remote sensing Article Reevaluating Mare Moscoviense And Its Vicinity Using Chang’e-2 Microwave Sounder Data Zhiguo Meng 1,2,3,4, Shengbo Chen 1, Yongzhi Wang 1,2,3,* , Tianxing Wang 2 , Zhanchuan Cai 3, Yuanzhi Zhang 4 , Yongchun Zheng 4 and Shuo Hu 1 1 College of Geoexploration Science and Technology, Jilin University, Changchun 130026, China; [email protected] (Z.M.); [email protected] (S.C.); [email protected] (S.H.) 2 State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100101, China; [email protected] 3 State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China; [email protected] 4 Key Laboratory of Lunar and Deep-space Exploration, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China; [email protected] (Y.Z.); [email protected] (Y.Z.) * Correspondence: [email protected]; Tel.: +86-4318-850-2362 Received: 5 December 2019; Accepted: 5 February 2020; Published: 6 February 2020 Abstract: Mare Moscoviense (148◦E, 27◦N) is one of the few large maria on the lunar farside, with the thinnest crust and a positive gravity anomaly. In this paper, the Chang’E-2 Microwave Sounder (CELMS) data was employed to study the microwave thermal emission features of mare basalts in Moscoviense Basin. The time angle and linear interpolation method are used to generate the brightness temperature (TB) maps at noon and night, as well as the TB difference (dTB) map. The obtained important results are as follows. (1) A new geologic map is generated with the TB and dTB maps using the maximum likelihood method, which gives a new expression about the basaltic units in Mare Moscoviense compared to the optical results; (2) the substrate temperature of Moscoviense Basin is likely warmer than what we know; (3) unit Ihtm (a Late (?) Imbrian, mid- to high-Ti, high-Fe basalt) is re-understood as two independent volcanic features with their own fissures; (4) the dTB maps firstly indicate that the depth lunar regolith is homogeneous in the highlands surrounding Mare Moscoviense, at least in the microwave domain, and secondly that there exists a special material bringing about the low dTB anomaly in the shallow layer of the east highlands. The results will be of great significance to better understand the basaltic volcanism of the Moon. Keywords: Mare Moscoviense; microwave thermal emission; basaltic units; basaltic volcanism; CELMS data 1. Introduction Moscoviense Basin, centered at (148.12◦E, 27.28◦N), is a multi-ringed impact basin on the lunar farside located within the Feldspathic Highland Terrain (Figure1a). It hosts abundant mare deposits, which cover an area of 35,000 km2 within the 420 km diameter basin (Figure1b) [ 1–4]. It was reported that Mare Moscoviense has some of the thinnest crust on the entire Moon and a large positive gravity anomaly for a basin in this size [5–7]. Therefore, observations and studies of the lunar regolith in Moscoviense Basin with the passive microwave data will be of great significance to improve understanding the basaltic volcanism on the lunar farside. Up to now, five different volcanic units across the Basin were recognized, which were mainly based on spectral properties and composition, namely, high-Ti basalts, low-Ti basalts, ancient mare units, cryptomare, and highland basin soils [8,9]. Using Clementine data, Craddock et al. [10] revealed the unique, complex nature of the geology in Mare Moscoviense and found the evidence for large-scale Remote Sens. 2020, 12, 535; doi:10.3390/rs12030535 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 535 2 of 22 pyroclastic material on the northern lunar farside through the analysis of compositions and the ages of basalt units. Mainly dependent on the iron and titanium abundances (FTA) (Figure1c), Mare Moscoviense is divided into four broad units, namely: Im (an Early(?) Imbrian, very low-Ti, low-Fe basalt), IltmRemote (an Imbrian-age,Sens. 2020, x, x FOR PEER low-Ti, REVIEW mare basalt), Ikm (an Imbrian low-Ti basalt that partially2 of 22 fills the crater Komarov), and Ihtm (a Late(?) Imbrian, mid- to high-Ti, high-Fe basalt) by Kramer et al. [9] or large-scale pyroclastic material on the northern lunar farside through the analysis of compositions named Ehtmand (Eratosthenian the ages of basalt mid units. to Mainly high-Ti, dependent high-Fe on the basalt) iron and by Morotatitanium abundances et al. [11] (FTA) (Figure (Figure1d). Through analysis of the1c), Mare previously Moscoviense mapped is divided mare into units four broad using units, the namely Chandrayaan-1: Im (an Early(?) Moon Imbrian, Mineralogy very low- Mapper (M3) data, ThaisenTi, low-Fe et basalt al. suggested), Iltm (an Imbrian that the-age, oldest low-Ti, maremare basalt unit), Im Ikm was (an likelyImbrian the low remnant-Ti basalt that of the impact melt sheet, whichpartially wasfills the important crater Komarov to explore), and Ihtm the ( geologica Late(?) Imbrian, context mid of- to initial high-Ti, lunar high- crustalFe basalt) development by Kramer et al. [9] or named Ehtm (Eratosthenian mid to high-Ti, high-Fe basalt) by Morota et al. [11] and modification(Figure 1d [12). ].Through Pieters analysis et al. of found the previously that there mapped were mare several units using small the exposuresChandrayaan- of1 Moon three separate but distinctiveMineralogy rock types Mapper along (M3) data, the innermostThaisen et al. Moscoviensesuggested that the Basin oldest ring mare usingunit Im M3 was datalikely andthe detected olivine, orthopyroxene,remnant of the andimpact Mg-rich melt sheet, spinel which along was important the southwest to explore sectorthe geologic of the context peak-ring of initial[ lunar13]. Bhatt et al. crustal development and modification [12]. Pieters et al. found that there were several small re-classifiedexposures the previously of three separate mapped but unitdistinctive Iltm rock using type Chandrayaan-1s along the innermost visible-near Moscoviense infraredBasin ring (VIS-NIR) data and namedusing M3 the data northern and detected part olivine, as unit orthopyroxene Ivltm (Imbrian, and Mg very-rich low spinel Ti along and verythe southwest low Fe) sector because of its lower titaniumof the and peak the-ring absolute [13]. Bhatt model et al. re- ageclassified [14]. the The previously different mapped understanding unit Iltm using of Chandrayaan the basaltic-1 units also presents thevisible difficulty-near infrared in studying (VIS-NIR the) data basaltic and named volcanism the northern by part only as usingunit Ivltm the (Imbrian spectral very data. low Ti and very low Fe) because of its lower titanium and the absolute model age [14]. The different This isunderstanding mainly because of the the basaltic penetration units also presents depth ofthe thedifficulty used in spectral studying datathe basaltic is rather volcanism low, by only several microns belowonly the using lunar the spectral regolith data. [15 –18]. In such depth, the regolith can easily be affected by the space weathering andThis the is impactmainly because ejecta the from penetration nearby, depth or evenof the used far awayspectral regionsdata is rather [14 ,low,15]. only The several impact events microns below the lunar regolith [15–18]. In such depth, the regolith can easily be affected by the space from Titov craterweathering (150.28 and the◦E, impact 28.55 ejecta◦N) withinfrom nearby the, or Basin even far floor away and regions abundant [14,15]. The craters impact outsideevents from have clearly altered the surfaceTitov crater compositions (150.28°E, 28.55°N) and within topography, the Basin floor which and isabundant inevitable craters to outside affect have the clearly understanding altered of the basaltic unitstheof surface Mare compositions Moscoviense. and topograp Howhy, to which mitigate is inevitable such to disadvantages affect the understanding and of to the better basaltic explore the regolith thermophysicalunits of Mare Moscoviense features in. How situ to is mitigate a critical such problem disadvantages in current and to better lunar explore studies. the regolith thermophysical features in situ is a critical problem in current lunar studies. (a) (b) (c) (d) Figure 1. (aFigure) Position 1. (a) Position of Mare of MoscovienseMare Moscoviense (Green (Green rectangle)rectangle) on onthe thelunar lunar farside. farside. (b) A mosaic (b) Aof mosaic of optical images from Lunar Reconnaissance Orbiter Camera (LROC), which was downloaded from optical images from Lunar Reconnaissance Orbiter Camera (LROC), which was downloaded from https://astrogeology.usgs.gov /search/map/Moon/LRO/LROC_WAC; the green circle is an identified circle related to the rim of Mare Moscoviense. (c) Iron and titanium abundances (wt.%), which was estimated with the Lucey model [19] based on Clementine ultraviolet-visible (UV-VIS) data downloaded from https://astrogeology.usgs.gov/search/map/Moon/Clementine/UVVIS.(d) Geologic map [9] of Mare Moscoviense. Remote Sens. 2020, 12, 535 3 of 22 It is known that the observations in the microwave domain can eliminate well the aforementioned disadvantages [15–18]. For the first time, the Microwave Sounder (CELMS) onboard Chang’E-1/2 (CE-1/2) missions was used to measure the thermal emission of the lunar regolith in microwave domain. For the absence of the water on the Moon, the signal in microwave range can penetrate the regolith up to 10 to 20 times the wavelength [15,16]. The wavelength of the microwave used by the CELMS instrument is about 10 cm at 3.0 GHz, 3.85 cm at 7.8 GHz, 1.55 cm at 19.35 GHz, and 0.81 cm at 37 GHz. Correspondingly, the penetration is about 1 m–2 m at 3.0 GHz, 38.5 cm to 75 cm at 7.8 GHz, 15.5 cm to 31 cm at 19.35 GHz, and 8.1 cm to 16.2 cm at 37 GHz, which is decided by the FTA of the lunar regolith [15–18].