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