Mercury's spectrophotometric properties: Update from the Mercury Dual Imaging System observations during the third MESSENGER flyby

Presented here are analyses of the photometric measurements acquired by the imaging system on the MESSENGER spacecraft during its three flybys of Mercury, in particular the dedicated sequence of photometric measurements obtained during the third flyby. A concise, analytical approach is adopted for characterizing the effects of scattered light on the images. This approach works well for wavelengths shorter than 700 nm but breaks down at the longer wavelengths where the scattering behavior of the imaging system is more complex. Broadband spectral properties are commensurate with ground-based observations for spectra acquired at phase angles less than 110°; photometric corrections to a common illumination and viewing geometry provide consistent results for those phase angles. No phase reddening is apparent in the image-derived spectra. A bolometric albedo of 0.081 is derived over the wavelength range of the imaging system.     

Photometry of Mercury from SOHO/LASCO and Earth: The Phase Function from 2 to 170°

CCD observations of Mercury were obtained with the large angle spectrometric coronograph (LASCO) on the solar and heliospheric observatory spacecraft, near superior and inferior solar conjunctions. Whole disk photometry was extracted from the orange and blue filter images and transformed to V magnitudes on the UBV system. The LASCO data were combined with ground-based, V-filter photometry acquired at larger elongation angles. The resulting photometric phase function covers the greatest span of angles to date and is the first wide-range function to be obtained since the era of visual observation. We analyzed the data using a polynomial fit and a Hapke function fit, and derived the following photometric results. Mercury's fully lit brightness, adjusted to a distance of 1.0 AU from the Sun and observer, was found to be V=−0.694(±0.030), which is more luminous than previously measured. The corresponding geometric albedo is 0.142(±0.005). The phase integral is 0.478(±0.005) and resulting spherical albedo is 0.068(±0.003). The upper limit of a possible rotational brightness variation is about 0.05 magnitude. Mercury's brightness surges by more than 40% between phase angles 10 and 2°, while the illuminated fraction of the disk increases by less than 1%. A set of coefficients for Hapke's function that fit most of the phase curve includes h=0.065±0.002 indicating that Mercury and the Moon have similar regolith compaction states and particle size distributions, and θ-bar=16°±1° implying a macroscopically smoother surface than the Moon. However, we found other solutions that fit the observations nearly as well with significantly smaller and larger values of h, and with values of θ-bar around 25°. The wide range for θ-bar is due to the inability of the model to fit the photometry obtained at large phase angles.     

A comparison of the ultraviolet to near-infrared spectral properties of Mercury and the Moon as observed by MESSENGER

Measurements of the disk-integrated reflectance spectrum of Mercury and the Moon have been obtained by the MESSENGER spacecraft. A comparison of spectra from the two bodies, spanning the wavelength range 220-1450 nm, shows that the absolute reflectance of Mercury is lower than that of the nearside waxing Moon at the same phase angle with a spectral slope that is less steep at visible and near-infrared wavelengths. We interpret these results and the lack of an absorption feature at a wavelength near 1000 nm as evidence for a Mercury surface composition that is low in ferrous iron within silicates but is higher in the globally averaged abundance of spectrally neutral opaque minerals than the Moon. Similar conclusions have been reached by recent investigations based on observations from both MESSENGER and Mariner 10. There is weak evidence for a phase-reddening effect in Mercury that is slightly larger in magnitude than for the lunar nearside. An apparent absorption in the middle-ultraviolet wavelength range of the Mercury spectrum detected from the first MESSENGER flyby of Mercury is found to persist in subsequent observations from the second flyby. The current model of space weathering on the Moon, which also presumably applies to Mercury, does not provide an explanation for the presence of this ultraviolet absorption.     

Photometric analysis of the nucleus of Comet 81P/Wild 2 from Stardust images

The disk-resolved flyby images of the nucleus of Comet 81P/Wild 2 collected by Stardust are used to perform a detailed study of the photometric properties of this cometary nucleus. A disk-integrated phase function from phase angle 11° to about 100° is measured and modeled. A phase slope of 0.0513 ± 0.0002 mag/deg is found, with a V-band absolute magnitude of 16.29 ± 0.02. Hapke’s photometric model yields a single-scattering albedo of 0.034, an asymmetry factor of phase function −0.53, a geometric albedo 0.059, and a V-band absolute magnitude of 16.03 ± 0.07. Disk-resolved photometric modeling from both the Hapke model and the Minnaert model results in 11% model RMS, indicating small photometric variations. The roughness parameter is modeled to be 27 ± 5° from limb-darkening profile. The modeled single-scattering albedo and asymmetry factor of the phase function are 0.038 ± 0.004 and −0.52 ± 0.04, respectively, consistent with those from disk-integrated phase function. The bulk photometric properties of the nucleus of Wild 2 are comparable with those of other cometary nuclei. The photometric variations on the surface of the nucleus of Wild 2 are at a level of or smaller than 15%, much smaller than those on the nucleus of Comet 19P/Borrelly and comparable or smaller than those on the nucleus of Comet 9P/Tempel 1. The similar photometric parameters of the nuclei of Wild 2, Tempel 1, and the non-source areas of fan jets on Borrelly may reflect the typical photometric properties of the weakly active surfaces on cometary nuclei.     

Whole-disk spectrophotometric properties of Mercury: Synthesis of MESSENGER and ground-based observations

Disk-integrated and disk-resolved measurements of Mercury’s surface obtained by both the Mercury Dual Imaging System (MDIS) and the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft were analyzed and compared with previous ground-based observations of Mercury at 11 wavelengths. The spectra show no definitive absorption features and display a red spectral slope (increasing reflectance with increasing wavelength) typical of space-weathered rocky surfaces. The MDIS spectra show evidence of phase reddening, which is not observed in the MASCS spectra. The MDIS spectra are commensurate with ground-based observations to within 10%, whereas the MASCS spectra display greater discrepancies with ground-based observations at near-infrared wavelengths. The derived photometric calibrations provide corrections within 10% for observations taken at phase angles less than ∼100°. The derived photometric properties are indicative of a more compact regolith than that of the lunar surface or of average S-type asteroids. The photometric roughness of the surface is also much smoother than the Moon’s. The calculated geometric albedo (reflectance at zero phase) is higher than lunar values. The lower reflectance of immature units on Mercury compared with immature units on the Moon, in conjunction with the higher geometric albedo, is indicative of more complicated grain structures within Mercury’s regolith.     

Photometric modeling of Asteroid 5535 Annefrank from Stardust observations

In this paper the Stardust disk-integrated phase curve at phase 47.2-134.6° of the Asteroid 5535 Annefrank, combined with groundbased observations (at phase 2.3-18.3°), are fit with Hapke’s photometric model. We confirm Newburn et al.’s (Newburn, R.L. et al. [2003]. J. Geophys. Res. 108 (E11), 5117. doi:10.1029/2003JE002106) observation that Annefrank exhibits a steep phase curve. This manifests itself in an unusually high fit surface roughness parameter of 49°. The single particle scattering albedo is 0.62, also high for an S-asteroid, while the fit phase function is more forward scattering than the typical S-asteroid being nearly isotropic with an asymmetry parameter of −0.09. The fit opposition surge width (h = 0.015) is typical of S-asteroids. However these fits assume a spherical shape to the asteroid. Li et al. (Li, J., A’Hearn, M.F., McFadden, L.A. [2004]. Icarus, 415-431) have shown that this assumption may lead to significant errors particularly at high phase angles leading to higher modeled single particle scattering albedos, macroscopic roughnesses and more forward scattering phase functions than actually exhibited. Our results confirm this finding—fitting only the data below 90° phase yields lower particle albedos (0.41) and roughnesses (20°) and more backscattering particles (−0.19) than the fit including the high phase angle data. Overall Annefrank appears to be on the bright side but otherwise is typical for an S-type asteroid suggesting that it may be a recent collisional fragment with a relatively immature surface which has had relatively little time to be weathered.     

Cassini/VIMS observation of an Io post-eclipse brightening event

During the Cassini-Jupiter flyby, VIMS observed Io at different phase angles, both in full sunlight and in eclipse. By using the sunlight measurements, we were able to produce phase curves in the visual through all the near infrared wavelengths covered by the VIMS instrument (0.85-5.1 μm). The phase angle spanned from ∼2° to ∼120°. The measurements, done just after Io emerged from Jupiter's shadow, show an increase of about 15% in Io's reflectance with respect to what would be predicted by the phase curve. This behavior is observed at wavelengths >1.2 μm. Moreover, just after emergence from eclipse an increase of about 25% is observed in the depth of SO₂ frost bands at 4.07 and 4.35 μm. At 0.879<λ<1.04 μm the brightening is 10-24%. Below λ=0.879 μm the brightening, if present, should be less than the precision of our measurements (∼5%). Apparently, these observations are not explained neither by a diverse spatial distribution of SO₂ on the Io' surface nor by atmospheric SO₂ condensation on the surface during the eclipse.     

Europa's opposition surge in the near-infrared: interpreting disk-integrated observations by Cassini VIMS

Near-infrared observations of Europa's disk-integrated opposition surge by Cassini VIMS, first published in Fig. 4 of Brown et al. (2003, Icarus, 164, 461), have now been modeled with the commonly used Hapke photometric function. The VIMS data set emphasizes observations at 16 solar phase angles from 0.4° to 0.6°—the first time the <1° phase “heart” of Europa's opposition surge has been observed this well in the near-IR. This data set also provides a unique opportunity to examine how the surge is affected by changes in wavelength and albedo: at VIMS wavelengths of 0.91, 1.73, and 2.25 μm, the geometric albedo of Europa is 0.81, 0.33, and 0.18, respectively. Despite this factor-of-four albedo range, however, the slope of Europa's phase curve at <1° phase is similar at all three wavelengths (to within the error bars) and this common slope is similar to the phase coefficient seen in visible-light observations of Europa. The two components of the opposition surge—involving different models of the physical cause of the surge—are the Shadow Hiding Opposition Effect (SHOE) and the Coherent Backscatter Opposition Effect (CBOE). Because of sparse VIMS phase coverage, it is not possible to constrain all the surge parameters at once in a Hapke function that has both SHOE and CBOE; accordingly, we performed separate Hapke fits for SHOE-only and CBOE-only surges. At 2.25 μm, where VIMS data are somewhat noisy, both types of surges can mimic the slope of the VIMS phase curve at <1° phase. At 0.91 and 1.73 μm, however—where VIMS data are “cleaner”—CBOE does a noticeably poorer job than SHOE of matching the VIMS phase coefficient at <1° phase; in particular, the best CBOE fit insists on having a steeper phase-curve slope than the data. This discrepancy suggests that Europa's near-IR opposition surge cannot be explained by CBOE alone and must have a significant SHOE component, even at wavelengths where Europa is bright.     

Properties of the Hermean Regolith: II. Disk-Resolved Multicolor Photometry and Color Variations of the “Unknown” Hemisphere

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°-340°) 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-μm absorption feature near 940 nm due to the presence of ferrous iron (Fe²⁺) in pyroxenes; and (3) no effect of phase reddening of the integrated disk is observed between phase angles of 63° and 84°.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.     

Deep Impact photometry of Comet 9P/Tempel 1

The photometric properties of the nucleus of Comet 9P/Tempel 1 are studied from the disk-resolved color images obtained by Deep Impact (DI). Comet Tempel 1 has typical photometric properties for comets and dark asteroids. The disk-integrated spectrum of the nucleus of Tempel 1 between 309 and 950 nm is linear without any features at the spectral resolution of the filtered images. At V-band, the red slope of the nucleus is 12.5±1% per 100 nm at 63° phase angle, translating to B-V=0.84±0.01, V-R=0.50±0.01, and R-I=0.49±0.02. No phase reddening is confirmed. The phase function of the nucleus of Tempel 1 is constructed from DI images and earlier ground-based observations found from the literature. The phase coefficient is determined to be β=0.046±0.007 mag/deg between 4° and 117° phase angle. Hapke's theoretical scattering model was used to model the photometric properties of this comet. Assuming a single Henyey-Greenstein function for the single-particle phase function, the asymmetry factor of Tempel 1 was fitted to be g=−0.49±0.02, and the corresponding single-scattering albedo (SSA) was modeled to be 0.039±0.005 at 550 nm wavelength. The SSA spectrum shows a similar linear slope to that of the disk-integrated spectrum. The roughness parameter is found to be 16°±8°, and independent of wavelength. The Minnaert k parameter is modeled to be 0.680±0.014. The photometric variations on Tempel 1 are relatively small compared to other comets and asteroids, with a ∼20% full width at half maximum of albedo variation histogram, and ∼3% for color. Roughness variations are evident in one small area, with a roughness parameter about twice the average and appearing to correlate with the complex morphological texture seen in high-resolution images.     

Properties of the Hermean regolith: IV. Photometric parameters of Mercury and the Moon contrasted with Hapke modelling

A comparison of the photometric properties of Mercury and the Moon is performed, based on their integral phase curves and disk-resolved 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°, 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.     

Global photometric properties of Asteroid (4) Vesta observed with Dawn Framing Camera

Dawn spacecraft orbited Vesta for more than one year and collected a huge volume of multispectral, high-resolution data in the visible wavelengths with the Framing Camera. We present a detailed disk-integrated and disk-resolved photometric analysis using the Framing Camera images with the Minnaert model and the Hapke model, and report our results about the global photometric properties of Vesta. The photometric properties of Vesta show weak or no dependence on wavelengths, except for the albedo. At 554 nm, the global average geometric albedo of Vesta is 0.38 ± 0.04, and the Bond albedo range is 0.20 ± 0.02. The bolometric Bond albedo is 0.18 ± 0.01. The phase function of Vesta is similar to those of S-type asteroids. Vesta’s surface shows a single-peaked albedo distribution with a full-width-half-max ∼17% relative to the global average. This width is much smaller than the full range of albedos (from ∼0.55× to >2× global average) in localized bright and dark areas of a few tens of km in sizes, and is probably a consequence of significant regolith mixing on the global scale. Rheasilvia basin is ∼10% brighter than the global average. The phase reddening of Vesta measured from Dawn Framing Camera images is comparable or slightly stronger than that of Eros as measured by the Near Earth Asteroid Rendezvous mission, but weaker than previous measurements based on ground-based observations of Vesta and laboratory measurements of HED meteorites. The photometric behaviors of Vesta are best described by the Hapke model and the Akimov disk-function, when compared with the Minnaert model, Lommel–Seeliger model, and Lommel–Seeliger–Lambertian model. The traditional approach for photometric correction is validated for Vesta for >99% of its surface where reflectance is within ±30% of global average.     

Lunar photometric properties at wavelengths 0.5-1.6 μm acquired by SELENE Spectral Profiler and their dependency on local albedo and latitudinal zones

The lunar photometric function, which describes the dependency of the observed radiance on the observation geometry, is used for photometric correction of lunar visible/near-infrared data. A precise photometric correction parameter set is crucial for many applications including mineral identification and reflectance map mosaics. We present, for the first time, spectrally continuous photometric correction parameters for both sides of the Moon for wavelengths in the range 0.5-1.6 μm and solar phase angles between 5° and 85°, derived from Kaguya (SELENE) Spectral Profiler (SP) data. Since the measured radiance also depends on the surface albedo, we developed a statistical method for selecting areas with relatively uniform albedos from a nearly 7000-orbit SP data set. Using the selected data set, we obtained empirical photometric correction parameter sets for three albedo groups (high, medium, and low). We did this because the photometric function depends on the albedo, especially at phase angles below about 20° for which the shadow hiding opposition effect is appreciable. We determined the parameters in 160 bands and discovered a small variation in the opposition effect due to the albedo variation of mafic mineral absorption. The consistency of the photometric correction was checked by comparing observations made at different times of the same area on the lunar surface. Variations in the spectra obtained were lower than 2%, except for the large phase angle data in mare. Lastly, we developed a correction method for low solar elevation data, which is required for high latitude regions. By investigating low solar elevation data, we introduced an additional correction method. We used the new photometric correction to generate a 1° mesh global lunar reflectance map cube in a wavelength range of 0.5-1.6 μm. Surprisingly, these maps reveal that high latitude (?75°) regions in both the north and south have much lower spectral continuum slopes (color ratio r_1547.7nm/r_752.8nm ? 1.8) than the low and medium latitude regions, which implies lower degrees of space weathering.     

The structure of mercury’s regolith from photometric data

The integral photometric properties of Mercury and the Moon were studied, and the similarities in the reflectance characteristics of small surface areas determined by centimeter-sized irregularities were found. The relations obtained were used to pattern the surface of the Hermean soil on a specified scale. In a first approximation, the resulting image shows the character of the photometric relief of Mercury and the fine-grained structure of the surface in a range of phase angles, where the shadow function of centimeter-sized irregularities most affects the brightness of the mantle material. Probable variations of the photometric-relief types were revealed in processing the images selected from a series obtained from the Mariner 10 flyby. According to the data analysis, three main types of photometric relief were distinguished, which correspond to different morphological types of the Hermean surface formations. The relationship between the morphological types and the properties of the surface structure of the Hermean mantle material was revealed via morphological studies.Translated from Astronomicheskii Vestnik, Vol. 38, No. 6, 2004, pp. 504–512.Original Russian Text Copyright © 2004 by Molodensky     

Photometric relief of the previously uninvestigated surface of mercury

The results of investigations of the surface relief of Mercury with a classical photometric method are reported. A subject of the photometric method is the intensity of light reflected by the surface of the planet. The main data for the photometric study are the high-resolution images of Mercury received from the MESSENGER space station during its first flyby over Mercury. The images of the surface of Mercury were downloaded from the NASA web-site (http://messenger.jhuapl.edu) and converted to a digital form for photometric measurements. The reflectance characteristics of the surface were calculated according to the model of a three-dimensional scattering phase function (Shevchenko, 1979; 2004a; 2006). From the photometric processing of the space-borne images, the reflectance of four morphologic types of the surface structure of Mercury was determined. With the Hapke model of the bidirectional reflectance (Hapke, 2001), the structural inhomogeneity of the regions of the morphologic forms with centimeter-sized rough-ness was estimated.     

Opposition polarimetry and photometry of S- and E-type asteroids

The first results of the observational program devoted to simultaneous investigation of asteroid polarimetric and photometric opposition phenomena are presented. UBVRI polarimetric and V-band photometric observations of the S-type Asteroid 20 Massalia and the E-type Asteroids 214 Aschera and 620 Drakonia were carried out in 1996-1999 down to phase angles of 0.08°, 0.7°, and 1.2°, correspondingly. The S-type Asteroid 20 Massalia is characterized by the pronounced brightness opposition surge with an amplitude larger than that observed for the E-type asteroids. A sharp peak of negative polarization at small phase angles was not observed for this asteroid. The value of polarization degree at phase angle α<1° is less than 0.5% for both S and E types. The negative polarization branches of S and especially E-asteroids have an asymmetrical shape. The phase angle at which the polarization minimum occurs is close to the angle at which non-linear increase begins in the asteroid magnitude phase curves. A relation of the observed effects to the mechanism of coherent backscattering is discussed.     

The icy Galilean satellites: ultraviolet phase curve analysis

Ultraviolet disk-integrated solar phase curves of the icy galilean satellites Europa, Ganymede, and Callisto are presented, using combined data sets from the International Ultraviolet Explorer (IUE), Hubble Space Telescope (HST), and the Galileo Ultraviolet Spectrometer. Global, disk-integrated solar phase curves for all three satellites, in addition to disk-integrated solar phase curves for Europa's leading, trailing, jovian, and anti-jovian hemispheres, are modeled using Hapke's equations for 7 broadband UV wavelengths between 260 and 320 nm. The sparse coverage in solar phase angle, particularly for Ganymede and Callisto, and the noise in the data sets poorly constrain some of the photometric parameter values in the model. However, the results are sufficient for forming a preliminary relationship between the effects of particle bombardment on icy surfaces and photometric scattering properties at ultraviolet wavelengths. Callisto exhibits a large UV opposition surge and a surface comprised of relatively low-backward scattering particles. Europa's surface displays a dichotomy between the jovian and anti-jovian hemispheres (the anti-jovian hemisphere is more backward scattering), while a less pronounced hemispherical variation was detected between the leading and trailing hemispheres. Europa's surface, with the exception of the trailing hemisphere region, appears to have become less backscattering between the late-1970s-early-1980s and the mid-1990s. These results are commensurate with the bombardment history of these surfaces by magnetospheric charged particles.     

Observations with the Visual and Infrared Mapping Spectrometer (VIMS) during Cassini's flyby of Jupiter

The Cassini Visual and Infrared Mapping Spectrometer (VIMS) is an imaging spectrometer covering the wavelength range 0.3-5.2 μm in 352 spectral channels, with a nominal instantaneous field of view of 0.5 mrad. The Cassini flyby of Jupiter represented a unique opportunity to accomplish two important goals: scientific observations of the jovian system and functional tests of the VIMS instrument under conditions similar to those expected to obtain during Cassini's 4-year tour of the saturnian system. Results acquired over a complete range of visual to near-infrared wavelengths from 0.3 to 5.2 μm are presented. First detections include methane fluorescence on Jupiter, a surprisingly high opposition surge on Europa, the first visual-near-IR spectra of Himalia and Jupiter's optically-thin ring system, and the first near-infrared observations of the rings over an extensive range of phase angles (0-120°). Similarities in the center-to-limb profiles of H⁺₃ and CH₄ emissions indicate that the H⁺₃ ionospheric density is solar-controlled outside of the auroral regions. The existence of jovian NH₃ absorption at 0.93 μm is confirmed. Himalia has a slightly reddish spectrum, an apparent absorption near 3 μm, and a geometric albedo of 0.06±0.01 at 2.2 μm (assuming an 85-km radius). If the 3-μm feature in Himalia's spectrum is eventually confirmed, it would be suggestive of the presence of water in some form, either free, bound, or incorporated in layer-lattice silicates. Finally, a mean ring-particle radius of 10 μm is found to be consistent with Mie-scattering models fit to VIMS near-infrared observations acquired over 0-120° phase angle.     

The opposition surge of Enceladus: HST observations 338-1022 nm

Hubble Space Telescope (HST) Wide-Field Planetary Camera (WFPC2) observations at phase angles in the range α=0.26°-6.4° obtained at every opposition and near quadrature between October 1996 and December 2002 reveal the opposition effect of Enceladus. We present a photometric analysis of nearly 200 images obtained through the five broadband UVBRI filters (F336W, F439W, F555W, F675W, and F814W) and the F785LP and F1042M filters from which we generate mutually consistent solar and rotational phase curves. Our solar phase curves reveal a dramatic, sharp increase in the albedo (from 0.11 mag in the F675W filter to 0.17 mag in the F785LP filter) as phase angles decrease from 2° to 0.26°. A slight opposition effect is evident in data from the F1042M filter (λ_eff=1022 nm); however, the smallest phase angle currently available for observations from this filter is α=0.58°. With the addition of data from the F255W filter we demonstrate the wavelength dependence of the albedo of the trailing hemisphere from 275 to 1022 nm. Our rotation curves show that the trailing hemisphere is ∼0.06 mag brighter than the leading when observed at wavelengths between 338 and 868 nm and 0.11 mag brighter than the leading at 1022 nm. We have supplemented the phase curve from the F439W filter (λ_eff=434 nm) with Voyager clear filter (λ_eff=480 nm) observations made at larger phase angles (α=13°-43°) to produce a phase curve with the most extensive phase angle coverage possible to date. This newly expanded range of phase angles enhances the ability of the Hapke photometric model (Hapke B., 2002, Icarus 157, 523-534) to relate physical characteristics of the surface of Enceladus to the manner in which incident light is reflected from it. We present Hapke 2002 model fits to solar phase curves from each UVBRI filter as well as from the F785LP and F1042M filters. Geometric albedos derived from these model fits range from p=0.92±0.01 at 1022 nm to p=1.41±0.03 at 549 nm, necessitating an increase of about 20% from previously derived values. Our Hapke fits demonstrate that the opposition surge of Enceladus is best described by a model which combines both moderate shadow-hiding and narrow coherent backscattering components.     

Laboratory Study of the Bidirectional Reflectance of Powdered Surfaces: On the Asymmetry Parameter of Asteroid Photometric Data

We performed laboratory experiments on the bidirectional reflectance of powdered surfaces of dunite, graphite, and Allende (CV3) and Gao (H5) meteorites. The particle size of each sample varied from tens to hundreds of micrometers. The absolute bidirectional reflectance was determined for two ranges of phase angle: from 2° to 80° at 0° angle of incidence and from 2° to 155° at 75° angle of incidence. The phase angle was incremented every 1° in between 2° and 5° and every 10° between 10° and 150°. We determined the Hapke parameters and found that the values of the asymmetry parameter retrieved for most of the samples were positive when the coverage of the phase angle was wide, 2° to 155°, although the values derived from remote sensing instruments on asteroid flyby missions have been negative or nearly zero. Among our sample surfaces, only those of graphite, Allende, and Gao with particle sizes of hundreds micrometers show negative or nearly zero values. The single-scattering albedos determined for the Gao samples are comparable to the values of S-class asteroids, while those for the Allende samples are much larger than those of C-class asteroids.