Characterization of Previously Unidentified Lunar Pyroclastic Deposits Using Lunar Reconnaissance Orbiter Camera Data J

Home , Agatharchides (crater), Aitken (crater), Alexander (crater), Anderson (crater), Ball (crater), Birt (crater)

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, E00H25, doi:10.1029/2011JE003893, 2012

Characterization of previously unidentified lunar pyroclastic deposits using Lunar Reconnaissance Orbiter Camera data J. Olaf Gustafson,1 J. F. Bell III,2,3 L. R. Gaddis,4 B. R. Hawke,5 and T. A. Giguere5,6 Received 1 July 2011; revised 31 March 2012; accepted 14 April 2012; published 8 June 2012.

[1] We used a Lunar Reconnaissance Orbiter Camera (LROC) global monochrome Wide-angle Camera (WAC) mosaic to conduct a survey of the Moon to search for previously unidentified pyroclastic deposits. Promising locations were examined in detail using LROC multispectral WAC mosaics, high-resolution LROC Narrow Angle Camera (NAC) images, and Clementine multispectral (ultraviolet-visible or UVVIS) data. Out of 47 potential deposits chosen for closer examination, 12 were selected as probable newly identified pyroclastic deposits. Potential pyroclastic deposits were generally found in settings similar to previously identified deposits, including areas within or near mare deposits adjacent to highlands, within floor-fractured craters, and along fissures in mare deposits. However, a significant new finding is the discovery of localized pyroclastic deposits within floor-fractured craters Anderson E and F on the lunar farside, isolated from other known similar deposits. Our search confirms that most major regional and localized low-albedo pyroclastic deposits have been identified on the Moon down to 100 m/pix resolution, and that additional newly identified deposits are likely to be either isolated small deposits or additional portions of discontinuous, patchy deposits. Citation: Gustafson, J. O., J. F. Bell III, L. R. Gaddis, B. R. Hawke, and T. A. Giguere (2012), Characterization of previously unidentified lunar pyroclastic deposits using Lunar Reconnaissance Orbiter Camera data, J. Geophys. Res., 117, E00H25, doi:10.1029/2011JE003893.

1. Introduction [Gaddis et al., 1985, 1998, 2003, 2011; Hawke and Head, 1980; Hawke et al., 1989]. The deposits are globally dis- [2] Dark-mantle deposits (DMDs) have been observed at tributed, but are most frequently observed in the highlands many locations on the Moon, and were described and adjacent to major maria or along fissures within floor-frac- mapped by several early investigators [e.g., Carr, 1966; 2 tured craters. They range in area from less than 10 km to Schmitt et al., 1967; Pohn and Wildey, 1970; Wilhelms and 2 over 50,000 km . McCauley, 1971]. Characterized primarily by their low [3] Although volumetrically small compared to mare albedo, smooth surfaces, and mantling relationship to volcanism [e.g., Head, 1976; Head and Coffin, 1997], lunar underlying terrain features, they have been widely inter- pyroclastic volcanism nevertheless represents an important preted as explosively emplaced volcanic or pyroclastic aspect of volcanic activity on the Moon [e.g., Wilson and deposits [El-Baz, 1973; Adams et al., 1974; Head, 1974; Head, 1981, 1983]. A more complete understanding of the Heiken et al., 1974; Rosanova et al., 1998]. As more com- distribution, composition, and geologic setting of pyroclastic plete and detailed photographic coverage of the Moon has deposits across the Moon could provide important con- become available, the number of identified DMDs has straints on the timing, duration, and location of volcanic increased, with well over 100 deposits identified to date activity as well as insights into mechanisms of magma ascent and eruption and the composition and mineralogy of 1Department of Earth and Atmospheric Sciences, Cornell University, magma source regions. Identifying and characterizing the Ithaca, New York, USA. full range of compositions represented by pyroclastic 2 Astronomy Department, Cornell University, Ithaca, New York, USA. deposits is an important part of efforts to understand the 3School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA. inventory of lunar surface materials. In addition, the unique 4Astrogeology Program, U.S. Geological Survey, Flagstaff, Arizona, composition and physical characteristics of lunar pyroclastic USA. deposits make them valuable as potential raw materials for 5Hawaii Institute of Geophysics and Planetology, University of Hawaii, extraction of desirable resources such as iron, titanium, Honolulu, Hawaii, USA. 6 oxygen, and volatile elements [Hawke et al., 1990; Coombs Intergraph Corporation, Kapolei, Hawaii, USA. et al., 1998]. Corresponding author: J. O. Gustafson, Department of Earth and [4] Previous investigators have placed lunar pyroclastic Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USA. deposits into two general groups: “regional” and “localized” ([email protected]) [Gaddis et al., 1985, 2003]. Regional deposits are typically ©2012. American Geophysical Union. All Rights Reserved. larger, are found along the margins of maria, and are thought to have formed during Hawaiian-style fire-fountaining

E00H25 1of21 E00H25 GUSTAFSON ET AL.: NEW PYROCLASTIC DEPOSITS E00H25 eruptions [Heiken et al., 1974; Wilson and Head, 1981; groups based on the steepness of the spectral continuum Gaddis et al., 1985; Weitz et al., 1998]. The regional slope and the position, depth, and shape of the 1-mm band deposits are commonly widely distributed and are often [Hawke et al., 1989; Coombs and Hawke, 1992]. These associated with a fissure or the source crater for a sinuous spectral groups were interpreted to represent varying con- rille, and possibly formed during an earlier phase of lunar tributions of highlands wall rock, basaltic plug rock, and volcanic eruptions in which the maria were emplaced [Head, juvenile basaltic magma to the mixture of clasts emplaced at 1974]. Dating of pyroclastic material collected from regional each location. deposits at Montes Apenninus (Apollo 15) and Taurus- [7] Pyroclastic deposits have also been examined using Littrow (Apollo 17) indicated formation ages of 3.35– Earth-based and satellite-based radar [e.g., Zisk et al., 1974; 3.62 Ga [Spangler et al., 1984] and 3.48 Ga [Tera and Gaddis et al., 1985; Campbell et al., 2008; Carter et al., Wasserburg, 1976], respectively. Localized deposits are 2009, 2010; Trang et al., 2010]. Radar provides an alterna- smaller features interpreted to result from vulcanian-style tive approach for studying the properties of the lunar surface eruptions caused by the explosive release of gases that that is complementary to spectroscopic methods. The radar accumulated under a solidified magma plug [Head and signal is sensitive to both the composition of the surface and Wilson, 1979; Hawke and Head, 1980; Hawke et al., the presence of surface or subsurface scatterers on the scale 1989]. Although there is no rigorous division in size of the radar wavelength. The ability to detect such scatterers between the two groups, regional deposits have surface areas at the centimeter to meter scale has direct applicability to that are typically greater than 1000 km2, while localized identifying and characterizing smooth textured, relatively deposits are typically only a few tens to hundreds of km2 in block free pyroclastic deposits. The coverage and spatial size. Localized pyroclastic deposits have been identified in a resolution of the radar data sets produced to date has limited variety of geologic settings. Many occur in floor-fractured their application to studies of relatively large and thick craters, but others are found on or near major maria or pyroclastic deposits. Consequently, radar data were not adjacent to smaller mare deposits, and some are found as employed in this study, which focused on identifying smal- isolated deposits in the highlands [Gaddis et al., 1985; ler and typically thinner pyroclastic deposits. However, as Hawke et al., 1989; Coombs and Hawke, 1992; Rosanova more complete radar coverage is acquired at higher resolu- et al., 1998; Gaddis et al., 2000, 2003]. The ages of tions, it may provide a valuable tool for characterizing unsampled regional and localized pyroclastic deposits are smaller pyroclastic deposits as well. difficult to assess, in part because their unconsolidated [8] Recent lunar missions have resulted in a vast increase nature is thought to have reduced artificially the number of in both the quantity and quality of remote sensing data. small craters observed on their surfaces [Lucchitta and Increased image resolution and coverage have enabled a Sanchez, 1975]. Stratigraphic evidence at sites such as the more thorough and detailed search for potential pyroclastic floor of Alphonsus crater suggests that these deposits post- deposits. For this study, we used Lunar Reconnaissance date upper Imbrian-aged crater floor deposits [McCauley, Orbiter Camera (LROC) data to search for potential local- 1969], but their exact ages are not at all clear. ized pyroclastic deposits that have not been previously [5] Soil samples collected during the Apollo missions identified. Our analysis takes advantage of the high resolu- contained glassy to microcrystalline beads, typically less tion, global coverage provided by LROC as well as our than 0.1 mm in diameter, which have been interpreted to increased ability to compare multiple data sets as each new have formed in regional pyroclastic eruptions [e.g., Heiken mission is flown to the Moon. The goals of this effort were et al., 1974; Pieters et al., 1974; Arndt et al., 1984]. The to assess the utility of the LROC data in locating and mineral compositions of the beads reflect a primitive, oliv- studying pyroclastic deposits; to estimate the prevalence of ine-rich (picritic) composition, likely indicating both an uncatalogued low-albedo pyroclastic deposits on the Moon; origin deep within the lunar mantle (300–400 km) and to help compile a more complete catalog of pyroclastic rapid ascent and eruption [Delano, 1986; Shearer and deposits; and to characterize the properties and possible Papike, 1993; Papike et al., 1998]. In addition, the sur- origins of these features. faces of the beads are often coated or enriched with a variety of volatile elements (for example, S, In, Cd, Zn, Ga, Ge, Au, 2. Methods Pb, and Na) inferred to have been present in the eruption cloud [e.g., Baedeker et al., 1974; Meyer et al., 1975; Butler, [9] The Lunar Reconnaissance Orbiter (LRO) Wide-angle 1978]. Measurements of the concentrations of volatiles Camera (WAC) acquires multispectral images at two ultra- trapped in the interior of a variety of volcanic glasses have violet (UV) and five visible (VIS) wavelengths (320, 360, 415, 565, 605, 645, and 690 nm) at a resolution of revealed that H2O, F, S, and Cl are frequently present [Saal et al., 2008]. 400 m/pix in the UV and 75 m/pix in the visible during the mission’s primary mapping orbit phase. The LRO [6] Earth-based telescopic observations have identified a Narrow Angle Camera (NAC) produces monochrome ima- variety of spectral classes among regional and localized pyroclastic deposits. Regional deposits generally exhibit a ges at a resolution of 0.5 m/pix [Robinson et al., 2010] broad 1-mm absorption feature with a shape and position during the same mission periods. We examined a prelimi- that are consistent with a mixture of pyroxene with olivine nary 100 m/pix global monochrome (645 nm) WAC mosaic and/or (based on their low albedos) iron-bearing volcanic of the Moon [Robinson et al., 2011; Denevi et al., 2011; Sato glasses [e.g., Gaddis et al., 1985]. Variations in the strength et al., 2011; Speyerer et al., 2011] for dark-toned deposits of the 1-mm feature may be related in part to the degree of with morphologic indicators of pyroclastic origin. Six key crystallinity of the pyroclastic beads [Weitz et al., 1998]. criteria employed in the identification were: (1) Material Localized pyroclastic deposits have been classified into three exhibits smooth texture, and is block-free in high-resolution

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Table 1. Summary of Newly Identified Lunar Pyroclastic Deposits ID Name Area (km2) Latitude (deg) Longitude (deg) Region Geologic Setting 1 Anderson E 81a 17.1N 173.6E Freundlich-Sharonov Floor-fractured highlands crater 2 Anderson F 31a 16.4N 174.1E Freundlich-Sharonov Floor-fractured highlands crater 3 Buys-Ballot 440 19.6N 175.4E Freundlich-Sharonov Basalt-flooded highlands crater 4 Kopff 1100 18.0S 89.8W Orientale Floor-fractured highlands crater 5 Schluter 210 5.2S 83.0W Orientale Basalt-flooded highlands crater 6 Birt E 540 20.9S 9.8W Nubium Maria 7 Walther A 660 32.4S 0.7E Nubium Highlands crater 8 Tobias Mayer NW 170 19.2N 31.1W Mt. Carpatus Maria 9 Tobias Mayer N 680 18.2N 28.8W Mt. Carpatus Maria 10 Tobias Mayer SW 370 14.0N 30.7W Mt. Carpatus Maria 11 Gay-Lussac N 1400 14.9N 20.7W Mt. Carpatus Highlands on basin rim 12 Gay-Lussac NE 760 14.8N 18.4W Mt. Carpatus Highlands on basin rim aTotal area of multiple small deposits.

images; (2) material mantles and subdues subjacent terrain; Planet database (http://www.mapaplanet.org) [e.g., Garcia (3) deposit exhibits diffuse margins; (4) material does not et al., 2010]. These data provide compositional information flood adjacent topographic lows; (5) deposit surface does not that can help delineate the extent of dark mantle deposits by exhibit lava flow textures; and (6) deposit occurs in associ- differentiating them on the basis of iron content from subja- ation with rilles, fractures, or other possible vents [e.g., cent materials, especially if the mantling material is thin or Gaddis et al., 1985; Hawke et al., 1989]. shading effects obscure albedo differences. The 1 mm 2+ [10] Our search focused initially on locations in the LROC absorption band in CSR data results primarily from Fe targeting database [e.g., Gaddis et al., 2009] which had present in components such as orthopyroxene, clinopyrox- characteristics consistent with potential pyroclastic deposits. ene, olivine, or iron-bearing volcanic glass [Pieters and This database was open for target entry by a wide range of Englert, 1993]. Although the spectral resolution of the CSR individuals based on both published and unpublished target characteristics. Key information often used to identify possible pyroclastic deposits included prior mapping, unusual albedo, and proximity to fractures, rilles, or possible vents. Table 2. Other Potential Lunar Pyroclastic Deposits Examined These deposits typically were not well enough resolved in Location Latitude (deg) Longitude (deg) Assessment previous data sets to assess their mode of emplacement. For the purpose of this survey, we avoided well-studied loca- Agatharchides E 21.1S 32.9W Inconclusive Artem’ev 10.5N 145.2W Inconclusive tions that have previously been identified as pyroclastic Artem’ev 11.9N 145.3W Inconclusive deposits. In addition to this targeted search, we also exam- Artem’ev 12.2N 142.2W Inconclusive ined the new global WAC mosaic using a gridded search Bonpland 7.74S 17.1W Inconclusive between 60N and 60S latitude; higher latitude locations Clausius 37S 44.2W Inconclusive Crisium, Mare 17.2N 62.4E Inconclusive were excluded due to unfavorable illumination geometry. Crisium, Mare 10N 59E Unlikely For locations that met our preliminary morphologic criteria, Gartner M 53.2N 39.3E Inconclusive we processed and examined WAC color mosaics [Robinson Gay-Lussac 13.3N 23.3W Inconclusive et al., 2010] and/or high-resolution NAC images (if avail- Golitsyn B 20.9S 100.4W Inconclusive able for the target area) to look at additional details of Hell B 30S 4W Inconclusive Hell HB 30S 3.3W Inconclusive mantling relationships, deposit textures, rock abundances, Heraclides A 41.2N 34.2W Inconclusive and the possible presence of volcanic vents. Heraclides A 41.1N 35.3W Inconclusive [11] We also examined Clementine spectral reflectance Hertzsprung basin 0.9N 120.7W Unlikely (CSR) data at ultraviolet, visible and near-infrared wavelengths Hohmann Q 25.7S 95.2W Unlikely Langmak 11.3S 119.5E Inconclusive (415, 750, 900, 950, 1000 nm) for candidate pyroclastic deposits Langmak 9.8S 117.5E Unlikely (http://www.lpi.usra.edu/lunar/tools/clementine/instructions/ Le Monnier 25.8N 29.5E Inconclusive UVVIS_DIM_Info.html). The position of the filters in the Lehmann E 37.2S 56W Inconclusive spectrum was chosen to maximize information about Loewy 23.1S 32.4W Inconclusive absorption features related to common lunar minerals such as Loewy 22S 32.8W Inconclusive Lucretius 4.5S 116.5W Inconclusive plagioclase feldspar, orthopyroxene, clinopyroxene, and Lucretius 1.8S 111.8W Inconclusive olivine [e.g., Tompkins and Pieters, 1999]. We reviewed Marius Hills 13.6N 55.8W Inconclusive Clementine UVVIS “natural” color composite images (red = Michelson 7.6N 122.6W Inconclusive 1000 nm, green = 900 nm, blue = 415 nm) to assist in Orientale, Mare 24.9S 95.2W Inconclusive Palmieri 27.3S 46.6W Inconclusive delineating the extent of dark-toned material. We used the Plato 51N 13.4S Inconclusive 750 nm reflectance data to estimate the Lambert albedo to Schroter 1.5N 9.9W Inconclusive compare the albedo of candidate pyroclastic deposits with Secchi A 0.68N 40.5E Unlikely that of surrounding deposits. We also used the UVVIS color Vaporum 14.4N 0.7E Inconclusive ratio composite images and derived iron abundance images Vieta T 33.4S 58.4W Inconclusive Vieta T 33.6S 57.2W Inconclusive available at 100 m/pixel from the PDS/USGS Map-A-

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Figure 1. Global distribution of pyroclastic deposits. Base map is the global WAC mosaic [Speyerer et al., 2011] overlain on the global WAC DTM [Scholten et al., 2011]. O = Orientale Basin, MC = Montes Carpatus, N = Mare Nubium, FS = Freundlich-Sharonov Basin. Red boxes indicate the location of regional WAC mosaic images presented below.

data does not allow for differentiation among these possible follow the contours of the preexisting topography [e.g., components, the band depth at 950 nm has been shown to be Wilhelms, 1987]. This can be seen when a younger deposit correlated to the total abundance of these mafic components embays older, higher terrain within depressions or valleys, [e.g., Lucey et al., 2000]. The CSR color ratio composite for example. In contrast, pyroclastic fall deposits mantle the images are created by displaying the 750/415 nm ratio as red, preexisting topography, covering both high and low areas the 750/950 nm ratio as green, and the 415/750 nm ratio as [Wilson and Houghton, 2000]. Fall deposits can also pro- blue. Generally, spectrally mature highland deposits appear duce diffuse margins as the thickness of material emplaced red, and “fresh” (spectrally immature) highlands exposures tapers off with increasing distance from the source vent [e.g., appear blue. Basaltic mare-type deposits appear yellow to Wilson and Head, 1981]. The bedrock block abundance at orange (iron-rich, lower-Ti) or blue (iron-rich, higher-Ti) the surface of a deposit is also useful for distinguishing lava [McEwen et al., 1994; Pieters et al., 1994]. It is important to flows from pyroclastic deposits. For example, fine-grained note, however, that individual colors do not correspond to pyroclastic deposits will likely contain blocks only very unique compositions, and regions containing heterogeneous close to the source vent or where impacts have excavated compositions and maturity states can appear complex in the blocky bedrock from beneath the pyroclastic layer. In a ratio images. The derived iron abundance images are created similar manner, the presence of flow textures such as ripples, using the algorithms of Lucey et al. [2000] as modified by ropy surfaces, and lobes provides evidence of emplacement Lawrence et al. [2002]. Weight percent FeO is presented as either impact melt or an effusive, rather than pyroclastic, using a gray scale ranging from 0 to 25 wt.%. deposit [Schaber, 1973; Wilhelms, 1987]. Care must be [12] The morphology and sharpness of deposit margins taken when evaluating these factors, however. Fall deposits provide important clues to their mode of emplacement. shed from slopes can follow topographic contours Effusive lavas often pool in topographic lows, resulting in [Wilhelms, 1987], and impact gardening can “smear” the smooth, level deposits with relatively sharp margins that edges of effusive deposits, complicating efforts to

Table 3. Previously Identified Pyroclastic Deposits Shown on Figures ID Name Area (km2) Latitude (deg) Longitude (deg) Region Geologic Setting Reference C Montes Carpatus 354 14.6N 25.4W Imbrium Highlands on basin rim Gaddis et al. [2003] A Alphonsus 288a 13.8S 2.9W Nubium Floor-fractured highlands crater Gaddis et al. [2003] L Lassell Crater 192 15.4S 7.9W Nubium Highlands crater Gaddis et al. [2003] F Fra Mauro F 85 7.1S 16.8W Nubium Highlands adjacent to maria Weitz and Head [1999] aTotal area of multiple small deposits.

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Figure 2. WAC DTM of Freundich-Sharonov basin [Scholten et al., 2011] (NASA/ASU/DLR).

differentiate between processes. In addition, ballistically features. The remaining locations are considered to be emplaced impact melt can “splash” over uneven terrain, probable pyroclastic deposits based on a qualitative assess- resulting in an appearance similar to pyroclastic mantling ment of the six criteria listed above. These are presented as [e.g., Cintala and Grieve, 1998]. examples and not as a comprehensive list of remaining [13] Volcanic vent characteristics can provide important pyroclastic deposits. evidence regarding the mode of eruption. For effusive eruptions, source vents are often buried by the late stage 3. Results and Discussion lavas [Greeley, 1976; Wilhelms, 1987]. When they are visible, they typically are associated with rilles or domes. In [15] Preliminary review of approximately 125 dark-toned contrast, while pyroclastic source vents can also be associ- deposits resulted in a list of 47 locations that warranted ated with these features, they are more often observed as closer inspection. Of these 47 deposits, 12 were assessed as irregular craters or depressions similar to maars on Earth, probable newly identified pyroclastic deposits (Table 1) and often in association with fractures or graben [Head and 35 were assessed as inconclusive or unlikely (Table 2). Wilson, 1979]. There is some overlap in the morphologies Probable or possible pyroclastic deposits are grouped geo- of both kinds of eruptive styles, however, and both types of graphically as follows: the farside Freundlich-Sharonov volcanic source vents/regions can be obscured by subse- basin, Orientale basin, Mare Nubium region, and the Montes quent eruptions or by impact ejecta. Carpatus region (Figure 1). The global distribution of these potential pyroclastic deposits is presented in Figure 1 along [14] After performing an initial screening of candidate pyroclastic deposits using the data sets and criteria described with the locations of many previously identified pyroclastic above, we eliminated many from further consideration deposits as compiled by Gaddis et al. [2003]. Previously because they lacked sufficient evidence of pyroclastic identified pyroclastic deposits shown on more detailed fig- emplacement (classified as “unlikely”) or the available data ures throughout the paper are identified by letters and listed did not support a conclusive interpretation (classified as in Table 3. “inconclusive”). Other potential mechanisms for formation 3.1. Freundlich-Sharonov Basin of irregular or patchy dark deposits include partially obscured mare material or partially exhumed cryptomaria 3.1.1. Anderson E and F [e.g., Antonenko et al., 1995], small mare ponds, and impact [16] Anderson E and F are Nectarian craters with dia- melt [e.g., Cintala and Grieve, 1998]. Many of the locations meters of 28 km and 49 km, respectively, located in the classified as inconclusive had high incidence angle illumi- highlands on the farside east of Mare Moscoviense nation that was not optimal for assessment of albedo (Figures 1 and 2). They lie near the center of the 600 km

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to the typical simple crater profile. Numerous small dark deposits are revealed in the WAC image mosaic (Figure 3a), with many of the dark spots associated with craters that appear to be endogenic based on several of the criteria established by Head and Wilson [1979]: their irregular shape, alignment along floor fractures, lack of rays, and lack of a raised rim. Some of these deposits may have been obscured by the emplacement of highlands material in the form of crater rays crossing Anderson E and F. The dark material surrounding these possible vents has mantled the hummocky floor material of both Anderson E and F. Typical Lambert albedo of these deposits ranges from 0.201 to 0.213, compared to 0.262–0.267 for the surrounding crater floor deposits (Table 4). The enhanced mafic signature of the dark mantling material can be seen in the CSR ratio image (Figure 3b). Green colors seen across the center of Anderson E and at the spots in the southern portion of Anderson F indicate mixing of highland debris with mafic materials. Approximately 10 possible vents have been identified in Anderson E, to the north, and at least four possible vents have been identified in Anderson F. Examples of possible volcanic source fissures are indicated on the NAC image in Figure 4. We have concluded that these are likely pyroclastic deposits based on the dark tone, association with possible vents located along floor fractures, areal distribution around the possible vents, and their mafic signature. [17] The raised floors and the system of floor fractures within Anderson E and F suggest post-impact intrusion of lava beneath the crater floor [e.g., Schultz, 1976; Stewart- Alexander, 1978]. The dark-mantle deposits in Anderson E and F appear to be Alphonsus-type deposits erupted from a series of small vents aligned with the fracture system, suggesting their origin in volatile-rich vulcanian-style eruptions containing relatively small amounts of juvenile material [e.g., Head and Wilson, 1979]. The two small deposits within the southern portion of Anderson F may represent gas-rich eruptions that emplaced very little juvenile material; the green colors in the CSR ratio image suggest that these deposits are a mixture of mafic minerals and highlands material that could represent wall rock eroded from the volcanic vents [e.g., Hawke et al., 1989]. The presence of such volcanic features within the central farside highlands indicates that magma ascent and eruption have occurred Figure 3. (a) WAC “natural” color mosaic of Anderson E even within this region of relatively thick crust [e.g., Kaula and F craters (R = 645 nm, G = 605 nm, B = 565 nm). Con- et al., 1974; Zuber et al., 1994], likely due to localized centrations of dark material are visible along several of the crustal thinning (10 km thickness) related to the formation floor fractures in both craters. Box shows the location of of the Freundlich-Sharonov basin [Wieczorek et al., 2006]. Figure 4 (NASA/GSFC/ASU). (b) Clementine color ratio However, this is the only such occurrence that we have image of Anderson E and F craters (R = 750/415 nm, G = located, and it appears to represent an endpoint in the con- 750/950 nm, B = 415/750 nm). The yellow mafic signature tinuum of eruption styles, where the eruption was just barely correlates well with the low-albedo features in Figure 3a. able to reach the surface but could not transport enough Green colors seen across the center of Anderson E and at magma to the surface to form an effusive deposit [e.g., Head the spots in southern Anderson F indicate mixing of high- and Wilson, 1979]. land debris with mafic materials (USGS). 3.1.2. Buys-Ballot [18] Buys-Ballot is an unusual oblong Imbrian impact crater consisting of a roughly circular northern part (D = 55 km) and a narrower lobe extending an additional 20 km diameter Freundlich-Sharonov basin (Figure 2). The unusual to the south, giving it a pear shape [Schultz et al., 1998; van Buys-Ballot feature, discussed below, is located approxi- der Bogert et al., 1998]. It has a central ridge running along mately 100 km to the north. Anderson E and F both exhibit a the long axis and contains a mare-like flat floor deposit in series of floor fractures aligned primarily concentric to the the northern portion. Immediately south and southeast of crater rims, and both crater floors appear raised with respect Buys-Ballot is a dark deposit (Lacus Luxuriae) that consists

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Table 4. Clementine 750 nm Reflectance Measurements and Estimated Lambert Albedo (Æ1s) 750 nm Reflectance Lambert Albedo ID Name Area (km2) Latitude (deg) Longitude (deg) Pyroclastic Deposit Adjacent Units Pyroclastic Deposit Adjacent Units 1 Anderson E 81a 17.1N 173.6E 0.192 Æ 0.002 0.255 Æ 0.001 0.201 Æ 0.002 0.267 Æ 0.001 2 Anderson F 31a 16.4N 174.1E 0.204 Æ 0.003 0.252 Æ 0.002 0.213 Æ 0.004 0.262 Æ 0.002 3 Buys-Ballot 440 19.6N 175.4E 0.170 Æ 0.002 0.231 Æ 0.003 0.181 Æ 0.003 0.245 Æ 0.003 4 Kopff 1100 18.0S 89.8W 0.139 Æ 0.002 0.206 Æ 0.002 0.146 Æ 0.002 0.217 Æ 0.002 5 Schluter 210 5.2S 83.0W 0.142 Æ 0.004 0.196 Æ 0.003 0.143 Æ 0.004 0.197 Æ 0.003 6 Birt E 540 20.9S 9.8W 0.096 Æ 0.001 0.115 Æ 0.001 0.103 Æ 0.001 0.123 Æ 0.001 7 Walther A 660 32.4S 0.7E 0.170 Æ 0.002 0.222 Æ 0.005 0.202 Æ 0.002 0.263 Æ 0.006 8 Tobias Mayer NW 170 19.2N 31.1W 0.097 Æ 0.001 0.118 Æ 0.001 0.103 Æ 0.001 0.125 Æ 0.002 9 Tobias Mayer N 680 18.2N 28.8W 0.100 Æ 0.001 0.123 Æ 0.003 0.106 Æ 0.002 0.130 Æ 0.003 10 Tobias Mayer SW 370 14.0N 30.7W 0.101 Æ 0.001 0.122 Æ 0.004 0.104 Æ 0.001 0.126 Æ 0.005 11 Gay-Lussac N 1400 14.9N 20.7W 0.105 Æ 0.003b 0.150 Æ 0.003 0.109 Æ 0.004b 0.155 Æ 0.003 12 Gay-Lussac NE 760 14.8N 18.4W 0.118 Æ 0.002b 0.149 Æ 0.004 0.122 Æ 0.002b 0.154 Æ 0.004 aTotal area of multiple small deposits. bDark streaks within mantled area.

of a very dark-toned central region surrounded by lobes of chambers where subsequent eruption could occur [e.g., Head lighter-toned deposits to the west, south, and east (Figure 5a). and Wilson, 1979]. We believe this is a plausible alternative The southern portion of Lacus Luxuriae appears to be rela- to the Schultz et al. [1998] hypothesis that the mafic deposits tively smooth-surfaced and constrained primarily by local within Buys-Ballot and Lacus Luxuriae were deposited by topography; we interpret this region as effusively emplaced the crater-forming impactor. mare basalt. In contrast, the dark-toned material to the north, where we identify a candidate pyroclastic deposit, appears to 3.2. Orientale mantle a portion of the highlands north of Lacus Luxuriae as 3.2.1. Kopff well as hummocky terrain south of Buys-Ballot and a portion [20] Kopff is a 41-km Imbrian-aged impact crater located of the southern wall of the crater itself (Figure 5b). The dark in the eastern portion of the Orientale basin, immediately tone of this mantling material can be seen in the CSR UVVIS east of Mare Orientale but within the Montes Rook ring image (Figure 6a); a typical Lambert albedo within the deposit (Figures 1 and 7) [Scott et al., 1977; Whitten et al., 2011]. is 0.181, compared to 0.245 for the surrounding highlands Scott et al. [1977] mapped mare materials in the floor of deposits (Table 4). The derived iron image (Figure 6b) shows Kopff as being Eratosthenian/Imbrian in age, indicating that that this area contains relatively iron-rich material similar in they formed at the same general time as the mare deposits in composition to the smoother deposits within the southern and eastern portions of Lacus Luxuriae. We have identified this as a likely pyroclastic deposit based on the observed mantling of the crater wall, hummocky terrain, and adjacent highlands by the dark-toned, iron-rich material. [19] Buys-Ballot most likely formed from an oblique impact coming from the north, and the dark-toned deposit south of the crater (Lacus Luxuriae) has been interpreted as residual material from a mafic-rich impactor deposited along the downrange rim of the impact crater [Schultz et al., 1998]. These authors cited the region’s remoteness from known volcanic activity in support of a non-volcanic interpretation of this feature. The relative paucity of volcanic deposits over much of the lunar farside is generally attributed at least in part to greater crustal thickness and higher surface elevation relative to the nearside, which would inhibit the ascent of magma to near-surface depths [e.g., Head and Wilson, 1979, 1992; Zuber et al., 1994]. However, farside volcanic deposits are found in basins (e.g., South Pole–Aitken, Moscoviense, Orientale) where excavation and crustal thinning have occurred as a result of the basin-forming impacts. Although the Freundlich-Sharonov basin is not as deep as the mare- containing farside basins [Scholten et al., 2011], the evidence for pyroclastic eruptions at Anderson E and F (section 3.1.1) Figure 4. Portion of NAC image showing detailed view of supports the possibility of volcanism at Buys-Ballot as well. dark deposits and irregular craters that are possible volcanic This would suggest that the depth of basin excavation was vents (e.g., arrows) within Anderson E. Location of this sufficient to allow magma ascent to near-surface magma image is shown by box on Figure 3a (NASA/GSCF/ASU).

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Figure 5. (a, b) Portion of LROC WAC mosaic showing irregular Buys-Ballot crater and the adjacent dark deposit (Lacus Luxuriae) (NASA/GSFC/ASU). Dashed line in Figure 5b depicts the approximate area of the pyroclastic deposit based on mantling of the southern wall of Buys-Ballot, hummocky terrain in the northern portion of Lacus Luxuriae, and highlands north of Lacus Luxuriae (NASA/GSFC/ASU).

Mare Orientale to the west and Lacus Veris to the north and Head et al., 2002], there are several pyroclastic deposits east. A number of pyroclastic deposits have been previously east and northeast of Kopff (e.g., Schluter A, Hedin, and identified in the vicinity of Orientale basin; besides the well- Riccioli) [Coombs and Hawke, 1992]. Possible pyroclastic known Orientale dark ring deposit to the southwest [e.g.,

Figure 6. (a) Clementine UVVIS “natural” color image of Lacus Luxuriae. The dark-toned material extends across the uneven terrain outlined by the dashed line (USGS). (b) Clementine derived iron abundance for Lacus Luxuriae. Values range from 0 wt% FeO (black) to 25 wt% FeO (white). The mantling deposit outlined by the dashed line is elevated in iron compared to surrounding highlands units, with concentrations similar to those seen at the margins of the smooth, effusive deposits (USGS).

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Figure 7. (a, b) WAC “natural” color image of Kopff crater (R = 645 nm, G = 565 nm, B = 415 nm). Crater diameter is 41 km (NASA/GSFC/ASU). Dashed line in Figure 7b indicates extent of pyroclastic deposit (NASA/GSFC/ASU). (c) Clementine derived iron image of Kopff crater. Virtually the entire floor and much of the crater wall, as well as a portion of the rim to the south, exhibits elevated iron compared to the surrounding highlands. Values range from 0 wt% FeO (black) to 25 wt% FeO (white) (USGS). (d) Clementine color ratio image of Kopff crater. Green and orange hues across the crater floor and walls indicated widely distributed mafic material. R = 750/415 nm, G = 750/950 nm, B = 415/750 nm (USGS).

deposits at Schluter, approximately 300 km northeast of color ratio composite images (Figures 7c and 7d) indicates a Kopff, are discussed below. compositional affinity between the dark-toned floor material [21] There are a series of rilles (Rimae Kopff) in the floor and the dark-toned material along the southern rim, which is of the crater, as well as several irregular craters and depres- more mafic (orange) in the ratio image and higher in iron sions concentrated in the southern portion of the crater floor than adjacent rim deposits. Although the iron abundance is (Figure 7a). A dark-toned deposit visible on the southern highest in the portions of the crater floor that appear to floor of Kopff appears to extend onto the rim of the crater, contain effusive deposits, virtually the entire floor and much suggesting a pyroclastic component to this deposit. A typical of the crater wall exhibits elevated iron compared to the Lambert albedo of the mantling material on the rim is 0.146, surrounding highlands (Figure 7c). In a similar manner, the compared to 0.217 for the surrounding highlands deposits CSR color ratio image exhibits green and orange hues across (Table 4). Inspection of the Clementine derived iron and much of the crater floor and walls, indicating widely

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floor materials indicate subsequent modification by volcanic and/or tectonic processes; however, the unusual conditions of its formation make it difficult to differentiate impact melt materials from volcanic materials. The occurrence of a pyroclastic deposit in such a setting may be unique. 3.2.2. Schluter [23] Schluter is an 89-km Imbrian-aged impact crater located northeast of Orientale basin along the western limb of the Moon (Figure 1). It lies within the continuous Orientale ejecta blanket just outside the Montes Cordillera ring [Scott et al., 1977]. The crater has a central peak surrounded by hummocky floor material, which has been flooded by mare flows in the northern half of the crater (Figure 11). Scott et al. [1977] mapped the mare materials as being Eratosthenian/Imbrian in age, indicating they formed at the same general time as the mare deposits in Mare Orientale to the west and Lacus Autumni to the southwest. There is a network of rilles on the crater floor, some of which appear to be partially inundated by mare materials. A number of pyroclastic deposits have been previously identified in this region of the Moon [e.g., Coombs and Hawke, 1992; Gaddis et al., 2003]. Coombs and Hawke [1992] mapped pyroclastic deposits within Schluter A and at Mare Autumni, both located within 75 km south/southeast of Schluter. They also mapped pyroclastic deposits associated with mare fill deposits within the craters Hedin and Riccioli northeast of Schluter. [24] We have identified three areas of additional possible pyroclastic deposits within Schluter: two associated with the mare materials in the eastern and northeastern portions of the crater floor (#5b, #5c) and a third possible deposit just west Figure 8. NAC mosaic of southern floor and rim of Kopff of the central peak (#5a) (Figure 11). Deposit #5a appears as crater. The dark mantling material shown in Figure 7b cov- dark mantling surrounding a depression and fracture that ers hummocky terrain on the crater floor as well as the crater could be a source vent. The primary diagnostic feature of wall and rim. A bright crater ray deposit can be seen crossing these pyroclastic deposits is their mantling relationship to the the darker crater floor deposits. The approximate location of hummocky floor materials. A typical Lambert albedo of the the diffuse edge of the pyroclastic deposit is shown by the mantling material is 0.143, compared to 0.197 for the sur- dashed line. Inset boxes show location of closer NAC view rounding light-toned crater floor deposits (Table 4). The shown in Figure 9 (NASA/GSFC/ASU). mantling material appears to be similar in composition to the surrounding mare fill, as shown by the CSR ratio and derived iron images (Figures 12a and 12b). [25] Schluter is one of a number of highland craters con- distributed mafic material that is not confined to the regions taining basalt flows with possible pyroclastic mantling (e.g., of smooth effusive deposits (Figure 7d). NAC mosaics of the Petavius, Humboldt). This could be a relatively common southern floor and rim indicate that dark material mantles the occurrence on the Moon, but identification of mantling on ridges and hummocky terrain (Figures 8 and 9). Partially dark-toned mare deposits is often difficult due to the lack of buried large boulders indicate the presence of a thick layer of a strong albedo contrast and the generally smooth topogra- fine-textured regolith (Figure 9). NAC images of the crater phy. Mantling of the effusive deposits by pyroclastic mate- floor show deep, irregular pits within several rilles (Figures 8 rials indicates that the explosive volcanism came at the end and 10) which are possible source vents for the pyroclastic of the period of volcanic activity, perhaps due to stalling and material (e.g., Figure 10b). We have identified this as a likely degassing of the final magma pulse beneath the surface. pyroclastic deposit based on 1) the observed mantling of the 3.3. Nubium hummocky terrain on the crater floor and the southern crater wall and rim by dark-toned, mafic, iron-rich material, and [26] Mare Nubium, on the south-central nearside, exhibits 2) the association of these deposits with possible volcanic little evidence of the pyroclastic activity that is associated vents aligned with fractures on the crater floor. with other nearside maria such as Serenitatis, Imbrium, [22] Kopff crater has a somewhat unusual morphology Vaporum and Humorum. The most notable pyroclastic fea- (shallow floor, no central peak, subdued rim, lack of ejecta tures in this region are the dark-halo craters of Alphonsus, rays) that has led some to suggest that it may have been the type locality for localized pyroclastic deposits [Head and formed by impact into partially molten Orientale basin Wilson, 1979]. Previously recognized deposits also include materials [e.g., Spudis et al., 1984]. The raised and fractured localized deposits at Fra Mauro, Lassell crater, Airy, and

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Figure 9. Mantling material on the wall of Kopff crater (location shown on Figure 8). Note subdued topography in the more level terrain at the bottom of the image; thick lobes of fine-textured material mov- ing downslope (upward) have partially buried the large boulders (arrows). Large crater southwest of the image center is an older crater mantled by the dark material (NASA/GSFC/ASU).

Hell [Weitz and Head, 1999; Gaddis et al., 2003]. We 3.3.1. Birt E identified additional pyroclastic deposits in the maria at Birt [27] Birt E (Figure 14) is a small deposit associated with a E and in the highlands southeast of Nubium at Walther A linear rille. Possible pyroclastic mantling was identified (Figure 13). during geologic mapping of this region [Holt, 1974]. The

Figure 10. (a) NAC mosaic of southern floor and rim of Kopff crater showing deep pits within the rille network. (b) NAC image of a pit that is a possible volcanic vent (NASA/GSFC/ASU).

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Figure 11. WAC “natural color” image of Schluter crater (R = 645 nm, G = 605 nm, B = 565 nm). Deposit #5a appears as dark mantling surrounding a depression and fracture that could be a source vent. Deposits #5b–5c are man- Figure 13. Portion of LROC WAC global mosaic showing tling layers covering portions of hummocky terrain on the Mare Nubium with the locations of recognized pyroclastic crater floor (NASA/GSFC/ASU). deposits Alphonsus (A), Lassell (L), and Fra Mauro (F) along with the newly identified pyroclastic deposit at Birt E (#6). The potential deposit at Walther A is off the image to the lower right (NASA/GSFC/ASU). Birt E DMD surrounds the crater Birt E (D = 5 km), which is the largest of several pits located along Rima Birt I. The concentric nature of the deposit suggests that this is the west of Birt E. A typical Lambert albedo of the dark-toned likely source for at least some of the dark mantle material. deposit is 0.103, compared to 0.123 for the surrounding However, the morphology of the dark-toned deposit does not maria (Table 4). In the Clementine color ratio composite rule out the possibility that additional material may have image (Figure 14b), this deposit appears orange, suggesting erupted from the second linear rille located immediately a lower titanium concentration than the surrounding bluer

Figure 12. (a) Clementine color ratio image of Schluter crater (R = 750/415 nm, G = 750/950 nm, B = 415/750 nm). Green colors seen at deposit #5a indicate mixing of highland debris with mafic material. Orange and yellow colors at deposits #5b–5c indicate the hummocky terrain within the mare deposits has been covered by mafic material (USGS). (b) Clementine derived iron abundance for Schluter. Values range from 0 wt% FeO (black) to 25 wt% FeO (white). Relatively high iron values are associated with the mantling deposits in all three locations (#5a–5c) (USGS).

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Figure 14. (a) Portion of LROC WAC global mosaic showing dark mantle deposit and possible vent in Mare Nubium (Birt E crater) (NASA/GSFC/ASU). (b) Clementine color ratio image of Birt E crater (R = 750/415 nm, G = 750/950 nm, B = 415/750 nm). The strongest mafic signature (yellow-orange) is well correlated with the low albedo feature, but also extends further east (USGS).

mare deposits. We have identified this as a likely pyroclastic abundance images (Figures 16a and 16b), due to its rela- deposit based on the dark tone, the association with a likely tively mafic and iron-rich composition. The Walther A vent, and the contrast in composition with the surrounding DMD is somewhat unusual among potential pyroclastic maria. deposits in that it occurs in the highlands, away from major [28] The Birt E crater and nearby rilles are similar to fea- effusive deposits, with no significant associated crater floor tures found in association with many mare deposits. These fractures and no obvious candidate source vents. have been interpreted as volcanic vents and lava channels [30] We favor an interpretation of the origin of the dark- related to the effusive eruptions which emplaced the maria toned material at Walther A as pyroclastic, rather than [e.g., Head, 1974]. It is likely that early stage vents and rilles impact melt or excavated cryptomaria, for several reasons. were buried by subsequent eruptions; therefore features such The asymmetric distribution of the dark-toned material is not as Birt E and Rima Birt I must have been products of late consistent with an origin as ejecta from Walther A. The stage volcanic activity. Similar to the deposits in western DMD also mantles the topography formed by younger Montes Carpatus described below, the Birt E pyroclastic impacts into the Walther A ejecta blanket, seen most clearly deposit may be a late-stage volcanic deposit formed by the in the two 3–4 km craters immediately north and northeast of release of volatiles during cooling of residual magma after Walther A (arrows, Figure 15b), indicating that the dark emplacement of the regional mare basalt flows. In this deposit is significantly younger than Walther A itself. model, the pyroclastic material would contain a large com- Impact melt tends to form irregular, patchy deposits in ponent of basaltic plug rock fragmented and erupted by the topographic lows rather than continuous lobes, and would be accumulated gas pressure [e.g., Hawke et al., 1989]. similar in composition to the surrounding terrain [Hawke 3.3.2. Walther A and Head, 1977; Bray et al., 2010]. Impact melt deposits [29] Walther A is an 11-km Imbrian-aged impact crater also degrade relatively quickly, and are therefore not typi- located in the south-central highlands southeast of Mare cally seen in association with Imbrian-aged craters such as Nubium (Figure 1). It was formed in the hummocky pre- Walther A. The darkest portion of the DMD is centered near Imbrian floor of the larger (D = 135 km) crater Walther the north rim of Walther A (Figure 15c), and a possible (Figure 15a) [Pohn, 1972]. To the south and west of Walther source of the pyroclastic material is within the irregular A, the floor of Walther consists of a smooth plains unit. The craters and depressions found there. We have identified this floor of Walther A itself is slightly hummocky and does not as a likely pyroclastic deposit based on the observed man- exhibit evidence of significant, if any, resurfacing by mare tling of the hummocky terrain on the crater floor, the basalt. There is a thin, diffuse dark mantling deposit visible northern crater wall and rim, and the terrain northeast of in the WAC image (Figures 15b and 15c) across most of the Walther A by dark-toned, mafic, iron-rich material. floor, walls, and rim of the crater, and extending in east and northeast-trending lobes for 1–2 crater diameters. A typical 3.4. Montes Carpatus Lambert albedo of the mantling material is 0.202, compared [31] The Montes Carpatus are located along the southern to 0.263 for the nearby Walther crater floor deposits border of Mare Imbrium, immediately north of Copernicus (Table 4). The extent of this material is seen more clearly on crater (Figure 17). They are part of the raised rim structure of the Clementine color ratio composite and derived iron the Imbrium basin, and have experienced significant

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Figure 15. (a) Portion of global WAC mosaic showing the location of Walther A within the crater Walther. Dashed line in Figures 15a and 15b show approximate extent of the DMD. (b) WAC “natural” color image of Walther A crater (R = 645 nm, G = 605 nm, B = 415 nm). Inset box shows the location of the NAC image in Figure 15c. Arrows indicate large craters formed into the Walther A ejecta blanket that have been mantled by the dark-toned material. (c) Detailed NAC view of a portion of typical dark mantled surface north of Walther A. The dark deposit appears to be partially covered by lighter highlands ejecta (NASA/GSFC/ASU).

resurfacing by Copernican ejecta [Schmitt et al., 1967]. deposit (Figure 17, locations C1–C4) has previously been Extensive regional pyroclastic deposits are located east and identified within Montes Carpatus [Gaddis et al., 1985], one southeast of Montes Carpatus at Sinus Aestuum and Rima of several locations in the region mapped by Schmitt et al. Bode [e.g., Gaddis et al., 2003]. A localized pyroclastic [1967] as volcanic materials of possible pyroclastic origin.

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Figure 16. (a) Clementine color ratio image of Walther A crater (R = 750/415 nm, G = 750/950 nm, B = 415/750 nm). (b) Clementine derived iron abundance image of Walther A crater; values range from 0wt% FeO (black) to 25 wt% FeO (white). In both images the crater location is marked by a circle, and the approximate location of the DMD is outlined with a dashed line. Low albedo unit in Figure 15 is nearly coincident with the region of enhanced mafic composition, indicated by yellow colors in Figure 16a and the lightest shades in Figure 16b (USGS).

[32] The dark mantle deposits in the vicinity of Montes and mantle hills that appear to be remnant blocks of highland Carpatus occur as irregular patches, either within the high- material (Figures 18 and 19). Deposit #8 mantles blocks of lands or on the maria adjacent to the highlands (Figure 17). highland material that were previously identified as a mare The irregular shape of these deposits is in part due to the dome [Schaber, 1973]. However, close inspection of low- presence of bright rays (apparently superimposed) of high- sun Apollo images indicates that what might appear to be a land debris from Copernicus, which appear to partially dome with a central pit is actually two separate highland obscure larger areas of dark material. Deposits near Tobias blocks (Figure 18b). Deposit #9 mantles numerous small Mayer (#8–10) occur within generally flat effusive deposits hills that lack summit craters and appear to also be remnants

Figure 17. Portion of LROC WAC global mosaic showing Montes Carpatus area with pyroclastic deposits labeled. Previously identified Montes Carpatus deposit (see Table 3) is subdivided into locations C1– C4. Locations #8–12 are newly identified deposits (see Table 1): (#8–10) Montes Carpatus; (#11–12) Copernicus. Approximate boundaries of newly identified deposits are marked by dashed lines. Note patchy appearance of darker maria and pyroclastic deposits interspersed with lighter highlands and Coper- nicus ejecta (NASA/GSFC/ASU).

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Figure 18. (a–c) Portions of Apollo orbital photo A17-M-2450 showing the Montes Carpatus region at low sun angle. White arrows mark features previously identified as mare domes [Schaber, 1973]. Deposits #8–9 are labeled with their approximate boundaries outlined, although the dark toned material cannot be seen at this illumination angle. These images highlight the large blocks and hills of highlands material within the mare deposit that have been mantled by the dark-toned material seen in Figures 19a and 19b. Although the two adjacent highland blocks within deposit #8 (Figure 18b) had previously been described as a mare dome with a central pit, in this image it can be seen that these are two individual blocks rising separately above the mare surface (NASA/JSC/ASU). of highland material (Figure 18c). Deposit #10 mantles a locations near Tobias Mayer as likely pyroclastic deposits mixture of small hills and hummocky terrain (Figure 19c). based primarily on the dark tone and the observed mantling Typical Lambert albedo of these deposits ranges from of the blocks of highland material preserved among the mare 0.103 to 0.106, compared to 0.125–0.130 for the surround- deposits. ing light-toned streaks (Table 4). All these deposits exhibit [33] Deposits near Gay-Lussac (#11–12) occur within an enhanced mafic signature in the CSR ratio images the mountains immediately north of Copernicus (Figures 17 (Figure 19), although the contrast in composition with the and 20). The dark material is somewhat patchy with mostly surrounding mare materials is limited. We identified these diffuse margins, but occasionally more defined edges are

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Figure 19. LROC WAC images of Montes Carpatus deposits #8–10 located near Tobias Mayer (see Figure 17). (a) Deposit #8 surrounds and mantles blocks of highlands material. (b) Deposit #9 covers an area containing several blocks and hills of highlands material, as well as an irregular depression. (c) Deposit #10 also mantles an area of highland hills and hummocks. Dashed lines depict approximate area of mantling material (NASA/GSFC/ASU). (d–f) Clementine color ratio images of the same deposits. Each exhibits darker orange colors indicating an enhanced mafic signature, but the contrast with adjacent mare materials is limited (R = 750/415 nm, G = 750/950 nm, B = 415/750 nm; USGS).

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Figure 20. (a) WAC mosaic of Montes Carpatus deposits #11–12 located near Gay-Lussac (see Figure 17). (b) Clementine color ratio images of Gay-Lussac deposits. Both deposits exhibit an enhanced mafic signature, as indicated by the orange colors, although the deposits have a mottled appearance due to the complex character of the terrain (R = 750/415 nm, G = 750/950 nm, B = 415/750 nm; USGS).

seen (Figure 21a). These deposits occur in hilly terrain with [34] The pyroclastic deposits we identified in Montes steep slopes, many of which exhibit layers and streaks of Carpatus are similar in nature to previously identified dark material. In places, this material also appears to have Montes Carpatus pyroclastic deposits (Figure 17, locations been obscured by or mixed with Copernicus ejecta. Typical C1–C4) [Gaddis et al., 1985] in that they appear as some- Lambert albedo of these deposits ranges from 0.109 to what irregular, patchy dark-toned areas among superposed 0.122, compared to 0.154–0.155 for the surrounding light- brighter-toned Copernicus ray materials. This morphology is toned streaks (Table 4). CSR ratio images of the Gay-Lussac not consistent with a model where each dark deposit erupted deposits reveal that the dark mantle material has an from a single localized vent. These exposures are therefore enhanced mafic signature compared to the surrounding likely part of more widespread, overlapping deposits that unmantled highlands (Figure 20b), although the deposits have been partially obscured by the highland debris. The two have a mottled appearance in the ratio images due to the locations near Gay-Lussac within the massifs of Montes complex character of the terrain. We identified these loca- Carpatus (Figure 20) could be exposures of pyroclastic tions near Gay-Lussac as likely pyroclastic deposits based material emplaced around the perimeter of Mare Imbrium on the observed mantling of the hilly terrain by dark-toned from vents within the basin, similar to previously identified material, the presence of very dark streaks on many of the deposits such as those at Mons Huygens [Hawke et al., steep slopes, and the enhanced mafic composition compared 1979]. The locations farther west near Tobias Mayer to adjacent deposits. (Figure 19) are associated with smooth effusive deposits as

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Figure 21. (a) NAC view of dark mantling material within Gay-Lussac deposit #12. Dark deposit is somewhat patchy with mostly diffuse margins, but occasionally more defined edges are seen (example shown by arrows). Location of Figure 21b is shown by box. (b) NAC image of darker portion of the mantling deposit (NASA/GSFC/ASU).

well as features such as irregular craters, depressions, and confirms that most major regional and localized pyroclastic domes that are also likely volcanic in origin. Similar to the deposits have likely been identified on the Moon down to Birt E deposit discussed above, these pyroclastic deposits 100 m/pix resolution, and that additional newly identified could be late-stage volcanic materials erupted after dark-toned deposits are likely to be either isolated small emplacement of the regional mare basalt flows. deposits or additional portions of discontinuous, patchy deposits. Based on the locations where we identified previ- 4. Conclusions and Future Work ously unidentified pyroclastic deposits, the greatest potential for identification of additional pyroclastic deposits is likely [35] Our examination of new LROC WAC images of to be in regions with other volcanic constructs associated 47 low-albedo deposits resulted in identification of 12 new with mare deposits (such as near Tobias Mayer west of locations of probable pyroclastic deposits not previously Montes Carpatus), highland locations along the margins of cataloged. Newly identified pyroclastic deposits were gen- maria (such as near Gay-Lussac north of Copernicus), and erally found in settings similar to previously identified smaller floor-fractured craters that have not yet been thor- deposits, for example at the margins of major maria and oughly imaged at higher resolution, particularly on the far- within floor-fractured highland craters. However, a signifi- side (such as at Anderson E and F). cant new finding is the discovery of potential localized [37] A common theme among these new and previously pyroclastic deposits within floor-fractured craters Anderson identified pyroclastic deposits is their frequent association E and F and adjacent to nearby Buys-Ballot on the lunar with effusive lava flows. It is possible that future, more farside, isolated from other known deposits. The presence of detailed examination of many volcanic deposits in LRO data volcanic deposits within this region of relatively thick crust and future improved data sets will find that hybrid deposits is possibly the result of localized crustal thinning related to exhibiting elements of both effusive and pyroclastic the Freundlich-Sharonov basin. This evidence of volcanic emplacement are common, and perhaps even the norm, on activity in a region generally devoid of volcanism provides the Moon. Testing this hypothesis will yield important additional constraints for models of the composition, struc- insights into lunar magmatic and volcanic processes, par- ture, and thermal history of the crust and upper mantle in this ticularly the role of volatiles in various styles of lunar vol- region of the farside. canism. We plan to examine this issue by conducting [36] Many of the 47 potential locations screened were detailed studies of a number of pyroclastic deposits incor- eliminated from consideration based on inconclusive evi- porating multispectral WAC data for compositional assess- dence regarding their mode of emplacement. Additional ment, high-resolution NAC data for morphologic studies, optical imaging, or analyses of other data sets such as and topographic data, in order to assess the frequency with radar, imaging spectroscopy, or thermal inertia, could which pyroclastic and effusive deposits occur together as result in identification of additional pyroclastic deposits, well as the geologic settings under which this is most com- especially lighter-toned deposits. However, our search also mon. Additional future work will focus on characterization

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