Hansteen Mons: An LROC geological perspective

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Hansteen Mons: An LROC geological perspective Joseph M. Boyce a,∗, Thomas A. Giguere a, B. Ray Hawke a,1, Peter J. Mouginis-Mark a, Mark S. Robinson b, Samuel J. Lawrence b, David Trang a, Ryan N. Clegg-Watkins c,d a

Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI, 96822, USA School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, USA c Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA d Planetary Science Institute, Tucson, AZ, 85719, USA b

a r t i c l e

i n f o

Article history: Received 1 July 2015 Revised 17 February 2016 Accepted 11 August 2016 Available online xxx Keywords: Moon, surface Silicic Geological processes Geologic mapping

a b s t r a c t Mons Hansteen is a relatively high-albedo, well-known lunar “red spot” located on the southern margin of Oceanus Procellarum (2.3°S, 50.2°W). It is an arrowhead-shaped (∼ 25 km on a side), two-layer mesa with a small cone-shaped massif on its north edge formed by three morphologically and compositionally distinct geologic units. These units were emplaced in three phases over nearly 200 million years. The oldest (∼3.74 Ga), Hilly–Dissected unit, composed of high-silica, and low-FeO content materials formed a low, steep sided mesa. The materials of this unit erupted mainly from vents along northeastand northwest-trending sets of fractures. The Pitted unit, which comprises the upper-tier mesa, is composed of high-silica and even lower-FeO content materials. This material was erupted at ∼ 3.5 Ga from numerous closely spaced vents (i.e., pits) formed along closely spaced northeast-southwest-trending sets of fractures. At nearly the same time, eruptions of lower silica and higher FeO materials occurred on the north flank of Mons Hansteen at the intersection of two major fractures to produce the North Massif unit. The eruptions that created the North Massif units also produced materials that thinly blanketed small areas of the Hilly-Dissected and Pitted units on the north flank of Mons Hansteen. Also at nearly the same time (i.e., ∼ 3.5 Ga), basalt flows formed the surrounding mare. Each unit of Mons Hansteen appears to be mantled by locally derived ash, which only modestly contaminated the other units. The morphology of Mons Hansteen (especially the Pitted unit) suggests a style of volcanism where only a relatively small amount of material is explosively erupted from numerous, closely spaced vents. © 2016 Elsevier Inc. All rights reserved.

1. Introduction Unraveling the nature of lunar “red spots”, such as Mon Hansteen (MH), has major implications for lunar thermal history and crustal evolution (Hagerty, 2006; Jolliff et al., 2011), thus providing crucial information for understanding the early Moon. Lunar red spots are characterized by a relatively high albedo and a strong absorption in the UV (Wood and Head, 1975; Head and McCord, 1978). Some early workers presented evidence that at least some red spots were produced by non-mare or highlands volcanism and suggested a connection with KREEP basalts or even

∗

Corresponding author. E-mail addresses: [email protected] (J.M. Boyce), thomas.giguere@ intergraph.com (T.A. Giguere), [email protected] (P.J. Mouginis-Mark), [email protected] (M.S. Robinson), [email protected] (S.J. Lawrence), [email protected] (D. Trang), [email protected] (R.N. Clegg-Watkins). 1 Deceased.

more evolved highlands compositions such as dacite or rhyolite (Malin, 1974; Wood and Head, 1975; Head and McCord, 1978). One of these red spots, Mons Hansteen (IAU, 1976, see website at planetarynames.wr.usgs.gov) also known as Hansteen α (e.g., see Wagner et al., 2010) or Hansteen Alpha (e.g., see Hawke et al., 2003) are hereafter referred to as Mons Hansteen is a relatively high-albedo, polygonal, arrowhead-shaped, mesa that measures ∼25 km on a side. It is located on the southern margin of Oceanus Procellarum adjacent to the craters Billy and Hansteen at 12.3°S, 50.2°W (Fig. 1). New high-resolution observations from the Lunar Reconnaissance Orbiter (LRO) mission Lunar Reconnaissance Orbiter Camera (LROC), Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) images and image mosaics provide the means to significantly advance our understanding of the geology and morphology of this volcanic center. The objective of this study is to characterize geologic units of MH, and to determine their morphology, extent, distribution, age, composition, and geologic history. Recently

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Fig. 1. Mons Hansteen is located at 12.3°S, 50.2°W on the southern margin of Oceanus Procellarum near the craters Hansteen and Billy. Base image on the left is a full Moon telescopic view, and the one on at right is a mosaic of LROC WAC images (from LROC Quickmap). North is at the top in both images. Location of Fig. 4 indicated by white box.

Fig. 2. Oblique view from the east looking west of Mons Hansteen. The dotted lines trace the centers of the broad valleys developed in the Hilly-Dissected unit. North is to the right. LROC image M1154506530.

acquired meter scale images from the LROC cameras (Robinson et al., 2010) (Fig. 2) combined with datasets from previous missions (e.g., Clementine, Kaguya) enable new detailed mapping and discovery of three geologic units in MH, revealing its history and how it compares with other lunar red spot volcanic centers. 2. Background McCauley (1973) described MH as being a steep-sided, bulbous, very bright dome of material exhibiting a hackly surface. He also identified several small, linear, smooth-walled depressions at the crests of gentle individual highs and interpreted these depressions as probable volcanic vents. Wood and Head (1975) noted that MH Please

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has a distinctive surface texture, color, and albedo compared to the nearby highlands and adjacent mare units. Wagner et al., (2010) mapped MH from Lunar Orbiter IV images and described a flat summit region reminiscent of a mesa. They noted that the summit area, as well as the flanks, appears much more rugged than the Gruithuisen domes (Head and McCord, 1978; Chevrel et al., 1999) which are characteristic of level summits. Further, Wagner et al., (2010) identified two small, distinct areas on the summit region of MH, and measured superimposed crater frequency, but the low-resolution images they used prevented them from detailed geologic mapping of this feature. From two distinct areas of the summit they found two statistically significant crater distributions, with cratering model ages of 3.74 and 3.55 Ga Mons:

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(a model age of 3.67 for the sum of the two areas). Based on these measurements, Wagner et al., (2010) suggested that MH is Upper Imbrian age, clearly postdating the highlands materials, and predating the surrounding mare materials, confirming earlier results by Wood and Head [1975]. The younger age of 3.55 Ga measured on its summit could be connected to active mare volcanism in the area between 3.5 and 3.6 Ga., but Wagner et al., (2010) did not map these two count areas as separate geologic units. Remote sensing estimates and geomorphic analysis suggest that MH is composed of low-iron and silica-rich rock, and likely represents an evolved lunar lithology presently thought to be analogous to terrestrial granites and felsites, although the origin and emplacement of evolved silicic lithologies on the Moon remains unknown (Hawke et al., 2003; Glotch et al., 2010; Greenhagen et al., 2010; Paige et al., 2010; Glotch et al., 2011; Haggerty, 2006; Jolliff et al., 2011). Hawke et al., (2003) and Wagner et al., (2010) noted that if Mons Hansteen was present prior to the formation of Billy and Hansteen craters, it should have been covered with FeO- and TiO2 -rich ejecta since it is within one crater diameter of the rim crest of each crater. Since it is not, they concluded that MH was emplaced on top of the FeO-rich ejecta deposits, consistent with the model crater age of Wagner et al., (2010). Recent research using Clementine, Lunar Prospector (LP), and Lunar Reconnaissance Orbiter (LRO) data have provided strong evidence that some red spots, including the MH, are dominated by Th and silica-rich, highly evolved highlands lithologies (Hawke et al., 20 03; Lawrence et al., 20 05; Hagerty et al., 20 06; Glotch et al., 2010; Greenhagen et al., 2010; Glotch et al., 2011; Hawke et al., 2011; 2012; Ashley et al., 2016). For example, Clementine UV-VIS images were used to produce FeO, TiO2 , and optical maturity maps of the MH region utilizing the algorithms of Lucey et al. (20 0 0a; b). Mare units in this region exhibit FeO abundances > 16 wt%, and TiO2 values range between 4 wt% and 8 wt%. In sharp contrast, much lower FeO and TiO2 values are exhibited by Mons Hansteen where FeO values range from 5 wt% to 9 wt% and TiO2 of < 1 wt%. In the central portion of MH, the surface materials have an average FeO value of 6.9 wt% and an average TiO2 value of 0.5 wt% (Hawke et al., 2003). Hawke et al. (2003) suggested that since this central region would be less contaminated by debris from the surrounding mare units thrown there by impacts, its composition may most closely approximate that of the underlying lithology; i.e., ejecta from the Imbrian-aged craters Billy and Hansteen. However, this explanation is unlikely because their map does not show evidence of contamination from MH on the surrounding mare surface. Hence, the contamination would have to be unidirectional (from mare to MH and to higher elevations), and would be substantially greater than noted as occurring elsewhere along the maria/highlands boundary (Logan et al., 1972). Lawrence et al., (2005) and Hagerty et al., (2006) used forward modeling of LP Gamma Ray Spectrometer data to show that the Th abundance at MH is not 6 ppm, but could possibly range from ∼20 to ∼25 ppm. This is consistent with Th abundances measured in evolved lunar lithologies such as granites and felsites. Subsequently, Glotch et al., (2010) based on thermal emission signatures measured by the LRO Diviner Lunar Radiometer Experiment (Diviner) found that Si-rich materials are more abundant near the center of MH and in the high terrain SW of the center (Fig. 3). Glotch et al., (2010) suggest that the lower values on the margins of the feature may be the result of contamination by mare debris transported to the slopes of the dome by impacts in the surrounding mare. Recently, Kiefer et al., (2016) used gravity modeling based on high-resolution Gravity Recovery and Interior Laboratory (GRAIL) mission observations to suggest that MH is composed of relatively low-density, felsic materials (bulk density of the crust beneath MH of 150 0–20 0 0 kg −3 ). This is similar to their findings for the Please

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Fig. 3. LROC WAC images mosaic with the LRO Diviner Standard Christiansen Feature Value (silica) map superposed. The white indicates areas of relatively highsilica content. (Image from LROC Quickmap mosaic).

Gruithuisen domes. They note that to be consistent with their density observations silica-rich magmas required, can be produced either by 1) silicate liquid immiscibility (Hagerty et al., 2006), or 2) crustal melting induced by basaltic underplating. They favored the basaltic underplating mechanism because it can produce a crustal rhyolitic composition magma that is consistent with the FeO content of MH materials as well as their inferred bulk density of MH. Kiefer et al., (2016) also suggest that basaltic underplating is supported by model melting calculations that indicate that partial melting of KREEP basalt (driven by the heat from the intrinsic radioactivity of KREEP and from mare basaltic intrusions) should produce significant volumes of rhyolitic magma with the right range in FeO, as well as a high thorium abundance like that observed at MH and other felsic domes (Hagerty et al. 2006). Each red spot volcanic complex appears to have its own unique shape. For example, Mons Hansteen is a two-layer mesa with multiple vents and one satellite cinder cone. The Compton–Belkovich Volcanic Complex (Jolliff et al., 2011; Chauhan et al., 2015), which is approximately the same size as MH, is a broad area of elevated topography with a range of volcanic features (e.g., irregular collapse depressions, and a variety of size domes). The Lassell Massif complex may also be a layered volcanic complex of about the same size (Ashley et al., 2013; 2016). In contrast, the Gruithuisen domes include two relatively large elongate domes and a small dome (Chavrel et al., 1999). The detailed geologic history like that presented here for MH has yet to be completed for each of these red spot volcanic centers, but would help us to understand why these volcanic centers are so different.

3. Regional context The Mons Hansteen is located on the southern margin of Oceanus Procellarum centered at ∼ 12.5°S and 50W (Fig. 1). This complex formed in highlands materials and is Imbrian age (McCauley, 1973; Wagner et al., 2010). It began to form after the nearby impact craters Hansteen and Billy (∼ 3.9 Ga), but before the mare flooded its flanks at ∼ 3.5 Ga. Mons:

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Fig. 5. Oblique view, looking west, of the wrinkle ridge (arrowed) extending into the southeastern side of the Mons Hansteen from the mare to the southeast. Width of image in foreground is ∼6.6 km. North is on the right in this image. The image is a portion of LROC image M1154506530, its location is shown in Fig. 2.

Fig. 4. Cumulative size-frequency distribution (CSFD) curves of impact craters, and subdued circular and quasi-circular pits on the geologic units of the MH as well as the adjacent mare to the east. The Hilly-Dissected unit crater counts are in closed squares (390 crater between 100 m and 1.48 km dia., in 153 km2 area); the Pitted unit are in closed circles (186 craters between 100 m and 900 m dia., in 126 km2 area); the North Massif unit craters are in inverted closed triangle (40 craters between 100 m and 500 m dia., in 24.6 km2 area); the mare east of MH are in open circles (97 craters between 143 m and 510 m dia., in 134 km2 area), subdued pits on the Pitted units are crosses (48 pits between 267 m and 1.3 km, in 126 km2 area); and subdued pits on the Hilly-Dissected unit are x s (24 pits between 240 m and 750 m dia., in 153 km2 area). The best fit model age production functions are plotted for 3.5, 3.74 and 4.0 Ga and the theoretical crater equilibrium curve (see from Michael and Nuekum, 2010). Note the divergence of the CSFD curves on all the units of MH from lunar impact crater production functions < ∼0.5 km diameter. The dashed line is the average for all these units. The linear nature of these curves below ∼0.5 km crater diameter suggest a process that degrades and erases smaller crater faster than larger ones such as would occur in areas mantled by particulate material. Also note that the CSFD of the subdued pits on both units also do not follow the lunar impact crater production function.

3.1. Highlands Highlands in the MH region are composed of high albedo material characterized by rugged, hilly or hummocky, furrowed morphologies (McCauley, 1973; Wagner et al., 2010). The highland units in the MH region were mapped as undivided terra material by Wilhelms and McCauley (1971). Later McCauley (1973) mapped the highlands south of Hansteen crater near MH as principally radial outer rim material of crater Hansteen. McCauley (1973) and Wilshire (1973) assigned the highlands material in the Hansteen area as belonging to the Imbrian system, and Wagner et al., (2010) used crater counts to derive a model age for it of ࣡ 3.91 Ga. The closest large, old craters to MH, Billy and Hansteen (both ∼ 45 km diameter), were assigned by McCauley (1973) an Imbrian age. This is consistent with the crater model ages of 3.88 Ga, for Please

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Billy crater and 3.87 Ga for Hansteen crater measured by Wagner et al., (2010). Imbrian age and possibly older grabens form sets of structures that trend northeast-southwest (i.e., concentric with the Humorum basin in the area of MH), as well as sets of northwest-southeast trending structures that formed parallel with the edge of Oceanus Procellarum. The northwest-southeast trending structures most likely were created by tectonic stresses following the formation of the Procellarum or Imbrium basin (Wilhelms, 1987; Solomon and Head, 1979; 1980), while the northeast-southwest trending structures of MH are probably related to the formation of Humorum basin, Billy crater, and Hansteen crater. 3.2. Maria Hiesinger et al., (2003) identified several distinct basalt types in the Hansteen region based on multispectral analyses of mare using color data of the Clementine UVVIS-camera. The basalts surrounding MH have FeO values in the 14 to 18 wt.% range, and TiO2 values fall in the 2 to 6 wt.% range. These mare materials are the youngest volcanic deposits in the area evidenced by their embayment of all other materials, i.e., all highlands units, older crater materials, Hansteen or Billy, (Whitford-Stark and Head, 1980) as well as the units of MH. Wilhelms and McCauley (1971) assigned Eratosthenian ages to most of the mare materials in the MH area. Later, Wagner et al., (2010) found that mare materials in the Hansteen region could be classified into three time-stratigraphic units based on crater counts, with most being late Imbrian age. They found that individual areas in Oceanus Procellarum near MH range in model ages from 3.67 Ga to 3.35 Ga. We also counted impact craters on the mare east of MH using LROC NAC images as a base and found an impact crater model age of ∼ 3.5 Ga (Fig. 4), consistent with the ages found by Wagner et al., (2010) and that found by Boyce (1976) based on crater degradation. Northeast and northwest trending wrinkle ridges have developed in the mare around MH. LROC images indicate that two of these structures extend northwest across the mare south of MH discernable on the flank of MH for a short distance (Fig. 5), indicating that the stress field that formed these structures existed after the formation of the mare and MH. Another set of Mons:

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Fig. 7. Geologic units and structures of Mons Hansteen. The map on the left shows the geologic units of Mons Hansteen. The map on the right shows the locations of examples of structures (black lines) that cut Mons Hansteen (white) and mare (wrinkle) ridges in the surrounding mare (short lines with small closed circles). North is at the top in both figures.

Fig. 6. Topographic contour map of Mons Hansteen and the surrounding area superposed on a LROC WAC image mosaic (from LROC Quickmap). The contour interval is 100 m and the numbers (black with white backgrounds) are in kilometers. The data for this map is from Kaguya Terrain Mapper Camera images.

wrinkle ridges intersects MH on its northwest side (Figs. 2 and 6). These wrinkle ridges are on a line that connects with the wrinkle ridges in the south. Both sets likely formed by the same stress system because they are the same trend. In addition, a northeast-southwest trending wrinkle ridge intersects with Mons Hansteen on its northeast side (Figs. 2 and 6).

3.3. Mons Hansteen Mons Hansteen is a polygonal, arrowhead-shaped, mesa that measures ∼25 km on a side (Fig. 2). Its maximum relief above the surrounding mare is over 900 m with its base at ∼1900 m below the global mean surface elevation (Fig. 6). Our mapping has found that it is composed of three major geologic units: the Hilly–Dissected unit, the Pitted unit, and the North Massif unit (Fig. 7), described in Section 4. Although previously called a volcanic dome (McCauley, 1973) or a mountain of smoothly increasing elevation, LROC NAC-derived topographic data shows that MH is actually a two-layer mesa (Figs. 6 and 8). This shape is unique among red spot volcanic complexes on the Moon. The bottom layer of the MH mesa rises sharply (in places, with slopes ∼35 °) from the surrounding mare surface (at ∼ –1900 m) to an average height of ∼ 525 m (i.e., ∼ –1375 m) above the surrounding mare surface. The top layer of MH is an elongate mesa located in the north-central portion, and has an average height of ∼ 650 m (i.e., – 1250) above the surface of the surrounding mare. It is capped by a small hill whose summit is at –924 m. Obvious impact craters are found on these mesas. But in addition to these impact craters, both mesas contain subdued circular to quasi-circular pits (Fig. 2) whose cumulative size frequency distributions differ (i.e ., higher negative slopes) substantially from that of the crater production function of lunar impact craters. The population density of these subdued pits is greatest on the top mesa (Fig. 4). The subdued pits are typically Please

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Fig. 8. LROC WAC low-sun angle mosaic form LROC Quickmap showing MH (top), and the locations of three cross sections (A-A’, B-B’ and C-C’) across it (bottom). This mosaic (north at the top) and topographic profiles shows that MH is composed of two progressively smaller, mesa-like layers stacked one on the other. The bottom mesa is a rugged layer that includes small hills, valleys, and circular to elongate pits and whose top is at ∼ 500 m (i.e., −1350 m) above the surrounding mare. The top mesa has an oval shape with a peak just left of its center at an elevation of −924 m. Topographic data are derived from a combination of LROC high-resolution digital terrain model (DTM) at 100 m/pixel (LROC WAC Global Lunar DTM 100 m) with a vertical accuracy of 10 m, 2.0 m post spacing and an RMS error relative to two LOLA tracks of 4.73 m (data are available at the LROC website at lroc.sese.asu.edu) and from topographic derived from Kaguya images. Vertical exaggeration 7x.

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Fig. 9. High-resolution NAC image of a small area in the Pitted unit showing that the ridges between the pits are relatively smooth and rounded, and the floors are typically broad, gently rounded to nearly flat. Topographic profiles (A – A’, and B – B’) have also been constructed using NAC DEM data across a portion of the Pitted Unit (see insert for location). These profiles show that most pits are shallow saucer shaped (i.e., relatively steep slopes on the sides and gently downward curving, bowl-shaped floors). The image is a portion of the LROC NAC M1127462982LC. North is at the top of the image.

a few meters to tens of meters in depth and range from a few hundred meters to over a kilometer diameter (Fig. 9). In addition to the two mesa structure of MH, a small, rounded, relatively low-albedo massif ∼ 150 m high and ∼ 5 km across is located on the north edge of MH (Fig. 10). A shallow northeast trending trough ∼ 1 km wide separates this massif from the rest of MH to the south. To the north, this massif is superposed on highlands materials. Two elongate pits sit atop this massif. Mons Hansteen is cut by crossing sets of northeast-southwest trending and southeast-northeast trending structures (Fig. 7). Particularly on the lower mesa, these structures commonly form shallow, graben-like troughs, none of which show evidence of lateral displacement along their lengths (Figs. 2 and 10). Where the northeast-southwest trending structures cross the upper mesa, they commonly show coincidental alignment with chains of the largest of the subdued pits. The cumulative size frequency distribution (CSFD) (Fig. 4) and their alignment along structures suggest that the subdued pits are most likely to be volcanic vents instead of impact craters. Two of the major northeast-southwest trending structures intersect with the large elongate, cleft-like pits found on the southwest flank of MH suggesting they also may be vents. The most northern of these northeast-southwest structures also intersect with a major northeast-southwest trending structure at the pits on the summit of the massif located on the north flank of MH. In addition, none of the structures on MH can be traced onto the surrounding mare or highlands nor does any of their directions match any trends of the wrinkle ridges developed in the mare around MH. This suggests that the stress field that produced the structures found on MH were produced by a different stress field than the stress field that produced the wrinkle ridges, and was likely a result of deformation associated with MH volcanism. Hawke et al., (2003) provided an overview of remote-sensing and morphology derived geochemistry of MH to suggest that it is a relatively high-silica, low-FeO and low-TiO2 mountain interpreted as volcanic. They used the steep flank morphology of MH to infer Please

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that the lavas must have been of high viscosity (SiO2 -rich) in order to form it’s nearly 35° slopes. This is also consistent with LRO Diviner estimates of silica content suggest that MH is composed of relatively high silica materials (Fig. 3). Hawke et al., (2003) suggested that a high SiO2 content made more geochemical sense than an exceptionally aluminous magma that would also produce high viscosity and steep slopes. Based on this inference, they speculated that MH may be composed of dacitic or rhyolitic formed by extrusions of relatively viscous lavas at low rates. In this overview they also mapped the distribution of FeO and TiO2 based on Clementine UVVIS and Earth-based near - IR reflectance spectra. They reported that the average FeO value of MH is 6.9 ± 0.5 wt.%, but that it might be composed of more than one geochemical unit with a possible unit in the central MH composed of low-FeO materials surrounded by higher-FeO material. The boundary between these possible geochemical units are similar to the boundaries of the heavily pitted area on the upper mesa and the less pitted, hummocky terrain of the lower mesa (see Section 4). Hawke et al., (2003) commented that material ejected from impact craters on the mare may land on MH and mix with the material on MH to be an important contaminant and that it could potentially affect the ability to measure accurate compositional values on MH. We suggest that the effects of such contamination are not severe enough to prevent compositional mapping and identification of units of different composition on MH. The effects of contamination from impact transport on remote sensing measurements was evaluated by Logan et al., (1972) at the Apollo 14 and 15 landing sites and found to be minimal at distances of a kilometer or two from a contamination source. To evaluate the finding of Logan et al., (1972), and to test this at MH. We turn to the FeO map (Fig. 11) produced by Hawke et al., (2003) based on Clementine data as a base for this evaluation. This was approached by mapping the change in FeO wt.% of the mare from the edges of MH progressively outward to where FeO content becomes relatively uniform, and unchanging. We assume that the effects of contamination from low-FeO materials blasted off MH by small impact craters are minimal from this point outward. Likewise, the effects of contamination from small impacts on the mare onto MH from the mare/MH border inward should be similar with distance to contamination in the opposite direction or even possibly less because MH is at higher elevation than the mare. We constructed profiles of FeO wt.% (Fig. 12) with distance along the lines (lines 1 – 2 and 3 – 4) shown in Fig. 11. The locations of these profiles were chosen because the geology is simplest (e.g., no nearby highlands), the relief between MH and the mare is at a minimum, and relatively large craters are far enough away to contribute little materials (i.e., thickness of ejecta declines roughly as a power of –3 relative to crater radii, Melosh, 1989). These profiles (Fig. 12) show a zone of continuously increasing value of FeO wt.% from about 1 km in the interior of MH outward onto the mare where at a distance of ∼ 1 to 2 km from the mare/MH boundary become nearly constant. We suggest that this indicates that significant contamination extends away from the mare/MH boundary in both directions for approximately 1 – 2 km. In order to add more detail and confidence to this assessment, we have also produced histograms of FeO wt.% values in sample areas (each with ∼ 13 km2 area) on the mare (D though G) along those profiles, as well as for sample areas containing the Pitted unit, Hilly and Dissected unit, and North Massif (sample areas A, B and C respectively). These are plotted in Fig. 13, and show (1) an increase in FeO wt.% of the mare surface along profile 1 – 2 eastward from the mare/MH boundary from area E to area F, with only a small increase in FeO from F to G (outward of G all mare has a FeO content of > ∼ 17.7 wt.%, similar to F and G), and (2) a similar trend along profile 3 – 4 where the FeO content from sample areas B to D significantly increases from ∼ 8.5 wt.% Mons:

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Fig. 10. These images show Mons Hansteen under different lighting conditions. These images were used to identify structures and their trends as well as the units shown in Fig. 7. Top left is a high-sun angle LROC WAC image mosaic from LROC Quickmap (note low-albedo surface around North Massif outlined in white dots), and at top right is a low-sun angle LROC WAC image mosaic (sun on the right). The lower two images have been constructed from DEM data (from Kaguya Terrain Mapper Camera data) in order to assess illumination effects on identification of structures (white arrows show solar illumination direction). The lower left is a shaded relief image, illumination from top, Sun elevation 15° above horizon. Lower right is a shaded relief image, illumination from right, Sun elevation 15° above horizon. North is at the top of all images.

to nearly the average mare values. Based on these sample areas and the distribution of FeO values on the FeO map of Hawke et al., (2003) we suggest that a zone of significant contamination extends outward in both directions approximately 1 – 2 km from the mare/MH boundary. It should also be noted that if pyroclastic material was erupted from MH these results suggest that it would have no more of a contamination effect than did impact transport. In an effort to identify and define major composition units on MH, we have used this assessment of the extent of contamination as a guide to analyze the compositional data produced by Hawke et al., (2003). As a starting point, we looked at FeO wt.% values in a sample area within each of the two compositional units on the main massif of MH (i.e., sample areas A, and B in Fig. 11) shown in the maps of Hawke et al., (2003). The locations of these sample areas were chosen far enough away from the mare/MH boundary to reduce, as much as possible, contamination from the mare and/or the other MH units. The histograms of FeO wt.% values within these two sample areas (Fig. 13) show that sample area A has FeO content of ∼6.7 wt.% and sample area B has FeO content

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of 8.5 wt.%. The North Massif unit (sample area C) is also evaluated here, and has the FeO content of ∼ 10.4 wt.% FeO). The surface materials of North Massif unit are distinctly different from those in the other two MH units (i.e., A and B) and from materials of surrounding mare of ∼ 17.7 wt.% FeO. The North Massif unit was not recognized by Hawke et al., (2003) to be part of MH, likely because of its lower albedo and higher FeO contents as well as its location on the edge of the main edifice of MH. The histograms of the individual sample areas in MH (Fig. 13) are leptokurtic and show little to no overlap suggesting that each contains materials of different and distinct compositions. In addition, the histograms of FeO content in the sample areas in the eastern mare (i.e., areas D, E, F, and G in Fig. 11) are clearly different from the sample areas on MH. Consequently, we suggest there are at least three distinct compositional units on MH, and that the map distribution of these units shows a reasonable correlation with the three units defined on the basis of geomorphology. This correlation suggests the Pitted unit is composed of low-FeO material, the Hilly-Dissected unit is composed of comparatively

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Fig. 11. A map of the FeO distribution for MH and the surrounding area. The black line on the map is the approximate boundary of MH with the other units in the area. The white boxes are locations of sample areas A, B, C, D, E, F, and G; which are equal in area (13 km sq.). Sample areas A, B, and C are all on MH. Sample area A has a mean FeO value of ∼ 6.7 wt.%, with σ = 0.41; sample area B has a mean value of ∼ 8.5 wt.% FeO, with σ = 0.54; sample Area C has a ∼ 10.4 wt.% FeO, with σ = 0.45. Sample areas D, E, F, G, and F are all on the mare on the eastern side of MH. Sample area G (the mare on line 1 – 2) has a mean values of ∼ 17.7 wt.% FeO, with σ = 0.13; sample area F has a mean value of 16.9 wt.% FeO with σ + 0.15; sample area E has a mean value of ∼ 15.6 wt.% FeO, with σ + 0.49. Sample area D (the mare on line 3 – 4), which is about a kilometer from the edge of Mons Hansteen has a mean value of ∼ 16.6 wt.% FeO, with σ = 0.20 (data from Hawke et al., 2003). Lines 1 -2 and 3 -4 are the locations of profiles plotted in Fig. 12..

Fig. 12. Profiles (top profile is lines 1 – 2, and bottom is 3 – 4) of FeO values from the map of Hawke et al. (2003) shown in Fig. 11. The FeO values on this figure are ranges. The closed triangles are the high values of the range and gray circle are the low values of the range. A dotted line is drawn along the top value. The location of the mare/MH boundary is marked with a vertical solid line. The top profile (i.e., 1 – 2) starts in the Pitted unit, extends through a narrow band containing the HillyDissected units (the two are separated by a dashed line), and for 12 km onto the mare east of MH. This profile suggests that contamination may be important out to 2 km from the boundary. The bottom profile (i.e., 3 – 4) crosses a portion of the southeastern part of MH that only includes the Hilly-Dissected unit and extends for 6 km onto the mare. This profile shows a transition zone between the two units that suggests that contamination is important only ∼ 1 km on either side of the boundary.

higher FeO materials, and the North Massif unit is composed of an even higher FeO content material, but still substantially lower than the surrounding mare. 4. Geologic Units; and chronology Recently acquired high-resolution, high-quality data from missions like LRO (principally LROC) provide superb quality imaging, topographic, and remote sensing data that support new detailed geologic mapping of MH. Based on these new data, our mapping has found that MHVC is comprised of three major geologic units (Fig. 7) associated with volcanism; (1) the Hilly-Dissected unit, (2) the Pitted unit, and (3) the North Massif unit. These will be discussed below in order of their age and stratigraphic position. 4.1. Hilly–Dissected unit The Hilly–Dissected unit is characterized by low hills, scarps, mesas, valleys, troughs, and various shaped depressions that are likely to be volcanic vents. It is composed of relatively low-iron (B in Fig. 11), and relatively high-silica content material (Hawke et al., 2003; Hawke et al., 2011). It comprises most of the lower mesa of the main edifice and surrounds the Pitted unit. This unit butts up against highlands materials on the southwestern and northeastern sides of Mons Hansteen, while on its southern, northwestern, and eastern sides it is embayed by mare materials. The Hilly-Dissected Please

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Fig. 13. Histograms of FeO values (binned in 0.25 wt.% increments) in areas sampled in Fig. 8 and the entire surface of MH. Note that the units on Mons Hansteen (A, B, and C) are distinct from one another and from those on the mare (D, E, F, and G). We also suggest that the reason sample area E shows the lowest values of FeO compared to the other mare sample areas is because of greater effects of contamination of material ejected by small impact craters from MH.

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Fig. 14. Oblique view (looking southeastward) of the Pitted unit on Mons Hansteen. Note the dichotomy in morphology of the relatively large pits (P) and the small impact craters (IC). Image of LROC NAC DTM HANSTEENAL mosaic. The scene is ∼ 9 km across.

unit is the oldest stratigraphic unit of MH based on an impact crater model age suggesting it is the oldest unit, and the cross cutting relationship of structures. Several types on pits and depressions are found on this unit distinguished by the different morphology and size frequency distributions. For example, subdued circular to quasi-circular pits (similar to those in the Pitted unit, see below) are common in this unit. Most of these pits are located in the southern part of the unit. None show evidence of ejecta deposits, or have obvious associated lava flows (these pits will be discussed in great detail below). As well as the subdued pits, the Hilly-Dissected unit contains several steep-sided (∼ 30° based on LROC NAC digital terrain model [DTM]) cleft-shaped, elongate depressions (Fig. 2). These elongate depressions are unique to this unit, and unlike secondary impact crater chains, they are each one long continuous pit, instead of a series of connected pits. Their shape and occurrence in a volcanic complex make the probable volcanic vents. Though lava flows are not observed emanating from any of them the surface in the immediate vicinity of the two on the southwest flank of MH (Fig. 6) is smoother than elsewhere on MH and may be blanketed by ash. The largest of these steep-sided depressions is located on the southwestern edge of MH. Its long axis is in a line with a lineament that cuts across the northwest flank of MH and intersects with the two elongate subdued pits on top of the North Massif (Fig. 7). Impact craters were also identified based on their morphology (see next section) and counted to derive a model crater age for the Hill-Dissected unit (Fig. 4). The subdued pits were excluded from these counts. These counts suggest an impact crater model age of ∼ 3.74 Ga (error of + 75 million years, - 150 million years) for this unit, consistent with the model age determined by Wagner et al., (2010) using Lunar Orbiter IV images. In addition, the age reported here is based on the population of craters > ∼ 0.5 km diameter where the CSFD of these craters follows the lunar impact crater production function. However, the CSFD of craters below that size forms a curve with a substantially different slope (lower negative) suggesting that these craters may have been affected by a process that degrades small craters at a greater rate than larger ones. More will be discussed about the possible cause of this below. Relatively broad valleys and ridges, presumably controlled by faulting, cut the Hilly-Dissected Unit with the broadest ones commonly terminating at the boundary with the Pitted unit (see Figs. 2 and 7). This suggests that these features predate the Pitted unit. These are most common along the southern part of MH in this unit with most striking in a northwest-southeast, or northeast-southwest direction. However, there are lineaments that strike northwestward along these large broad valleys on the southeast side of the Hilly–Dissected unit and continue on a line with ridges that cross the Pitted unit (Fig. 7) and are on a line Please

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Fig. 15. The interior slopes of most pits (“P”), especially those in the Pitted unit, change abruptly at the pit floors, and then form a gently curving bowl shape. LROC image number M1127462982LC. North is at the top.

that intersects the two pits on the North Massif Unit. These two lineaments also appear to be members of a set of closely-spaced (∼ 1–2 km apart) parallel structures that trend ∼ N 340° W, S 160° E) that cross MH. Consequently, there are structures that appear to cross the Hilly-Dissected unit and extend across the Pitted unit, suggesting that these structures were active even after the Hilly-Dissected unit formed (Fig. 7). In addition, a line of low ridges extends northwestward across the southeastern part of the Hilly–Dissected unit, and terminate at the southern edge of the Pitted unit (Fig. 2). The most northerly of these ridge segments cuts across the northeastern major trending valley on the southern edge of the Pitted unit, but terminates at that unit suggesting that the ridge developed after the valley, but most likely before the Pitted unit. 4.2. Pitted unit The Pitted Unit is a high-albedo, low-iron (∼ 6.7 wt.%, A in Fig. 11), relatively high-silica area located in the north central portion of MH. This unit contains an area of ∼110 km2 located mainly above an elevation of ∼ 500 m (relative to datum). It is characterized by closely-spaced, commonly overlapping and nested, subdued circular to irregular-shaped depressions (Fig. 14) similar to those found in the Hilly-Dissected unit. Similar to the Hilly-Dissected unit, none of the pits in the Pitted unit exhibit ejecta deposits, nor have associated lava flows. The average diameter of the pits is ∼ 0.6 km, but they range from ∼0.14 km to > ∼ 1.4 km in diameter. Fig. 9 shows topographic profiles plotted on a high-resolution NAC image of a small area in the Pitted unit. The ridges between the pits are relatively smooth and rounded, and the floors are typically broad, gently rounded to nearly flat (Fig. 15). However, the interior slopes of the pits can be relatively steep (20° to 25° based on the NAC DTM). These characteristics suggest that the unit may be blanketed by particulate material, such as a volcanic ash, that drapes over ridges and pools in low places. McCauley (1973) also suggested that these pits are of volcanic origin because some of them have elongate or irregular shapes. This interpretation is consistent with their lack of ejecta deposits, irregular outlines, subdued topography, and their association with the small domical mounds on this unit reported by Hawke et al., (2014). Mantling of the pits by fine material (discussed in Mons:

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detail later), such as ash, also suggests that volcanism may have produced these pits, although an impact or collapse origin cannot be completely ruled out. The cumulative size-frequency distribution of the pits in the Pitted unit, and those in the Hilly–Dissected unit, are plotted in Fig 4. This figure shows that the CSFDs of the pits exhibits marked differences from the impact crater production function with the CSFD of subdued pits > ∼0.5 km in diameter (on both units) showing steeper distribution curves than those of impact crater production functions at similar diameters. These data suggest that the pits are not impact craters, but more likely to be related to volcanism. But, if these closely-spaced pits are volcanic vents, then they represent a new style of volcanic eruption. Some pits on MH are circular, have relatively steep interior slopes, raised rims, and exhibit ejecta blankets, and hence, appear to be impact craters (Fig. 14). Their CSFD is shown in Fig. 4 and suggests a model age for the Pitted unit of ∼ 3.50 Ga (with an error of + 0.150 million years and – 1.0 billion years). This model age is also consistent with the model age for MH found by Wagner et al., (2010) using Lunar Orbiter IV images. It should be noted that considering the error in crater density age measurements (which was used here to estimate age), this unit could have been emplaced only a few tens of million years after the Hilly-Dissected unit, but also could have had been emplaced as long as a billion years later, hence, while shorter gaps between episodes would be more geologically reasonable, longer gaps cannot be ruled out. 4.3. North massif unit The North Massif units is a small, dome-shape massif (∼ 6 km × 4 km, with ∼ 150 m relief) separated from the main edifice of MH on its northeast flank by a shallow trough (Figs. 7 and 10). Like the other two units, the surface of the North Massif unit appears to be mantled, but, in contrast, has a lower albedo. A pair of shallow, elongate pits, also likely to be volcanic vents, has formed atop this massif (Fig. 16). They have formed at the intersection of two major lineaments on the MH. The long axis of these pits is on a line with the long axis of the northern most of the elongate depressions (another vent) on the western edge of MH. These pits are also on lines connecting ridges in the Pitted units and the faults in the largest valley in the southern part of the Hilly-Dissected unit. The massif is surfaced by lower-albedo, relatively higher-iron (∼10.4% FeO, C in Fig. 11), higher TiO2 (∼1–3 wt.%), and lower silica (based on albedo and diviner data, see Fig. 3) material compared with the other two units of MH (Hawke et al., 2003; Glotch et al., 2010). The low-albedo material from this unit blankets the surface of the Hilly–Dissected and Pitted units in the near vicinity of the massif (Figs. 7 and 10). This material is likely to be the result of ash erupted from the pits on the North Massif and suggests that the North Massif unit is younger than the other two units. The model crater age of North Massif unit is difficult to constrain because its surface area is too small to contain enough reasonable size craters (i.e.,> ∼ 0.5 km diameter required to obtain a crater production function) for an accurate model age estimate using crater counts. However, we have counted impact craters on this unit to assess shape of the CSFD and what it may reveal about processes that may have affected the crater population. The resultant CSFD is shown in Fig. 4 and is strikingly similar to those of the other MH units for crater diameters < ∼ 0.5 km. This suggests that the craters on this unit are not a production function, but are also likely being affecting by the same process that affects the small craters on other units. The density of the craters on North Massif, although they are not a production function they are still similar to those on the other units at those sizes. We suggest that likely means that the North Massif unit is likely to Please

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Fig. 16. Elongate possible volcanic vents (arrows) at the summit of the North Massif. Pitted unit is toward the bottom of the image (portion of LROC NAC image M1127462982LC). The area at lower left is thinly mantled by relatively low-albedo materials likely from North Massif (see high sun-angle image in Fig. 10).

be similar in age to those units. Considering that material from North Massif unit appears to be thinly blanketing a small area of Hilly–Dissected and Pitted units on the northeast flank of MH, it can be inferred that North Massif unit is most probably slightly younger than the Pitted unit but still ∼ 3.5 Ga. 4.4. Pyroclastic mantle There is evidence that the surface of the MH is mantled by a layer of particulate material that may have been produced by pyroclastic volcanism. This is suggested by the subdued topography of MH, the nature of the CSFD of its superposed impact craters as well as their detailed morphology, weathering of blocks from beneath a layer of smooth material at the tops of some slopes, and the nature of reflected light from MH. The morphology of the small relatively fresh impact craters on MH provides evidence that MH is covered by a layer of particulate material similar to regolith. These craters on MH commonly exhibit interior benches (Fig. 17), as do those on the surrounding younger mare. Interior benches are rings of material on the interior slopes of small impact crater that Oberbeck and Quaide (1967) suggest are caused by the effects of different strength of materials in layered targets. The geometry of the benches in the craters on the surface of MH is shown in Fig. 17 and suggests a low-strength layer overlying a stronger substrate. Depth estimates based on expected crater shapes suggests that the low-strength surface layer that produced these benches is ∼ 9–11 m thick. This thickness is Mons:

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Fig. 17. Two relatively fresh, small impact craters on Hilly-Dissected unit of Mons Hansteen that exhibit interior benches. Such benches are indications of a target that includes a surface layer of low-strength material ∼ 9–11 m thick. Image is portion of the LRO LROC NAC image M1127462982LC.

∼ 3 times thicker than expected for the regolith produced by the flux of impacts on the surface of MH alone (Moore et al., 1980), and 3 to 4 times that of the regolith on the mare just east of MH measured using the same technique. The shapes of impact crater size-frequency distributions (CSFD) can provide information about surface processes. In the case of the geologic units of MH, their CSFD (Fig. 4) show that craters with diameters > ∼ 0.5 km follow the lunar crater impact production function, but at ∼ 0.5 km diameter (down to 0. 1 km) craters of these units do not. Instead, these small craters decrease in abundance continuously and more rapidly with decreasing crater size relative to the production function. This particular shape of the small crater CSFD curve can be caused by either a constant, slow deposition of material in crater bottoms or a mantle of particulate materials, both of which will obliterate small craters rapidly and larger one slowly (Hartmann, et al., 1981, p. 1052). We suggest that the latter is more likely on the moon. An additional argument for pyroclastics is the surface brightness of MH. Along with the high reflectance observed in all wavelengths, the MH region has been observed to have relatively high reflectance in the visible wavelengths (Whitaker, 1972; Wood and Head, 1975; Hawke et al., 2003). Clegg et al., (2014, 2015) conducted a photometric analysis of several silicic regions of the moon and derived a single-scattering albedo (w) which is dependent on grain size and composition, for each region, allowing for the direct comparison of each region corrected for the effects of viewing geometry and phase angle. Although Compton-Belkovich had the highest single scattering albedo values (0.59 +/–1.0 w), MH also exhibited very high values (0.47 +/–1.0 w) compared to other silicic areas and the Apollo landing sites. Apollo and Luna soil compositions correlate with reflectance and w values such that more reflective soils (and therefore soils with a higher w) have higher plagioclase contents and lower mafic mineral content and Clegg et al., (2015) found that silicic regions plot along the extrapolation of landing site data to low mafic contents. Elevated w values for MH indicate a lower mafic component and a higher plagioclase or silica-rich component on the surface, which is what would be expected for a silicic pyroclastic. The pits and the other topography at the meter to tens of meters scale on all geologic units of MH are subdued (low slopes and smooth, rounded topography), in contrast to the superposed impact craters that exhibit much crisper topography (Fig. 15). This Please

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morphology is unlike topography whose subdued morphology is caused by age where the superposed impact craters show a spectrum of maturity, with the morphology of oldest of these consistent with the degree of terrain softening of the underlying terrain. This morphology is most characteristic of terrain that has been mantled soon after its formation with subsequent exposure to background impact bombardment. This results in subdued terrain with superposed impact craters that exhibit morphologically fresher shapes like that of the surface of MH. Hawke et al., (2011) noted numerous areas on MH with high block (i.e., rocks, and boulders) densities associated with steep slopes and impact craters, although some blocks are also found on flat terrain. The locations of areas of relatively high rock abundance on steep slopes on the MHVC generally correlates with the concentrations of ≥ 1 m rocks shown in the LRO Diviner rock abundance map (Fig. 18). The areas around these patches of rocks appear to be relatively rock-free. A visual examination of these rocky areas on MH using LROC NAC images shows that the places along slopes where these blocks are concentrated are also places where blocks appear to be weathering from under a layer of smooth material (Fig. 19). Judging by the size of the largest blocks compared with the thickness of the smooth material this layer of smooth material is > ∼ 8–10 m thick along its edges. We suggest that this smooth material is volcanic ash. Based on the observations discussed above, MH is most likely mantled by at least 8–10 m of particulate materials. In addition, considering the results of our contamination assessment (in Section 3.3), the degree of contamination of the surrounding mare by material from MH is consistent with transport by impacts, although emplacement due to explosive eruption cannot be ruled out. Hence, we suggest that the mantle was likely produced by the volcanism associated with development of the MH before emplacement of the surrounding mare. 5. Geologic history of the Mons Hansteen After the formation of Hansteen and Billy craters at around 3.9 Ga, volcanism at MH began at ∼3.74 Ga (error of + 75 million years, - 150 million years) with eruption of relatively high-silica, low-iron (mean of ∼8.5% wt. FeO) materials. The materials erupted at this time may have been vented from elongate, cleft-shaped pits as well as nearly circular pits. This phase of volcanism produced the Hilly–Dissected Unit and although we can find no evidence of lava flow lobes associated with formation of this unit it is possible that this early volcanism was effusive in style comparable to the viscous, high-silica lavas proposed for the Gruithuisen domes, and Lassell Massif (Chevrel et al., 1999; Wilson and Head, 2003; Ivanov and Head, 2015; Ashley et al., 2016). These eruptions appear to have occurred along intersecting sets of northwest-southeast and northeast-southwest trending faults and grabens that acted as conduits for the magma to reach the surface. These structures were probably formed as a result of doming caused by intrusion of magma beneath this center. None of these structures can be traced into the surrounding mare suggesting that activity along them ceased before the mare was emplaced. In addition, extension along some of these structures produced relatively wide (a few hundred meter to a few kilometers) fault-controlled valley and troughs. These terminate against the younger Pitted unit suggesting that they formed during this first episode of MH formation. The emplacement of the Hilly-Dissected unit was followed by eruption of relatively high-silica and an even lower iron (mean of ∼6.7% wt. FeO) material to form the Pitted unit. These materials cover the Hilly-Dissected unit just northeast of the center of the present MH forming the top mesa and peak of the edifice. Emplacement of the Pitted unit occurred at ∼ 3.50 Ga (error of + 150 million years, − 1.0 billion years), but considering error Mons:

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Fig. 18. Outlined in white is Mons Hansteen with an LROC WAC Quickmap mosaic image on the left and a Diviner surface rock abundance map on the right. The high concentration of ≥ 1 m blocks occur on the slopes and fresh craters (arrows). North is at the top of both images.

Fig. 19. Top: Oblique view looking west across the Pitted unit (North is on the right and area is ∼ 3 km across). Arrows indicate areas where rocks are prominent on steep slopes. Bottom: LROC NAC image showing blocks on the slopes of a ridge in the Hilly-Dissected unit. Blocks, dominantly in the size range of ∼8 m to ∼2 m, appear to be bleeding out onto the surface. We suggest that the top of the ridge is surfaced by a nearly 8 m thick layer of smooth material that is likely to be ash. Top image is a portion of a LROC image M1154506530LR. The bottom image is a portion of LROC M166182355LC of MH (north is at the top of this image).

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in the model age estimates it could have been emplaced nearly continuously with the Hill-Dissected unit or over a billion years later. Eruption of these materials appears to have been through numerous closely-spaced, overlapping, nearly circular pits, mainly along closely-spaced, northwest-southeast trending fractures. Although the period of time over which the individual pits formed cannot be resolved, it is likely that they were active at different times suggesting a magma source that migrated from one locality to another, not unlike the mode of formation for a field of cinder cones on Earth (e.g., the San Francisco field; Settle, 1979). This also raises the question about the total volume of pyroclastics that comprise the Pitted unit. There is ∼150 m to 200 m of relief between this unit and the Hilly-Dissected Unit, so at one extreme all of this elevation might be associated with late-stage pyroclastic eruptions, while at the other extreme, pyroclastic volcanism may have only produced the 9–11 m mantle that presently covers both units. Soon after the emplacement of the Pitted units, relatively lowsilica and high-iron (mean of ∼10.4 wt.% FeO) material produced a small cone shaped edifice on the north east flank of MH. This produced the North Massif unit. The material from these eruptions also thinly blankets the area around North Massif including a small area of the northernmost side of the Hilly–Dissected and Pitted units. This suggests that the North Massif unit is younger than the Hill–Dissected unit and the Pitted unit, but may approximately be the same age the Pitted unit, or a model crater age of ∼ 3.5 Ga (but this age is only loosely constrained). The North Massif unit sits at the intersection of major faults that cut MH. These faults do not extend into the mare suggesting that if there was movement along them associated with volcanism at North Massif, which is likely, then these structures and the North Massif unit are older than the mare. At about the same time as the eruption of North Massif materials and the Pitted unit, mare basalts flooded the vicinity around MH. The model age for this emplacement is ∼ 3.5 Ga and may suggest a genetic relationship between these units. Following mare emplacement, northeast and northwest sets of wrinkle ridges developed. The strikes of these structures are also different than those of the sets of structures produced by extension that cut MH. These two types of structure are likely not related if wrinkle ridges are, indeed, compressional structures produced by regional

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stresses much later than the structures of MH, which appear to be extensional features produced by stress associated with volcanism. Each geologic unit of MH is mantled, probably by volcanic ash. This mantling likely occurred soon after their emplacement because each unit remains geochemically distinct based on remote sensing measurements. In addition, if the mantling material was distributed widely, then cross-contamination would be greater than we have observed on the units of MH as well as on the surrounding mare. This suggests that the mantle on each unit is composed of materials distributed only short distances from their source vents. 6. Summary and conclusions The volcanism at MH produced three major geologic units during three episodes of volcanism. This volcanism began at ∼ 3.74 Ga, relatively soon after the formation of Hansteen and Billy impact craters, and produced the Hilly–Dissected unit. This unit makes up the low, steep-sided lower mesa of the edifice. It is composed of high-silica, low-FeO content materials that are mainly from vents along northeast, and intersecting northwest trending sets of fractures. This fracture system was likely produced by doming caused by the volcanic activity under MH. None of these fractures (or later ones) extend into the surrounding mare. The emplacement of the Hilly-Dissected unit was followed at ∼ 3.5 Ga by volcanism in the north central part of the mesa. These eruptions produced a smaller mesa mapped as the Pitted unit on top the older, larger, lower mesa. The magma that formed the Pitted unit was high in silica and even lower in FeO than the Hilly–Dissected unit. This material was mainly erupted from numerous vents (i.e., pits) along closely spaced northeast-southwest trending sets of fractures. The close spacing of the vents in the Pitted unit may represent a new style of low-volume eruptions. Shortly afterward, lower silica and higher FeO materials were erupted on the north flank of MH at the intersection of two major fractures, to produce the North Massif unit. These eruptions produced a small cone, and thinly mantled the north flank of Hilly–Dissected and Pitted units. At about the same time, ∼ 3.5 Ga, the surrounding mare was emplaced, flooding the base of MH. Wrinkle ridges were subsequently formed in the mare, but their strikes are different than the structures in MH. This suggests that the structures of MH and the wrinkle ridges were produced by different stress fields and at different times, hence likely they had different origins. In addition, each unit of MH appears to be mantled by volcanic ash. These ash deposits also appear to have only modestly contaminated the other units suggesting that although explosive volcanism occurred at MH it was likely not particularly violent. Acknowledgements We dedicate this manuscript to our late friend and colleague Dr. B. Ray Hawke, who had a love for all things lunar for his entire career, and a specific long-term interest in Hansteen Alpha dating back many decades. B. Ray was instrumental in the targeting of many of the LROC data sets used here and we will sorely miss his encyclopedic knowledge of lunar geology and his willingness to share it with others. We would like to thank James Ashley, and an anonymous reviewer for their thoughtful comments and help to make this a much better contribution. We would also like to acknowledge NASA’s support for coauthors MSR and SJL of ASU through a LRO/LROC contract. References Ashley, J.W., Robinson, M., Stopar, J., Glotch, T., Hawke, B., Lawrence, S., Greenhagen, B., Paige, D., 2013. The lassell massif—evidence for complex volcanism on the Moon. Lunar Planet. Sci. XXXXIV Abstract 2504.

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