Mapping Lunar Mare Basalt Units in Mare Imbrium As Observed with the Moon Mineralogy Mapper (M³)

Planetary and Space Science 104 (2014) 244–252

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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, respectively), which are indicative of pyroxene and olivine abundance variations. 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). M³ was These spectral parameters allowed mapping basalt units by designed to obtain visible to near-infrared (420–3000 nm) reflectance visually identifying units with different color, and thus different data in 85 or 260 spectral bands. In this study, global mode observations compositions. Due to the high density of mineralogical variations, with a resolution of 140 m/pixel and 85 spectral bands during the and the noise of the data, a statistical approach to map the units OpticalPeriodOP1Bareused(Boardman et al., 2011). The data delivered was not possible to map coherent lava units. Basalt flows were to the Planetary Data System (PDS), and used in this study, have been separately mapped in each of the CC images, thus resulting in map-projected (Boardman et al., 2011), and a thermal (Clark et al., 2011) slight differences in the definition of basalt boundaries and and a photometric correction have been applied (Besse et al., 2013). different numbers of subunits. For instance, the mapping of CC1 Figs. 1 and 2ashowtheM³ mosaic of the Imbrium basin, which consists resulted in limited number of basalt units mostly because of a of 61 individual strips acquired from February 5, 2009 to February 9, lower signal-to-noise ratio for this CC, and because of the limited 2009. All observations were acquired during the same Optical Period variation of the 1000 nm band location between maficminerals OP1B in order to limit temperature variations of the detector (Isaacson (Fig. 1a). Redder units in the CC1 (Fig. 1a) are indicative of absorption et al., 2013). Additional cross-track corrections as described in Besse band center at wavelengths shorter than 1000 nm, and consistent with et al. (2013) were performed to reduce image-to-image brightness low-Ca pyroxenes. High-Ca pyroxenes exhibit a band center close to variations of the mosaic. The proposed ground truth correction from 1000 nm (Adams, 1974; Cloutis and Gaffey, 1991) and thus, regions Isaacson et al. (2013) is focused on feldspathic mature soils, thus it is not with a higher abundance of high-Ca pyroxenes will appear with a used in this study. green-yellow hue. On the other hand, a stronger blue hue corresponds to a stronger absorption at 1250 nm, which is most likely indicating the 2.2. Spectral parameters presence of olivine. The CC2 mainly highlights the differences between Ca- and Mg- In order to map the basalt units, different spectral parameters pyroxenes (Fig. 1b). Ca-pyroxenes exhibit band centers at wavelengths were chosen based on the diagnostic absorption bands of lunar longer than 2100 nm, whereas Mg-pyroxenes have band centers at 246 F. Thiessen et al. / Planetary and Space Science 104 (2014) 244–252

Fig. 1. Comparison of color composite CC1 and CC2. Large and spectrally bright craters are mapped separately in grey and were excluded from the basalt units. The surrounding highlands and kipuckas inside the Imbrium basin are also shown in grey. Black strips correspond to portion of the lunar surface not observed with M³ during OP1B. (a) CC1 (red: BD950, green: BD1050, blue: BD1250), 39 basalt units could be identiﬁed. (b) CC2 (red: BD1900, green: BD2300, blue: R750), 46 basalt units were identiﬁed.

Fig. 2. (a) M³ color composite image (CC3) of the Imbrium basin (red: IBD1000, green: IBD2000, blue: R750 nm). Numbers indicate the basalt units mapped in this work. Large and spectrally bright craters are mapped separately in grey and were excluded from the basalt units. The surrounding highlands and kipuckas inside the Imbrium basin are also shown in grey. Dark strips correspond to portion of the lunar surface not observed with M³ during OP1B. The white cross and arrow indicate the landing site of Chang'e 3 and the Yutu rover. (b) Eratosthenian basalt flows from Schaber (1973) with flow phases I-III. wavelengths shorter than 2100 nm (Adams, 1974; Cloutis and Gaffey, subunits from CC1 and CC2, if they were not already mapped in 1991). Thus, green hues have a higher abundance of Ca-pyroxenes CC3. In Fig. 2a, mare basalts that appear redder have stronger (green channel: BD 2300), red hues are more consistent with Mg- absorption bands at 1000 nm (red channel) and weaker absorp- pyroxenes (red channel: BD 1900). Although the CC2 composite shows tions at 2000 nm (green channel). A yellow hue indicates a different numerous spectral variations, it mainly highlights the compositional relative strength of the two absorption bands with a stronger variations in pyroxene. contribution of the 2000 nm band. Any hues between yellow and Unit boundaries presented in Fig. 2a are based on an integrated red are thus characteristic of variations of the relative strengths approach that combines the mapping performed on three different between the two IBD. Areas with a blue hue are mostly character- color composite images, CC1, CC2 and CC3. In order to highlight ized by a high reflectance at 750 nm, where mafic absorptions are pyroxene and olivine variations simultaneously, the observation of very limited. Thus, areas with blue hues represent most likely non- CC3 is highly relevant since it maps the mineralogical variations of mare materials. In fact, most of these areas are correlated with pyroxenes and olivine. Therefore, the resulting mapping consists ejecta from large impacts (e.g., Copernicus) that have excavated primarily of boundaries from the CC3 with added boundaries and feldspathic materials that are now superposed on the mare basalts. F. Thiessen et al. / Planetary and Space Science 104 (2014) 244–252 247

3.2. Basalt units ejecta contamination might have contributed to the observation of a high number of small basalt units in this area. In total, 83 spectral units were identified within the Imbrium basin, and are summarized in Table 1. These units include basalts, 3.3. Spectral data basalts mixed with ejecta, and the ejecta blankets of large craters only. The largest basalt units occur in the north and north-western Fig. 3 compares the spectra of five different mare units (units 21, part of the basin, with unit 80 being the largest (i.e., 81268 km²). 36, 35, 80, and 41), before and after continuum removal. These units Units 19 and 32, with the prominent blue hues (Fig. 2a), corre- are chosen to highlight basalt units of Fig. 2a with different color hues spond to areas containing large amounts of ejecta contamination from and variations in mineralogical composition: (1) area 41 exhibits a the crater Aristillus and Copernicus, respectively. These blue hues dark red hue, (2) area 80 is orange, (3) area 35 is bright yellow, (4) area correspond to a lack of mafic absorptions characteristic of highlands 36 has a yellow hue, and (5) area 21 displays a light orange hue. material surrounding the Imbrium basin. Due to the ejecta contam- Furthermore, these basalt units are adjacent to each other and show inationinthesouth-westernpartoftheImbriumbasin,spectral major changes within a few kilometers. Moreover, units 21, 35, and 36 boundaries of the basalt units were more difficult to identify. Thus, are stratigraphically older low-titanium basalts defined by Wilhelms

Table 1 Characteristics of the basalt units mapped in Fig. 2a with their respective sizes. Kipuckas and large craters mapped within the units (e.g., light grey in Fig. 2a) were excluded in the calculated area size.

Unit Description Area [km²] Unit Description Area [km²] Unit Description Area [km²]

1 Basalt 5893 29 Basalt 536 57 Basalt 842 2 Basalt 13970 30 Basalt 5194 58 Basalt 1264 3 Basalt 6783 31 Basalt 27997 59 Basalt 1217 4 Basalt 4161 32 Ejecta 73920 60 Basalt / ejecta 3592 5 Basalt 2329 33 Basalt 44704 61 Basalt 1286 6 Basalt 1355 34 Basalt 4736 62 Basalt 4383 7 Ejecta 2026 35 Basalt 4119 63 Basalt 11301 8 Basalt 79084 36 Basalt 17397 64 Basalt 1147 9 Basalt 1551 37 Basalt 1060 65 Basalt 27151 10 Basalt 1151 38 Basalt 1230 66 Basalt 27359 11 Basalt 1069 39 Basalt 2429 67 Basalt 2284 12 Basalt 194 40 Basalt 661 68 Basalt 10380 13 Basalt 768 41 Basalt 52012 69 Basalt 12099 14 Basalt Plato 3046 42 Basalt 445 70 Basalt 1371 15 Basalt Plato 2905 43 Basalt 507 71 Basalt 38340 16 Basalt 10018 44 Basalt 586 72 Basalt 55683 17 Basalt 4197 45 Basalt 1178 73 Basalt 768 18 Basalt 12500 46 Basalt 280 74 Basalt 460 19 Ejecta 53750 47 Basalt 2337 75 Basalt 1590 20 Basalt 13187 48 Basalt 1977 76 Basalt 8101 21 Basalt 14082 49 Basalt 12262 77 Basalt 4304 22 Basalt 4997 50 Basalt 12932 78 Basalt 734 23 Basalt 10956 51 Basalt / ejecta 9901 79 Basalt 422 24 Basalt 9273 52 Basalt / ejecta 8231 80 Basalt 81268 25 Basalt 8583 53 Basalt 5957 81 Basalt 187 26 Basalt 7603 54 Basalt / ejecta 590 82 Basalt 414 27 Basalt 1684 55 Basalt / ejecta 1893 83 Basalt 215 28 Basalt 1009 56 Basalt / ejecta 10801

Fig. 3. (a) Spectroscopic comparison of five distinctive basalt units (i.e., 21, 35, 36, 80, and 41 in Fig. 2a). The spectra are averaged over the entire mapped units with the exclusion of large impact craters, kipuckas, bright ejecta, and clusters of craters that might excavate materials from underneath the basalt units, and therefore change the spectral signal. (b) Continuum removed spectra using a straight-line approximation (i.e., 730–1620 nm and 1620–2580 nm see Besse et al. (2014)). 248 F. Thiessen et al. / Planetary and Space Science 104 (2014) 244–252 and McCauley (1971), whereas units 41 and 80 represent younger variations in hues from yellow to red are correlated to an increase in Eratosthenian high-titanium basalts (Wilhelms and McCauley, 1971). olivine content. The younger high-titanium basalts (i.e., 41 and 80) were also observed and characterized by Pieters (1978) as spectrally blue basalts due to their high UV/VIS ratio, which to a first order is consistent with higher 4. Discussion titanium content. Except for area 41 and 80, all spectra show similar band locations 4.1. Comparison with previous work near 1000 and 2000 nm (Fig. 3b). The spectra of unit 41 and 80 show a shift toward longer wavelengths of the 1000 nm absorption bands as Our larger basalt units agree with the previous boundaries well as weaker absorptions near 2000 nm. As previously proposed by defined by Hiesinger et al. (2003). However, Hiesinger et al. (2003) Staid et al. (2011), these changes in the M3 reflectance data are defined only 30 units within the Imbrium basin, whereas this analysis consistent with variations in the olivine abundances between major identified 83 units. This difference is likely due to a wider spectral units of the younger high-titanium basalts in this region (i.e., 1000 nm range and better spectral resolution of the M³ data, since Hiesinger band shifted to longer wavelengths, and an important change in the et al. (2000, 2003) focused only on the UV/VIS region. The two maps of relative strength of the two absorption bands). Olivine absorptions spectral mare units in the Imbrium basin are compared in Fig. 4. Fig. 4b are easily covered by pyroxene absorptions since olivine is less shows a more detailed examination of one region with the basaltic absorbing (Pieters et al., 1980). Thus, the distinctive shapes of the units (i.e., basalt units 21, 35, 36, 80, and 41 in Fig. 2a) described in the spectra from areas 41 and 80 indicate that the olivine abundance is spectroscopic analyses (i.e., Section 3.3). likely to be relatively low in comparison to pyroxene. However, other From this new map several conclusions can be drawn: (1) the lunar minerals such as plagioclase also have diagnostic absorptions large basalt units were previously correctly identified in UV/VIS near 1200 nm that could be confused with olivine. Nevertheless, images (Hiesinger et al., 2000, 2003) and based on morphology plagioclase absorptions at 1250 nm appear very weak in comparison (Schaber, 1973), (2) more spectral boundaries are found in this with the mafic silicate absorptions, which is due to the small amount work, and previous units could be subdivided into smaller units of Fe2þ and the consequent low absorption coefficient (Crown and given the better spectral resolution of M³, (3) spectral differences Pieters, 1987). Studies have shown that the plagioclase absorption are mapped more accurately using simultaneously the absorption changes the spectra only if plagioclase comprises more than 85 vol% bands at 1000 and 2000 nm and thus, improve the definition of (Crown and Pieters, 1987; Cheek et al., 2013), which is very unlikely for unit boundaries, and (4) a few boundaries identified by Hiesinger this basaltic region. Thus, it appears that unit 41 and 80 are enriched in et al. (2000, 2003) cross units without any visible spectral boundary olivine compared to the three other units. Minor differences in olivine in the M³ images. content might also be inferred for the three lower-titanium Imbrian units, with unit 35 being the least olivine-rich, unit 21 the most 4.2. Relationship with the stratigraphy olivine-rich and unit 36 in the middle. This interpretation is based on the slight widening of the 1000 nm band from unit 35 to 21, and Our unit boundaries are similar to the flow boundaries pro- the observation of weaker absorptions around 2000 nm. However, posed by Schaber (1973) (Fig. 2b). Although Schaber (1973) did not because these changes are subtle, associations with higher olivine map the older Imbrian-aged basalt flows, most of the boundaries content should be made with caution. Moreover, small changes in of the younger Eratosthenian basalts match with the spectral the shape of the 2000 nm band can be caused by variations in the boundaries defined in our study. For instance, the proposed phase thermal correction, photometric correction, and other effects (e.g., III of the volcanic activity corresponds to unit 66, phase II to unit grain size) (Clarketal.,2011,Besseetal.,2013). However, the fact that 41, and phase I to unit 80 (located in the northern part of the the two absorption bands do not show identical changes, in particular basin). However, the basalt flows of phase I and II south-east of in intensity, tends to limit the contribution of artifacts. Thus, the crater Euler are divided in several subunits in this work, based observation of the five spectra leads to the interpretation that the on the new mapping of subtle spectral differences. This area is

Fig. 4. (a) Comparison of basalt units from Hiesinger et al. (2003) (blue lines) and this work (black lines). (b) Detailed view of the region highlighted by the black square in Fig. 4a containing also the spectral units investigated in Section 3.3. F. Thiessen et al. / Planetary and Space Science 104 (2014) 244–252 249 covered by ejecta from Copernicus and thus, these subunits might Section 3.3). Thus, taking the stratigraphic relationship and the either: (1) belong to a larger unit, which could not be identified spectral characteristics of M³ observations into account, it appears in this study due to the ejecta contamination, or (2) these subunits that the volcanic activity of the Imbrium basin was less olivine- might indicate the final products of late-stage volcanism in this rich at the beginning of phase I and that the olivine abundance area with higher and variable olivine content, and smaller units. increased until the end of phase III. High Ca-pyroxene units, such These smaller high-titanium subunits are located close to crater as unit 8 are not described in Schaber (1973), but they were mapped Euler, which is suspected to be close to the source of lava flows as stratigraphically older basalts by Wilhelms and McCauley (1971), (Schaber, 1973). The observation that basalt units are getting andwerethusemplacedbeforethephaseIbasalts.Thisisalso smaller and thus, less voluminous close to the suspected source consistent with absolute model ages from Hiesinger et al. (2000). is consistent with the observations from Schaber (1973), since his These observations imply that the older Imbrian aged basalt units are mapped basalt flows decrease in length and volume with time. less olivine rich, whereas the younger basalt units (phases I-III from Although, the petrological model from Head and Wilson (1992) Schaber, 1973) exhibit higher olivine abundances, which increase predicts large-scale flows at the end of volcanism, the authors also with the emplacement of the stratigraphic youngest flows. This is suggested that the flows could decrease in volume during the consistent with other spectral observations within Oceanus Procel- Eratosthenian due to the overall cooling of the Moon. This might larum that show two important volcanic episodes with the first be due to the inability of magma to overcome greater compres- being richer in low/high Ca-pyroxene, and the second being enriched sional horizontal stress and thus, to reach the surface (Head and in olivine (Staid et al., 2011; Besse et al., 2011). Such observations are Wilson, 1992). The morphological (Schaber, 1973) and mineralo- supported by absolute model ages derived by Hiesinger et al. (2000, gical [this work] properties of Imbrium flows are consistent with 2003, 2011), which indicate that olivine abundances increase as the shorter and less voluminous flows with time. basalts are getting younger. Fig. 2a-b highlight not only the stratigraphic relationships, but more importantly the variations in spectral properties between 4.3. Relationship with the topography older low-titanium basalts and younger high-titanium basalts. The youngest flows (phase II-III of Schaber, 1973) with a redder hue Fig. 5 highlights the topography at kilometer scale from the (stronger 1000 nm vs. 2000 nm IBD) superpose older flows (phase Lunar Orbiter Laser Altimeter (LOLA) of the Imbrium basin with I), which exhibit an orange hue (stronger absorptions at both, 1000 the updated mare basalt units superposed. The boundaries of the and 2000 nm). Moreover, the stratigraphically older low-titanium spectrally defined basalt units generally do not match topography basalts in the north and east of the basin show a yellow hue due to at this scale. The basalt units south of the crater Euler (i.e., brown strong absorptions at both 1000 nm and 2000 nm (compare with color and units 56, 57, 58, 59, 60, 61, and 62) occur at the highest

Fig. 5. Lunar Orbiter Laser Altimeter image displaying the elevations within of the Imbrium basin with the updated mare basalt units of this work superposed. The lowest areas of the basin are represented by a pink hue, whereas brown areas indicate the highest points of the basin. The location of the crater Euler is indicated by the black arrow. The surrounding highlands, and kipuckas are shown in grey. The color bar indicates the elevation in meters relative to the lunar geoid deﬁned as the equipotential surface whose mean equatorial radius is 1737.4 km. LOLA image is sourced from the PDS LOLA node. 250 F. Thiessen et al. / Planetary and Space Science 104 (2014) 244–252

Fig. 6. (a) Chang'e 3 landing site (grey arrow) within the younger darker high-Ti basalts. Regions 1 and 2 are the locations of spectra presented in Fig. 6b (full lines). Spectra of the same two spectral units (regions 3 and 4) were also taken farther away from the unit boundaries (dashed lines in Fig. 6b), they exhibit the same characteristics. (b) Continuum-removed spectra of both units showing that the high-Ti unit has band minimum shifted to longer wavelengths. elevation of the basin. This part of the basin is the suggested source of Fig. 6b) are presented in Fig. 6b, along with spectra of the same the high-Ti lava flows mapped by Schaber (1973),whichthenflowed units, which are located further away (regions 3 and 4 in Fig. 6a, from the highest elevation in the south-western part of the basin to dashed lines in Fig. 6b). The high-Ti unit (i.e., regions 2 and 4, and the lower elevated north-eastern part of the basin (Fig. 2b). Thus, these grey lines, Fig. 6b) shows absorption bands minima at 1000 and three flow phases show a general trend to follow the general slope of 2000 nm located at longer wavelengths relative to the low-Ti unit the basin. Another possible correlation with the topography is shown (i.e., regions 1 and 3, and black lines, Fig. 6b). The band minima are for unit 6, which appears to cover a portion of a wrinkle ridge. In fact, consistent with a higher abundance of olivine and/or clinopyrox- Daket et al. (2013) argued that the ridge has been reactivated by late ene in the high-Ti unit. Moreover, the relative band strength of the volcanism, which could explain the relationship between this unit and 1000 and 2000 nm absorption bands is consistent with an increase the topography. However, the proposed age of 2.97 Ga (Daket et al., of olivine content for the high-Ti unit, as already stated at a larger 2013) is not younger than the 2.96 Ga of unit 80 (Hiesinger et al., scale observations in the Imbrium basin (e.g., this work) and 2000), although color trends in Fig. 2a suggest that unit 80 is olivine Oceanus Procellarum (e.g., Staid et al., 2011) using M3, and other richer, and therefore, younger than unit 6. Nevertheless, at a kilometer data. The spectra of the two units located farther away from the scaletherearenoobviousrelationshipsbetweenthebasaltunitsand Chang'e 3 landing site are consistent with those closer to the the topography of the basin. This absence of correlation is highlighted landing site, showing that contamination of the units (e.g., spectral by wrinkle ridges, which form a topographically higher inner ring mixing due to regolith formation) is limited close to the boundary. within the basin. These ridges intersect basalt units (e.g., units 76 and Although the landing site lies in high-Ti basalts that appear to be 72 or units 35, 33, 25, and 21) without any preferential association enriched in olivine relative to surrounding low-Ti units, other with their spectral boundaries, suggesting an absence of correlation (younger) regions of the western high-Ti basalts appear to have between topography and spectrally defined basalt units. significantly higher concentrations of olivine than these basalts (e.g. Staid et al., 2011; Besse et al., 2011). 4.4. Implications for the Chang-e 3 lunar landing in the Imbrium basin

In December 2013, the Chinese Chang'e 3 lunar mission landed in the Imbrium basin close to the crater Laplace F (coordinates are 5. Conclusions 441 N, 19.51W). The current location of the rover in Imbrium is of great interest since it is located very close to the high and low-Ti Using M³ observations, a new map of basalt units in Mare mare basalts boundary identified in several studies (e.g., Schaber, Imbrium has been produced. The boundaries are based on spectral 1973; Staid et al., 2001). This boundary corresponds to properties, that is, the 1000 and 2000 nm absorption bands. On the separation of units 80 and 8 of this study (Fig. 2a), and is the basis of our new spectral map, we conclude: indeed characteristic of variations in titanium content and/or olivine (Fig. 2a). This boundary could also be easily identified in (1). The comparison with previous work generally shows good the 2860 nm map of Fig. 6a, suggesting that the texture, and/or the agreement in the definition of units with the flows of Schaber albedo of these two units are not the same because thermal (1973) and the spectral units of Hiesinger et al. (2000, 2003, contribution is important at this wavelength (Clark et al., 2011). 2011). The Chang'e 3 landing site is located south of the boundary, within (2). Eighty-three units were mapped; this is a significant increase the high-Ti mare basalts. Average spectra of two regions charac- in the number of units attributed to the improved spectral teristics of each unit (Fig. 6a, regions 1 and 2, and solid lines in resolution and range of M³ observations. F. Thiessen et al. / Planetary and Space Science 104 (2014) 244–252 251

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