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1.
We present results of a campaign to map much of the Moon’s near side using the 12.6-cm radar transmitter at Arecibo Observatory and receivers at the Green Bank Telescope. These data have a single-look spatial resolution of about 40 m, with final maps averaged to an 80-m, four-look product to reduce image speckle. Focused processing is used to obtain this high spatial resolution over the entire region illuminated by the Arecibo beam. The transmitted signal is circularly polarized, and we receive reflections in both senses of circular polarization; measurements of receiver thermal noise during periods with no lunar echoes allow well-calibrated estimates of the circular polarization ratio (CPR) and the four-element Stokes vector. Radiometric calibration to values of the backscatter coefficient is ongoing. Radar backscatter data for the Moon provide information on regolith dielectric and physical properties, with particular sensitivity to ilmenite content and surface or buried rocks with diameter of about one-tenth the radar wavelength and larger.Average 12.6-cm circular polarization ratio (CPR) values for low- to moderate-TiO2 mare basalt deposits are similar to those of rough terrestrial lava flows. We attribute these high values to abundant few-centimeter diameter rocks from small impacts and a significant component of subsurface volume scattering. An outflow deposit, inferred to be impact melt, from Glushko crater has CPR values near unity at 12.6-cm and 70-cm wavelengths and thus a very rugged near-surface structure at the decimeter to meter scale. This deposit does not show radar-brightness variations consistent with levees or channels, and appears to nearly overtop a massif, suggesting very rapid emplacement. Deposits of similar morphology and/or radar brightness are noted for craters such as Pythagoras, Rutherfurd, Theophilus, and Aristillus. Images of the north pole show that, despite recording the deposition of Orientale material, Byrd and Peary craters do not have dense patterns of radar-bright ejecta from small craters on their floors. Such patterns in Amundsen crater, near the south pole, were interpreted as diagnostic of abundant impact melt, so the fraction of Orientale-derived melt in the north polar smooth plains, 1000 km farther from the basin center, is inferred to be much lower.  相似文献   

2.
Franklin Hadley Cocks 《Icarus》2010,206(2):778-779
Because of their cryogenic temperatures, analysis indicates that permanently shadowed polar lunar craters may have substantially higher levels of 3He than sunlit lunar surfaces and are conservatively estimated to contain as much as 50 ppb or more.  相似文献   

3.
Twenty-one lunar craters have radar bright ring appearances which are analogous to eleven complete ring features in the earth-based 12.5 cm observations of Venus. Radar ring diameters and widths for the lunar and Venusian features overlap for sizes from 45 to 100 km. Radar bright areas for the lunar craters are associated with the slopes of the inner and outer rim walls, while level crater floors and level ejecta fields beyond the raised portion of the rim have average radar backscatter. We propose that the radar bright areas of the Venusian rings are also associated with the slopes on the rims of craters.The lunar craters have evolved to radar bright rings via mass wasting of crater rim walls and via post impact flooding of crater floors. Aeolian deposits of fine-grained material on Venusian crater floors may produce radar scattering effects similar to lunar crater floor flooding. These Venusian aeolian deposits may preferentially cover blocky crater floors producing a radar bright ring appearance.We propose that the Venusian features with complete bright ring appearances and sizes less than 100 km are impact craters. They have the same sizes as lunar craters and could have evolved to radar bright rings via analogous surface processes.  相似文献   

4.
Radar imaging results for Mercury's non-polar regions are presented. The dual-polarization, delay-Doppler images were obtained from several years of observations with the upgraded Arecibo S-band (λ12.6-cm) radar telescope. The images are dominated by radar-bright features associated with fresh impact craters. As was found from earlier Goldstone-VLA and pre-upgrade Arecibo imaging, three of the most prominent crater features are located in the Mariner-unimaged hemisphere. These are: “A,” an 85-km-diameter crater (348° W, 34° S) whose radar ray system may be the most spectacular in the Solar System; “B,” a 95-km-diameter crater (343° W, 58° N) with a very bright halo but less distinct ray system; and “C,” an irregular feature with bright ejecta and rays distributed asymmetrically about a 125-km source crater (246° W, 11° N). Due south of “C” lies a “ghost” feature (242° W, 27° S) that resembles “A” but is much fainter. An even fainter such feature is associated with Bartok Crater. These may be two of the best mercurian examples of large ejecta/ray systems observed in an intermediate state of degradation. Virtually all of the bright rayed craters in the Mariner 10 images show radar rays and/or bright rim rings, with radar rays being less common than optical rays. Radar-bright craters are particularly common in the H-7 quadrangle. Some diffuse radar albedo variations are seen that have no obvious association with impact ejecta. In particular, some smooth plains regions such as the circum-Caloris plains in Tir, Budh, and Sobkou Planitiae and the interiors of Tolstoj and “Skinakas” basins show high depolarized brightness relative to their surroundings, which is the reverse of the mare/highlands contrast seen in lunar radar images. Caloris Basin, on the other hand, appears dark and featureless in the images.  相似文献   

5.
The Lunar CRater Observation and Sensing Satellite mission (LCROSS) impacted the moon in a permanently shadowed region of Cabeus crater on October 9th 2009, excavating material rich in water ice and volatiles. The thermal and spatial evolution of LCROSS ejecta is essential to interpretation of regolith properties and sources of released volatiles. The unique conditions of the impact, however, made analysis of the data based on canonical ejecta models impossible. Here we present the results of a series of impact experiments performed at the NASA Ames Vertical Gun Range designed to explore the LCROSS event using both high-speed cameras and LCROSS flight backup instruments. The LCROSS impact created a two-component ejecta plume: the usual inverted lampshade “low-angle” curtain, and a high speed, high-angle component. These separate components excavated to different depths in the regolith. Extrapolations from experiments match the visible data and the light curves in the spectrometers. The hollow geometry of the Centaur led to the formation of the high-angle plume, as was evident in the LCROSS visible and infrared measurements of the ejecta. Subsequent ballistic return of the sunlight-warmed ejecta curtain could scour the surface out to many crater radii, possibly liberating loosely bonded surface volatiles (e.g., H2). Thermal imaging reveals a complex, heterogeneous distribution of heated material after crater formation that is present but unresolved in LCROSS data. This material could potentially serve as an additional source of energy for volatile release.  相似文献   

6.
From radar images of Mercury's poles and MESSENGER Neutron Spectrometer (NS) measurements obtained during the spacecraft's flybys of Mercury, predictions of neutron count rates and their uncertainties are calculated for Mercury's north polar region as of the end of the MESSENGER primary orbital mission. If Mercury's poles contain large amounts of water ice, as has been suggested on the basis of the radar data, then during the one-year-long orbital mission the NS should detect signals indicative of excess polar hydrogen with a significance of at least 4σ, where σ is the standard deviation derived from Poisson counting statistics. If the polar deposits are not enriched with hydrogen, but are dominated by other elements, such as sulfur, then the MESSENGER neutron measurements should be able to confirm the absence of deposits having surface concentrations in excess of 50 wt% H2O on permanently shadowed floors of craters near Mercury's north pole. Because of the large spatial footprint of the NS data, individual polar deposits will not be spatially resolved, but longitudinal asymmetries may be detected if residual systematic uncertainties are sufficiently low.  相似文献   

7.
Abstract— We use Mars Orbiter Laser Altimeter (MOLA) topographic data and Thermal Emission Imaging System (THEMIS) visible (VIS) images to study the cavity and the ejecta blanket of a very fresh Martian impact crater ?29 km in diameter, with the provisional International Astronomical Union (IAU) name Tooting crater. This crater is very young, as demonstrated by the large depth/diameter ratio (0.065), impact melt preserved on the walls and floor, an extensive secondary crater field, and only 13 superposed impact craters (all 54 to 234 meters in diameter) on the ?8120 km2 ejecta blanket. Because the pre‐impact terrain was essentially flat, we can measure the volume of the crater cavity and ejecta deposits. Tooting crater has a rim height that has >500 m variation around the rim crest and a very large central peak (1052 m high and >9 km wide). Crater cavity volume (i.e., volume below the pre‐impact terrain) is ?380 km3 the volume of materials above the pre‐impact terrain is ?425 km3. The ejecta thickness is often very thin (<20 m) throughout much of the ejecta blanket. There is a pronounced asymmetry in the ejecta blanket, suggestive of an oblique impact, which has resulted in up to ?100 m of additional ejecta thickness being deposited down‐range compared to the up‐range value at the same radial distance from the rim crest. Distal ramparts are 60 to 125 m high, comparable to the heights of ramparts measured at other multi‐layered ejecta craters. Tooting crater serves as a fresh end‐member for the large impact craters on Mars formed in volcanic materials, and as such may be useful for comparison to fresh craters in other target materials.  相似文献   

8.
We use conventional numerical integrations to assess the fates of impact ejecta in the Saturn system. For specificity we consider impact ejecta launched from four giant craters on three satellites: Herschel on Mimas, Odysseus and Penelope on Tethys, and Tirawa on Rhea. Speeds, trajectories, and size of the ejecta are consistent with impact on a competent surface (“spalls”) and into unconsolidated regolith. We do not include near-field effects, jetting, or effects peculiar to highly oblique impact. Ejecta are launched at velocities comparable to or exceeding the satellite's escape speed. Most ejecta are swept up by the source moon on time-scales of a few to several decades, and produce craters no larger than 19 km in diameter, with typical craters in the range of a few km. As much as 17% of ejecta reach satellites other than the source moon. Our models generate cratering patterns consistent with a planetocentric origin of most small impact craters on the saturnian icy moons, but the predicted craters tend to be smaller than putative Population II craters. We conclude that ejecta from the known giant craters in the saturnian system do not fully account for Population II craters.  相似文献   

9.
R.J. Soare  J.S. Kargel  F. Costard 《Icarus》2007,191(1):95-112
We have identified a number of gullies that could be aqueous in origin near or at the rim of several impact craters in Utopia Planitia and western Elysium Planitia (30.0°-59.0° N; 241.0°-291.0° W). Based on the sharpness of their incisions and the general absence of superposed craters, we ascribe a relatively recent origin to the gullies. Scalloped depressions are commonplace throughout the region, as well as on the crater walls, rims and floors near the areas of gully issuance. Occasionally, the depressions cross-cut the gully debris-aprons, suggesting that the formation of some depressions is even more recent than that of the gullies. Previous research has proposed that the depressions are collapse basins formed by thermokarst processes. On Earth, thermokarst landforms occur in areas of low gradient topography where the permanently frozen ground (permafrost) is ice rich and has undergone a change in thermal equilibrium. This change can be triggered by long-term or episodic/cyclic climate change and accompanying rises in mean temperatures towards ∼0 °C as well as by rises in seasonally sustained summer temperatures well above ∼0 °C. In order to explain the origin of the rim or near-rim gullies we invoke high obliquity and the possibility that this region of Mars experienced obliquity-driven rises in temperature, atmospheric pressure and humidity sufficient to keep surface water and near-surface ground-ice stable for extended periods of time. We propose that gully formation is closely related to local freeze-thaw processes that, in turn, generate a thermokarst landscape (of which the gullies are a part). This geological and climatological scenario comprises the following steps:
1.
An inundation of meltwater at high obliquity (due to the thawing of an atmospherically-deposited snowpack or ice-sheet) and the subsequent saturation of the underlying regolith to tens of metres of depth.
2.
Loss of water on the surface, perhaps as obliquity decreases slightly, followed by the progressive freezing of the saturated regolith; this creates an aggrading mass of ice-rich regolith.
3.
Obliquity-induced temperature rises that engender the thaw, drainage and partial evaporation of the near-surface, ice-rich regolith.
4.
Localised formation of thermokarst collapse-basins (alases), as water is evacuated from these basins.
5.
Formation of gullies near, or at, some impact-crater rims as the result of meltwater migration from nearby alases through the thawed regolith to the areas of gully issuance.
Although the plains' materials in this region are in part very old (possibly Hesperian or even Noachian), the mantling deposits and their deformation by thermokarst processes appears to be relatively young. This suggests that recent climatic conditions could have been episodically warmer and wetter than had been previously thought.  相似文献   

10.
Reta F. Beebe 《Icarus》1980,44(1):1-19
The simple-to-complex transition for impact craters on Mars occurs at diameters between about 3 and 8 km. Ballistically emplaced ejecta surround primarily those craters that have a simple interior morphology, whereas ejecta displaying features attributable to fluid flow are mostly restricted to complex craters. Size-dependent characteristics of 73 relatively fresh Martian craters, emphasizing the new depth/diameter (d/D) data of D. W. G. Arthur (1980, to be submitted for publication), test two hypotheses for the mode of formation of central peaks in complex craters. In particular, five features appear sequentially with increasing crater size: first flat floors (3–4 km), then central peaks and shallower depths (4–5 km), next scalloped rims (? km), and lastly terraced walls (～8 km). This relative order indicates that a shallow depth of excavation and an unspecified rebound mechanism, not centripetal collapse and deep sliding, have produced central peaks and in turn have facilitated failure of the rim. The mechanism of formation of a shallow crater remains elusive, but probably operates only at the excavation stage of impact. This interpretation is consistent with two separate and complementary lines of evidence. First, field data have documented only shallow subsurface deformation and a shallow transient cavity in complex terrestrial meteorite craters and in certain surface-burst explosion craters; thus the shallow transient cavities of complex craters never were geometrically similar to the deep cavities of simple craters. Second, the average depths of complex craters and the diameters marking the transition from simple to complex craters on Mars and on three other terrestrial planets vary inversely with gravitational acceleration at the planetary surface, g, a variable more important in the excavation of a crater than in any subsequent modification of its geometry. The new interpretation is summarized diagrammatically for complex craters on all planets.  相似文献   

11.
We numerically calculate the probability and area of permanent shadowing as a function of the selenographic latitude as well as the total area of the permanently shadowed surface for various hierarchical models of the lunar surface. The permanently shadowed area is shown to rapidly increase with increasing number of hierarchical surface levels. For a two-level model of the lunar relief, where the surface of craters is complicated by a random small-scale relief with a Gaussian distribution of heights and slopes, the area of the doubly shadowed regions of the lunar surface is approximately an order of magnitude smaller than the area of the singly shadowed regions. A comparison of the permanently shadowed area calculated by using averaged statistical relations and data on the actual distribution of craters near the lunar poles shows almost complete agreement.  相似文献   

12.
Observations of 3.5- and 12.6-cm radar echoes from the rings of Saturn suggest that no significant difference in scattering properties exists in this wavelength interval. The echoes are largely unpolarized at both wavelengths, and yield a radar cross section at 3.5 cm of 7.32 ± 0.84 × 109 km2 for each polarization. The combined radar cross sections for both polarizations correspond to 1.37 ± 0.16 times the optically observed projected A- and B-ring areas (excluding that part of the rings shadowed by the planet). The shape of the echo spectrum is compatible with a homogeneous ring scattering model, except in having excess power at frequencies near the center of the spectrum. A number of possible explanations for the observed scattering properties are explored.  相似文献   

13.
Wenzhe Fa  Mark A. Wieczorek 《Icarus》2012,218(2):771-787
The inversion of regolith thickness over the nearside hemisphere of the Moon from newly acquired Earth-based 70-cm Arecibo radar data is investigated using a quantitative radar scattering model. The radar scattering model takes into account scattering from both the lunar surface and buried rocks in the lunar regolith, and three parameters are critically important in predicting the radar backscattering coefficient: the dielectric constant of the lunar regolith, the surface roughness, and the size and abundance of subsurface rocks. The measured dielectric properties of the Apollo regolith samples at 450 MHz are re-analyzed, and an improved relation among the complex dielectric constant, bulk density and regolith composition is obtained. The complex dielectric constant of the lunar regolith is estimated globally from this relation using the regolith composition derived from Lunar Prospector gamma-ray spectrometer data. To constrain the lunar surface roughness and abundance of subsurface rocks from radar data, nine regions are selected as calibration sites where the regolith thickness has been estimated using independent analysis techniques. For these sites, scattering from the lunar surface and buried rocks cannot be perfectly distinguished, and a tradeoff relationship exists between the size and abundance of buried rocks and surface roughness. Using these tradeoff relations as guidelines for globally representative parameters, the regolith thickness of four regions over the lunar nearside is inverted, and the inversion uncertainties caused by calibration errors of the radar data and model input parameters are analyzed. The regolith thickness of the maria is generally smaller than that of highlands, and older surfaces have thicker regolith thicknesses. Our approach cannot be applied to regions where the surface roughness is very high, such as with young rocky craters and regions in the highly rugged highlands.  相似文献   

14.
We report here on a survey of distal fine-grained ejecta deposits on the Moon, Mars, and Venus. On all three planets, fine-grained ejecta form circular haloes that extend beyond the continuous ejecta and other types of distal deposits such as run-out lobes or ramparts. Using Earth-based radar images, we find that lunar fine-grained ejecta haloes represent meters-thick deposits with abrupt margins, and are depleted in rocks ?1 cm in diameter. Martian haloes show low nighttime thermal IR temperatures and thermal inertia, indicating the presence of fine particles estimated to range from ∼10 μm to 10 mm. Using the large sample sizes afforded by global datasets for Venus and Mars, and a complete nearside radar map for the Moon, we establish statistically robust scaling relationships between crater radius R and fine-grained ejecta run-out r* for all three planets. On the Moon, r* ∼ R−0.18 for craters 5-640 km in diameter. For Venus, radar-dark haloes are larger than those on the Moon, but scale as r* ∼ R−0.49, consistent with ejecta entrainment in Venus’ dense atmosphere. On Mars, fine-ejecta haloes are larger than lunar haloes for a given crater size, indicating entrainment of ejecta by the atmosphere or vaporized subsurface volatiles, but scale as R−0.13, similar to the ballistic lunar scaling. Ejecta suspension in vortices generated by passage of the ejecta curtain is predicted to result in ejecta run-out that scales with crater size as R1/2, and the wind speeds so generated may be insufficient to transport particles at the larger end of the calculated range. The observed scaling and morphology of the low-temperature haloes leads us rather to favor winds generated by early-stage vapor plume expansion as the emplacement mechanism for low-temperature halo materials.  相似文献   

15.
Optical and near-IR signatures of water ice on the Moon's surface were sought in the permanently shadowed regions near its poles. Significant amounts of multiply-scattered radiation partly illuminate primary shadows cast by craters and other features. If there is water ice in the permanently shadowed regions of the Moon's surface, its spectral signature should appear in this multiply-scattered light. This investigation can be done most effectively with observations obtained by spacecraft, because most selenocentric positions occupied by the Earth will also be occupied by the Sun at some point in time, and because the lunar poles are seen only obliquely to a terrestrial observer. Images obtained by Clementine are particularly well-suited to this task, because the spacecraft's polar orbit allowed images of the poles to be acquired on nearly every orbit, resulting in literally thousands of images taken within a few degrees of each pole, and because the filters on the ultraviolet-visual camera (UVVIS) and the near infrared camera (NIR) occur at major absorption bands or within important continuum features of water ice. Approximately 5800 images obtained by the UVVIS camera and 1800 images obtained by the NIR camera were calibrated and combined into coadded mosaics to create multispectral maps of the lunar poles with the highest possible signal-to-noise. Unfortunately, analysis of our UVVIS mosaics indicates that any possible signal from multiply-scattered light in primary shadows was overwhelmed by instrumental stray light. For the NIR camera, we were able to determine the normalized reflectance of several regions that were identified by Margot et al. (1999, Science284, 1658-1660) as permanent shadows. We have identified one permanently shadowed crater with a 1.5-μm band spectral signature indicative of between 2.5 and 21% fractional coverage of H2O frost. However, the same region shows a 2.0 μm spectral signature that is inconsistent with the presence of any water.  相似文献   

16.
We investigate the depth, variability, and history of regolith on asteroid Vesta using data from the Dawn spacecraft. High‐resolution (15–20 m pixel?1) Framing Camera images are used to assess the presence of morphologic indicators of a shallow regolith, including the presence of blocks in crater ejecta, spur‐and‐gully–type features in crater walls, and the retention of small (<300 m) impact craters. Such features reveal that the broad, regional heterogeneities observed on Vesta in terms of albedo and surface composition extend to the physical properties of the upper ~1 km of the surface. Regions of thin regolith are found within the Rheasilvia basin and at equatorial latitudes from ~0–90°E and ~260–360°E. Craters in these areas that appear to excavate material from beneath the regolith have more diogenitic (Rheasilvia, 0–90°E) and cumulate eucrite (260–360°E) compositions. A region of especially thick regolith, where depths generally exceed 1 km, is found from ~100–240°E and corresponds to heavily cratered, low‐albedo surface with a basaltic eucrite composition enriched in carbonaceous chondrite material. The presence of a thick regolith in this area supports the idea that this is an ancient terrain that has accumulated a larger component of exogenic debris. We find evidence for the gardening of crater ejecta toward more howarditic compositions, consistent with regolith mixing being the dominant form of “weathering” on Vesta.  相似文献   

17.
J.E. Baron  G.L. Tyler  R.A. Simpson 《Icarus》2003,164(2):404-417
Radar scattering from the icy galilean satellites is marked by unusually high backscatter cross sections and polarization ratios at wavelengths λ0=3.5-70 cm. The persistence of exotic scattering behavior over this large a wavelength range suggests that the responsible mechanisms remain at least partially effective as the wavelength approaches or exceeds the size of individual scatterers. We examine two models previously analyzed in the geometrical optics limit—radar glory from buried craters (Eshleman, 1986, Science 234, 587-590) and refraction scattering from subsurface lenses (Hagfors et al., 1985, Nature 315, 637-640)—at wavelength scales using three-dimensional finite-difference time-domain (FDTD) numerical simulations. We include craters with rough walls and lenses with random inclusions of heterogeneous material. For hemispherical craters spanning up to 3λ0 in diameter, we observe none of the exotic backscatter behavior attributed to the geometrical optics models. Nonspherical refraction scatterers can produce circular polarization ratios μC>1 and linear polarization ratios μL=0.5-0.8 at diameters as small as ∼λ0, but the density of such inclusions must be high if refraction scattering alone is to account for the measured cross sections.  相似文献   

18.
A. Mantz  R. Sullivan  J. Veverka 《Icarus》2004,167(1):197-203
Images of Eros from the NEAR Shoemaker spacecraft reveal bright and dark albedo features on steep crater walls unlike markings previously observed on asteroids. These features have been attributed to the downslope movement of space-weathered regolith, exposing less weathered material (Science 292 (2001) 484; Meteor. Planet. Sci. 36 (2001) 1617; Icarus 155 (2002) 145). Here we present observations of the interiors of large craters (>1 km in diameter) to test this hypothesis and constrain the origin of the features. We find that bright regions in these craters correspond to steep slopes, consistent with previous work. The geographic distribution of craters with albedo variations shows no pattern and does not resemble the distribution of ponds, another phenomenon on Eros attributed to regolith movement. Shadows and other indications of topography are not observed at feature boundaries, implying that the transported layer is ?1 m thick. The presence of multiple bright and dark units on long slopes with sharp boundaries between them suggests that mobilized regolith may be halted by frictional or other effects before reaching the foot of the slope. Features on crater walls should darken at the same rate as bright ejecta deposits from crater formation; the lack of observed, morphologically fresh craters with bright interiors or ejecta suggests that the albedo patterns are younger than the most recently formed craters greater than about 100 m in diameter. Smaller or micrometeorite impacts, which would not necessarily leave evident deposits of bright ejecta, remain possible causes of albedo patterns. Although their effectiveness is difficult to assess, electrostatic processes and thermal creep are also candidates.  相似文献   

19.
Distributions of boulders ejected from lunar craters   总被引：1，自引：0，他引：1  
We investigate the spatial distributions of boulders ejected from 18 lunar impact craters that are hundreds of meters in diameter. To accomplish this goal, we measured the diameters of 13,955 ejected boulders and the distance of each boulder from the crater center. Using the boulder distances, we calculated ejection velocities for the boulders. We compare these data with previously published data on larger craters and use this information to determine how boulder ejection velocity scales with crater diameter. We also measured regolith depths in the areas surrounding many of the craters, for comparison with the boulder distributions. These results contribute to understanding boulder ejection velocities, to determining whether there is a relationship between the quantity of ejected boulders and lunar regolith depths, and to understanding the distributions of secondary craters in the Solar System. Understanding distributions of blocky ejecta is an important consideration for landing site selection on both the Moon and Mars.  相似文献   

20.
Polarimetric measurements were collected at different areas of the surface of Mercury, and for the whole disk in six wavelengths. The curves of polarization are compared with telescopic observations of the Moon and laboratory studies of minerals and returned lunar samples. The negative branch of polarization proves that Mercury's surface is almost everywhere covered by a regolith layer of fines of the lunar type, also made of dark and adsorbing material, and most probably of the same impact generated origin. The polarization maximum of Mercury is reproduced by lunar samples of fines of intermediate albedo corresponding to the lightest regolith found in the Apollo explored maria.The albedo of Mercury at phase angle 5° deduced from telescopic photometry is to be corrected by a factor of 1.20 and the best “polarimetric” values of albedos are 0.130 at λ = 0.585μm, 0.119 at λ = 0.520 μm, 0.093 at λ = 0.379μm and 0.087 at λ = 0.354μm. The contrast between light and dark-lined regions at the surface of Mercury is most probably much fainter than between the maria and continents on the Moon.The molecular atmosphere of Mercury, if any, has a surface pressure probably smaller than 2 × 10?4 bars.  相似文献   

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