Characterization of High-priority Landing Sites for Robotic Exploration Missions in the Apollo Basin, Moon

The South Pole–Aitken (SPA) basin is the oldest and largest visible impact structure on the Moon, making it a high priority science site for exploration missions. The 492 km diameter Apollo peak-ring basin is one of the youngest and largest basins within the SPA basin. We selected three regions of interest (ROIs) in the Apollo basin for which the landing and operational hazards are minimized and evaluated their science and in situ resource utilization (ISRU) potential. We examined topography, slope, crater density, rock abundance, geologic mapping, mineralogy, and inferred subsurface stratigraphy within each ROI. The results show that the terrain is safe for landing without precision landing (within a few hundred meters). The mare materials have high ISRU potential with relatively high FeO (∼16–20 wt%) and TiO2 (∼3–10 wt%) contents. Two robotic exploration mission architectures were examined for their scientific potential: (1) lander and rover with a dedicated payload suite and (2) the same architecture with sample return capability. In situ observations can address six of seven National Research Council concepts (1–3, 5–7) and Campaigns 1 and 5 of the European Space Agency’s Strategy for Science at the Moon.

Several mission concepts with 14 day durations are designed for short-distance (3.7 and 10.8 km), medium-distance (22.5 and 12.7 km), and long-distance (28.8 and 37 km) traverses of the Schrödinger basin (e.g., Potts et al. 2015;Steenstra et al. 2016;Allender et al. 2019, and additional references therein).More ambitiously, Steenstra et al. (2016) proposed a humanassisted robotic sample return mission concept in support of ESA's HERACLES human-assisted sample return mission concept.This was based on a 3 yr mission plan where two robotic mission traverses are planned with an ESA-specified total traverse distance of 100-300 km.
On 2019 January 3, the Chinese Chang'E-4 mission became the first to land on the farside of the Moon, in Von Kármán crater (the northwest portion of the SPA) (Huang et al. 2018;Jia et al. 2018;Wu et al. 2019).Initially, the primary landing site for the Chang'E-4 robotic mission was the 492 km diameter Apollo basin (Figure 1), in the northeast quadrant of the SPA basin (Wang & Liu 2016), also a location of interest for NASA's Constellation Program (Gruener & Joosten 2009).Additionally, the Endurance-R mission concept is targeting the Apollo basin with a rover traversing across the SPA basin interior from the Poincaré to the Apollo basin (Keane et al. 2021).
As the Apollo basin has remained a potential landing site for other future robotic, nonpolar exploration missions inside the SPA (Jawin et al. 2019), we studied the exploration potential of this basin, building on work done mapping this part of the SPA basin (Ivanov et al. 2018).We preselected three regions of interest (ROIs), located on both central and southern mare deposits, based on the inspection of terrain properties, hazards (e.g., varying topography and roughness, slopes, crater density, and rock abundance), scientific rationale, and high FeO and TiO 2 contents (Figures 2 and 3).Then, we performed a series of detailed geospatial and geostatistical analyses for each ROI using various optical, topographical, and spectral data sets in ArcGIS.Finally, we investigated how many key lunar science objectives of the US National Research Council (National Research Council 2007) and Campaigns 1 and 5 of the European Space Agency's Strategy for Science at the Moon (European Space Agency 2019a) could be addressed and satisfied in such candidate landing sites in the Apollo basin.
Taking previous mission and concept studies into account, we chose exploration zones of similar size (2.5, 5, and 10 km) around each candidate landing site (e.g., Kring & Durda 2012;Karachevtseva et al. 2013;Potts et al. 2015;Qiao et al. 2015;Steenstra et al. 2016;Karachevtseva et al. 2017;Allender et al. 2019).We investigated the hazards, science, and in situ resource utilization (ISRU) potentials for a nominal mission phase using two robotic exploration mission scenarios: (1) lander and rover assets with a dedicated payload suite and (2) the same architecture with additional sample return capability.This paper is focused specifically on geoscience-related themes for landing site characterization.

Science Rationale: What Do We Know?
The SPA and Apollo basins exhibit a diverse suite of geological settings where numerous fundamental scientific questions can be addressed, which are summarized in Sections 2.1 and 2.2.

SPA Basin
The SPA basin is an elliptical-shaped giant impact structure, with a diameter of 2400 by 2050 km and a depth of approximately 13 km (Garrick-Bethell & Zuber 2009;Smith et al. 2010).Moreover, the SPA basin is the oldest observable impact structure on the Moon (4.26 ± 0.03 Gyr, Hiesinger et al. 2012;4.31 +0.019/−0.021Gyr, Orgel et al. 2018).
Four distinct, approximately concentric mineralogical zones characterize the SPA basin (Lucey et al. 1998a;Jolliff et al. 2000;Pieters et al. 2001;Lawrence et al. 2002;Gibson & Jolliff 2011;Moriarty & Pieters 2018): (1) the SPA compositional anomaly in the SPA basin center, which exhibits Ca and Fe-rich pyroxenes; (2) an orthopyroxene annulus (OPX-A), dominated by Mg-rich pyroxenes; (3) a heterogeneous annulus (HET-A), which shows a mixture of Mg-rich pyroxenes and feldspathic materials; and (4) an SPA exterior with feldspathic composition (see Figure 9 of Moriarty & Pieters 2018).These mineralogical findings provide a framework for understanding of the formation, evolution, and subsequent geologic processes of the SPA basin and the early lunar crust and mantle.
As a consequence of such a large basin-forming impact event, the crustal thickness beneath SPA is less than 20 km, with local thicknesses of <5 km beneath the Apollo and Poincaré basins (Wieczorek et al. 2013).Additionally, the northwest of the SPA basin exhibits slightly enhanced thorium concentrations, while the floor of SPA is relatively low in thorium (Moriarty et al. 2021b).Thorium is a heat-producing element due to its radioactivity and could increase volcanic activity (Wieczorek & Phillips 2000).However, despite the thin crustal thickness, the presence of volcanic deposits across the SPA basin floor is minor compared to the nearside of the Moon (Yingst & Head 1999;Moriarty & Pieters 2015, 2016a;Whitten & Head 2015;Pasckert et al. 2018;Meyer et al. 2020;Chuang et al. 2022;Moriarty et al. 2022).There are several observations of pyroclastic deposits ranging from small (as in the Apollo basin and the center of the SPA basin) to large local deposits (as in the Oppenheimer crater and Schrödinger basin; Wilhelms et al. 1979;Gaddis et al. 2003;Borst et al. 2012;Kramer et al. 2013) and indicating a deeply seated mantle source around 400 km depths.
Moreover, other than the central peak/peak-ring structures of Schrödinger basin and Zeeman crater, a significant amount of olivine has not yet been detected within the SPA basin, the suggested main mineral phase of the lunar mantle that was expected to be exposed due to the SPA and subsequent basinforming impact events (Yamamoto et al. 2012;Kramer et al. 2013).The absence of extensive olivine signatures suggests the presence of a mafic-rich primary crust or cumulate pile at the depth of excavation (Potter et al. 2018), which is in good agreement with the findings of Vaughan & Head (2014).Melosh et al. (2017) used three-dimensional numerical impact simulations and demonstrated that the SPA basin-forming event must have sampled the upper mantle to depths of 100 km.Observations from Moriarty et al. (2021b) show that the SPA impact melt is dominated by low-Ca pyroxene-dominated assemblages and suggest a stratified upper mantle at the time of the SPA formation.

Apollo Basin
The Apollo basin (36.09°S, 159.69°W) is the largest (492 km diameter; Baker et al. 2011) and one of the youngest well-  From a mineralogical point of view, the Apollo basin overlays two mineralogical zones (Moriarty & Pieters 2018, their Figure 9): the HET-A and OPX-A terrains.Baker & Head (2015) defined the Apollo basin as the "Class C = 95% plagioclase (+ pyroxene)" spectral type.However, they did not detect crystalline or shocked plagioclase on any outcrops.Furthermore, Potter et al. (2018) suggested that the absence of pure anorthosite is due to the magnitude of the impact, which shocked the crust and erased the 12.5 μm anorthosite band.However, the peak ring is dominated by pyroxenes (Klima et al. 2011;Moriarty & Pieters 2014;Baker & Head 2015), which might originate from the lower crust or could be explained by the presence of a differentiated SPA melt sheet (Hurwitz & Kring 2014;Vaughan & Head 2014).According to Melosh et al. (2017) and Moriarty et al. (2021b), the Apollo basin is located on the proximal ejecta blanket of the SPA basin, which had a thickness of up to 30 km; thus, it is unlikely that the SPA impact melt sheet extended out to the Apollo basin area.Moreover, the Th abundance maps show that the formation of the Apollo basin excavated entirely through the SPA ejecta blanket, exposing substrate crustal materials.Therefore, due to the deep excavation depths of both the SPA and Apollo basin-forming impacts and mineralogical observations, the potential to find diverse rock types in the Apollo basin is high, which could allow us to better understand the internal structure and composition of the Moon.
The first geologic maps of the Apollo basin at 1:5M scale were compiled based on Apollo and Lunar Orbiter IV imagery by Stuart-Alexander (1978) and Wilhelms et al. (1979;Figure 3(e)).Recently, higher-resolution data sets from the Lunar Reconnaissance Orbiter Wide Angle Camera (LROC WAC) have allowed for the Apollo basin to be mapped at 1:500 K scale (Ivanov et al. 2018).It was found that two major classes of landforms are present: (1) impact craters and related features and (2) plains-forming terrains of volcanic (dark plains) and impact (light plains) origin (Wilhelms et al. 1979;Thiessen et al. 2012;Meyer et al. 2016;Ivanov et al. 2018;Pasckert et al. 2018;Meyer et al. 2020).
The basin floor is mainly covered by four major mare basalt provinces (Pasckert et al. 2018, their Figure 5) with absolute model ages (AMAs) ranging from 2.30 to 3.45 Gyr (central mare province: 2.93 Gyr, southern mare province: 3.31 and 3.45 Gyr; Pasckert et al. 2018) or from 2.44 Gyr (southern mare province) to 2.49/3.51Gyr (central mare province; Haruyama et al. 2009).These mare deposits have enhanced FeO and TiO 2 contents, among the highest in the SPA basin, and are thus attractive for ISRU activities (Figures 3(f   genetically linked 70% of all light plains, including those in the Apollo basin, to the Imbrium and Orientale basin-forming events.

Selection Criteria for ROIs
Typically, landing site assessments are based on criteria including (1) terrain properties (e.g., topography, roughness, slopes, crater density, and rock abundance), (2) landing precision and operations constraints, (3) scientific interest, and (4) ISRU potential, which has become one of the driving forces to return to the lunar surface.
In order to find the most suitable candidate landing sites for a future robotic mission in the Apollo basin (Figure 3(a)), we selected three ROIs on the central and southern mare deposits, where most of the following criteria were fulfilled (Figure 3): (1) flat, smooth terrain with relatively gentle slopes of <7°; (2) relatively low crater density and (3) rock abundance; (4) scientific diversity; and (5) high FeO (>18 wt%) and TiO 2 (>5 wt%) contents for ISRU.

Terrain Properties
A relatively flat and smooth terrain (Figure 3(b)) is one of the main requirements for landing safety and the surface navigation and operation of a mobile asset.The distribution of slopes (Figure 3(c)) at baselines relevant to landing and operations safety (e.g., Luna-25, <7°, Djachkova et al. 2017;Chang'E-3, <7°, Wu et al. 2014) is a key element in landing site assessment; thus, we considered <7°slopes at a 7 m baseline.On a generally flat mare deposit, steep slopes (>25°; Basilevsky et al. 2014) are mostly associated with fresh craters, as craters generally become shallower and slopes become gentler with time (Fassett 2016).Thus, we avoided large craters (D > 2.5 km) within a 10 km exploration zone from the proposed candidate landing sites.We set this threshold value based on the range of values from previous missions (e.g., Apollo landing sites, <1 km, and ChangE'3, <∼6.7 km, Basilevsky et al. 2015;ChangeE'4, 3.6 km, Huang et al. 2018).Additionally, the crater rims and proximal ejecta of relatively fresh craters within ∼1-1.5 crater radii are often characterized by higher rock abundances, which are challenging for the mobility of a rover (Figure 3(d)).In general, mare deposits are less rocky than highlands, typically <0.5%, but fresh, small craters are typically "rocky" (D > hundreds of m), showing more elevated rock concentrations with an areal fraction of less than 10% (Bandfield et al. 2011).

Landing Precision and Operations Constraints
The landing precision of a landing system and the resulting uncertainties on the exact landing point could influence the fulfillment of the science and ISRU objectives of a mission.Some of those objectives, e.g., finding water ice in localized areas, could require a landing system with capabilities for landing within hundreds of meters, while other requirements, e.g., targeting mare deposits with high TiO 2 and FeO in tens of km-scale areas, will not need a precision landing.Thus, we do not consider landing precision and operations in our study.

Scientific Potential
These ROIs address several scientific objectives as outlined in various strategy documents, including the Lunar Exploration Roadmap developed by the Lunar Exploration Analysis Group (Lunar Exploration Analysis Group 2016a), the Advancing Science of the Moon: Report of the Specific Action Team released by the Lunar Exploration Analysis Group (2017), the Lunar Science for Landed Missions Workshop Findings Report (Jawin et al. 2019) built upon the US National Research Council (National Research Council 2007) report, and Crawford et al. (2012), which provide guidelines for the scientific challenges and opportunities enabled by robotic and human exploration of the Moon.Based on the scientific potential of the Apollo basin, we selected three ROIs that provide a diverse geologic history and a variety of rock types (Figure 3(e)), which could be potentially visited by a wellequipped lander and rover within a 10 km exploration zone.
The National Research Council (2007) report prioritizes eight science concepts and goals for lunar exploration.In this study, we consider concepts 1-7 as we focus on addressing lunar surface and interior goals (concept 8 addresses the lunar exosphere).We summarize the National Research Council (2007) concepts and goals, Campaigns 1 and 5 of the European Space Agency's Strategy for Science at the Moon (European Space Agency 2019a), and Strategic Knowledge Gaps (Lunar Exploration Analysis Group 2016b) relevant for the ROIs in Table 1 and Figures 15 and 16.

ISRU Potential
Robotic missions will target landing sites with high ISRU potential to allow the characterization of volatiles (e.g., H 2 O) and mineral resources (e.g., ilmenite, FeTiO 3 ) that can be extracted from the regolith and utilized to produce fuel, oxygen, and building materials via ISRU (e.g., Allen et al. 2012;Allen 2015;Ehresmann et al. 2017;van der Bogert et al. 2021).Theme 1 of NASA's Strategic Knowledge Gaps (SKG) summarizes the gaps in exploration science, technology development, and necessary measurements needed to close ISRU-related gaps in knowledge (Lunar Exploration Analysis Group 2016b; Table 1).One of the priority areas for the space resources activities of the ESA Space Resources Strategy (European Space Agency 2019b) is to establish the resource potential of regolith and pyroclastic deposits across the lunar surface by the identification and characterization of at least one nonpolar deposit.These goals are being pursued through a commercial ISRU technology development program (e.g., van der Bogert et al. 2021).To allow the characterization of mare basalt regolith for potential ISRU, we preselected ROIs where the highest FeO and TiO 2 values occur in the Apollo basin.

Data
To evaluate the potential scientific return of each proposed ROI, we used available data sets from previous lunar missions, such as the Lunar Reconnaissance Orbiter (LRO), Kaguya, Clementine, Chandrayaan-1, and mission concept studies (Kring & Durda 2012); these data sets were integrated in ArcGIS 10.  2010).We used the Kaguya TC images as the photobase for geologic mapping at a scale of 1:50,000 for the central and southern portions of the Apollo basin, thus providing a higherresolution geologic map than those of Ivanov et al. (2018) and Poehler et al. (2021).Then, to derive crater densities, we mapped craters >70 m in diameter (resolvable within 10 pixels or more in Kaguya TC) in various degradation states (Wu et al. 2014) across the selected areas.
Geologic mapping and terrain hazards were assessed via the merged LRO Lunar Orbiter Laser Altimeter (LOLA) and Kaguya TC Digital Elevation Model (DEM) global mosaic (60 m pixel −1 ; Barker et al. 2016) and the Kaguya TC DEM global mosaic (7 m pixel −1 ; Haruyama et al. 2008Haruyama et al. , 2012)), respectively.Detailed slope maps were derived from the TC DEM by applying the Surface Slope tool in ArcMap.For both safety and scientific purposes, we used the LRO Diviner Lunar Radiometer Experiment 236 m pixel −1 global rock abundance product.Rock abundance can be described in terms of the fraction of each pixel covered by exposed rocks (Bandfield et al. 2011).Numerous small young craters (D > hundreds of m in diameter) expose abundant rocks on lunar maria, typically <0.5% rock concentration values.The distribution of "rocky craters" helps to identify hazards for landing and mobility of rovers on the lunar surface.
For assessing mare compositions and evaluating ISRU potential, we determined FeO and TiO 2 contents using both the Clementine Ultraviolet/Visible Spectrometer (UVVIS) ∼400 m pixel −1 global maps (Lucey et al. 1998b;Lucey et al. 2000) and Kaguya Multiband Imager (MI) ∼60 m pixel −1 global data (Lemelin et al. 2016) for comparison.To extract the minimum, maximum, and average FeO and TiO 2 abundances (wt%) from Clementine UVVIS and Kaguya MI global raster data sets, we generated 50 random measurement points within the 10 km exploration zone of each ROI.

Regolith Thickness Estimation
We estimated the uppermost regolith layer thickness in the ROI using all LROC NAC images available up to 2018 February.The coverage was almost complete (see Figure 10), but a few images had poor data quality.Regolith thickness was derived using the method from Quaide & Oberbeck (1968).Small (D < 250 m), relatively fresh concentric craters (CCs) are strongly affected by variations in target properties (porosity, rock strength, and layering), regolith protolith (Head &Wilson 2020), andimpact velocity (e.g., van der Bogert et al. 2017).According to Quaide & Oberbeck (1968), CCs form when the impactor hits the lunar regolith-mare interface.We used the following equation: where k is an empirical constant (0.86), D f is the diameter of the inner concentric ring, D a is the outer rim-to-rim diameter of a crater, and tan(α) is the corresponding slope of the inner walls of fresh craters (31°± 2°).To map CCs, we primarily considered NAC images with smaller incidence angles (<55°) because of the proper differentiation between the CCs and normal flat-floored craters (Fa et al. 2014), but we also used images with higher incidence angles to fill gaps in image coverage.The distribution of CCs is sparse in the ROI.We used the ordinary kriging geostatistical technique (Oliver & Webster 1990) to interpolate the obtained regolith thickness values from each measurement point at the distance of the half of the mean second-closest neighbor distance within the 10 km 1.The bombardment history of the inner solar system is uniquely revealed on the Moon.2. The structure and composition of the lunar interior provide fundamental information on the evolution of a differentiated planetary body.3. Key planetary processes are manifested in the diversity of lunar crustal rocks.4. Lunar volatiles increase our understanding of the composition state and distribution of volatiles in the lunar polar regions.5. Lunar volcanism provides a window into the thermal and compositional evolution of the Moon.6.The Moon is an accessible laboratory for studying the impact process on planetary scales.exploration zone (Figure 10).The regolith thickness was estimated by measuring the inner and outer rim diameter of the small CCs after Quaide & Oberbeck (1968).However, based on their hydrocode simulations of CCs, Prieur et al. (2018) suggest that (1) smaller strength discontinuities, such as impact melt sheets within the regolith/target, and (2) low-impact velocities are sufficient to form CCs, so these features may not exclusively represent regolith thickness effects.Thus, the estimated regolith thicknesses should be taken with caution.

Mineralogy
To infer the local stratigraphy, we analyzed the mineralogy of materials exposed by impact crater ejecta on the mare floor and basin material based on the premise that smaller craters expose shallow stratigraphic units, while larger craters excavate deeper units (Figure 11).We developed a python algorithm that generated a red, green, and blue (RGB) color-composite mosaic of M 3 data covering the Apollo basin (Torres et al. 2022).As M 3 has a wide spectral range (430-3000 nm), it enabled the analysis of the 1 and 2 μm regions, which are relevant for detecting lunar mafic minerals and plagioclase (e.g., Moriarty & Pieters 2016b;Martinot et al. 2018).The developed algorithm first extracts the reflectance of each pixel at a given wavelength, then derives the absorption band depth of each pixel with the continuum removal method modified after Martinot et al. (2018)-a straight line between two tie points at 730 and 1620 nm was used to remove the continuum.Note that we used the simplified version of the Martinot et al. (2018) technique by fixing the tie points instead of searching for individual tie points pixel by pixel due to the limitations in the computation power.Finally, the algorithm derives the integrated band depth of the 1 μm (789-1308 nm) and 2 μm (1658-2498 nm) regions for each pixel.For the RGB color -composite mosaic of M 3 data to be sensitive to mafic composition, we used the integrated band depth at 1 μm (red), integrated band depth at 2 μm (green), and reflectance at 1580 nm (blue; Besse et al. 2011).Consequently, the red and yellow colors are indicative of high-Ca pyroxenes (basaltic material), green suggests the presence of low-Ca pyroxenes (noritic material), and blue represents anorthositic or low-Febearing materials.We can distinguish between old and freshly exposed mare, as well as feldspathic terrain materials (Figure 11(a)).We selected seven measurement points related to diverse geologic features (Figure 11(a)): (1) the central mare province; (2) a fresh crater rim; (3) an "island" (kipuka) within ROI 1; (4) the distal ejecta from an unnamed crater overlaying Resnik crater; (5) the inner crater wall of "Crater 1," the largest impact crater on the central mare province; (6) the proximal ejecta from an unnamed crater overlaying Resnik crater; and (7) freshly exposed material by a cluster of small craters.Moreover, we compared the M 3 spectra from seven key locations with olivine,8 augite,9 and pigeonite10 from the PDS spectral library (Figure 11(b)).

Relative Stratigraphy
We studied the relative stratigraphy within the ROIs with two different approaches.The local and regional stratigraphy can be examined via observations of compositional variations within all impact crater ejecta (not limited to CCs) located on the mare floor using the Chandrayaan M 3 pyroxene map and Kaguya MI data sets.The diversity of mineralogical signatures could suggest chemical variations with depth.To estimate the minimum thickness of specific strata, we measured the diameter of the craters using NAC images and calculated the crater excavation depth (H exc ) using the equation from Huang et al. (2018) and Melosh (1989): where H exc is the maximum excavation depth and D is the final crater diameter.It is important to note that the lack of availability of a full range of crater sizes (from hundreds of meters to kilometers) limits our ability to document mineralogical variations with depth.
To establish a relative stratigraphy for the geological units within the overall region of the Apollo basin, we used AMA data of large impact craters from Ivanov et al. (2018) and Fortezzo et al. (2020), as well as light plains deposits from Thiessen et al. (2012) and Meyer et al. (2016Meyer et al. ( , 2020)).Additionally, we placed impact structures without currently measured AMAs based on their degradation state (Ivanov et al. 2018) in the stratigraphic column from the direct vicinity of the Apollo basin and its interior.Both kinds of stratigraphic analyses provide geological context for further studies, as well as mission planning and operations, in the Apollo basin.

Terrain Hazards: Slopes, Craters, and Rock Abundance
The selection of landing sites for lunar missions takes into consideration several key factors, including hazard mitigation.In this case, all proposed landing sites are situated on top of mare deposits, which have relatively flat terrain, ensuring safe and stable landing conditions.Additionally, the crater densities and rock abundances in these regions are low, reducing the risk of potential hazards.
Slope analysis based on 7 m baseline Kaguya TC DEM data reveals that the slopes in the proposed landing areas generally do not exceed 5°, making them suitable for landing and subsequent operations, although a few fresh craters exhibit steeper slopes exceeding 25°(Figure 4).
Within the 10 km exploration zones surrounding the landing sites, the number of craters with diameters of 70 m or larger is estimated to be less than ∼1000.The average crater diameter ranges from 128 m in ROI 1 to 150 m in ROI 3, as indicated in Table 2 and Figure 5.The largest crater diameters are: ROI 1 (588 m), ROI 2 (983 m), and ROI 3 (2.3 km).Furthermore, the depths of crater excavation vary, ranging from approximately 5.9 m for the lower limit of the considered crater diameter range up to 195 m at ROI 3 for the largest crater observed there (Table 2).A relationship between crater age and boulder densities is observed, where younger craters tend to have higher boulder densities on their rims and within their proximal ejecta radial to the crater centers compared to older craters.In terms of areal rock density, ROI 1 exhibits the lowest value at 3.5%, while ROI 3 demonstrates the highest rock abundance at 14.1% within the 10 km exploration zone (Figure 6, Table 3).
These findings provide valuable insights for mission planning and resource assessment at the proposed landing sites.The relatively flat terrain, low crater densities, and rock abundances contribute to the favorable conditions for safe landing and operations.2021), but this study generated higher-resolution maps.Here, we identified three major classes of landforms that we mapped in the central and southern Apollo basin at 1:50,000 mapping scale (Figures 7 and 8).A correlation chart of the mapped geologic units is shown in Figure 9.The following geologic unit groups are defined in Ivanov et al. (2018) and Wilhelms et al. (1979) and used in our map as well.0-11.9 0-9.9 0-10.3During the pre-Nectarian/Nectarian period, the Apollo basin formed in the northeastern section of the SPA basin rim (e.g., Ivanov et al. 2018) and on the proximal ejecta blanket of the transient crater cavity of the SPA basin (Melosh et al. 2017;Moriarty et al. 2021b).It is distinguished by its intricate peakring basin structure (NpNbm) and the presence of hummocky terrain (NpNbh).The hummocky terrain is interpreted by Ivanov et al. (2018) to be an impact melt located within the inner peak ring and between the inner peak ring and the outer ring.Subsequently, a series of craters (NpNc) formed in the area, such as Onizuka, Resnik, Jarvis, Oppenheimer, Chaffee, and Chawla craters.These craters have significantly altered the landscape of the Apollo basin, and they typically exhibit extensive degradation and lack ejecta deposits.

Science Potential
During the Imbrian period, light plains deposits (Ip) were formed, originating from various older geological units and notably including material from the Orientale impact event, as suggested by Meyer et al. (2016Meyer et al. ( , 2020)).Ivanov et al. (2018) estimated AMAs for three large light plains deposits and concluded a range from ∼3.80 to ∼3.72 Gyr suggesting different sources of origin.These deposits are found overlaying other geological units and are primarily preserved in flat, lowlying areas.The surface of the light plains deposits exhibits a high density of impact craters and possesses a higher albedo    (Moriarty & Pieters 2014).Of the two hypotheses, the latter is favored due to the lack of visible ejecta over the central mare deposit.This suggests that Dryden crater predates the mare flooding in the central area of the Apollo basin.
The younger Ec exhibit distinct rim morphologies with sharp crests and lack prominent ejecta rays.In contrast, the youngest Cc display well-preserved impact features with crisp morphologies, prominent ejecta rays, and the presence of secondary craters (e.g., Wilhelms et al. 1979).These Cc are observed to overlay or cut across all other geologic units in the area.Both mare provinces are influenced by larger Cc.In the southern mare deposit, the ejecta from the Chaffee S crater, which is approximately 20 km in diameter and located in the northwest quadrant of the mapping area (Figure 8), affects the region.However, the ejecta deposit of the Chaffee S crater is poorly visible and presents challenges in confident mapping.Likewise, the central mare province is affected by an unnamed crater that has a well-preserved ejecta deposit overlaying Resnik crater (Figure 7).This distinct ejecta deposit can be traced back to the unnamed crater as its source.These observations provide insights into the impact history of the region, with the younger Ec and Cc exhibiting different morphological characteristics and affecting the geological units in distinctive ways.
In summary, ROI 1 offers a diverse range of geologic materials within the 10 km exploration zone, making it an intriguing area for a rover mission (Figure 9(a)).The region contains three distinct units: Im, Cc, and NpNbh.The Im unit represents the central mare province and is characterized by its formation during the Imbrian period.The Cc unit corresponds to the youngest Copernican craters, displaying well-preserved impact features with prominent ejecta rays and secondary craters.These craters have left their mark on the landscape, superposing other geological units.The NpNbh unit is particularly interesting.It is believed to have an impact melt origin and may contain materials derived from the SPA basin and the Apollo impact melt breccia, as suggested by Wilhelms et al. (1979) and Ivanov et al. (2018).However, other studies (e.g., Melosh et al. 2017;Moriarty et al. 2021b) suggest that there is no SPA material to be found in the Apollo basin.This unit forms isolated "islands" known as kipukas within the mare deposit, and two such kipukas are located within ROI 1.Additionally, it is possible that the NpNbh unit can be found within the ejecta deposit of the unnamed crater on top of Resnik crater, largely overlapping the central mare deposit and in part of ROI 1.The geologic diversity within ROI 1, encompassing Im, Cc, and the intriguing NpNbh unit, presents an exciting opportunity for scientific exploration and further understanding of the lunar geologic history.
ROI 2 is situated within the Im unit, specifically in an area where the ejecta deposits from various Cc merge, originating from both the northeast and southwest directions, particularly evident in the southwestern part of the 10 km exploration zone (Figure 9(b)).It is worth noting that these Copernican-aged ejecta deposits, similar to ROI 1, may have excavated the NpNbh unit, potentially exposing its material through the impact process.
ROI 3 is located within the Im unit, in close proximity to the outer ring of the Apollo basin, known as the NpNbm unit (Figure 9(c)).This location shares a similar geologic setting with the Apollo 17 landing site, which was situated in the Taurus-Littrow Valley on the eastern edge of Mare Serenitatis, surrounded by remnants of the Serenitatis basin massif.
Compared to ROI 1 and ROI 2, ROI 3 exhibits less disturbance from reworked materials.This implies that a landing site in ROI 3 retains a more pristine and less altered geologic environment, making it an intriguing location for further exploration and study.The geologic similarity to the Apollo 17 landing site suggests that ROI 3 may offer valuable insights into the lunar geology and contribute to our understanding of the formation and evolution of the Apollo basin.

Regolith Thickness, Mineralogy, and Subsurface Stratigraphy
The lunar surface is predominantly covered by a layer of fine-grained regolith, which is the product of continuous impact processes.Our research has revealed that the average thickness of the regolith varies between 4.6 and 8.3 m, as shown in Table 4 and Figure 10.These findings align well with previous studies conducted by Quaide & Oberbeck (1968), Fa et al. (2014), Huang et al. (2018), andFu et al. (2020).
Analysis of M 3 data does not indicate the presence of distinct lava flows, suggesting that each mare province in the Apollo basin experienced a single magmatic event (Figure 11(a)).Alternatively, M 3 data might only indicate distinct lava flows if they were substantially different in composition, as the data are affected by several artifacts (e.g., striping, image boundaries).Additionally, we have observed a lack of diversity in pyroxene mineralogy, which is in contrast to the variations in pyroxenes observed in the Von Karman crater due to exposure by small and large craters (Huang et al. 2018), which could be interpreted by significantly more mixing events from local crater ejecta blankets.Figure 11(a) illustrates a mature mare deposit (marked as number 1) characterized by red color.Small yellow craters (numbers 2 and 7) exhibit high-calcium clinopyroxene (HCP) mineralogy, representing fresh and immature basaltic material (number 5).The ejecta ray of an unnamed large Copernican-aged crater overlaying Resnik crater displays similar mineralogy to the NpNbh geologic unit, marked in blue (numbers 3, 4, and 6).These areas exhibit higher reflectance at 1580 nm, consistent with feldspathic material.The variations observed in the Kaguya RGB composite image demonstrate contrasts in titanium and iron contents, as well as surface maturity (Figure 11(c)).Larger craters, such as "Crater 1," exhibit distinct composition and/or represent the illuminated crater walls and/or relatively "fresher" material (depicted in red).However, it is important to approach the interpretation of these results with caution due to the potential diversity in preimpact geology, including intrusions of different compositions.These variations could lead to erroneous interpretations compared to our current understanding of the structure of the lunar crust, as suggested by Martinot et al. (2018).

Central Apollo Mare Province
Within the central mare province where ROI 1 and ROI 2 are situated, we interpret a subsurface stratigraphy or sequence of impact events (Figure 12(a)) based on our observations.The upper part of the stratigraphy consists of a layer of fine-grained regolith, ranging from 1.1 to 24 m in ROI 1 and 18.5 m in ROI 2. This thickness is in line with the general understanding of regolith thickness, which is estimated to be around 20 m (Fa et al. 2015).The upper regolith layer becomes thicker toward the high-albedo Copernican ejecta rays that overlay the mare deposit (Figures 10(a We have observed a mixed zone within this upper regolith layer, where the fine-grained regolith and HCP mafic material (depicted in yellow in Figure 11(a)) are exposed by small, fresh impact craters.These craters have excavated material between approximately 5.9 m and the maximum estimated regolith thickness.
The next layer in the stratigraphy is the mare deposit, which reaches a maximum depth of over 380 m.However, in the case of "Crater 1," instead of encountering impact melt or feldspathic material, mare materials have been excavated.Additionally, the ejecta from a large, unnamed, Copernicanaged crater overlaying Resnik crater in the northeastern part of the central mare province has exposed a feldspathic lithology known as the NpNbh unit (Figure 7).
Underlying the mare layer, we expect to find Imbrian ejecta layers from surrounding larger craters such as Dryden crater (3.69 billion yr; Upper Imbrian, according to Ivanov et al. 2018) and Borman V crater (Lower Imbrian).However, we cannot distinguish their ejecta deposits overlaying the mare deposit.
Beneath the Ic ejecta deposits, we anticipate the presence of light plains material.These light plains do not overlap stratigraphically with Dryden and Borman V craters, suggesting that these Imbrian large craters formed after the deposition of the light plains.The light plains may originate from the Orientale and Imbrium basin-forming events.
The subsequent layer in the stratigraphy should consist of ejecta deposits from various nearby Nectarian large craters, including Onizuka, Resnik, Jarvis, and Oppenheimer craters.The relative stratigraphy of these subdued and degraded craters is not clear without detailed analysis, such as AMA analysis, but they clearly formed before the formation of the mare deposit.
Finally, at the bottom of the stratigraphy, we expect to find the impact melts and breccias of the Apollo and SPA basins.
These proposed stratigraphies provide insight into the subsurface composition and sequence of impact events within the central mare province of the Apollo basin.However, further analysis and investigations, including in situ sampling and analysis, are necessary to validate and refine this proposed stratigraphy.

Southern Apollo Mare Province
In the southern mare province, Figure 12(b) presents a proposed subsurface stratigraphy.The upper part of the stratigraphy consists of a layer of fine-grained regolith, ranging from 1.2 to 19.9 m at ROI 3. The thickness of the upper regolith layer increases toward the center of the mare province, away from the outer rim of the Apollo basin.This accumulation of ejecta deposits on top of the mare is evident, with the ejecta of the Copernican Chaffee S crater serving as an example.
Similar to the central mare province, there is a mixed zone within the upper regolith layer, where the fine-grained regolith and HCP mafic material (depicted in yellow in Figure 11(a)) M 3 spectra from seven key locations (indicated in panel (a)) compared with PDS spectral library data.Red: mature mare material; yellow: "fresh," immature mare deposit; blue: feldspathic material.The ejecta of the large, unnamed Copernican-aged crater overlaying Resnik crater is a prominent feature even on spectral data.This ejecta deposit superposes "Crater 1" (marked with a white arrow in Figure 11) in the northeast-southwest direction (blue; number 4).Crater 1 is 4491 m in diameter with a calculated excavation depth of 377 m (Huang et al. 2018).Crater 1 might indicate exhumation of fresh mare material (yellow; number 5) similar to the small, fresh craters (numbers 2 and 7).are excavated by small, fresh impact craters.This mixed zone extends from approximately 5.9 m to the maximum estimated regolith thickness.
The next layer in the stratigraphy is the mare material, which, based on the estimated excavation depths of the largest crater, is at least ∼200 m thick.Notably, there are no large craters overlaying the mare deposit that exhibit different mineralogical compositions.
Beneath the mare deposit, it is expected that the older impact melt and breccia from Maksutov crater (3.71 billion yr; Upper Imbrian according to Ivanov et al. 2018) should be located.However, Maksutov crater is situated outside of the Apollo basin, so the ejecta layer within the Apollo basin must be relatively thin, on the scale of centimeters.
The subsequent possible strata should be composed of the light plains deposits, which do not overlie Maksutov crater and must be older than it.Beneath the Imbrian ejecta materials, impact breccia from various nearby Nectarian large craters, such as Chaffee, Oppenheimer, and Chawla craters, are anticipated.However, the relative stratigraphy of these craters remains ambiguous.At the bottom of the stratigraphy, the impact melts and breccias from the SPA and Apollo basins are expected to be present.
It is important to note that this proposed subsurface stratigraphy is based on available data and interpretations.Further investigations and analysis, including in situ sampling and analysis, are necessary to validate and refine this proposed stratigraphy within the southern mare province of the Apollo basin.

ISRU Potential
Based on our findings, the Apollo basin within the SPA basin exhibits some of the highest FeO and TiO 2 contents.
These high abundances are associated with the Imbrian mare deposits, as illustrated in Figures 13 and 14, and supported by data provided in Tables 5 and 6.
Among the three ROIs, ROI 2 displays both the highest average and maximum FeO and TiO 2 contents.This suggests a significant presence of Fe and Ti in the mare deposits within ROI 2. On the other hand, ROI 1 has the lowest average and minimum FeO and TiO 2 values among all the ROIs but still exhibits relatively high FeO abundances.
These results highlight the variation in FeO and TiO 2 abundances within different regions of the Apollo basin.The high FeO and TiO 2 contents are characteristic of the Imbrian mare deposits found within the basin.The variations observed among the ROIs provide insights into the compositional diversity within the basin and contribute to our understanding of the lunar volcanism in this region.

Discussion
In this section, we discuss the open science questions and how these could be addressed by two robotic exploration mission scenarios with (1) lander and rover assets with a dedicated payload suite and (2) the same architecture with additional sample return capability.The following science objectives could be addressed by a robotic exploration mission at all ROIs.
1. Determine the age of the Apollo and SPA basins.
Performing in situ detailed geochemical, isotopic, and petrologic analysis of lunar materials with additional sample return capability would allow us to advance our understanding of the bombardment history of the Moon.2. Determine the composition, age, and diversity of mare basalts on the farside of the Moon.A detailed compositional analysis of the lunar surface materials, particularly the mare deposits, could help to better understand their mineralogy, elemental composition, and variations in FeO and TiO 2 abundances.This analysis would provide insights into the magmatic evolution of the Apollo basin and the farside of the Moon.
3. Characterize the diversity and origin of rocks related to various geologic units.The investigation of the subsurface stratigraphy, including the layers of regolith, mare deposits, impact melt, and breccias, would contribute to our understanding of the formation and deposition of different units, the potential presence of older geologic materials beneath the Imbrian mare deposits, and the geologic evolution of the Apollo basin.4. Characterize small impact craters at various degradation states and impact gardening processes in the Apollo  basin.By studying the distribution, morphology, and ages of craters within the ROIs, the mission can help unravel the recent impact history of the Apollo basin.This analysis would shed light on the chronology of recent impact events, the morphology of secondary craters, and the effects of cratering processes on the lunar surface.5. Determine the thickness of the lunar crust, mantle, and core beneath the Apollo basin.Investigating the interior structure could reveal the physical and thermal evolution of the Moon, especially focusing on the early magma ocean history and thermal asymmetries of the nearside and farside of the Moon.6. Determine the thickness, physical properties, and chemical composition of the regolith and megaregolith in the Apollo basin.A robotic mission can study the characteristics of the regolith, including its grain size distribution, physical properties, composition, and thermal behavior.These investigations would contribute to our understanding of regolith processes, such as impact gardening and regolith maturity, and provide valuable data for future lunar surface operations and resource utilization.
To maximize the science return, a carefully selected payload package for both the rover and lander can provide valuable data and address various scientific goals.Here is a breakdown for a proposed payload suite for the rover and lander, along with the potential scientific objectives they can address.Rover Payload Suite.The National Research Council (2007) goals that can be addressed by the proposed instruments are given in Table 7.These goals include objectives such as understanding lunar geology and the history of the Moon, studying the lunar interior and its structure, characterizing lunar resources, and enabling future human exploration and habitation.
By combining the capabilities of the rover and lander payloads, this mission architecture provides a comprehensive suite of instruments to address a wide range of scientific objectives, contributing to our understanding of the Moon's bombardment history, regolith processes, internal structure, volcanic evolution, and resources.
The National Research Council (2007) goals that could be addressed by a robotic exploration mission with lander and rover capability at the candidate landing sites in the Apollo basin are evaluated in Figure 15.Altogether, 12 plus 9 scientific goals from the National Research Council (2007) report could be or may be addressed, respectively.Science questions and goals from Campaign 5 of the European Space Agency's Strategy for Science at the Moon (European Space Agency 2019a) could be covered in the Apollo basin by a robotic exploration mission scenario.
With sample return capability, additional National Research Council (2007) goals could be addressed (Figure 16).Altogether, we identified 19 plus 5 scientific goals from the National Research Council (2007) that could be or may be addressed, respectively.Campaigns 1 and 5 of the European Space Agency's Strategy for Science at the Moon (European Space Agency 2019a) could be targeted by a sample return mission to the Apollo basin.Almost all National Research Council (2007) concepts would be addressed, seven more goals than without sample return capability.
Determining the duration of the lunar basin-forming epoch and calibrating the lunar chronology are high priorities in the    Additionally, the Apollo basin offers the opportunity to sample previously unsampled geochemical terrains within the SPA basin.This inner peak-ring basin formed at the boundary of SPA and the anorthositic highlands provides a cross section to constrain magma ocean crystallization processes, impact melt sheet differentiation, and volcanic processes on the farside of the Moon.Sampling the lunar mare on the farside would provide a unique opportunity to constrain the volcanic processes, reveal the difference in crustal and thermal evolution, and better understand the formation of the nearside and farside dichotomy boundary.
ESA's technology development, the European Large Logistics Lander (the EL3 lander, named Argonaut), will be supporting human exploration of the Moon throughout the end of the 2020s and the 2030s.Argonaut will be capable of delivering a combination of cargo items, scientific payloads, and small robotic assets (rovers) to the lunar surface, including retrieving samples from the surface and bringing them back to Earth for analysis (European Space Agency 2022).Sending cargo missions to the lunar surface every 2 or 3 yr could greatly enhance the diversity of samples collected from the Moon.

Conclusions
On the basis of the studied data, all ROIs fulfill the general hazard requirements, science interest, and ISRU potential for a robotic exploration mission, such as Chang'E-6 (Zeng et al. 2023), to the Apollo basin.
The main focus of this study is on the central and southern mare provinces in the Apollo basin.We selected three equally suitable ROIs based on terrain properties, science, and ISRU potentials.We highlighted exploration zones of 2.5, 5, and 10 km from each proposed landing site.Furthermore, we discussed each landing site in the context of key lunar science objectives from the National Research Council (2007) and Campaigns 1 and 5 of the European Space Agency's Strategy for Science at the Moon (European Space Agency 2019a), which could be addressed by robotic exploration missions with (1) lander and rover assets with a dedicated payload suite and (2) the same architecture with additional sample return capability.We conducted a series of geospatial data analyses.We investigated the topography, slopes, crater density, rock abundance, geology, and mineralogy and studied the subsurface stratigraphy of each ROI.
The studied ROIs are generally smooth, with average slopes of <5°at baselines of 7 m, with the exception of steeper slopes (>25°) associated with fresh craters.High local rock abundances associated with these fresh craters still could allow for good traversability.The proposed ROIs reflect geologically complex areas, where mare deposits are covered by younger, Copernican-aged ejecta material with various thicknesses, compositions, and distributions.These ejecta materials have low FeO and TiO 2 contents representing nonmare deposits.The origin of that material could be reexcavated SPA and Apollo impact melt, which is the key sample type to address the lunar cataclysm hypothesis.In addition, the mare deposits have high ISRU potential with relatively high FeO and TiO 2 contents ranging from 16 to 20 wt% and 3 to 10 wt%, respectively.Additionally, all ROIs provide the opportunity to directly sample material from the structure of the Apollo basin in the form of kipukas (ROIs 1 and 2), which are embayed hills by the mare deposits, or the outer basin rim of the Apollo basin could be sampled near a candidate landing site (ROI 3).
In situ observations with advanced rover and lander payload capabilities can help to address six of seven National Research Council concepts (1-3, 5-7), as well as Campaigns 1 and 5 of the European Space Agency's Strategy for Science at the Moon (European Space Agency 2019a).However, there is a great loss in science benefit without the sample return capability.A sample return mission could address 19 plus 5 scientific goals of the National Research Council (2007), while an in situ robotic exploration mission could aim to study 12 plus 9 scientific goals of the National Research Council (2007) in the Apollo basin, seven fewer goals than with samples brought back to our laboratories.To test the cataclysm hypothesis (Tera et al. 1974;Ryder 2002) and establish an accurate lunar chronology (concept 1), as well as study the diversity of lunar crustal rocks (concept 3), and lunar farside volcanism (concept 5; National Research Council 2007), a sample return mission scenario would be greatly beneficial.Thus, a successful Chang'E-6 sample return mission can achieve multiple science goals.
and content of which are described in Supporting Table A1.The data presented in this study will be available in the TRR170-DB Repository (10.35003/PI5HHE).
(a) and (b); Ivanov et al. 2018).The modeled crustal thickness beneath the Apollo basin is less than ∼5 km, which is the thinnest in the entire SPA basin (Wieczorek et al. 2013).Baker & Head (2015) calculated a preimpact crustal thickness in the range of 27.0-35.3km and a maximum depth of excavation of 39 km.Similarly, numerical modeling from Potter et al. (2018) gives estimates for preimpact crustal thickness in the range of 25-40 km and a depth of excavation of ∼32 km.
)-(h); Kring & Durda 2012; Pasckert et al. 2018).Additionally, Gaddis et al. (2003) described a small pyroclastic deposit (42 km 2 ) on the floor of a 20.5 km diameter crater on the outer ring of the Apollo basin.Based on low optical maturity and the lower returned Mini-RF radar signal and slightly lower FeO abundance than the average green glass composition of 19.5 wt% FeO, Trang et al. (2010) suggested that pyroclastics in the Apollo basin could be a new type of pyroclastic material.Thiessen et al. (2012) mapped and derived AMAs from 3.4 to 3.8 Gyr for light plains deposits in the SPA, while Ivanov et al. (2018) report AMAs for three large fields of light plains in the northeast portion of SPA (outside of the Apollo basin) ranging from ∼3.72 to ∼3.80 Gyr.Moreover, Meyer et al. (2016, 2020) mapped the light plains deposits at 1:300K scale around the Orientale basin and globally, respectively, and

Figure 5 .
Figure 5. Maps of crater densities overlaid on the Kaguya TC 7 m pixel −1 mosaic on which craters >70 m in diameter were measured.(a) ROI 1, (b) ROI 2, and (c) ROI 3. The circles indicate exploration zones of 2.5, 5, and 10 km from the proposed landing sites (yellow star).The maps are in Lambert azimuthal equal-area projection, centered at −152.0°W, −35.8°S.

( a )
Copernican craters ( Cc) have crisp, well-preserved impact morphologies with prominent ejecta rays and secondary craters and overlay or cut across all other geologic units.(b) Eratosthenian craters ( Ec) have sharp-crested rim morphologies without prominent ejecta rays.(c) Imbrian craters ( Ic) are well-preserved large craters with complex morphologies and subdued ejecta deposits.(d) Pre-Nectarian and Nectarian craters ( NpNc) are heavily degraded large craters without obvious ejecta deposits.

Figure 7 .
Figure 7. Geologic map of the central mare province of the Apollo basin overlaid on Kaguya TC and LROC WAC at 1:50,000 mapping scale.ROI 1 (upper) and ROI 2 (lower) are indicated with circular exploration zones of 2.5, 5, and 10 km from the proposed landing sites (yellow star).The maps are in Lambert azimuthal equalarea projection, centered at −152.0°W, −35.8°S.

Figure 8 .
Figure 8. Geologic map of the southern mare province of the Apollo basin overlaid on Kaguya TC and LROC WAC at 1:50,000 mapping scale.ROI 3 is indicated with circular exploration zones of 2.5, 5, and 10 km from the proposed landing site (yellow star).Same legend as in Figure 7.The maps are in Lambert azimuthal equal-area projection, centered at −152.0°W, −35.8°S.

Figure 9 .
Figure 9. Geologic maps of ROIs overlaid on the Kaguya TC image, with exploration zones of 2.5, 5, and 10 km from the proposed sites (yellow star).(a) ROI 1 has access to study Im; Copernican-aged ejecta (Cc), which might excavate SPA and Apollo impact melt from the subsurface; and remnants of NpNbh with putative impact melt origin.(b) ROI 2 could sample different Copernican-aged ejecta rays (Cc), potential SPA and Apollo impact melt materials, and Imbrian mare deposit.(c) ROI 3 could investigate Imbrian mare deposits, as well as NpNbm.(d) Geologic correlation chart of geologic units.The maps are in Lambert azimuthal equal-area projection, centered at −152.0°W, −35.8°S.

Figure 10 .
Figure 10.Maps of regolith thickness estimations for each ROI ((a), (b), and (c)) using Quaide & Oberbeck (1968).The data were derived by measuring the diameter of the inner and outer rim of CCs (d) using LROC NAC 1 m pixel −1 data and applying the kriging technique to interpolate the results to the larger area.Points mark the measured values.The circles indicate exploration zones of 2.5, 5, and 10 km from the proposed landing site (yellow star).LROC NAC coverage as of 2018 February is overlaid over Kaguya TC data.The maps are in Lambert azimuthal equal-area projection, centered at −152.0°W, −35.8°S.

Figure 11 .
Figure 11.Chandrayaan M 3 pyroxene spectral map and Kaguya MI overlaid on the LROC WAC mosaic showing the central mare province region in the Apollo basin.No distinct lava flows can be identified.(a) Chandrayaan M 3 pyroxene spectral map showing the overview of the central mare province in the Apollo basin.(b)M 3 spectra from seven key locations (indicated in panel (a)) compared with PDS spectral library data.Red: mature mare material; yellow: "fresh," immature mare deposit; blue: feldspathic material.The ejecta of the large, unnamed Copernican-aged crater overlaying Resnik crater is a prominent feature even on spectral data.This ejecta deposit superposes "Crater 1" (marked with a white arrow in Figure11) in the northeast-southwest direction (blue; number 4).Crater 1 is 4491 m in diameter with a calculated excavation depth of 377 m(Huang et al. 2018).Crater 1 might indicate exhumation of fresh mare material (yellow; number 5) similar to the small, fresh craters (numbers 2 and 7).(c) Kaguya MI spectral ratio map exhibiting the central mare province (light blue) and basin material (yellow) in the Apollo basin.The maps are in Lambert azimuthal equal-area projection, centered at −152.0°W, −35.8°S.M 3 image numbers: M3G20090718T054612, M3G20090718T014252, M3G20090717T211502, M3G20090717T171142, M3G20090717T124352, M3G20090717T084032, M3G20090717T041548, M3G20090717T000932.
Figure 11.Chandrayaan M 3 pyroxene spectral map and Kaguya MI overlaid on the LROC WAC mosaic showing the central mare province region in the Apollo basin.No distinct lava flows can be identified.(a) Chandrayaan M 3 pyroxene spectral map showing the overview of the central mare province in the Apollo basin.(b)M 3 spectra from seven key locations (indicated in panel (a)) compared with PDS spectral library data.Red: mature mare material; yellow: "fresh," immature mare deposit; blue: feldspathic material.The ejecta of the large, unnamed Copernican-aged crater overlaying Resnik crater is a prominent feature even on spectral data.This ejecta deposit superposes "Crater 1" (marked with a white arrow in Figure11) in the northeast-southwest direction (blue; number 4).Crater 1 is 4491 m in diameter with a calculated excavation depth of 377 m(Huang et al. 2018).Crater 1 might indicate exhumation of fresh mare material (yellow; number 5) similar to the small, fresh craters (numbers 2 and 7).(c) Kaguya MI spectral ratio map exhibiting the central mare province (light blue) and basin material (yellow) in the Apollo basin.The maps are in Lambert azimuthal equal-area projection, centered at −152.0°W, −35.8°S.M 3 image numbers: M3G20090718T054612, M3G20090718T014252, M3G20090717T211502, M3G20090717T171142, M3G20090717T124352, M3G20090717T084032, M3G20090717T041548, M3G20090717T000932.

Figure 12 .
Figure 12.Based on this study, a proposed theoretical subsurface stratigraphy in the central (a) and southern (b) mare provinces.
lunar science community.The minimum requirement to solve the impact bombardment history of the Earth-Moon system and recalibrate the lunar chronology curve might only be possible by returning samples from several key geologic features.Collecting impact melt-bearing samples could help to constrain the age of the Apollo basin, one of the youngest impact basins within the SPA basin, and possibly the age of SPA as well.

Figure 15 .
Figure 15.National Research Council (2007) science concepts and goals assessed for a robotic exploration mission with rover and lander assets in the Apollo basin.The highest-ranked National Research Council (2007) science concepts and goals cannot be fully addressed without sample return capability.

Figure 16 .
Figure 16.National Research Council (2007)  science concepts and goals in the case of a robotic exploration mission to the lunar surface with a rover and lander with sample return capability.The highest-ranked science concepts and goals can be achieved with sample return capability.

Table 1
Summary of Lunar Geoscience and ISRU Knowledge Gaps   National Research Council (2007) lunar ISRU production efficiency 2: measure the actual efficiency of ISRU processes in the lunar environment using technology (e.g., produce and store small quantities of hydrogen and oxygen from lunar regolith, demonstrate disposal of heated regolith after processing, process at high temperature to test techniques for extracting metals (e.g., Fe, Al) from regolith)

Table 2
Crater Statistics of Craters >70 m in Diameter within the Three ROIs

Table 4
Regolith Thickness Estimated Using CCs and the Equation from Quaide &

Table 5
FeO and TiO 2 Weight Percent of Mare Deposits Based on 400 m pixel −1

Table 6
FeO Weight Percent of Mare Deposits Based on Kaguya MI 60 m pixel −1 Passive and active seismology package.Includes both passive and active seismometers to study lunar seismic activity.This package enables the characterization of moonquakes, impact-induced vibrations, and other seismic events, helping to understand the Moon's internal structure, crustal thickness, and potential tectonic activity.2. Heat flow experiment.Measures the thermal gradient and heat flux from the lunar surface to provide insights into the Moon's thermal history and heat transfer processes.This experiment aids in understanding the thermal properties of the regolith, the thermal conductivity of the subsurface, and the overall heat flow from the interior.3. Drill.Allows for the collection of subsurface samples, helping to characterize the subsurface stratigraphy and the structure of the megaregolith and potentially uncovering materials from different geological epochs.The drill provides access to deeper layers and helps in understanding the vertical variations in composition and geologic history.4. ISRU demonstration package.Demonstrates the extraction and utilization of lunar resources and explores technologies and processes for resource extraction, refining, and utilization, paving the way for sustainable lunar operations.

Table 7
National Research Council (2007) Goals Addressed or Possibly Addressed with the Proposed Instruments