 ,

(2)

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Fig. 2. Albedo (750 nm) (left column) and color ratio (750/415 nm) (right column) images of the Copernicus crater, Helmet formation, and Hansteen Alpha, respectively, 1 – 3 rows.

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Fig. 3. Shaded topography map produced with GLD100 (Scholten et al., 2012) data (left column) and phase-curve slope η(415 nm) distribution (right column) for the same areas as have been shown in Fig. 2.

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Fig. 4. Distributions of LROC WAC reflectance at λ = 604 nm (a, c, and e) and Diviner rock abundance (b,d, and f) for Copernicus (upper row), Hansteen Alpha (middle row, and Helmet (low row).

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Fig. 5. Distribitions of OMAT (left column) and FeO abundance (right column) distributions for the same areas as in Fig. 2.

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Fig. 6. Distributions of the TiO2 (left column) and oxygen (right column) abundances for the regions under study.

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of oxygen and Si. To assess the oxygen content we used a technique suggested by Shkuratov et al. (2005). This is an interpolation of available Lunar Prospector gamma-ray spectrometer (GRS) data from low to high spatial resolution, using Clementine UVVIS spectral reflectance images. The idea is to apply low-resolution GRS measurements as ground truth to establish relationships linking formally optical data and geochemical information, thereby maximizing the respective correlation coefficients with varying the coefficients a, b, c, h, f, e, and g in the following regression equation (Shkuratov et al., 2005):

log P = aAR + bCBR + cCIR1 + hCIR2 + f CIR3 + eD + g,

Fig. 7. Two M3 spectra of the sites pointed out in Fig. 8. The wavelengths used for the calculation of the parameters bend1, bend2, and bend3 are indicated.

where A(λ) is albedo (%), х = 0.08, у = 1.19, and z = 0.42. Formula for the parameter OMAT was obtained using the correlation diagram А(750 nm) – С(950/750 nm)



OMAT =

 (A(750 nm ) − x )2 +

A(950 nm ) −y A(750 nm )

2 .

(3)

Structure characteristics also were studied for the three selected areas. We built shaded topography obtained from the topography data set GLD100 (Scholten et al., 2012). Moreover, we also estimated the optical micro-roughness. Distributions of this characteristic with a resolution of approximately 100 m were found from LROC data using all available WAC images for the selected regions in order to find the phase function for each point of each scene with the algorithm described in detail by Korokhin et al. (2014, 2015, 2016). This algorithm requires the exploitation of the following presentation of the lunar phase function:

A(α , i, e ) = A0 exp(−η

√

α )D(α , i, e),

(4)

where A is the apparent albedo (radiance factor) (Hapke, 1993), A0 is the normal albedo, η is the parameter of phase-curve slope, and D is Akimov’s disk function (Shkuratov et al., 2011; Velokodsky et al., 2011):

D(α , β , γ ) = cos

α 2

cos

π α

(cos β )α/(π −α ) γ− , π −α 2 cos γ

(5)

where the so-called photometric (luminance) coordinates α , β , and γ are the phase angle, photometric latitude and longitude, respectively. These angle sets can be expressed through the incident and emergent angles i and e, and the azimuth α that is the angle between the planes of scattering and incidence (e.g., Hapke, 1993; Shkuratov et al., 2011). The parameters A0 and η were calculated using the least-squares method for several tens of source WAC images (Korokhin et al., 2015, 2016). We call η the optical microroughness, since it characterizes the intensity of the shadowing effect in regoliths. The lunar red spots, e.g., the Gruithuisen Domes, the Lassell massif, the formation Hansteen Alpha, Helmet, Montes Riffaeus and others are considered to have rhyolite composition (e.g., Malin, 1974; Hawke et al., 2003). This implies a higher abundance

(6)

where P is a chemical parameter, e.g., oxygen abundance, in weight (%), AR = A(750 nm) in (%), CBR = A(415 nm)/A(750 nm), CIR1 = A(900 nm)/A(750 nm), CIR2 = A(950 nm)/A(750 nm), CIR3 = A(10 0 0 nm)/A(750 nm), and D = A(750 nm)A(10 0 0 nm)/[A(900 nm)]2 . Eq. (6) together with the coefficients presented in Table 2 by Shkuratov et al. (2005) allows one to derive maps of oxygen content with spatial resolutions corresponding to the resolution of Clementine data. Of course, oxygen is not a chromophore, and its determination with optical measurements easily can be criticized. On the other hand, the O-abundance correlates with Fe and Ti. This provides a rather high correlation coefficient 0.75 with optical data (Shkuratov et al., 2005), and therefore, such an approach appears to have merit. 3. Results and discussion For comparison, we show distributions of some of the parameters mentioned above in Figs. 2–6 for the crater Copernicus and the formations Helmet and Hansteen Alpha. We study these three areas to confirm the hypothesis that the Copernicus red spot can be a residual of an extrusion of a red material, perhaps, of rhyolitelike composition, which was involved in the impact formation process of the crater. The extrusion body could be partially melted, crushed, and ejected to the crater vicinity. The color asymmetry (Fig. 2b) of the red ejecta can be related, e.g., to the eccentricity of the crater formation impact relative to the extrusion (Shkuratov et al., 2015). We also note the complexity of the impact event, drawing attention to a symmetric slight red ring located immediately out of the crater rim (Fig. 2b). Fig. 2b,d, and f shows that the red excess values of Copernicus are very similar for the other anomaly areas. There is no topography asymmetry in the ejecta blanket distribution related to the red spot for the crater Copernicus. The map of slopes calculated from the relief data set GLD100 (Scholten et al., 2012) clearly demonstrates this fact in Fig. 3a. As distinct from the formations Helmet and Hansteen Alpha (Fig. 3c and e), the Copernicus red spot is not visible in topography at all. This confirms that the red material is a portion of the target involved in the crater formation as the materials of other composition. Microstructure characteristics influence the phase curve of the lunar surface, see Eq. (4). We use all available images to find the phase function for each point of the scene, and note that η is the parameter of phase-curve slope. For each point of the lunar surface fragment, the parameter η was calculated for the wavelength 415 nm, since the albedo of the lunar regolith in blue light is small and, hence, the influence of multiple scattering of light on η is minimal. Resulting maps are presented in Fig. 3b,d and f. The map in Fig. 3b shows that the color anomaly area in the crater Copernicus does not reveal any peculiarity in η, since the area has micro-roughness and albedo like those of its surroundings. In contrast, the Helmet and Hansteen Alpha formations exhibit a distinct difference of η from their mare surroundings. However, this not an anomaly, but is related rather to the inverse correlation between the phase-curve slope and albedo (Shkuratov et al., 2011). Thus, we may conclude

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Fig. 8. Distributions of the spectral bends determined with the Chandrayaan-1 M3 spectrometer for the Copernicus crater. Panels (a) – (d) correspond, respectively, to brightness distribution and bends: 0.95, 2.2, and 2.8 μm. A mosaic of the two high-Sun M3 images M3G20090610T030313 and M3G20090610T070604 is used.

that the Copernicus red asymmetry is rather old, otherwise it could be detected using this parameter, as in the case of surfaces in and around young craters (e.g., Shkuratov et al., 2012). This result has been confirmed by LRO Diviner data. The Diviner instrument allowed an assessment of rock abundance using measurements of the thermal-inertia characteristic of the lunar surface. The maps of the abundance in comparison with the brightness distributions are shown in Fig. 4. Higher concentrations of rocks are observed for young craters and in and near terraces and landslides of the crater Copernicus. The red spots, including the Copernicus color feature, do not reveal themselves in this parameter. The parameter OMAT (Lucey et al. 1995, 20 0 0) was found from Clementine UVVIS images and shown in Fig. 5a,c and e for the

three scenes. We find very slight red asymmetry in the distribution of this parameter calculated by Eq. (3) for the crater Copernicus. This result demonstrates that the asymmetry probably was not formed during the evolution of the lunar surface, but rather arose in the process of the crater formation. The red spot Helmet has somewhat higher maturity degree than that of neighboring areas. Hansteen Alpha demonstrates nearly the same optical maturity as that of its surroundings. All these structure characteristics clearly show that the red spots including the Copernicus spot are rather old formations. Using the approach of Lucey et al. (1995; 20 0 0) and the Clementine mosaics at λ = 415 and 750 nm, we calculate the FeO abundances, and using LROC WAC data we found TiO2

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Fig. 9. The same as in Fig. 8 for the formation Hansteen Alpha. Chandrayaan-1 M3 high-Sun images M3G20 090612T10160 0 are used.

abundances (Korokhin et al., 2015, 2016). We present maps of these abundances in Figs. 5b,d and f and 6a,c, and e, respectively. All three formations reveal low values of these parameters. They are more typical for highlands than for maria, especially, within the red anomalies. The asymmetry of Fe and Ti concentrations for the Copernicus crater is clearly seen. We also apply the algorithm suggested by Shkuratov et al. (2005) to assess the abundance of oxygen, the most important lunar element. Fig. 6b,d, and f shows the oxygen distributions for the studied areas. All the images appear reasonable, revealing higher O concentration in the red spots. The Copernicus asymmetry also is seen very clearly. High content of oxygen is a key indicator of chemically evolved rock types. This strongly supports the hypothesis on the rhyolite/dacite composition of the formation, although the Lunar Prospector gamma-ray data show rather ambiguously that the red spots are enriched in the element thorium (Hagerty et al., 2006). Exploiting the approach suggested for the oxygen determination (Shkuratov et al., 2005), we have made the prognosis map of Si, which resembles the oxygen distribution, as has been anticipated. Using Chandrayaan-1 M3 spectral data, we study characteristics of bands in the lunar IR spectra. There are different methods to estimate the depth of an absorption band in the spectra of powdered (regolith-like) surfaces. For instance, this can be estimated with spectral slopes of the band wings, or using continuum interpolations, linear or non-linear (e.g., Bhattacharya et al., 2013). We here use the parameter bend, which does not use artificial procedures related to interpolation or extrapolation. The bend is the

ratio A(λ1 )A(λ3 )/[A(λ2 )]2 , where λ1 and λ3 are the wavelengths of the band beginning and ending, respectively, and the wavelength λ2 is the band center. Fig. 7 displays M3 spectra of the two sites pointed out in Fig. 8a. We here demonstrate the wavelengths used for the determination of the parameters bend1, bend2, and bend3. Figs. 8–10 represent maps of the following band bends: A750 A1109 /(A950 )2 , A1548 A2537 /(A2218 )2 , and A3 · A1 /(A2 )2 , where A1 = (A2657 + A2697 + A2737 )/3, A2 = (A2777 + A2817 + A2856 )/3, and A3 = (A2896 + A2936 + A2976 )/3 for the three regions under study. The bands at λ = 0.95 μm and 2.2 μm can be associated with ions Fe2+ in pyroxenes and olivines. All these bands are produced by the crystal field mechanism (Burns, 1993). The 2.8 μm band also can be attributed to the spectral feature of cations Fe2+ in some oxides. For instance, an admixture of bivalent iron may produce a plainly seen band in spinels (Cloutis et al., 2004) and even glasses (Boulos et al., 1997). On the other hand, the longest wavelength band of Fe2+ at 2.8 μm is usually hidden under strong absorption of water and hydroxyl. The OH/H2 O overlapping bands relate to the (O-H) stretching transitions (e.g., Stolper, 1982). For the region of the crater Copernicus we have built a mosaic of the two Chandrayyan-1 M3 high-Sun images (Fig. 8a). Fig. 8b–d displays distributions of bends calculated for the bands centered at 0.95, 2.2, and 2.8 μm. As can be seen, the depths of shortwavelength bands of the ions Fe2+ (crystal field transitions) are shallower inside the red spot than those for its surroundings. The same is observed for Hansteen Alpha and Helmet (Figs. 9 and 10). This may indicate that the regolith of these areas contains a large amount of glasses, e.g., obsidian-like glasses (a crystallized form

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Fig. 10. The same as in Figs. 8 and 9 for the formation Helmet. Chandrayaan-1 M3 high-Sun images M3G20 090611T0 01830 are used.

of obsidian is rhyolite), since crystal field splitting of the ion energy levels weakens in disordered structures. Unlike the 0.95 and 2.2 μm bands, the depth of the 2.8 μm band is deeper for the red-spot material than for the surroundings. This suggests that the spectral feature relates rather to OH/H2 O absorption than to Fe2+ . This is in agreement with results obtained earlier (Bhattacharya et al., 2013; Pathak et al., 2015). These NIR spectral features are not typical for the lunar surface. However, they are characteristic of pyroclastic regions; e.g., Aristarchus Plateau demonstrates a high abundance of oxygen (Shkuratov et al., 2005) and high content of H2 O molecules, perhaps of endogenic nature (Li and Milliken, 2015). Thus, among lunar materials, the orange glasses can be considered as a probable component of the regolith of the red spots. Raitala et al. (1999) applied a spectral mixing model (Shkuratov et al., 1999) to simulate an admixture of the orange glasses (RELAB spectra of an Apollo-17 sample) to typical highland regolith (spectra of Appolo16 samples). It was found that 20–50% of orange glasses in the regolith provide the observed values of the spectral bend at 0.95 μm.

However, we do not see pyroclastic sources inside Copernicus or in its immediate surroundings. Thus we can conclude that the red spot associated with the northwestern quadrant of the crater Copernicus has characteristics similar to those of classical red formations like Hansteen Alpha and Helmet. We have found only one significant difference. Fig. 11 shows the distributions of the Christiansen feature (CF) for the areas indicating variations of mineral composition. The CF value shifts to shorter wavelengths and is most sensitive to plagioclase/olivine abundance. The mean value is around 8.2 μm. The CF decreases to 7.3–7.8 μm for the highland material (plagioclase feldspar) and increases to greater than 8.7 μm in the case of olivine abundance (Glotch et al., 2010). The formations Helmet and Hansteen Alpha have lower CF values than those of their surroundings; whereas, the Copernicus asymmetric red anomaly demonstrates higher CF values in comparison with the material located to the east and south of the crater center. However, we note that the CF wavelengths are rather close to 8.2 μm both for the Copernicus red spot and Helmet. The situation with Hansteen Alpha is unclear,

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Fig. 11. Distributions of Diviner determination of the Cristiansen feature for the red spot in the crater Copernicus (a, b), formations Hansteen Alpha (c, d), and Helmet (e, f).

since the Diviner data used for this area have been obtained only at low Sun. The complex crater Copernicus is an example demonstrating a high occurrence of impact melt deposits. The impact events responsible for forming large craters may have generated melt that coated not only the crater floor, but the surroundings located within ∼1 crater radius from the rim (Melosh, 1996). The ejecta deposits of Copernicus contain a portion of melt and crushed bedrock material that may have composition and optical properties distinct from their distant surroundings. For instance, this may be the rhyolite-like material from an extrusion that was involved in the crater-formation event. Hence, the heterogeneity of the crater deposits may be due to the composition complexity of the target area. In spite of evidence for efficient mixing of impact melt in complex craters, Dhingra et al. (2013, 2015) document the mineral

heterogeneity in impact melt deposits on a scale of tens of kilometers in the Copernicus floor and northern wall that are spectrally distinct from melt in its immediate vicinity. Our data are consistent with this conclusion. The body of red material could even be beneath the surface, but then it was revealed by the crater formation impact. Another example of the opening-up of a possible extrusion hidden by latter lava floodings is probably found for the photometric anomaly in Mare Nubium (Korokhin et al., 2015, 2016). In order to introduce a geological context for the observed features associated with Copernicus in more detail, we have compiled a geological map (1:50,0 0 0 scale, Fig. 12) of the crater cavity and its immediate surroundings (6–13N, 17–24W). We divide the entire study area into three domains: crater floor, crater wall, and zone of contiguous ejecta. Three major units cover the floor of the crater. Hummocky plains (unit ph in Fig. 13a) form the most

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Fig. 12. Geological map of the crater Copernicus. The pattern of the striated ejecta around the crater may suggest that the crater was formed by an oblique impact in the direction from the northwest to the southeast. Thick white and dark gray lines at the NW sector of the crater outline the most prominent anomaly of the color ratio (750/415 nm) that is associated with the crater.

widespread unit that covers about three quarters of the floor, except for the northwestern quarter (Fig. 12). The most characteristic features of the unit are numerous smaller (hundred meters across) and larger (a few kilometers across) hills that usually are separated by small occurrences of flat, low-lying plains. We interpret this unit as a mixture of impact-generated blocks and impact melt with a strong dominance of the blocks. Smooth plains (unit ps in Fig. 13a) are less abundant and predominantly occur in the northwestern segment of the crater floor (main occurrence). The smaller fields of the smooth plains occur at the NE, SE, and SW edges of the floor near its contact with the crater walls. In general, smooth plains have the same albedo as that of the hummocky plains. The surface of the unit, however, is significantly smoother than the surface of the hummocky plains (Fig. 13a) because smooth plains contain a much smaller amount of impact-generated blocks. The boundary between the smooth and hummocky plains is gradational, without clear evidence of superposition of the smooth plains onto the hummocky plains, which would be expected if the smooth plains would represent late volcanic flows. The surface of the smooth plains is gently rolling, which is not typical of the common lava plains, and slightly fractured; it lacks any evidence for the source-like features of the plains materials. The higher albedo of the smooth plains, their gradational boundary with the hummocky plains, and the low FeO content collectively suggest that smooth plains represent occurrences of impact melt. The obvious concentration of the melt within the northwestern segment of the

floor may indicate that either the target materials consist of components with different melting temperature, or Copernicus formed due to an oblique impact, or both. The central massif of Copernicus (unit cm in Fig. 13a) occurs as two large and elevated mounds near the geometric center of the crater (Fig. 12). The walls of the crater consist of two units. The major unit, rugged wall materials (unit wr in Fig. 13b), constitutes the crater cavity wall (Fig. 12) and is characterized by the rough, blocky surface complicated by numerous steep scarps. The scarps outline individual blocks tens of kilometers long and several kilometers wide that represent terraces on the crater wall. Morphologically smooth plains (unit ws in Fig. 13b) often occur on top surfaces of the terraces (Fig. 13b). Again, the lack of sources and spectral characteristics of the plains, which are similar to those of the adjacent materials, suggest that materials of the smooth plains on the wall are accumulations of impact melt and/or the mass-wasted fine-grained materials. Three material/morphological units make up the surface outside of the crater rim (Fig. 12). The unit of sculptured terrain (unit ts in Fig. 13c) forms a relatively narrow (10–15 km wide) zone adjacent to the crater rim (Fig. 12). The surface of the unit consists of numerous low hills, knobs, and ridges that often have sharp edges. Because of this, the morphology of the unit appears to be crisp, which is in sharp contrast to the more gently rolling and soft-looking surfaces of the other ejecta units (Fig. 13c). The softlooking ejecta (unit es in Fig. 13c) form the most abundant unit of

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Fig. 13. Examples of units. (a) The crater floor consists of three units, smooth plains (ps), hummocky plains (hp), and the central massif of the crater (cm). Center of the image is at 9.90ºN, 19.90ºW. (b) Smooth plains (ws) and rugged wall materials (wr) compose the crater wall. Center of the image is at 8.58ºN, 20.27ºW. (c) Ejecta of the crater exhibit at least two facies: the proximal striated ejecta (est) and ejecta with softer morphology (es) that usually compose the distal portion of the ejecta blanket. A terrain with crisp-looking surface (sculptured terrain, ts) makes a relatively narrow zone around the crater rim. Center of the image is at 8.09ºN, 18.58ºW. (d) In some places, the outward edges of the striated ejecta show short spur-like extensions that likely represent splashes of partly liquefied materials. Center of the image is at 7.28ºN, 20.27ºW.

the ejecta blanket around Copernicus that extends for many tens of kilometers away from the crater rim (Fig. 12). The surface of the unit consists of radial elongated low ridges and troughs with chains and clusters of the Copernicus secondary craters. The secondary craters are mostly concentrated to the northeast, southeast and southwest of the crater at distances 60–80 km from the crater rim. This unit obviously represents the major portion of materials ejected by the Copernicus impact event. A specific facies of ejecta, which has a fluidized appearance, striated ejecta (unit est in Fig. 13c), occur in several zones around the rim of Copernicus (Fig. 12). Elongated and sometimes sinuous grooves oriented radially relative to the crater rim characterize the surface of the unit (Fig. 13c). In places, the outer edges of the unit suggest a series of splashes (Fig. 13d), indicating that the unit may have been formed by emplacement of materials that have been partly liquefied either acoustically, or due to an enhanced amount of impact melt, or both. In any case, this facies of the ejecta produce a specific pattern of deposits around the crater rim (Fig. 12)

that resemble the butterfly pattern of ejecta typical of oblique impacts (Herrick and Forsberg-Taylor, 2003). 4. Conclusion The red spots Helmet, Hansteen Alpha are considered to be old extrusions, possibly volcanoes. As for the Copernicus red spot, this, perhaps, is a similar formation that has been destroyed by the impact. A significant amount of the regolith of all these formations is made up perhaps of fragments of felsic rocks and glasses. We demonstrate that the material of the red asymmetry of the crater Copernicus probably has the same composition as the red spots Helmet and Hansteen Alpha. This was not formed during the long evolution of the lunar surface, but during the crater formation. We find several confirmations of the hypothesis that the Copernicus red spot can be a residual of a red material, possibly rhyolite, extrusion that was involved in the impact process, in particular, in target material melting. The red extrusion could be partially

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