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.     

Properties of the Hermean regolith: I. Global regolith albedo variation at 200 km scale from multicolor CCD imaging

Multicolor imaging of Mercury has been performed with the 0.5 m Swedish Vacuum Solar Telescope (SVST) on La Palma at five elongations from 1995 to 1999, resulting in a global Minnaert normalized map of the surface at 200 km resolution. Short exposure CCD imaging has been performed in the optical and near-infrared with broad- and intermediate band filters at wavelengths from 550 to 940 nm. Positions for 86 and morphological parameters for 63 bright albedo features on the Hermean surface have been determined. The distribution of bright albedo features is shown to be spatially uniform on the well known (i.e., observed by Mariner 10) and poorly known hemispheres, as well as for the global surface. The number densities of bright albedo features on the two hemispheres are very similar. This indicates that the late evolutionary history of the Hermean regolith has not varied on regional to global scales in terms of impacts generating bright ray craters, constituting approximately 70% of the detected bright albedo features. The locations of bright albedo features correspond well to those determined from nominal resolution and smeared (to the approximate resolution of the SVST data) Mariner 10 maps. Feature parameters (radius, center intensity and intensity gradient) have been determined and correlated with the geologic nature of a subset of observed features imaged by the Mariner 10 Vidicon camera. No difference in feature properties is apparent between the poorly known and well known hemispheres. Based on a comparitive study of Mariner 10 image data, ray craters tend to have higher center intensities and smaller intensity gradients than bright albedo features which are not ray craters. It is however concluded that it is not possible to uniquely determine the geologic nature of features with a high statistical significance, based on their morphological parameters at 200 km resolution. We do not find any general correlation between the locations of radar-bright and optically bright or dark albedo features. The surface contrast decreases from 35% to 25% over the wavelength range 550–940 nm. The range of feature contrasts is similar for all surface regions, except for the ray crater Kuiper, whose contrast to the mean surrounding surface is 50% at a wavelength of 750 nm. Kuiper is an extreme albedo feature also in terms of its center intensity and slope. The mean value of the Minnaert slope parameter for the global surface is determined to 0.76±0.10. A measured constant value of the Minnaert slope with wavelength indicates that the spectral slope for typical Hermean regolith should be linear over the wavelength range 550–940 nm.     

Constraining albedo,diameter and composition of near-Earth asteroids via near-infrared spectroscopy

We present a method to constrain the albedo and diameters of near-Earth asteroids (NEAs) based on thermal flux in their near-infrared spectra (0.7–2.5 μm) using the Standard Thermal Model. Near-infrared spectra obtained with the SpeX instrument on NASA Infrared Telescope Facility are used to estimate the albedo and diameters of 12 NEAs (1992 JE, 1992 UY4, 1999 JD6, 2004 XP14, 2005 YY93, 2007 DS84, 2005 AD13, 2005 WJ56, 1999 JM8, 2005 RC34, 2003 YE45, and 2008 QS11). Albedo estimates were compared with average albedo for various taxonomic classes outlined by Thomas et al. (Thomas, C.A. et al. [2011]. Astron. J. 142(3)) and are consistent with their results. Spectral band parameters, like band centers, are derived and compared to spectra of laboratory mineral mixtures and meteorites to constrain their composition and possible meteorite analogs. Based on our study we estimate the albedos and diameters of these NEAs and compare them with those obtained by other techniques such as ground-based mid-infrared, Spitzer thermal infrared and Arecibo radar. Our results are broadly consistent with the results from other direct methods like radar. Determining the compositions of low albedo asteroids is a challenge due to the lack of deep silicate absorption features. However, based on weak absorption features and albedo, we suggest possible meteorite analogs for these NEAs, which include black chondrites, CM2 carbonaceous chondrites and enstatite achondrites. We did not find any specific trends in albedo and composition among the asteroids we observed.     

A Huge Basin in the Unknown Portion of Mercury in the 250°–290° W Longitude Range

A basin with an interior about 1000 km across and an outer rim about 2000 km in diameter has been found on the side of Mercury that remained unknown after theMariner 10imaging in 1973–1975. In its size, which is as much as 1/2 of the Mercury's diameter, this formation is one of the largest basins on the terrestrial planets. The presence of other large features of global dimension on this side of the planet suggests their possible asymmetric allocation on Mercury, similar to that on the Moon. New methods of ground-based astronomical observations including data processing with a special software package have been used.     

Mercury's exosphere: A possible source for Na

Many ground-based observations of Na in Mercury's surface-bounded exosphere have been made and continued to be made in an effort to understand the sources, sinks, and distribution of Na around Mercury. These time consuming and costly efforts are made to better understand the physical processes on and around Mercury. A big step would be to discover an actual source of the Na from Mercury's crust because it is already known that meteorites and comets provide Na to the exosphere through impact. We provide ground-based CCD imagery obtained with small ground-based telescopes that show bright albedo features at locations coincident with enhanced Na emissions in Mercury's exosphere. We suggest these locations are sources for Na. We also provide a mechanism to test this hypothesis using in situ observations by instruments on the MESSENGER spacecraft during the three fly bys of Mercury that will occur in 2008 and 2009, and during the orbital mission which begins in 2011. It is necessary to prove that Na is delivered to the exosphere from one or more crustal source regions before exospheric Na can be used as a measure of the volatile content of Mercury used to infer formation and evolution from the primitive solar nebula. The same applies to other elements such as K which is known to be in Mercury's exosphere and S which is postulated to be present. We expound on the impact that the discovery of one or more source regions from Mercury's crust would have on our ability to discern between the three leading models of Mercury's formation and crustal evolution.     

The planet Mercury: Synthesis of resolved images of unknown part in the longitude range 250-290°W

Plans to send orbiter missions to Mercury (e.g., NASA's Messenger and ESA's BepiColombo) have prompted renewed efforts to investigate the surface of Mercury using ground-based remote sensing. While the highest resolution instrumentation optical telescopes (e.g. HST) cannot be used at small angular distances (<45°) from the Sun (Mercury's elongation never exceeds 28° seen from Earth), advanced ground-based astronomical techniques and modern processing software can be used to construct resolved images of the poorly known part of Mercury. Our observations of the planet presented here were carried out mainly in April and May, 2002, at evening elongation of the planet, at the Skinakas astrophysical observatory of Heraklion University (Crete, Greece). A synthesis of the acquired images of the hemisphere of Mercury, which was not observed by the Mariner 10 mission (1974-1975), is presented. A double rim basin with an internal diameter of about 1000 km and an external rim about 2000 km is suggested by the data. We present the observational method, the data analysis approach, and the resulting images.     

地基雷达技术及其在太阳系天体探测中的应用

人类利用雷达波进行天文研究距今已有40多年历史了.它是一种发射雷达波到目标天体,通过分析其回波特性来进行天文探测的技术.该文从地基雷达在太阳系天体探测中的应用出发,分析了地基雷达相比其他探测手段的优点;介绍了地基雷达的基本工作原理;给出了近年来地基雷达的发展情况和探测成果;最后从现有条件出发,探讨了我国开展地基雷达探测的设想.     

Resolved images of an unknown sector on the surface of Mercury

We took electronic photographs of Mercury on the side of the planet that was not photographed from the Mariner-10 spacecraft in 1973–1975 by the millisecond-exposure method in ground-based observations. Based on these photographs, we synthesized resolved images of the surface of unknown regions of the planet. The capabilities of the method are limited by the small angular size of the planetary disk (only 7.3 arcsec at average quadrature), specific difficulties of Mercury’s ground-based observations, their very limited duration, and the laboriousness of the subsequent computer-aided observational data processing. The millisecond-exposure method is complex, but a sufficient number of primary electronic photographs can be taken under good seeing conditions for the subsequent synthesis of Mercurian images with a resolution of no worse than the diffraction limit. A giant basin about 2000 km in diameter and other large structures are distinguished in the synthesized images of the planet. In the regions where radar data are available, these structures can be identified with previously found ones. In some measure, the synthesized images allow the relief of the longitude sector 210°–290° W to be reconstructed on Mercury. It can be asserted with caution that the large relief features are distributed asymmetrically over the surface of Mercury, much as observed on other terrestrial planets, the Moon, and many satellites of giant planets.     

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.     

Properties and Peculiarities of Mercury's Regolith. Disk-integrated Polarimetry in 2000–2002

On the basis of the data from ground-based polarimetric, photometric, and other observations, as well as from space measurements (Mariner 10), we survey the investigations of the properties and peculiarities of Mercury's regolith in detail. We also present the results of our own observations performed during three apparitions of the planet in 2000–2002. An analysis of the published data points to essentially more intensive maturation processes in the Hermean surface regolith compared to that on the lunar surface. In addition, the orbital characteristics of Mercury allow us to suppose that the intensity of its regolith maturation and, therefore, the optical properties of its surface can noticeably depend on the planetocentric longitude. Polarimetric observations of Mercury's surface (the planetocentric longitude range was 265°–330°) carried out in 2000–2002 with a 70-cm reflector actually detected a polarization degree varying with an amplitude of about 1.5%. To ascertain the nature of these variations, additional observations of Mercury in a maximally wide range of planetocentric longitudes of the viewed surface are required.     

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.     

The apparent lack of lunar-like swirls on Mercury: Implications for the formation of lunar swirls and for the agent of space weathering

Images returned by the MESSENGER spacecraft from the Mercury flybys have been examined to search for anomalous high-albedo markings similar to lunar swirls. Several features suggested to be swirls on the basis of Mariner 10 imaging (in the craters Handel and Lermontov) are seen in higher-resolution MESSENGER images to lack the characteristic morphology of lunar swirls. Although antipodes of large impact basins on the Moon are correlated with swirls, the antipodes of the large impact basins on Mercury appear to lack unusual albedo markings. The antipodes of Mercury’s Rembrandt, Beethoven, and Tolstoj basins do not have surface textures similar to the “hilly and lineated” terrain found at the Caloris antipode, possibly because these three impacts were too small to produce obvious surface disturbances at their antipodes. Mercury does have a class of unusual high-reflectance features, the bright crater-floor deposits (BCFDs). However, the BCFDs are spectral outliers, not simply optically immature material, which implies the presence of material with an unusual composition or physical state. The BCFDs are thus not analogs to the lunar swirls. We suggest that the lack of lunar-type swirls on Mercury supports models for the formation of lunar swirls that invoke interaction between the solar wind and crustal magnetic anomalies (i.e., the solar-wind standoff model and the electrostatic dust-transport model) rather than those models of swirl formation that relate to cometary impact phenomena. If the solar-wind standoff hypothesis for lunar swirls is correct, it implies that the primary agent responsible for the optical effects of space weathering on the Moon is solar-wind ion bombardment rather than micrometeoroid impact.     

New Cassini RADAR results for Saturn’s icy satellites

Cassini radar tracks on Saturn’s icy satellites through the end of the Prime Mission in 2008 have increased the number of radar albedo estimates from 10 (Ostro et al., 2006) to 73. The measurements sample diverse subradar locations (and for Dione, Rhea, and Iapetus almost always use beamwidths less than half the target angular diameters), thereby constraining the satellites’ global radar albedo distributions. The echoes result predominantly from volume scattering, and their strength is thus strongly sensitive to ice purity and regolith maturity. The combination of the Cassini data set and Arecibo 13-cm observations of Enceladus, Tethys, Dione, Rhea (Black et al., 2007), and Iapetus (Black et al., 2004) discloses an unexpectedly complex pattern of 13-to-2-cm wavelength dependence. The 13-cm albedos are generally smaller than 2-cm albedos and lack the correlation seen between 2-cm and optical geometric albedos. Enceladus and Iapetus are the most interesting cases. We infer from hemispheric albedo variations that the E-ring has a prominent effect on the 13-cm radar “lightcurve”. The uppermost trailing-side regolith is too fresh for meteoroid bombardment to have developed larger-scale heterogeneities that would be necessary to elevate the 13-cm radar albedo, whereas all of Enceladus is clean and mature enough for the 2-cm albedo to be uniformly high. For, Iapetus, the 2-cm albedo is strongly correlated with optical albedo: low for the optically dark, leading-side material and high for the optically bright, trailing-side material. However, Iapetus’ 13-cm albedo values show no significant albedo dichotomy and are several times lower than 2-cm values, being indistinguishable from the weighted mean of 13-cm albedos for main-belt asteroids, 0.15 ± 0.10. The leading side’s optically dark contaminant must be present to depths of at least one to several decimeters, so 2-cm albedos can mimic the optical dichotomy; however, it does not have to extend any deeper than that. The fact that both hemispheres of Iapetus look Asteroid-like at 13 cm means that coherent backscattering itself is not nearly as effective as it is at 2 cm. Since Iapetus’ entire surface is mature regolith, the wavelength dependence must involve composition, not structure. Either the composition is a function of depth everywhere (with electrical loss much greater at depths greater than a decimeter or two), or the intrinsic electrical loss of some pervasive constituent is much higher at 13 cm than at 2 cm. Ammonia is a candidate for such a contaminant. If ammonia’s electrical properties do not depend on frequency, and if ammonia is globally much less abundant within the upper one or two decimeters than at greater depths, then coherent backscattering would effectively be shut down at 13 cm, explaining the Asteroid-like 13-cm albedo.     

Physical characterization of the potentially hazardous high-albedo Asteroid (33342) 1998 WT24 from thermal-infrared observations

The potentially hazardous Asteroid (33342) 1998 WT₂₄ approached the Earth within 0.0125 AU on 2001 December 16 and was the target of a number of optical, infrared, and radar observing campaigns. Interest in 1998 WT₂₄ stems from its having an orbit with an unusually low perihelion distance, which causes it to cross the orbits of the Earth, Venus, and Mercury, and its possibly being a member of the E spectral class, which is rare amongst near-Earth asteroids (NEAs). We present the results of extensive thermal-infrared observations of 1998 WT₂₄ obtained in December 2001 with the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii and the ESO 3.6-m telescope in Chile. A number of thermal models have been applied to the data, including thermophysical models that give best-fit values of 0.35±0.04 km for the effective diameter, 0.56±0.2 for the geometric albedo, pv, and 100-300 J m⁻² s^−0.5 K⁻¹ for the thermal inertia. Our values for the diameter and albedo are consistent with results derived from radar and polarimetric observations. The albedo is one of the highest values obtained for any asteroid and, since no other taxonomic type is associated with albedos above 0.5, supports the suggested rare E-type classification for 1998 WT₂₄. The thermal inertia is an order of magnitude higher than values derived for large main-belt asteroids but consistent with the relatively high values found for other near-Earth asteroids. A crude pole solution inferred from a combination of our observations and published radar results is β=−52°, λ=355° (J2000), but we caution that this is uncertain by several tens of degrees.     

Measurement of the radius of Mercury by radio occultation during the MESSENGER flybys

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft completed three flybys of Mercury in 2008–2009. During the first and third of those flybys, MESSENGER passed behind the planet from the perspective of Earth, occulting the radio-frequency (RF) transmissions. The occultation start and end times, recovered with 0.1 s accuracy or better by fitting edge-diffraction patterns to the RF power history, are used to estimate Mercury's radius at the tangent point of the RF path. To relate the measured radius to the planet shape, we evaluate local topography using images to identify the high-elevation feature that defines the RF path or using altimeter data to quantify surface roughness. Radius measurements are accurate to 150 m, and uncertainty in the average radius of the surrounding terrain, after adjustments are made from the local high at the tangent point of the RF path, is 350 m. The results are consistent with Mercury's equatorial shape as inferred from observations by the Mercury Laser Altimeter and ground-based radar. The three independent estimates of radius from occultation events collectively yield a mean radius for Mercury of 2439.2±0.5 km.     

The surface of Mercury from ground-based astronomical observations

Recent ground-based astronomical short-exposure observations of Mercury have yielded more than 50000 electronic pictures of the planet at different phases and different positions relative to the Earth. The work was fulfilled in several observatories. The use of available and newly developed processing methods applied to large volumes of electronic frames allowed the images of a considerable portion of Mercury’s surface to be synthesized. We present the images of the 90°–180°W, 215°–280°W, and 50°–90°W sectors containing, among others, the longitudes not covered by spacecraft imaging. Along with the listed images, we present the results of recent observations of Mercury carried out on November 20–24, 2006 during the morning elongation at the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS) (Nizhnii Arkhyz, Karachai-Circassia, the Caucasus). The 265°–350°W longitude sector of Mercury was observed. The observations were made under good weather conditions. Among the main tasks of the new observations was obtaining a complete view of the S Basin. Previously, this basin had been investigated in fragments only by the actual solar illumination conditions. During the period of November 20–24, 2006, the S Basin was on the sunlit side of the planet. The complete image of the basin was obtained from the processing of a large number of electronic frames. The appearance of the S Basin is compared with the data on its relief acquired with radar methods. In this longitude sector, a number of other unusual surface features were found; among them, are a huge “Medallion” crater and other formations. The results considered in the present and earlier published studies are compared with the Mariner 10 data (1974–1975) and with the data received from the Messenger spacecraft during its first flyby of the planet (January 2008).     

Study of the unknown hemisphere of mercury by ground-based astronomical facilities

The short exposure method proved to be very productive in ground-based observations of Mercury. Telescopic observations with short exposures, together with computer codes for the processing of data arrays of many thousands of original electronic photos, make it possible to improve the resolution of images from ground-based instruments to almost the diffraction limit. The resulting composite images are comparable with images from spacecrafts approaching from a distance of about 1 million km. This paper presents images of the hemisphere of Mercury in longitude sectors 90°–180°W, 215°–350°W, and 50°–90°W, including, among others, areas not covered by spacecraft cameras. For the first time a giant S basin was discovered in the sector of longitudes 250°–290°W, which is the largest formation of this type on terrestrial planets. Mercury has a strong phase effects. As a result, the view of the surface changes completely with the change in the planetary phase. But the choice of the phase in the study using spacecrafts is limited by orbital characteristics of the mission. Thus, ground-based observations of the planet provide a valuable support.     

Electrical properties of Titan's surface from Cassini RADAR scatterometer measurements

We report regional-scale low-resolution backscatter images of Titan's surface acquired by the Cassini RADAR scatterometer at a wavelength of 2.18-cm. We find that the average angular dependence of the backscatter from large regions and from specific surface features is consistent with a model composed of a quasi-specular Hagfors term plus a diffuse cosine component. A Gaussian quasi-specular term also fits the data, but less well than the Hagfors term. We derive values for the mean dielectric constant and root-mean-square (rms) slope of the surface from the quasi-specular term, which we ascribe to scattering from the surface interface only. The diffuse term accommodates contributions from volume scattering, multiple scattering, or wavelength-scale near-surface structure. The Hagfors model results imply a surface with regional mean dielectric constants between 1.9 and 3.6 and regional surface roughness that varies between 5.3° and 13.4° in rms-slope. Dielectric constants between 2 and 3 are expected for a surface composed of solid simple hydrocarbons, water ice, or a mixture of both. Smaller dielectric constants, between 1.6 and 1.9, are consistent with liquid hydrocarbons, while larger dielectric constants, near 4.5, may indicate the presence of water-ammonia ice [Lorenz, R.D., 1998. Icarus 136, 344-348] or organic heteropolymers [Thompson, W.R., Squyres, S.W., 1990. Icarus 86, 336-354]. We present backscatter images corrected for angular effects using the model residuals, which show strong features that correspond roughly to those in 0.94-μm ISS images. We model the localized backscatter from specific features to estimate dielectric constant and rms slope when the angular coverage is within the quasi-specular part of the backscatter curve. Only two apparent surface features are scanned with angular coverage sufficient for accurate modeling. Data from the bright albedo feature Quivira suggests a dielectric constant near 2.8 and rms slope near 10.1°. The dark albedo feature Shangri-La is best fit by a Hagfors model with a dielectric constant close to 2.4 and an rms slope near 9.5°. From the modeled backscatter curves, we find the average radar albedo in the same linear (SL) polarization to be near 0.34. We constrain the total-power albedo in order to compare the measurements with available groundbased radar results, which are typically obtained in both senses of circular polarization. We estimate an upper limit of 0.4 on the total-power albedo, a value that is significantly higher than the 0.21 total albedo value measured at 13 cm [Campbell, D., Black, G., Carter, L., Ostro, S., 2003. Science 302, 431-434]. This is consistent with a surface that has more small-scale structure and is thus more reflective at 2-cm than 13-cm. We compare results across overlapping observations and observe that the reduction and analysis are repeatable and consistent. We also confirm the strong correlations between radar and near-infrared images.     

Albedo variations on the surface of mercury

Photoelectric photometric (slit) scans of Mercury have been obtained and combined with a man of the surface markings to yield relative normal albedoes over about one quarter of the planet's total surface at a wavelength of 0·45 microns. Maximum albedo ratios at a resolution of one fifth of the planetary diameter are not less than 2 to 1 and probably near 2·5 to 1. The corresponding average lunar value is 2·3 to 1. The blurring effects of seeing conditions on previous visual estimates of Mercury's albedo ratios are briefly discussed.