PUBLICATIONS Journal of Geophysical Research: Planets RESEARCH ARTICLE Global inventory and characterization of pyroclastic 10.1002/2013JE004480 deposits on Mercury: New insights into pyroclastic Key Points: activity from MESSENGER orbital data • We expand the catalog of pyroclastic deposits on Mercury from 40 to 51 Timothy A. Goudge1, James W. Head1, Laura Kerber2, David T. Blewett3, Brett W. Denevi3, • Stratigraphic relationships suggest 4 5 6 6 7 emplacement over a prolonged interval Deborah L. Domingue , Jeffrey J. Gillis-Davis , Klaus Gwinner , Jörn Helbert , Gregory M. Holsclaw , 3 3 7 3 8 • Deposits exhibit distinct spectral Noam R. Izenberg , Rachel L. Klima , William E. McClintock , Scott L. Murchie , Gregory A. Neumann , signatures with interdeposit variability David E. Smith9, Robert G. Strom10, Zhiyong Xiao10,11, Maria T. Zuber9, and Sean C. Solomon12,13 1Department of Geological Sciences, Brown University, Providence, Rhode Island, USA, 2Laboratoire de Météorologie Dynamique, Centre National de la Recherche Scientifique, Université Pierre et Marie Curie, Institut Pierre-Simon Laplace, Correspondence to: 3 4 T. A. Goudge, Paris, France, The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA, Planetary Science Institute, 5 [email protected] Tucson, Arizona, USA, Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘iatMānoa, Honolulu, Hawaii, USA, 6Institute of Planetary Research, Deutsches Zentrum für Luft- und Raumfahrt, Berlin, Germany, 7Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA, 8Solar System Exploration Division, Citation: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA, 9Department of Earth, Atmospheric and Planetary Sciences, Goudge, T. A., et al. (2014), Global 10 inventory and characterization of pyro- Massachusetts Institute of Technology, Cambridge, Massachusetts, USA, Lunar and Planetary Laboratory, University of clastic deposits on Mercury: New Arizona, Tucson, Arizona, USA, 11Planetary Science Institute, China University of Geosciences, Wuhan, China, 12Department insights into pyroclastic activity from of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, District of Columbia, USA, 13Lamont-Doherty MESSENGER orbital data, J. Geophys. Res. Earth Observatory, Columbia University, Palisades, New York, USA Planets, 119, 635–658, doi:10.1002/ 2013JE004480. Abstract We present new observations of pyroclastic deposits on the surface of Mercury from data Received 4 JUL 2013 Accepted 5 FEB 2014 acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging Accepted article online 12 FEB 2014 (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified Published online 28 MAR 2014 features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than ~400 nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits. 1. Introduction Multispectral images of Mercury acquired during three flybys by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft [Solomon et al., 2008] revealed a number of sites on the surface that were identified as pyroclastic deposits formed through explosive volcanic processes [Head et al., 2008, 2009; Murchie et al., 2008; Robinson et al., 2008; Blewett et al., 2009; Kerber et al., 2009, 2011]. These sites are all characterized by high-reflectance deposits with diffuse borders that are approximately centered on irregularly shaped, rimless pits. The deposits have a “red” spectral slope (i.e., reflectance increases with increasing wavelength) [Blewett et al., 2009; Kerber et al., 2009, 2011]. The central pits are interpreted to be the source vents for the pyroclastic deposits [Kerber et al., 2011]. GOUDGE ET AL. ©2014. American Geophysical Union. All Rights Reserved. 635 Journal of Geophysical Research: Planets 10.1002/2013JE004480 These pyroclastic deposits provide insight into the abundances, composition, and distribution of volatiles in Mercury’s interior [Kerber et al., 2009; Zolotov, 2011]. Moreover, their presence constitutes an important constraint on the formation of the inner solar system, because Mercury’s crust and mantle are not as volatile depleted as predicted by most earlier formation models for the innermost planet [Cameron, 1985; Benz et al., 1988; Boynton et al., 2007; Solomon et al., 2007; Kerber et al., 2009, 2011]. The possibility of explosive volcanism on Mercury had been suggested on the basis of analysis of Mariner 10 images [e.g., Rava and Hapke, 1987; Robinson and Lucey, 1997], but the large number of deposits identified from flyby images [Kerber et al., 2011] was surprising, and the distribution and geological setting of these deposits yielded new and important clues to Mercury’s complex geologic evolution. From Mercury Dual Imaging System (MDIS) [Hawkins et al., 2007] images acquired during MESSENGER’s three flybys of Mercury, Kerber et al. [2011] compiled a catalog of 40 pyroclastic deposits. The deposits are primarily located on the floors of impact craters and along the rim of the Caloris impact basin [Kerber et al., 2011]. Deposits were identified by high-reflectance, spectrally red terrain surrounding irregularly shaped, rimless pits. The radial extent of 35 of the 40 deposits was found to range from ~7 to 71 km [Kerber et al., 2011]. Together with a ballistic trajectory model, these radial extents were used to estimate the volatile content needed to emplace pyroclastic material to these distances from the source vent. Calculations indicated volatile contents of ~1600–16,000 ppm CO (or an equivalent amount of other volatile species) [Kerber et al., 2011], figures far larger than those previously hypothesized for Mercury’s interior [e.g., Boynton et al., 2007; Kerber et al., 2011]. Kerber et al. [2011] also used MDIS color images to show that none of the identified pyroclastic deposits displays a 1000 nm absorption feature in its reflectance spectrum that would be indicative of a crystal field effect produced by octahedrally coordinated Fe2+ bound in the structure of silicate minerals [Burns, 1993a]. Although MESSENGER flyby data were sufficient to recognize many of the pyroclastic deposits on Mercury [Head et al., 2008, 2009; Murchie et al., 2008; Robinson et al., 2008; Blewett et al., 2009; Kerber et al., 2009, 2011], the insertion of the MESSENGER spacecraft into orbit about Mercury on 18 March 2011 has provided images at higher spatial resolution and with more complete spatial coverage than were available during the three flybys, as well as such additional global data sets as spectral reflectance and topography. In this paper, we use orbital observations to augment the earlier catalog of pyroclastic deposits developed by Kerber et al. [2011] and to characterize those deposits in greater detail. Specific goals of this analysis are (1) to determine the morphometry of the vents associated with the pyroclastic deposits, (2) to assess whether these deposits tend to occur in specific settings or in association with specific geologic units, (3) to estimate the relative age of these deposits and their associated pyroclastic activity, and (4) to further our understanding of the spectral characteristics of the pyroclastic deposits. 2. Data Sets Used To address the goals of this study, we used three data sets from instruments on the MESSENGER spacecraft. The distribution and the morphometry of the deposits and associated vents were investigated with narrow- angle camera (NAC) and wide-angle camera (WAC) images obtained with MDIS. The spatial resolution of the utilized images ranged from ~15 to 200 m/pixel for the NAC and ~60 to 500 m/pixel for the WAC. Because of MESSENGER’s highly eccentric orbit and high northern periapsis [Solomon et al., 2007], the spatial resolution of MDIS NAC and WAC images depend on latitude; images at high northern latitudes have markedly higher spatial resolution than images of the southern hemisphere. However, a global campaign of mapping the surface of Mercury at ~250 m/pixel with MDIS has been completed, and the global MDIS NAC- and WAC-derived mosaic at ~250 m/pixel was also utilized in this study when no