Showing papers by "Brett W. Denevi published in 2014"

Resolved Hapke parameter maps of the Moon

Abstract: We derived spatially resolved near-global Hapke photometric parameter maps of the Moon from 21 months of Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) multispectral observations using a novel “tile-by-tile method” (1° latitude by 1° longitude bins). The derived six parameters (w,b,c,BS0,hS, andθp) for each tile were used to normalize the observed reflectance (standard angles i = g = 60°, e = 0° instead of the traditional angles i = g = 30°, e = 0°) within each tile, resulting in accurate normalization optimized for the local photometric response. Each pixel in the seven-color near-global mosaic (70°S to 70°N and 0°E to 360°E) was computed by the median of normalized reflectance from large numbers of repeated observations (UV: ∼50 and visible: ∼126 on average). The derived mosaic exhibits no significant artifacts with latitude or along the tile boundaries, demonstrating the quality of the normalization procedure. The derived Hapke parameter maps reveal regional photometric response variations across the lunar surface. The b, c (Henyey-Greenstein double-lobed phase function parameters) maps demonstrate decreased backscattering in the maria relative to the highlands (except 321 nm band), probably due to the higher content of both SMFe (submicron iron) and ilmenite in the interiors of back scattering agglutinates in the maria. The hS (angular width of shadow hiding opposition effect) map exhibits relatively lower values in the maria than the highlands and slightly higher values for immature highland crater ejecta, possibly related to the variation in a grain size distribution of regolith.

Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data

Abstract: We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified 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.

Images of surface volatiles in Mercury’s polar craters acquired by the MESSENGER spacecraft

Abstract: Images acquired by NASA's MESSENGER spacecraft have revealed the morphology of frozen volatiles in Mercury's permanently shadowed polar craters and provide insight into the mode of emplacement and evolution of the polar deposits. The images show extensive, spatially continuous regions with distinctive reflectance properties. A site within Prokofiev crater identified as containing widespread surface water ice exhibits a cratered texture that resembles the neighboring sunlit surface except for its uniformly higher reflectance, indicating that the surficial ice was emplaced after formation of the underlying craters. In areas where water ice is inferred to be present but covered by a thin layer of dark, organic-rich volatile material, regions with uniformly lower reflectance extend to the edges of the shadowed areas and terminate with sharp boundaries. The sharp boundaries indicate that the volatile deposits at Mercury's poles are geologically young, relative to the time scale for lateral mixing by impacts, and either are restored at the surface through an ongoing process or were delivered to the planet recently.

Characterization of space weathering from Lunar Reconnaissance Orbiter Camera ultraviolet observations of the Moon

Abstract: We investigate the effects of space weathering at ultraviolet wavelengths using a near-global seven-band (321–689 nm) mosaic from the Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC). We confirm that for moderate- to high-iron compositions (≳ 5 wt % FeO), the steeply positive UV slope at wavelengths <415 nm shallows with increasing exposure to space weathering. We measure these differences in LROC WAC data as variations in the 321/415 nm ratio, which has low values for fresh craters in the mare and moderate-iron highlands. For low-iron highland compositions, the break in slope occurs at shorter wavelengths, and it is instead the 321/360 nm ratio that increases with exposure to the space-weathering environment, whereas the 321/415 nm ratio appears to be largely controlled by the degree of shock experienced during the impact. The effects of shock may be more important at highland craters because modest shock pressures result in the solid-state transformation of plagioclase to its glass equivalent, maskelynite, and can help distinguish between primary shocked ejecta and locally exposed fresh material in rays. While all of the “fresh” craters we examined have UV spectral properties consistent with substantial alteration due to space weathering, the UV spectra of lunar swirls (magnetically shielded from the solar wind) are consistent with exposure of immature, crystalline material. Together these results suggest that lunar space weathering is dominated by the solar wind and “saturates” in the UV at Is/FeO values of ~40 (submature).

Nature of the "Orange" Material on Vesta From Dawn

Abstract: From ground-based observations of Vesta, it is well-known that the vestan surface has a large variation in albedo. Analysis of images acquired by the Hubble Space Telescope allowed production of the first color maps of Vesta and showed a diverse surface in terms of reflectance. Thanks to images collected by the Dawn spacecraft at Vesta, it became obvious that these specific units observed previously can be linked to geological features. The presence of the darkest material mostly around impact craters and scattered in the Western hemisphere has been associated with carbonaceous chondrite contamination [4]; whereas the brightest materials are believed to result from exposure of unaltered material from the subsurface of Vesta (in fresh looking impact crater rims and in Rheasilvia's ejecta and rim remants). Here we focus on a distinct material characterized by a steep slope in the near-IR relative to all other kinds of materials found on Vesta. It was first detected when combining Dawn Framing Camera (FC) color images in Clementine false-color composites [5] during the Approach phase of the mission (100000 to 5200 km from Vesta). We investigate the mineralogical and elemental composition of this material and its relationship with the HEDs (Howardite-Eucrite- Diogenite group of meteorites).

Spectra of the Wells lunar glass simulants: New old data for reflectance modeling

Abstract: Silicate glasses are an important constituent in the regolith of airless planetary bodies, and knowledge of glass reflectance characteristics is important for remote-sensing studies of the Moon, Mercury, and asteroids. We recovered reflectance spectra for 20 vacuum-melted lunar glass simulants measured by Wells (1977), which cover a wider range of Fe and Ti contents (0–17.5 wt % FeO and 0–15 wt % TiO2) and a wider wavelength range than those of the better-known Bell et al. (1976) study. We examine the spectra in terms of known absorptions, explore the relationship between ultraviolet spectral parameters and composition, and apply the Hapke radiative transfer model to predict the reflectance spectra of the Wells glasses. The imaginary part of the refractive index (k) at each wavelength was computed based on the Ti and Fe composition using the linear relationship presented by Wilcox et al. (2006) and with a new linear-exponential hybrid relationship. Comparison of the model spectra with the measured spectra reveals that the samples rich in Fe and Ti are best modeled by the linear relationship, because the linear model was developed using the Fe- and/or Ti-rich Bell et al. (1976) glasses. For Fe- and Ti-poor glasses, the hybrid model provides a better fit to the measured spectra, because this model for k is based on the wider compositional range of the Wells glasses. In the future, better linear model fits might be obtained if optical parameters were derived for a wider compositional range, from low-Fe/low-Ti to the higher-Fe/higher-Ti compositions of Apollo volcanic glasses.

Dark Surface Deposits in the North Polar Region of Mercury: Evidence for Widespread Small-Scale Volatile Cold Traps

Abstract: WIDESPREAD SMALL-SCALE VOLATILE COLD TRAPS. David A. Paige, Paul. O. Hayne, Matthew A. Siegler, David E. Smith, Maria T. Zuber, Gregory A. Neumann, Erwan M. Mazarico, Brett W. Denevi, Sean C. Solomon. Department of Earth, Planetary and Space Sciences, UCLA, Los Angeles, CA 90095, dap@moon.ucla.edu; Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109; Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA 02139; NASA Goddard Space Flight Center, Greenbelt, MD 20771; Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723; Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964; Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015.

The Photometric Properties of Vesta and the Implications

Abstract: Denevi, M. Hoffmann, A. Longobardo, S. Mottola, A. Nathues, V. Reddy, C.T. Russell, S.E. Schröder, Planetary Science Institute, jyli@psi.edu, Jet Propulsion Laboratory, California Institute of Technology, Istituto di Astrofisica e Planetologia Spaziali, INAF, Rome, Italy, Johns Hopkins University, Applied Physics Laboratory, Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany, Deutsches Zentrum für Luftund Raumfahrt (DLR), 12489 Berlin, Germany, Institute of Geophysics and Planetary Physics, University of California Los Angeles.