Planetary and Space Science 104 (2014) 244–252 Contents lists available at ScienceDirect Planetary and Space Science journal homepage: www.elsevier.com/locate/pss Mapping lunar mare basalt units in mare Imbrium as observed with the Moon Mineralogy Mapper (M³) F. Thiessen a,b, S. Besse a,n, M.I. Staid c, H. Hiesinger d a European Space and Technology Centre, Noordwijk, Netherlands b Leiden Observatory, Leiden University, Netherlands c Planetary Science Institute, Tucson, Arizona, USA d Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Germany article info abstract Article history: We examine compositional variations of volcanic units in the Imbrium basin using spectral observations Received 7 March 2014 from the Moon Mineralogy Mapper (M³) instrument on board the Chandrayaan-1 spacecraft. The Received in revised form spectral range of M³ reflectance measurements from 400 to 3000 nm is well-suited to study distinctive 30 September 2014 absorption bands near 1000 and 2000 nm resulting from mafic minerals in lunar basaltic flows. Eighty- Accepted 6 October 2014 three units with various mineralogical compositions were identified, and spectroscopic analyses were Available online 30 October 2014 used to map variations in olivine and pyroxene content within basalts emplaced in the Imbrium basin. Keywords: The results exhibit a more precise mapping of basaltic flow units with M³ data based on their better Moon mineralogy Mapper spatial and spectral range in comparison to previous available datasets. Nevertheless, there is a general Imbrium basin correlation between units mapped in this work and previous studies. Moreover, the results tend to Spectroscopy indicate an increase in olivine abundances in the stratigraphically younger high-Ti basalts compared to Lava flows the older low-Ti basalts. Therefore, on the basis of M³ data, we refine previous spectral maps that have been used, for example, to determine first order homogenous units for crater size-frequency distribution measurements. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction on albedo variations and surface roughness (Moore, 1965; Hackmann, 1966; Carr, 1966). This mapping resulted in the identification of large Lunar mare basalts cover 17% of the lunar surface (Wilhelms, basalt units. Stratigraphically, Wilhelms and McCauley (1971) divided 1987). They often occur in large impact structures, such as the the basalts into two units: younger Eratosthenian basalts and older Imbrium basin, which is the second largest basaltic area on the Imbrian basalts. The younger Eratosthenian high-titanium basalts cover Moon after Oceanus Procellarum. These western nearside basalts older low-titanium basalts, and also floodedsmallandlargeEratosthe- include the last extensive phase of lunar volcanism (Hiesinger, 2000, nian craters (Wilhelms, 1987). Although telescopic studies of the 2003), have high titanium-contents (e.g., Pieters, 1978; Wilhelms, younger Imbrium basalts demonstrated that they had similar albedo 1987), and are more iron-rich than other basaltic areas on the Moon and UV/VIS properties as other lunar basalts, the western high- (Staid and Pieters, 2001; Lawrence et al., 2002). titanium basalts were observed to have unusually strong and broad The Imbrium basin has a main ring of 1160 km in diameter 1000 nm absorption bands attributed to enrichment in olivine or FeO- (Spudis, 1993). The determined ages of the basin range from 3.92 Ga rich glass(Pieters et al., 1980). Subsequent studies of Clementine UV/VIS (Neukum, 1983)to3.77 Ga or 3.85 Ga (Stöffler and Ryder, 2001). data by Staid and Pieters (2001) confirmed the presence of olivine After the formation of the basin, volcanic materials started to fill the within these high-titanium basalts and further observed that the interior within the first100Maaftertheimpactevent(Hiesinger et al., olivine content appeared to increase in subsequent eruptions. Com- 2000). The duration of volcanism within the basin lasted for at least pared to the eastern maria (e.g., Tranquillitatis, Serenitatis), the western 1.5 Ga (Hiesinger et al., 2000). Geological mapping of the Imbrium mare basalts also exhibit a higher concentration of radioactive ele- basin began prior to the Apollo missions, when the Lunar Orbiter ments such as thorium and potassium (Lawrence et al., 1998), which missions delivered the first high resolution photographs (Head et al., may enable a longer thermal activity in the western maria (Soderblom 1978). Different units within the Imbrium basin were mapped based et al., 1977, Hiesinger et al., 2011). Recent observations by the Moon Mineralogy Mapper (M³)instrumentconfirm that these western high- titanium basalts show strong 1000 nm and weak 2000 nm ferrous n Corresponding author. Tel.: þ31 71 565 3677. bands, which are consistent with higher olivine abundances (Staid E-mail address: [email protected] (S. Besse). et al., 2011). Specifically, the spectral data from M³ indicate higher http://dx.doi.org/10.1016/j.pss.2014.10.003 0032-0633/& 2014 Elsevier Ltd. All rights reserved. F. Thiessen et al. / Planetary and Space Science 104 (2014) 244–252 245 abundances in olivine for the stratigraphically youngest flows (Staid mafic minerals. Basalts are, compared to the highlands, more enriched et al., 2011). in FeO and TiO2, they exhibit a higher pyroxene and/or olivine content, Schaber (1973) used low sun angle photographs obtained by and are less plagioclase-rich (Neal et al., 1992). Pyroxenes exhibit the Apollo 15 and 17 missions along with Lunar Orbiter images to distinctive absorption bands near 1000 and 2000 nm due to map discrete flow boundaries within Mare Imbrium. Three major ferrous iron (e.g., Adams, 1974; Cloutis and Gaffey, 1991; Burns, 1993), eruption phases were mapped extending from the south-western whereas olivine displays three overlapping characteristic absorption edge of the basin for 1200, 600, and 400 km, respectively. According to bands near 1000 nm. In pyroxenes, band centers shift to longer Schaber (1973),thesourceofthelavaflows was at the south-western wavelength as Ca and Fe substitute for Mg (e.g., Adams, 1974; Cloutis edge of Mare Imbrium close to crater Euler. The basalts in the western and Gaffey, 1991; Burns, 1993; Klima et al., 2011), whereas olivine portion of Mare Imbrium were interpreted as stratigraphically younger exhibitacomplexabsorptionbandcenteredat1050nmthatalsoshift than those in the eastern part (Schaber, 1973), consistent with later to longer wavelengths (Adams, 1975). Thus, in order to identify and crater size-frequency distribution model ages (e.g., Hiesinger et al. map the compositional variations of the basalt units, the analysis of 2000, 2003, 2011). In this later investigation, multispectral data from the 1000 and 2000 nm regions is needed. Integrated Band Depth (IBD) Clementine and Galileo were used to map basalt units based on their highlights the properties of the whole absorption band. The IBD1000 spectral properties. Furthermore, absolute model ages were derived represents the band depth between 789 and 1308 nm relative to a for those units, which indicate a long duration of volcanic activity and straight continuum, whereas the IBD2000 is the integrated band a large variety in composition. In total 30 different basalt units were depth between 1658 and 2498 nm with a straight continuum removal identified, with ages ranging from 2.01 to 3.57Ga (Hiesinger et al., (see also Klima et al., 2011; Besse et al., 2011). Moreover, we used 2000, 2003). Morota et al. (2011) investigated 10 of these units defined individual Band Depths (BD), which in contrary to the IBD represent by Hiesinger et al. (2000, 2003) with SELENE (Kaguya) data. Their the band depth at one particular wavelength. Furthermore, the model ages agree with the results of Hiesinger et al. (2000, 2003), reflectance (R) at 750 nm is used for the blue channel in the color although four of the ten investigated basalt units show up to 1Ga images due to limited absorptions of maficmineralsat750nm(e.g., younger model ages than determined by Hiesinger et al. (2000, 2003). feldspathic material). Previous spectral mapping of basalt units using Clementine and Three different color composite images (hereafter named CC) Galileo color ratio images were based on UV/VIS ratios with limited were created: (1) CC1: BD950 (red channel), BD1050 (green channel) spectral resolution (e.g., Hiesinger et al., 2000, 2003). In this analysis, and BD1250 (blue channel), (2) CC2: BD1900 (red channel), BD2300 high spectral and spatial resolution data from the M³ instrument were (green channel) and R750 (blue channel), and (3) CC3: IBD1000 (red used to study both the 1000 and 2000 nm absorption regions of mafic channel), IBD2000 (green channel) and R750 (blue channel). The CC1 is minerals within the Imbrium basin. We address the following ques- diagnostic of the 1000 nm region and mostly the variations between tions: (1) How many mineralogically different basalt units can be low and high Ca-pyroxenes (red and yellow hues, respectively), and defined? (2) Do we observe a regional distribution of basalts with olivine (green-blue hues). The parameters used in CC2 are helpful to similar compositions? (3) Do the results of this study agree with differentiate between Ca- and Mg-pyroxenes (green and red hues, previous analyses, and what are the implications for lunar volcanism respectively), whereas the CC3 is used to highlight the relative strength in the Imbrium basin? of the 1000 and 2000 nm bands (red and green-yellow hues, respec- tively), which are indicative of pyroxene and olivine abundance varia- tions. As shown by Staid et al. (2011), variations within CC3 highlight 2. Methods mineralogical differences within the younger high-titanium basalts and differentiate them from the older basalts within Mare Imbrium. 2.1. Moon Mineralogy Mapper (M³) M³ is an imaging spectrometer (Pieters et al., 2009) on Chandrayaan- 3. Results 1, India´s first planetary exploration mission (Goswami and Annadurai, 2009). Chandrayaan-1 was launched on October 22, 2008, and acquired 3.1. Mapping data with a suite of instruments from November 18, until contact with the spacecraft was lost on August 30, 2009 (Goswami, 2010).