Icarus 283 (2017) 254–267

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Icarus

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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 b 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

a r t i c l e i n f o a b s t r a c t

Article history: Mons Hansteen is a relatively high-albedo, well-known lunar “red spot” located on the southern mar- Received 1 July 2015 gin of Oceanus Procellarum (2.3 °S, 50.2 °W). It is an arrowhead-shaped ( ∼ 25 km on a side), two-layer

Revised 17 February 2016 mesa with a small cone-shaped massif on its north edge formed by three morphologically and com- Accepted 11 August 2016 positionally distinct geologic units. These units were emplaced in three phases over nearly 200 million Available online 18 August 2016 years. The oldest ( ∼3.74 Ga), Hilly–Dissected unit, composed of high-silica, and low-FeO content materials Keywords: formed a low, steep sided mesa. The materials of this unit erupted mainly from vents along northeast- Moon, surface and northwest-trending sets of fractures. The Pitted unit, which comprises the upper-tier mesa, is com- Silicic posed of high-silica and even lower-FeO content materials. This material was erupted at ∼ 3.5 Ga from Geological processes numerous closely spaced vents (i.e., pits) formed along closely spaced northeast-southwest-trending sets

Geologic mapping 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 ar- eas 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 more evolved highlands compositions such as dacite or rhyolite ( Malin, 1974; Wood and Head, 1975; Head and McCord, 1978 ). Unraveling the nature of lunar “red spots”, such as Mon One of these red spots, Mons Hansteen (IAU, 1976, see website Hansteen (MH), has major implications for lunar thermal history at planetarynames.wr.usgs.gov) also known as Hansteen α (e.g., and crustal evolution ( Hagerty, 2006; Jolliff et al., 2011 ), thus see Wagner et al., 2010 ) or Hansteen Alpha (e.g., see Hawke et al., providing crucial information for understanding the early Moon. 2003 ) are hereafter referred to as Mons Hansteen is a relatively Lunar red spots are characterized by a relatively high albedo and high-albedo, polygonal, arrowhead-shaped, mesa that measures a strong absorption in the UV ( Wood and Head, 1975; Head and ∼25 km on a side. It is located on the southern margin of Oceanus McCord, 1978 ). Some early workers presented evidence that at Procellarum adjacent to the craters Billy and Hansteen at 12.3 °S, least some red spots were produced by non-mare or highlands 50.2 °W ( Fig. 1 ). volcanism and suggested a connection with KREEP basalts or even New high-resolution observations from the Lunar Reconnais- sance Orbiter (LRO) mission Lunar Reconnaissance Orbiter Camera (LROC), Wide Angle Camera (WAC) and Narrow Angle Camera ∗ Corresponding author. (NAC) images and image mosaics provide the means to signifi- E-mail addresses: [email protected] (J.M. Boyce), thomas.giguere@ cantly advance our understanding of the geology and morphology intergraph.com (T.A. Giguere), [email protected] (P.J. Mouginis-Mark), of this volcanic center. The objective of this study is to characterize [email protected] (M.S. Robinson), [email protected] (S.J. Lawrence), [email protected] (D. Trang), [email protected] (R.N. Clegg-Watkins). geologic units of MH, and to determine their morphology, extent,

1 Deceased. distribution, age, composition, and geologic history. Recently http://dx.doi.org/10.1016/j.icarus.2016.08.013 0019-1035/© 2016 Elsevier Inc. All rights reserved. J.M. Boyce et al. / Icarus 283 (2017) 254–267 255

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 has a distinctive surface texture, color, and albedo compared to et al., 2010 ) ( Fig. 2 ) combined with datasets from previous mis- the nearby highlands and adjacent mare units. sions (e.g., Clementine, Kaguya) enable new detailed mapping and Wagner et al., (2010) mapped MH from Lunar Orbiter IV images discovery of three geologic units in MH, revealing its history and and described a flat summit region reminiscent of a mesa. They how it compares with other lunar red spot volcanic centers. noted that the summit area, as well as the flanks, appears much more rugged than the Gruithuisen domes ( Head and McCord, 1978; 2. Background Chevrel et al., 1999 ) which are characteristic of level summits. Further, Wagner et al., (2010) identified two small, distinct areas McCauley (1973) described MH as being a steep-sided, bulbous, on the summit region of MH, and measured superimposed crater very bright dome of material exhibiting a hackly surface. He also frequency, but the low-resolution images they used prevented identified several small, linear, smooth-walled depressions at the them from detailed geologic mapping of this feature. From two crests of gentle individual highs and interpreted these depressions distinct areas of the summit they found two statistically significant as probable volcanic vents. Wood and Head (1975) noted that MH crater distributions, with cratering model ages of 3.74 and 3.55 Ga 256 J.M. Boyce et al. / Icarus 283 (2017) 254–267

(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 pre- dating 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 anal- ogous 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 for- mation of Billy and Hansteen craters, it should have been covered with FeO- and TiO2 -rich ejecta since it is within one crater diam- eter 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 evi- dence that some red spots, including the MH, are dominated by Th Fig. 3. LROC WAC images mosaic with the LRO Diviner Standard Christiansen Fea- and silica-rich, highly evolved highlands lithologies ( Hawke et al., ture Value (silica) map superposed. The white indicates areas of relatively high- silica content. (Image from LROC Quickmap mosaic). 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 Gruithuisen domes. They note that to be consistent with their images were used to produce FeO, TiO2 , and optical maturity maps density observations silica-rich magmas required, can be produced of the MH region utilizing the algorithms of Lucey et al. (2000a; b). either by 1) silicate liquid immiscibility ( Hagerty et al., 2006 ), or > Mare units in this region exhibit FeO abundances 16 wt%, and 2) crustal melting induced by basaltic underplating. They favored

TiO2 values range between 4 wt% and 8 wt%. In sharp contrast, the basaltic underplating mechanism because it can produce a much lower FeO and TiO2 values are exhibited by Mons Hansteen crustal rhyolitic composition magma that is consistent with the < where FeO values range from 5 wt% to 9 wt% and TiO2 of 1 wt%. FeO content of MH materials as well as their inferred bulk density

In the central portion of MH, the surface materials have an av- of MH. Kiefer et al., (2016) also suggest that basaltic underplating erage FeO value of 6.9 wt% and an average TiO2 value of 0.5 wt% is supported by model melting calculations that indicate that

(Hawke et al., 2003). Hawke et al. (2003) suggested that since partial melting of KREEP basalt (driven by the heat from the this central region would be less contaminated by debris from the intrinsic radioactivity of KREEP and from mare basaltic intrusions) surrounding mare units thrown there by impacts, its composition should produce significant volumes of rhyolitic magma with the may most closely approximate that of the underlying lithology; right range in FeO, as well as a high thorium abundance like that i.e., ejecta from the Imbrian-aged craters Billy and Hansteen. observed at MH and other felsic domes ( Hagerty et al. 2006 ).

However, this explanation is unlikely because their map does not Each red spot volcanic complex appears to have its own unique show evidence of contamination from MH on the surrounding shape. For example, Mons Hansteen is a two-layer mesa with mul- mare surface. Hence, the contamination would have to be unidi- tiple vents and one satellite cinder cone. The Compton–Belkovich rectional (from mare to MH and to higher elevations), and would Volcanic Complex ( Jolliff et al., 2011; Chauhan et al., 2015 ), which be substantially greater than noted as occurring elsewhere along is approximately the same size as MH, is a broad area of elevated the maria/highlands boundary (Logan et al., 1972). topography with a range of volcanic features (e.g., irregular col-

Lawrence et al., (2005) and Hagerty et al., (2006) used forward lapse depressions, and a variety of size domes). The Lassell Massif modeling of LP Gamma Ray Spectrometer data to show that the complex may also be a layered volcanic complex of about the

Th abundance at MH is not 6 ppm, but could possibly range same size ( Ashley et al., 2013 ; 2016 ). In contrast, the Gruithuisen ∼ ∼ from 20 to 25 ppm. This is consistent with Th abundances domes include two relatively large elongate domes and a small measured in evolved lunar lithologies such as granites and felsites. dome ( Chavrel et al., 1999 ). The detailed geologic history like that

Subsequently, Glotch et al., (2010) based on thermal emission presented here for MH has yet to be completed for each of these signatures measured by the LRO Diviner Lunar Radiometer Exper- red spot volcanic centers, but would help us to understand why iment (Diviner) found that Si-rich materials are more abundant these volcanic centers are so different. 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 3. Regional context mare debris transported to the slopes of the dome by impacts in the surrounding mare. The Mons Hansteen is located on the southern margin of Recently, Kiefer et al., (2016) used gravity modeling based on Oceanus Procellarum centered at ∼ 12.5 °S and 50W ( Fig. 1 ). high-resolution Gravity Recovery and Interior Laboratory (GRAIL) This complex formed in highlands materials and is Imbrian age mission observations to suggest that MH is composed of relatively ( McCauley, 1973; Wagner et al., 2010 ). It began to form after the low-density, felsic materials (bulk density of the crust beneath nearby impact craters Hansteen and Billy ( ∼ 3.9 Ga), but before − MH of 150 0–20 0 0 kg 3). This is similar to their findings for the the mare flooded its flanks at ∼ 3.5 Ga. J.M. Boyce et al. / Icarus 283 (2017) 254–267 257

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.

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 Fig. 4. Cumulative size-frequency distribution (CSFD) curves of impact craters, and likely were created by tectonic stresses following the formation of subdued circular and quasi-circular pits on the geologic units of the MH as well as the Procellarum or Imbrium basin ( Wilhelms, 1987; Solomon and the adjacent mare to the east. The Hilly-Dissected unit crater counts are in closed Head, 1979; 1980), while the northeast-southwest trending struc- squares (390 crater between 100 m and 1.48 km dia., in 153 km 2 area); the Pitted tures of MH are probably related to the formation of Humorum unit are in closed circles (186 craters between 100 m and 900 m dia., in 126 km 2 area); the North Massif unit craters are in inverted closed triangle (40 craters be- basin, Billy crater, and Hansteen crater. tween 100 m and 500 m dia., in 24.6 km 2 area); the mare east of MH are in open circles (97 craters between 143 m and 510 m dia., in 134 km 2 area), subdued pits on 3.2. Maria the Pitted units are crosses (48 pits between 267 m and 1.3 km, in 126 km 2 area); and subdued pits on the Hilly-Dissected unit are x s (24 pits between 240 m and

750 m dia., in 153 km 2 area). The best fit model age production functions are plot- Hiesinger et al., (2003) identified several distinct basalt types ted for 3.5, 3.74 and 4.0 Ga and the theoretical crater equilibrium curve (see from in the Hansteen region based on multispectral analyses of mare Michael and Nuekum, 2010 ). Note the divergence of the CSFD curves on all the using color data of the Clementine UVVIS-camera. The basalts units of MH from lunar impact crater production functions < ∼0.5 km diameter. surrounding MH have FeO values in the 14 to 18 wt.% range, and The dashed line is the average for all these units. The linear nature of these curves TiO values fall in the 2 to 6 wt.% range. These mare materials below ∼0.5 km crater diameter suggest a process that degrades and erases smaller 2 crater faster than larger ones such as would occur in areas mantled by particulate are the youngest volcanic deposits in the area evidenced by their material. Also note that the CSFD of the subdued pits on both units also do not embayment of all other materials, i.e., all highlands units, older follow the lunar impact crater production function. 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 3.1. Highlands et al., (2010) found that mare materials in the Hansteen region could be classified into three time-stratigraphic units based on Highlands in the MH region are composed of high albedo crater counts, with most being late Imbrian age. They found that material characterized by rugged, hilly or hummocky, furrowed individual areas in Oceanus Procellarum near MH range in model morphologies ( McCauley, 1973; Wagner et al., 2010 ). The highland ages from 3.67 Ga to 3.35 Ga. We also counted impact craters on units in the MH region were mapped as undivided terra material the mare east of MH using LROC NAC images as a base and found by Wilhelms and McCauley (1971) . Later McCauley (1973) mapped an impact crater model age of ∼ 3.5 Ga ( Fig. 4 ), consistent with the highlands south of Hansteen crater near MH as principally the ages found by Wagner et al., (2010) and that found by Boyce radial outer rim material of crater Hansteen. McCauley (1973) and (1976) based on crater degradation. Wilshire (1973) assigned the highlands material in the Hansteen Northeast and northwest trending wrinkle ridges have devel- area as belonging to the Imbrian system, and Wagner et al., oped in the mare around MH. LROC images indicate that two (2010) used crater counts to derive a model age for it of ࣡ 3.91 Ga. of these structures extend northwest across the mare south of The closest large, old craters to MH, Billy and Hansteen (both MH discernable on the flank of MH for a short distance ( Fig. 5 ), ∼ 45 km diameter), were assigned by McCauley (1973) an Imbrian indicating that the stress field that formed these structures ex- age. This is consistent with the crater model ages of 3.88 Ga, for isted after the formation of the mare and MH. Another set of 258 J.M. Boyce et al. / Icarus 283 (2017) 254–267

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 su- perposed on a LROC WAC image mosaic (from LROC Quickmap). The contour inter- val 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, 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). ∼ and has an average height of 650 m (i.e., – 1250) above the This mosaic (north at the top) and topographic profiles shows that MH is composed surface of the surrounding mare. It is capped by a small hill whose of two progressively smaller, mesa-like layers stacked one on the other. The bottom summit is at –924 m. Obvious impact craters are found on these mesa is a rugged layer that includes small hills, valleys, and circular to elongate pits ∼ − mesas. But in addition to these impact craters, both mesas contain 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. subdued circular to quasi-circular pits (Fig. 2) whose cumulative Topographic data are derived from a combination of LROC high-resolution digital size frequency distributions differ (i.e ., higher negative slopes) terrain model (DTM) at 100 m/pixel (LROC WAC Global Lunar DTM 100 m) with a substantially from that of the crater production function of lunar vertical accuracy of 10 m, 2.0 m post spacing and an RMS error relative to two LOLA impact craters. The population density of these subdued pits is tracks of 4.73 m (data are available at the LROC website at lroc.sese.asu.edu) and greatest on the top mesa ( Fig. 4 ). The subdued pits are typically from topographic derived from Kaguya images. Vertical exaggeration 7x. J.M. Boyce et al. / Icarus 283 (2017) 254–267 259

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) sug-

gested 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 spec- ulated 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 ma- terials 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

Fig. 9. High-resolution NAC image of a small area in the Pitted unit showing that values on MH. We suggest that the effects of such contamination the ridges between the pits are relatively smooth and rounded, and the floors are are not severe enough to prevent compositional mapping and typically broad, gently rounded to nearly flat. Topographic profiles (A –A’, and B identification of units of different composition on MH. The effects –B’) have also been constructed using NAC DEM data across a portion of the Pit- of contamination from impact transport on remote sensing mea- ted Unit (see insert for location). These profiles show that most pits are shallow surements was evaluated by Logan et al., (1972) at the Apollo 14 saucer shaped (i.e., relatively steep slopes on the sides and gently downward curv- ing, bowl-shaped floors). The image is a portion of the LROC NAC M1127462982LC. and 15 landing sites and found to be minimal at distances of a North is at the top of the image. 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., a few meters to tens of meters in depth and range from a few (2003) based on Clementine data as a base for this evaluation. This hundred meters to over a kilometer diameter ( Fig. 9 ). was approached by mapping the change in FeO wt.% of the mare In addition to the two mesa structure of MH, a small, rounded, from the edges of MH progressively outward to where FeO content relatively low-albedo massif ∼ 150 m high and ∼ 5 km across is becomes relatively uniform, and unchanging. We assume that the located on the north edge of MH ( Fig. 10 ). A shallow northeast effects of contamination from low-FeO materials blasted off MH trending trough ∼ 1 km wide separates this massif from the rest by small impact craters are minimal from this point outward. of MH to the south. To the north, this massif is superposed on Likewise, the effects of contamination from small impacts on the highlands materials. Two elongate pits sit atop this massif. mare onto MH from the mare/MH border inward should be similar Mons Hansteen is cut by crossing sets of northeast-southwest with distance to contamination in the opposite direction or even trending and southeast-northeast trending structures ( Fig. 7 ). possibly less because MH is at higher elevation than the mare. We Particularly on the lower mesa, these structures commonly form constructed profiles of FeO wt.% ( Fig. 12 ) with distance along the shallow, graben-like troughs, none of which show evidence of lines (lines 1 –2 and 3 –4) shown in Fig. 11 . The locations of lateral displacement along their lengths ( Figs. 2 and 10 ). Where these profiles were chosen because the geology is simplest (e.g., the northeast-southwest trending structures cross the upper mesa, no nearby highlands), the relief between MH and the mare is at they commonly show coincidental alignment with chains of the a minimum, and relatively large craters are far enough away to largest of the subdued pits. The cumulative size frequency distri- contribute little materials (i.e., thickness of ejecta declines roughly bution (CSFD) ( Fig. 4 ) and their alignment along structures suggest as a power of –3 relative to crater radii, Melosh, 1989 ). These that the subdued pits are most likely to be volcanic vents instead profiles ( Fig. 12 ) show a zone of continuously increasing value of of impact craters. Two of the major northeast-southwest trending FeO wt.% from about 1 km in the interior of MH outward onto structures intersect with the large elongate, cleft-like pits found the mare where at a distance of ∼ 1 to 2 km from the mare/MH on the southwest flank of MH suggesting they also may be vents. boundary become nearly constant. We suggest that this indicates The most northern of these northeast-southwest structures also that significant contamination extends away from the mare/MH intersect with a major northeast-southwest trending structure at boundary in both directions for approximately 1 –2 km. the pits on the summit of the massif located on the north flank In order to add more detail and confidence to this assessment, of MH. In addition, none of the structures on MH can be traced we have also produced histograms of FeO wt.% values in sample onto the surrounding mare or highlands nor does any of their areas (each with ∼ 13 km 2 area) on the mare (D though G) along directions match any trends of the wrinkle ridges developed in the those profiles, as well as for sample areas containing the Pitted mare around MH. This suggests that the stress field that produced unit, Hilly and Dissected unit, and North Massif (sample areas the structures found on MH were produced by a different stress A, B and C respectively). These are plotted in Fig. 13 , and show field than the stress field that produced the wrinkle ridges, and (1) an increase in FeO wt.% of the mare surface along profile was likely a result of deformation associated with MH volcanism. 1 –2 eastward from the mare/MH boundary from area E to area Hawke et al., (2003) provided an overview of remote-sensing F, with only a small increase in FeO from F to G (outward of G and morphology derived geochemistry of MH to suggest that it is a all mare has a FeO content of > ∼ 17.7 wt.%, similar to F and G), relatively high-silica, low-FeO and low-TiO2 mountain interpreted and (2) a similar trend along profile 3 –4where the FeO content as volcanic. They used the steep flank morphology of MH to infer from sample areas B to D significantly increases from ∼ 8.5 wt.% 260 J.M. Boyce et al. / Icarus 283 (2017) 254–267

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 of 8.5 wt.%. The North Massif unit (sample area C) is also evaluated and the distribution of FeO values on the FeO map of Hawke here, and has the FeO content of ∼ 10.4 wt.% FeO). The surface et al., (2003) we suggest that a zone of significant contamination materials of North Massif unit are distinctly different from those extends outward in both directions approximately 1 –2 km from in the other two MH units (i.e., A and B) and from materials of the mare/MH boundary. It should also be noted that if pyroclastic surrounding mare of ∼ 17.7 wt.% FeO. The North Massif unit was material was erupted from MH these results suggest that it would not recognized by Hawke et al., (2003) to be part of MH, likely have no more of a contamination effect than did impact transport. because of its lower albedo and higher FeO contents as well as its In an effort to identify and define major composition units on location on the edge of the main edifice of MH. MH, we have used this assessment of the extent of contamination The histograms of the individual sample areas in MH ( Fig. 13 ) as a guide to analyze the compositional data produced by Hawke are leptokurtic and show little to no overlap suggesting that et al., (2003) . As a starting point, we looked at FeO wt.% values in each contains materials of different and distinct compositions. In a sample area within each of the two compositional units on the addition, the histograms of FeO content in the sample areas in main massif of MH (i.e., sample areas A, and B in Fig. 11 ) shown the eastern mare (i.e., areas D, E, F, and G in Fig. 11 ) are clearly in the maps of Hawke et al., (2003) . The locations of these sample different from the sample areas on MH. Consequently, we suggest areas were chosen far enough away from the mare/MH boundary there are at least three distinct compositional units on MH, and to reduce, as much as possible, contamination from the mare that the map distribution of these units shows a reasonable corre- and/or the other MH units. The histograms of FeO wt.% values lation with the three units defined on the basis of geomorphology. within these two sample areas ( Fig. 13 ) show that sample area A This correlation suggests the Pitted unit is composed of low-FeO has FeO content of ∼6.7 wt.% and sample area B has FeO content material, the Hilly-Dissected unit is composed of comparatively J.M. Boyce et al. / Icarus 283 (2017) 254–267 261

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 Hilly- Dissected 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 Fig. 11. A map of the FeO distribution for MH and the surrounding area. The black southeastern part of MH that only includes the Hilly-Dissected unit and extends for line on the map is the approximate boundary of MH with the other units in the 6 km onto the mare. This profile shows a transition zone between the two units area. The white boxes are locations of sample areas A, B, C, D, E, F, and G; which that suggests that contamination is important only ∼ 1 km on either side of the are equal in area (13 km sq.). Sample areas A, B, and C are all on MH. Sample area boundary. 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. . 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 mis- sions 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., Fig. 13. Histograms of FeO values (binned in 0.25 wt.% increments) in areas sam- 2003; Hawke et al., 2011 ). It comprises most of the lower mesa of pled in Fig. 8 and the entire surface of MH. Note that the units on Mons Hansteen the main edifice and surrounds the Pitted unit. This unit butts up (A, B, and C) are distinct from one another and from those on the mare (D, E, F, against highlands materials on the southwestern and northeastern 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 sides of Mons Hansteen, while on its southern, northwestern, and contamination of material ejected by small impact craters from MH. eastern sides it is embayed by mare materials. The Hilly-Dissected 262 J.M. Boyce et al. / Icarus 283 (2017) 254–267

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 Fig. 15. The interior slopes of most pits (“P”), especially those in the Pitted unit, distributions. For example, subdued circular to quasi-circular pits change abruptly at the pit floors, and then form a gently curving bowl shape. LROC image number M1127462982LC. North is at the top. (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 that intersects the two pits on the North Massif Unit. These two associated lava flows (these pits will be discussed in great detail lineaments also appear to be members of a set of closely-spaced below). As well as the subdued pits, the Hilly-Dissected unit ( ∼ 1–2 km apart) parallel structures that trend ∼ N 340 ° W, S 160 ° contains several steep-sided ( ∼ 30 ° based on LROC NAC digital E) that cross MH. Consequently, there are structures that appear terrain model [DTM]) cleft-shaped, elongate depressions ( Fig. 2 ). to cross the Hilly-Dissected unit and extend across the Pitted These elongate depressions are unique to this unit, and unlike unit, suggesting that these structures were active even after the secondary impact crater chains, they are each one long continuous Hilly-Dissected unit formed ( Fig. 7 ). pit, instead of a series of connected pits. Their shape and occur- In addition, a line of low ridges extends northwestward across rence in a volcanic complex make the probable volcanic vents. the southeastern part of the Hilly–Dissected unit, and terminate at Though lava flows are not observed emanating from any of them the southern edge of the Pitted unit ( Fig. 2 ). The most northerly of the surface in the immediate vicinity of the two on the southwest these ridge segments cuts across the northeastern major trending flank of MH ( Fig. 6 ) is smoother than elsewhere on MH and may valley on the southern edge of the Pitted unit, but terminates at be blanketed by ash. The largest of these steep-sided depressions that unit suggesting that the ridge developed after the valley, but is located on the southwestern edge of MH. Its long axis is in a most likely before the Pitted unit. line with a lineament that cuts across the northwest flank of MH and intersects with the two elongate subdued pits on top of the 4.2. Pitted unit North Massif ( Fig. 7 ). Impact craters were also identified based on their morphology The Pitted Unit is a high-albedo, low-iron ( ∼ 6.7 wt.%, A in (see next section) and counted to derive a model crater age for the Fig. 11 ), relatively high-silica area located in the north central Hill-Dissected unit ( Fig. 4 ). The subdued pits were excluded from portion of MH. This unit contains an area of ∼110 km 2 located these counts. These counts suggest an impact crater model age of mainly above an elevation of ∼ 500 m (relative to datum). It ∼ 3.74 Ga (error of + 75 million years, - 150 million years) for this is characterized by closely-spaced, commonly overlapping and unit, consistent with the model age determined by Wagner et al., nested, subdued circular to irregular-shaped depressions ( Fig. 14 ) (2010) using Lunar Orbiter IV images. In addition, the age reported similar to those found in the Hilly-Dissected unit. Similar to the here is based on the population of craters > ∼ 0.5 km diameter Hilly-Dissected unit, none of the pits in the Pitted unit exhibit where the CSFD of these craters follows the lunar impact crater ejecta deposits, nor have associated lava flows. The average diam- production function. However, the CSFD of craters below that size eter of the pits is ∼ 0.6 km, but they range from ∼0.14 km to > ∼ forms a curve with a substantially different slope (lower negative) 1.4 km in diameter. Fig. 9 shows topographic profiles plotted on a suggesting that these craters may have been affected by a process high-resolution NAC image of a small area in the Pitted unit. The that degrades small craters at a greater rate than larger ones. More ridges between the pits are relatively smooth and rounded, and the will be discussed about the possible cause of this below. floors are typically broad, gently rounded to nearly flat ( Fig. 15 ). Relatively broad valleys and ridges, presumably controlled However, the interior slopes of the pits can be relatively steep by faulting, cut the Hilly-Dissected Unit with the broadest ones (20 ° to 25 ° based on the NAC DTM). These characteristics suggest commonly terminating at the boundary with the Pitted unit that the unit may be blanketed by particulate material, such as a (see Figs. 2 and 7 ). This suggests that these features predate the volcanic ash, that drapes over ridges and pools in low places. Pitted unit. These are most common along the southern part of McCauley (1973) also suggested that these pits are of volcanic MH in this unit with most striking in a northwest-southeast, origin because some of them have elongate or irregular shapes. or northeast-southwest direction. However, there are lineaments This interpretation is consistent with their lack of ejecta deposits, that strike northwestward along these large broad valleys on the irregular outlines, subdued topography, and their association southeast side of the Hilly–Dissected unit and continue on a line with the small domical mounds on this unit reported by Hawke with ridges that cross the Pitted unit ( Fig. 7 ) and are on a line et al., (2014) . Mantling of the pits by fine material (discussed in J.M. Boyce et al. / Icarus 283 (2017) 254–267 263 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. Fig. 16. Elongate possible volcanic vents (arrows) at the summit of the North Mas-

A pair of shallow, elongate pits, also likely to be volcanic vents, sif. Pitted unit is toward the bottom of the image (portion of LROC NAC image has formed atop this massif ( Fig. 16 ). They have formed at the M1127462982LC). The area at lower left is thinly mantled by relatively low-albedo intersection of two major lineaments on the MH. The long axis of materials likely from North Massif (see high sun-angle image in Fig. 10 ). 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 be similar in age to those units. Considering that material from units and the faults in the largest valley in the southern part of North Massif unit appears to be thinly blanketing a small area of the Hilly-Dissected unit. Hilly–Dissected and Pitted units on the northeast flank of MH, it

The massif is surfaced by lower-albedo, relatively higher-iron can be inferred that North Massif unit is most probably slightly ∼ ∼ ∼ younger than the Pitted unit but still 3.5 Ga. ( 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., 4.4. Pyroclastic mantle 2010 ). The low-albedo material from this unit blankets the surface of the Hilly–Dissected and Pitted units in the near vicinity of the There is evidence that the surface of the MH is mantled by a massif ( Figs. 7 and 10 ). This material is likely to be the result of layer of particulate material that may have been produced by py- ash erupted from the pits on the North Massif and suggests that roclastic volcanism. This is suggested by the subdued topography the North Massif unit is younger than the other two units. of MH, the nature of the CSFD of its superposed impact craters The model crater age of North Massif unit is difficult to con- as well as their detailed morphology, weathering of blocks from strain because its surface area is too small to contain enough beneath a layer of smooth material at the tops of some slopes, reasonable size craters (i.e., > ∼ 0.5 km diameter required to obtain and the nature of reflected light from MH. a crater production function) for an accurate model age estimate The morphology of the small relatively fresh impact craters on using crater counts. However, we have counted impact craters MH provides evidence that MH is covered by a layer of particulate on this unit to assess shape of the CSFD and what it may reveal material similar to regolith. These craters on MH commonly exhibit about processes that may have affected the crater population. The interior benches ( Fig. 17 ), as do those on the surrounding younger resultant CSFD is shown in Fig. 4 and is strikingly similar to those mare. Interior benches are rings of material on the interior slopes of the other MH units for crater diameters < ∼ 0.5 km. This sug- of small impact crater that Oberbeck and Quaide (1967) suggest gests that the craters on this unit are not a production function, are caused by the effects of different strength of materials in but are also likely being affecting by the same process that affects layered targets. The geometry of the benches in the craters on the small craters on other units. The density of the craters on the surface of MH is shown in Fig. 17 and suggests a low-strength North Massif, although they are not a production function they layer overlying a stronger substrate. Depth estimates based on are still similar to those on the other units at those sizes. We expected crater shapes suggests that the low-strength surface layer suggest that likely means that the North Massif unit is likely to that produced these benches is ∼ 9–11 m thick. This thickness is 264 J.M. Boyce et al. / Icarus 283 (2017) 254–267

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 ter- rain 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 Fig. 17. Two relatively fresh, small impact craters on Hilly-Dissected unit of Mons where blocks appear to be weathering from under a layer of

Hansteen that exhibit interior benches. Such benches are indications of a target that smooth material ( Fig. 19 ). Judging by the size of the largest blocks includes a surface layer of low-strength material ∼ 9–11 m thick. Image is portion of the LRO LROC NAC image M1127462982LC. 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. ∼ 3 times thicker than expected for the regolith produced by the Based on the observations discussed above, MH is most likely flux of impacts on the surface of MH alone ( Moore et al., 1980 ), mantled by at least 8–10 m of particulate materials. In addition, and 3 to 4 times that of the regolith on the mare just east of MH considering the results of our contamination assessment (in measured using the same technique. Section 3.3 ), the degree of contamination of the surrounding mare The shapes of impact crater size-frequency distributions (CSFD) by material from MH is consistent with transport by impacts, can provide information about surface processes. In the case of the although emplacement due to explosive eruption cannot be ruled geologic units of MH, their CSFD ( Fig. 4 ) show that craters with out. Hence, we suggest that the mantle was likely produced by diameters > ∼ 0.5 km follow the lunar crater impact production the volcanism associated with development of the MH before function, but at ∼ 0.5 km diameter (down to 0. 1 km) craters of emplacement of the surrounding mare. these units do not. Instead, these small craters decrease in abun- dance continuously and more rapidly with decreasing crater size 5. Geologic history of the Mons Hansteen relative to the production function. This particular shape of the small crater CSFD curve can be caused by either a constant, slow After the formation of Hansteen and Billy craters at around deposition of material in crater bottoms or a mantle of particulate 3.9 Ga, volcanism at MH began at ∼3.74 Ga (error of + 75 million materials, both of which will obliterate small craters rapidly and years, - 150 million years) with eruption of relatively high-silica, larger one slowly ( Hartmann, et al., 1981 , p. 1052). We suggest low-iron (mean of ∼8.5% wt. FeO) materials. The materials erupted that the latter is more likely on the moon. at this time may have been vented from elongate, cleft-shaped pits An additional argument for pyroclastics is the surface bright- as well as nearly circular pits. This phase of volcanism produced ness of MH. Along with the high reflectance observed in all the Hilly–Dissected Unit and although we can find no evidence of wavelengths, the MH region has been observed to have relatively lava flow lobes associated with formation of this unit it is possible high reflectance in the visible wavelengths ( Whitaker, 1972; Wood that this early volcanism was effusive in style comparable to the and Head, 1975; Hawke et al., 2003 ). Clegg et al., (2014 , 2015 ) viscous, high-silica lavas proposed for the Gruithuisen domes, and conducted a photometric analysis of several silicic regions of the Lassell Massif ( Chevrel et al., 1999; Wilson and Head, 2003; Ivanov moon and derived a single-scattering albedo ( w ) which is depen- and Head, 2015; Ashley et al., 2016 ). These eruptions appear to dent on grain size and composition, for each region, allowing for have occurred along intersecting sets of northwest-southeast and the direct comparison of each region corrected for the effects of northeast-southwest trending faults and grabens that acted as viewing geometry and phase angle. Although Compton-Belkovich conduits for the magma to reach the surface. These structures had the highest single scattering albedo values (0.59 + /–1.0 w ), were probably formed as a result of doming caused by intrusion of MH also exhibited very high values (0.47 + /–1.0 w ) compared to magma beneath this center. None of these structures can be traced other silicic areas and the Apollo landing sites. Apollo and Luna into the surrounding mare suggesting that activity along them soil compositions correlate with reflectance and w values such ceased before the mare was emplaced. In addition, extension along that more reflective soils (and therefore soils with a higher w ) some of these structures produced relatively wide (a few hundred have higher plagioclase contents and lower mafic mineral content meter to a few kilometers) fault-controlled valley and troughs. and Clegg et al., (2015) found that silicic regions plot along the These terminate against the younger Pitted unit suggesting that extrapolation of landing site data to low mafic contents. Elevated they formed during this first episode of MH formation. w values for MH indicate a lower mafic component and a higher The emplacement of the Hilly-Dissected unit was followed by plagioclase or silica-rich component on the surface, which is what eruption of relatively high-silica and an even lower iron (mean of would be expected for a silicic pyroclastic. ∼6.7% wt. FeO) material to form the Pitted unit. These materials The pits and the other topography at the meter to tens of cover the Hilly-Dissected unit just northeast of the center of meters scale on all geologic units of MH are subdued (low slopes the present MH forming the top mesa and peak of the edifice. and smooth, rounded topography), in contrast to the superposed Emplacement of the Pitted unit occurred at ∼ 3.50 Ga (error of impact craters that exhibit much crisper topography ( Fig. 15 ). This + 150 million years, − 1.0 billion years), but considering error J.M. Boyce et al. / Icarus 283 (2017) 254–267 265

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.

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. Al- though 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 low- silica 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 mate- rials and the Pitted unit, mare basalts flooded the vicinity around Fig. 19. Top: Oblique view looking west across the Pitted unit (North is on the right MH. The model age for this emplacement is ∼ 3.5 Ga and may ∼ and area is 3 km across). Arrows indicate areas where rocks are prominent on suggest a genetic relationship between these units. Following mare 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, emplacement, northeast and northwest sets of wrinkle ridges appear to be bleeding out onto the surface. We suggest that the top of the ridge is developed. The strikes of these structures are also different than surfaced by a nearly 8 m thick layer of smooth material that is likely to be ash. Top those of the sets of structures produced by extension that cut image is a portion of a LROC image M1154506530LR. The bottom image is a portion MH. These two types of structure are likely not related if wrinkle of LROC M166182355LC of MH (north is at the top of this image). ridges are, indeed, compressional structures produced by regional 266 J.M. Boyce et al. / Icarus 283 (2017) 254–267 stresses much later than the structures of MH, which appear to be Ashely, J.W. , Ashley, J.W. , Robinson, M.S. , Stopar, J.D. , Glotch, T.D. , Ray Hawke, B. , extensional features produced by stress associated with volcanism. Lawrence, S.J. , Jolliff, B.L. , Hiesinger, H. , van der Bogert, C.H. , Greenhagen, B.T. , Giguere, T.A. , Paige, D.A. , 2016. The lassell massif —a silicic lunar volcano. Icarus

Each geologic unit of MH is mantled, probably by volcanic 273, 248–261 . ash. This mantling likely occurred soon after their emplacement Boyce, J.M. , 1976. Ages of flow units in the lunar nearside maria based on lunar because each unit remains geochemically distinct based on remote orbiter IV photographs. Seventh Lunar Sci. Conf., Geochim. et Cosmochim. Acta

3 (Suppl. 7), 2717–2729. sensing measurements. In addition, if the mantling material was Chevrel, S.D. , Pinet, P.C. , Head, J.W.III , 1999. Gruithuisen domes region: a candidate distributed widely, then cross-contamination would be greater for an extended nonmare volcanism unit on the moon. J. Geophys. Res. 104, than we have observed on the units of MH as well as on the 16515–16529 . surrounding mare. This suggests that the mantle on each unit is Chauhan, M., Bhattacharya, S., Saran, S., Chauhan, P., Dagar, A., 2015. Compton– Belkovich volcanic complex (CBVC): an ash flow caldera on the moon. Icarus composed of materials distributed only short distances from their 253, 115–129 . source vents. Clegg, R.N. , Jolliff, B.L. , Boyd, A. , Hawke, B.R. , 2014. Compositional constraints on lunar silicic volcanic regions using LROC NAC photometry. Lunar Planet. Sci. XXXXV Abstract 1777 .

6. Summary and conclusions Clegg, R.N. , Jolliff, B.L. , Coman, E. , 2015. Analysis of compositional variations at non– mare volcanic regions using LROC NAC photometry and spectra of glassy and The volcanism at MH produced three major geologic units dur- silicic mineral mixtures. Lunar Planet. Sci. XXXXVI Abstract 1256 . Glotch, T. , Lucy, P. , Banfield, J. , Greenhagen, B. , Thomas, I. , Elphic, R. , Bowles, N. , ∼ ing three episodes of volcanism. This volcanism began at 3.74 Ga, Wyatt, M. , Allen, C. , Hanna, K. , Paige, D. , 2010. Identification of highly silicic relatively soon after the formation of Hansteen and Billy impact features on the Moon. Science 329, 1510–1513 . craters, and produced the Hilly–Dissected unit. This unit makes up Glotch, T., Hagerty, J., Lucey, P., Hawke, B., Giguere, T., Arnold, J., Williams, J., Jol- liff, B., Paige, P., 2011. The Mairan domes: silicic volcanic constructs on the the low, steep-sided lower mesa of the edifice. It is composed of Moon. Geophys. Res. Lett. 38, L21204. doi: 10.1029/2011GL04 954 8 . high-silica, low-FeO content materials that are mainly from vents Greenhagen, B.T. , et al. ,2010. Global silicate mineralogy of the moon from the di- along northeast, and intersecting northwest trending sets of frac- viner lunar radiometer. Science 329, 1507–1509 .

Hagerty, J., Lawrence, D., Hawke, B., Vaniman, D., Elphic, R., Feldman, W., 2006. Re- tures. This fracture system was likely produced by doming caused fined thorium abundances for lunar red spots: implications for evolved, non- by the volcanic activity under MH. None of these fractures (or mare volcanism on the moon. J. Geophys. Res. 111, E06002 . later ones) extend into the surrounding mare. The emplacement Hartmann, W. , Strom, R. , Weidenschilling, S. , Blasius, K. , Woronow, A. , Dence, M. , of the Hilly-Dissected unit was followed at ∼ 3.5 Ga by volcanism Grieve, R., Diaz, J., Chapman, C., Shoemaker, G., Jones, K., 1981. Chronol- ogy of planetary volcanism by comparative studies of planetary cratering. In: in the north central part of the mesa. These eruptions produced a McGetchin, T., Pepin, R., Phillips, R. (Eds.), Basaltic Volcanism on the Terrestrial smaller mesa mapped as the Pitted unit on top the older, larger, Planets. Pergamon Press, New York, p. 1286 . lower mesa. The magma that formed the Pitted unit was high in Hawke, B., Lawrence, D., Blewett, D., Lucey, P., Smith, G., Spudis, P., Taylor, G., 2003. Hansteen Alpha: a volcanic construct in the lunar highlands. J. Geophys. Res 108 silica and even lower in FeO than the Hilly–Dissected unit. This (E7), 5069. doi: 10.1029/20 02JE0 02013 . material was mainly erupted from numerous vents (i.e., pits) along Hawke, B. , Giguere, T. , Lawrence, T. , Glotch, D. , Greenhagen, B. , Hagerty, J. , Braden, S. , closely spaced northeast-southwest trending sets of fractures. The Gaddis, L. , Tran, T. , Jolliff, B. , Lucey, P. , Stopar, J. , Peterson, C. , Paige, D. , Robin- son, M. , 2011. Hansteen Alpha: a silica volcanic construct on the moon. Lunar close spacing of the vents in the Pitted unit may represent a new Planet. Sci. XXXXII Abstract 1652 . style of low-volume eruptions. Shortly afterward, lower silica and Hawke, B. , Giguere, T. , Lawrence, T. , Glotch, T. , Greenhagen, B. , Hagerty, J. , Braden, S. , higher FeO materials were erupted on the north flank of MH at the Gaddis, L. , Jolliff, B. , Lucey, P. , Stopar, J. , Peterson, C. , Paige, D. , Robinson, M. the LROC Science Team, 2012. The geology and composition of hansteen alpha. Lu- intersection of two major fractures, to produce the North Massif nar Planet. Sci. XXXXIII Abstract 1754 . unit. These eruptions produced a small cone, and thinly mantled Hawke, B. , Giguere, T. , Lawrence, S. , Glotch, T. , Greenhagen, T. , Jolliff, B. , Lucey, P. , the north flank of Hilly–Dissected and Pitted units. At about the Stopar, J. , Peterson, C. , Paige, D. , Robinson, M. the LROC Science Team, 2014.

∼ Remote sensing studies of hansteen alpha. Lunar Planet. Sci. XXXXIII Abstract same time, 3.5 Ga, the surrounding mare was emplaced, flooding 1730 . the base of MH. Wrinkle ridges were subsequently formed in the Head, J.W. , McCord, T.B. , 1978. Imbrian-age highland volcanism on the Moon: the mare, but their strikes are different than the structures in MH. gruithuisen and mairan domes. Science 199, 1433–1436 .

This suggests that the structures of MH and the wrinkle ridges Hiesinger, H., Head III, J., Wolf, U., Jaumann, R., Neukum, G., 2003. Ages and stratig- raphy of mare basalts in oceanus Procellarum, Mare Nubium, Mare Cognitum, were produced by different stress fields and at different times, and Mare insularum. J. Geophys. Res. 108 (E7), 5065. doi: 10.1029/20 02JE0 01985 . hence likely they had different origins. In addition, each unit of Ivanov, M. , Head, J. , 2015. Lunar non-mare volcanism: topographic configuration,

MH appears to be mantled by volcanic ash. These ash deposits morphology, age and internal structure of the gruithuisen domes. Lunar Planet. Sci. XXXXVI Abstract 111 . also appear to have only modestly contaminated the other units Jolliff, B. , Wiseman, S. , Lawrence, S. , Tran, T. , Robinson, M , Sato, H. , Hawke, B. , suggesting that although explosive volcanism occurred at MH it Scholten, F. , Oberst, J. , Hiesinger, H. , van der Bogert, C. , Greenhagen, B. , was likely not particularly violent. Glotch, T. , Paige, D. , 2011. Non-mare silicic volcanism on the lunar farside at compton–belkovich. Nat. Geosci. 4, 566–571 . Kiefer, W. , Taylor, G. , Andrews-Hanna, J. , Head, J. , Jansen, J. , McGovern, P. , Robin- Acknowledgements son, K. , Wieczorek, M. , Zuber, M. , 2016. The bulk density of the small lunar volcanos gruithuisen delta and hansteen Alpha: implications for volcano com- position and petrogenesis. Lunar Planet. Sci. XXXXVI Abstract 1722 .

We dedicate this manuscript to our late friend and colleague Lawrence, D.J., Hapke, B., Hagerty, J., Elphic, R., Feldman, W., Prettyman, T., Van- Dr. B. Ray Hawke, who had a love for all things lunar for his entire iman, D., 2005. Evidence for a high-Th, evolved lithology on the Moon at career, and a specific long-term interest in Hansteen Alpha dating hansteen alpha. Geophys. Res. Lett. 32, L07201. doi: 10.1029/2004GL022022 .

Logan, L., Hunt, G., Balsano, S., Salisbury, J., 1972. Mid-infrared emission spectra of back many decades. B. Ray was instrumental in the targeting of apollo 14 and 15 soils and remote compositional mapping of the moon. Proc. many of the LROC data sets used here and we will sorely miss his Lunar Science Conf. 3, 3069–3076 . encyclopedic knowledge of lunar geology and his willingness to Lucey, P.G. , Blewett, D. , Jolliff, B. , 20 0 0a. Lunar iron and titanium abundance al- share it with others. We would like to thank James Ashley, and gorithms based on final processing of clementine ultraviolet- visible images. J. Geophys. Res. 105 (E8) 20,297-20,305 . an anonymous reviewer for their thoughtful comments and help Lucey, P.G. , Blewett, D. , Taylor, G. , Hawke, B. , 20 0 0b. Images of luna surface maturity. to make this a much better contribution. We would also like to J. Geophys. Res. 105 (E8) 20,377-20,386 . acknowledge NASA’s support for coauthors MSR and SJL of ASU Malin, M., 1974. Lunar red spots: possible pre-mare materials. Earth Planet. Sci. Lett. 21, 331–341 . through a LRO/LROC contract. McCauley, J.F. , 1973. Geologic map of the grimaldi quadrangle of the Moon. U.S. Geol. Survey, Misc. Invest. Ser. Map I-740 . References Melosh, H.J. , 1989. Impact Cratering. Oxford University Press, New York . Michael, G.G., Neukum, G., 2010. Planetary surface dating from crater size–

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