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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.04.008 Byline: J. Warell (a), A. Sprague (b), R. Kozlowski (c), D.A. Rothery (d), N. Lewis (b), J. Helbert (e), E. Cloutis (f) Keywords: Mercury, Surface; Mineralogy; Radiative transfer; Regoliths; Spectroscopy Abstract: The composition and chemistry of Mercury's regolith has been calculated from MESSENGER MASCS 0.3-1.3[mu]m spectra from the first flyby, using an implementation of Hapke's radiative transfer-based photometric model for light scattering in semi-transparent porous media, and a linear spectral mixing algorithm. We combine this investigation with linear spectral fitting results from mid-infrared spectra and compare derived oxide abundances with mercurian formation models and lunar samples. Hapke modeling results indicate a regolith that is optically dominated by finely comminuted particles with average area weighted grain size near 20[mu]m. Mercury shows lunar-style space weathering, with maturation-produced microphase iron present at [approximately equal to]0.065 wt.% abundance, with only small variations between mature and immature sites, the amount of which is unable to explain Mercury's low brightness relative to the Moon. The average modal mineralogies for the flyby 1 spectra derived from Hapke modeling are 35-70% Na-rich plagioclase or orthoclase, up to 30% Mg-rich clinopyroxene, Author Affiliation: (a) Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden (b) Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721-0091, USA (c) Department of Physics, Susquehanna University, 514 University Avenue, Selinsgrove, PA 17870, USA (d) Department of Earth and Environmental Sciences, The Open University, Milton Keynes MK7 6AA, UK (e) Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany (f) Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, Canada Article History: Received 16 August 2009; Revised 11 April 2010; Accepted 12 April 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.11.037 Byline: J. Warell, B.J.R. Davidsson Keywords: Mercury, Surface; Mineralogy; Radiative transfer; Regoliths; Spectroscopy Abstract: An implementation of Hapke's radiative transfer-based photometric model for light scattering in semi-transparent porous media is presented with special emphasis on the analysis of reflectance spectra of Mercury. The model allows intimate mixing of an arbitrary number of regolith components with varying modal abundances, modal chemistries and grain sizes, matured by microphase iron. Reflectance spectra of suites of silicates of varying grain sizes and chemistries are used to calculate the imaginary coefficient of the complex index of refraction as a function of chemistry, thus limiting the modeling effects of chemically atypical laboratory samples, and allowing controlled modeling of minerals with varying chemical compositions. The performance of the model in the visual to near-infrared wavelength range is evaluated for a range of chemically characterized silicate mixtures of terrestrial powders, meteorite powders, matured lunar return samples, and remotely sensed lunar spectra. Author Affiliation: Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden Article History: Received 16 August 2009; Revised 16 October 2009; Accepted 13 November 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.09.007 Byline: J. Warell (a), A.L. Sprague (b), J.P. Emery (c), R.W.H. Kozlowski (d), A. Long (d) Keywords: Mercury; Mineralogy; Regoliths; Surfaces; planets; Spectroscopy Abstract: We present infrared spectra of Mercury and the Moon in the wavelength range 0.7-5.3 [mu]m obtained with the SpeX spectrograph at the NASA Infrared Telescope Facility. The spectra were acquired from pole and terminator locations of Mercury's surface and of Mersenius C and the Copernicus central peak on the Moon. Spectra of both bodies were measured in close temporal succession and were reduced in the same manner with identical calibration stars to minimize differences in the reduction process. The Copernicus spectra display the expected absorption features due to mafic minerals in the near infrared and show spectral features in the SiO combination/overtone vibrational band region above 4 [mu]m. The spectra of Mercury from longitude 170[degrees] and north and south mid-latitudes display a 1-[mu]m absorption band indicative of high-Ca clinopyroxene, while a spectrum from longitude 260[degrees] and northern mid-latitudes does not. The Mercury spectra show a broad feature of low emittance over the full 3-5 [mu]m thermal infrared region, but no narrow features in this spectral range. The longitude 260[degrees] spectrum shows excess thermal emission around 5 [mu]m attributable to the existence of a thermal gradient in the insolated dayside regolith. The thermal-IR spectra suggest a significant difference in the compositional and/or structural properties of Mercury and the Moon that may be due to grain size, absorption coefficient, or the magnitude of near-surface thermal gradients. The results indicate that the composition of Mercury's surface is heterogeneous on regional scales, and that the near infrared wavelength range provides more discriminative information on the surface composition than the 2-4 [mu]m region, where the solar reflected and thermally emitted radiation contribute approximately equally to the observed flux of these bodies. Author Affiliation: (a) Institutionen for Astronomi och Rymdfysik, Uppsala Universitet, Box 515, SE-751 20 Uppsala, Sweden (b) Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721, USA (c) NASA Ames Research Center/SETI Institute, Moffet Field, CA 94035, USA (d) Physics Department, Susquehanna University, Selinsgrove, PA 17870, USA Article History: Received 5 April 2005; Revised 4 August 2005

We present new optical (0.4-0.65 [micro]m) spectra of Mercury and lunar pure anorthosite locations, obtained quasi-simultaneously with the Nordic Optical Telescope (NOT) in 2002. A comparative study is performed with the model of Lucey et al. (2000, J. Geophys. Res. 105, 20297-20305, and references therein) between iron-poor, mature, pure anorthosite (> 90% plagioclase feldspar) Clementine spectra from the lunar farside and a combined 0.4-1.0 [micro]m mercurian spectrum, obtained with the NOT, calculated for standard photometric geometry. Mercury is located at more extreme locations in the Lucey ratio-reflectance diagrams than any known lunar soil, specifically with respect to the extremely iron-poor mature anorthosites. Though quantitative prediction of FeO and Ti[O.sub.2] abundances cannot be made without a more generally applicable model, we find qualitatively that the abundances of both these oxides must be near zero for Mercury. We utilize the theory of Hapke (2002, Icarus 157, 523-534, and references therein), with realistic photometric parameters, to model laboratory spectra of matured mineral powders and lunar soils, and remotely sensed spectra of lunar anorthosites and Mercury. An important difference between fabricated and natural powders is the high value for the internal scattering parameter necessary to interpret the spectra for the former, and the requirement of rough and non-isotropically scattering surfaces in the modelling of the latter. The mature lunar anorthosite spectra were well modelled with binary mixtures of calcic feldspars and olivines, grain sizes of 25-30 [micro]m and a concentration of submicroscopic metallic iron (SMFe) of 0.12-0.15% in grain coatings. The mercurian spectrum is not possible to interpret from terrestrial mineral powder spectra without introducing an average particle scattering function for the bulk soil that increases in backscattering efficiency with wavelength. The observed spectrum is somewhat better predicted with binary mixture models of feldspars and pyroxenes, than with single-component regoliths consisting of either albite or diopside. Correct spectral reflectance values were predicted with a concentration of 0.1 wt% SMFe in coatings of 15-30 [micro]m sized grains. Since reasonable cosmogonical formation scenarios for Mercury, or meteoritic infall, predict iron concentrations at least this high, we draw the conclusion that the average grain size of Mercury is about a factor of two smaller than for average returned lunar soil samples. The 0.6-2.5 [micro]m spectrum of McCord and Clark (1979, J. Geophys. Res. 178, 745-747) is used to further limit the possible range of mineralogical composition of Mercury. It is found that an intimately mixed and matured 3 : 1 labradorite-to-enstatite regolith composition best matches both the optical and near-infrared spectra, yielding an abundance of ~ 1.2 wt% FeO and ~ 0 wt% Ti[O.sub.2]. Keywords: Mercury; Mineralogy; Regoliths; Surfaces, planets; Spectroscopy

A comparison of the photometric properties of Mercury and the Moon is performed, based on their integral phase curves and diskresolved image data of Mercury obtained with the Swedish Vacuum Solar Telescope. Proper absolute calibration of integral V-band magnitude observations reveals that the near-side of the Moon is 10-15% brighter than average Mercury, and 0-5% brighter for the 'bolometric' wavelength range 400-1000 nm. As shown, this is supported by recent estimates of their geometric albedos. Hapke photometric parameters of their surfaces are derived from identical approaches, allowing a contrasting study between their surface properties to be performed. Compared to the average near-side Moon, Mercury has a slightly lower single-scattering albedo, an opposition surge with smaller width and of marginally smaller amplitude, and a somewhat smoother surface with similar porosity. The width of the lobes of the single-particle scattering function are smaller for Mercury, and the backward scattering anisotropy is stronger. In terms of the double Henyey-Greenstein b-c parameter plot, the scattering properties of an average particle on Mercury is closer to the properties of lunar maria than highlands, indicating a higher density of internal scatterers than that of lunar particles. The photometric roughness of Mercury is well constrained by the recent study of Mallama et al. (2002, Icarus 155, 253-264) to a value of about 8[degrees], suggesting that the surfaces sampled by the highest phase angle observations (Borealis, Susei, and Sobkou Planitia) are lunar mare-like in their textural properties. However, Mariner 10 disk brightness profiles obtained at intermediate phase angles indicate a surface roughness of about twice this value. The photometric parameters of the Moon are more difficult to constrain due to limited phase angle coverage, but the best Hapke fits are provided by rather small surface roughnesses. Better-calibrated, multiple-wavelength observations of the integral and disk-resolved brightnesses of both bodies, and obtained at higher phase angle values in the case of the Moon, are urgently needed to arrive at a more consistent picture of the contrasting light scattering properties of their surfaces. Keywords: Mercury; Moon; Regoliths; Surfaces, planets

Disk-resolved reflectance spectra of the surface of Mercury (longitudes 240-300[degrees]), obtained in the visual (vis) and near-infrared (NIR) spectral region, are presented and analyzed. The observations were made at the 2.6-m Nordic Optical Telescope with the ALFOSC low-resolution spectrograph on 20 and 22 June 1999 in the wavelength range 520-970 nm with a footprint size of 700 km on the mid-disk of Mercury. A method which enables more accurate correction for telluric line absorptions and atmospheric extinction than that applied on previously published vis-NIR spectra of Mercury is introduced. The resulting reflectance spectra are remarkably linear, lack significant absorption features, and have optical slopes comparable to remotely sensed lunar pure anorthosites. The relation between spectral slope and photometric geometry found by Warell (2002, Icarus 156, 313-317) is confirmed and is explained as caused by strongly backscattering particles with embedded submicroscopic metallic iron in a mature regolith. With the theoretical maturation model of Hapke (2001, J. Geophys. Res. 106 (E5), 10039-10073) an abundance of 0.05-0.3 wt% submicroscopic metallic iron in the regolith for silicate grain sizes in the range 10-80 [micro]m is determined, implying a ferrous iron content in mafic minerals intrinsically lower than that of the lunar highlands. A binary crustal composition model with anorthite linearly mixed with pyroxene provides better spectral fits than a pure anorthitic composition. Comparison with mature lunar pure anorthosite spectra yields a confident upper limit to the FeO content of 3 wt% under the assumption that the surfaces are similarly matured, but this figure probably represents a considerable overestimate. The average mercurian regolith does not seem to be substantially more weathered than the most mature lunar highland soils in terms of abundance of submicroscopic metallic iron, indicating that a steady-state maturation level has been reached. However, the strong relation between optical spectral slope and photometric geometry may imply that the majority of regolith particles are more fine-grained than their lunar counterparts and that the regolith is admixed with complex agglutinate weathering products which are more abundant and more transparent than those of the lunar highlands. This is consistent with more energetic impacts and a higher rate of impact melt production in an iron-poor regolith. An observed relation between the spectral slope and latitude provides evidence that the Ostwald ripening process may be operating at equatorial latitudes on Mercury. Keywords: Mercury; Mineralogy; Regoliths; Surfaces, planets; Spectroscopy

Multicolor photometric observations of the 'unknown' hemisphere of Mercury have been performed with the Swedish Vacuum Solar Telescope on La Palma at maximal elongations from the Sun in 1997 and 1998. A set of six interference filters with central wavelengths from 450 to 940 nm were used. Multicolor photometry of Mercury was performed on disk-resolved images of the unknown hemisphere (longitudes 160 [degrees] -340 [degrees]) with a highest resolution of ~200 km (J. Warell and S. Limaye 2001, Planet. Space Sci. 49, 1531-1552). Disk-integrated spectrophotometry shows that (1) the spectrum of Mercury displays a linear slope from 650 to 940 nm, indicating that the average mercurian regolith is considerably more mature than relatively immature pure anorthosite regions on the Moon; (2) there is negative evidence for the presence of the putative 1-[micro]m absorption feature near 940 nm due to the presence of ferrous iron ([Fe.sup.2+]) in pyroxenes; and (3) no effect of phase reddening of the integrated disk is observed between phase angles of 63 [degrees] and 84 [degrees]. For the first time, disk-resolved spectrophotometry of Mercury's surface has been obtained, from which it is inferred that (4) the scattering properties of Mercury's regolith are more homogeneous than for the Moon and that there is no clear relation between reflectance and chemical properties at spatial scales of ~300 km on the unknown hemisphere and (5) there exists an inverse relation of spectral slope with emission angle which is larger for Mercury than for the Moon, indicating that the average mercurian regolith is more backscattering and that this effect increases with wavelength. Finally, from filter ratio images of Mercury's disk it is found that (6) no color variations larger that 2% with respect to the surroundings are detected at a spatial resolution of ~300 km. Key Words: Mercury; regoliths; planetary surfaces; terrestrial planets; spectrophotometry.