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1.
This investigation uses linear mixture modeling employing cryogenic laboratory reference spectra to estimate surface compositions and water ice grain sizes of Europa’s ridged plains and smooth low albedo plains. Near-infrared spectra for 23 exposures of ridged plains materials are analyzed along with 11 spectra representing low albedo plains. Modeling indicates that these geologic units differ both in the relative abundance of non-ice hydrated species and in the abundance and grain sizes of water ice. The background ridged plains in our study area appear to consist predominantly of water ice (∼46%) with approximately equal amounts (on average) of hydrated sulfuric acid (∼27%) and hydrated salts (∼27%). The solutions for the smooth low albedo plains are dominated by hydrated salts (∼62%), with a relatively low mean abundance of water ice (∼10%), and an abundance of hydrated sulfuric acid similar to that found in ridged plains (∼27%). The model yields larger water ice grain sizes (100 μm versus 50-75 μm) in the ridged plains. The 1.5-μm water ice absorption band minimum is found at shorter wavelengths in the low albedo plains deposits than in the ridged plains (1.498 ± .003 μm versus 1.504 ± .001 μm). The 2.0-μm band minimum in the low albedo plains exhibits a somewhat larger blueshift (1.964 ± .006 μm versus 1.983 ± .006 μm for the ridged plains).The study area spans longitudes from 168° to 185°W, which includes Europa’s leading side-trailing side boundary. A well-defined spatial gradient of sulfuric acid hydrate abundance is found for both geologic units, with concentrations increasing in the direction of the trailing side apex. We associate this distribution with the exogenic effects of magnetospheric charged particle bombardment and associated chemical processing of surface materials (the radiolytic sulfur cycle). However, one family of low albedo plains exposures exhibits sulfuric acid hydrate abundances up to 33% lower than found for adjacent exposures, suggesting that these materials have undergone less processing, thus implying that these deposits may have been emplaced more recently.Modeling identifies high abundances (to 30%) of magnesium sulfate brines in the low albedo plains exposures. Our investigation marks the first spectroscopic identification of MgSO4 brine on Europa. We also find significantly higher abundances of sodium-bearing species (bloedite and mirabilite) in the low albedo plains. The results illuminate the role of radiolytic processes in modifying the surface composition of Europa, and may provide new constraints for models of the composition of Europa’s putative subsurface ocean.  相似文献   

2.
New global maps of the five inner midsize icy saturnian satellites, Mimas, Enceladus, Tethys, Dione, and Rhea, have been constructed in three colors (UV, Green and near-IR) at resolutions of 1 km/pixel. The maps reveal prominent global patterns common to several of these satellites but also three major color features unique to specific satellites or satellite subgroups. The most common features among the group are first-order global asymmetries in color properties. This pattern, expressed on Tethys, Dione and Rhea, takes the form of a ∼1.4-1.8 times enhancement in redness (expressed as IR/UV ratio) of the surface at the center of the trailing hemisphere of motion, and a similar though significantly weaker IR/UV enhancement at the center of the leading hemisphere. The peak in redness on the trailing hemisphere also corresponds to a known decrease in albedo. These double hemispheric asymmetries are attributable to plasma and E-ring grain bombardment on the trailing and leading hemispheres, respectively, for the outer three satellites Tethys, Dione and Rhea, whereas as E-ring bombardment may be focused on the trailing hemisphere of Mimas due to its orbital location interior to Enceladus. The maps also reveal three major deviations from these basic global patterns. We observe the previously known dark bluish leading hemisphere equatorial band on Tethys but have also discovered a similar band on Mimas. Similar in shape, both features match the surface patterns expected for irradiation of the surface by incident MeV electrons that drift in a direction opposite to the plasma flow. The global asymmetry on Enceladus is offset ∼40° to the west compared to the other satellites. We do not consider Enceladus in detail here, but the global distribution of bluish material can be shown to match the deposition pattern predicted for plume fallback onto the surface (Kempf, S., Beckmann, U., Schmidt, S. [2010]. Icarus 206, 446-457. doi:10.1016/j.icarus.2009.09.016). E-ring deposition on Enceladus thus appears to mask or prevent the formation of the lenses and hemispheric asymmetries we see on the other satellites. Finally, we observe a chain of discrete bluish splotches along the equator of Rhea. Unlike the equatorial bands of Tethys and Mimas, these splotches form a very narrow great circle ?10-km wide (north-to-south) and appear to be related to surface disruption, exposing fresh, bluish ice on older crater rims. This feature is unique to Rhea and may have formed by impact onto its surface of orbiting material.  相似文献   

3.
Iapetus, one of the saturnian moons, has an extreme albedo contrast between the leading and trailing hemispheres. The origin of this albedo dichotomy has led to several hypotheses, however it remains controversial. To clarify the origin of the dichotomy, the key approach is to investigate the detailed distribution of the dark material. Recent studies of impact craters and surface temperature from Cassini spacecraft data implied that sublimation of H2O ice can occur on Iapetus’ surface. This ice sublimation can change the albedo distribution on the moon with time.In this study, we evaluate the effect of ice sublimation and simulate the temporal change of surface albedo. We assume the dark material and the bright ice on the surface to be uniformly mixed with a certain volume fraction, and the initial albedo distribution to incorporate the dark material deposits on the surface. That is, the albedo at the apex is lowest and concentrically increases in a sinusoidal pattern. This situation simulates that dark materials existed around the Iapetus’ orbit billions of years ago, and the synchronously rotating Iapetus swept the material and then deposited it on its surface. The evolution of the surface albedo during 4.0 Gyr is simulated by estimating the surface temperature from the insolation energy on Iapetus including the effect of Saturn’s eccentricity and Iapetus’ obliquity precession, and evaluating the sublimation rate of H2O ice from the Iapetus’ surface.As a result, we found that the distribution of the surface albedo changed dramatically after 4.0 Gyr of evolution. The sublimation has three important effects on the resultant surface albedo. First, the albedo in the leading hemisphere has significantly decreased to approach the minimum value. Second, the albedo distribution has been elongated along the equator. Third, the edge of the low albedo region has become clear. Considering the effect of ice sublimation, the current albedo distribution can be reconstructed from the sinusoidal albedo distribution, suggesting the apex-antapex cratering asymmetry as a candidate for the origin of the albedo dichotomy. From the model analysis, we obtained an important aspect that the depth of the turn-over layer where the darkening process proceeded for 4 Gyr should be an order of 10 cm, which is consistent with evaluation from the Cassini radar observations.  相似文献   

4.
We obtained time-resolved, near-infrared spectra of Io during the 60-90 min following its reappearance from eclipse by Jupiter on five occasions in 2004. The purpose was to search for spectral changes, particularly in the well-known SO2 frost absorption bands, that would indicate surface-atmosphere exchange of gaseous SO2 induced by temperature changes during eclipse. These observations were a follow-on to eclipse spectroscopy observations in which Bellucci et al. [Bellucci et al., 2004. Icarus 172, 141-148] reported significant changes in the strengths of two strong SO2 bands in data acquired with the VIMS instrument aboard the Cassini spacecraft. One of the bands (4.07 μm [ν1 + ν3]) observed by Bellucci et al. is visible from ground-based observatories and is included in our data. We detected no changes in Io’s spectrum at any of the five observed events during the approximately 60-90 min during which spectra were obtained following Io’s emergence from Jupiter’s shadow. The areas of the three strongest SO2 bands in the region 3.5-4.15 μm were measured for each spectrum; the variation of the band areas with time does not exceed that which can be explained by the Io’s few degrees of axial rotation during the intervals of observation, and in no case does the change in band strength approach that seen in the Cassini VIMS data. Our data are of sufficient quality and resolution to show the weak 2.198 μm (4549.6 cm−1) 4ν1 band of SO2 frost on Io for what we believe is the first time. At one of the events (June 22, 2004), we began the acquisition of spectra ∼6 min before Io reappeared from Jupiter’s shadow, during which time it was detected through its own thermal emission. No SO2 bands were superimposed on the purely thermal spectrum on this occasion, suggesting that the upper limit to condensed SO2 in the vertical column above Io’s surface was ∼4 × 10−5 g cm−2.  相似文献   

5.
The dramatic hemispheric dichotomy in albedo displayed by Saturn's moon Iapetus has intrigued astronomers for centuries. Here we report on far-ultraviolet observations of Iapetus' bright and dark terrains from Cassini. We compare the reflectance spectra of Iapetus's dark terrain, Hyperion and Phoebe and find that both Phoebe and Hyperion are richer in water ice than Iapetus' dark terrain. Spectra of the lowest latitudes of the dark terrain display the diagnostic water ice absorption feature; water ice amounts increase within the dark material away from the apex (at 90° W longitude, the center of the dark leading hemisphere), consistent with thermal segregation of water ice. The water ice in the darkest, warmest low latitude regions is not expected to be stable and may be a sign of ongoing or recent emplacement of the dark material from an exogenic source.  相似文献   

6.
The energy balance at the surface of an airless planetary body is strongly influenced by the bolometric Bond albedo and the surface thermal inertia. Both of these values may be calculated through the application of a thermal model to measured surface temperatures. The accuracy of either, though, increases if the value of the other is better constrained. In this study, we used the improved global bolometric Bond albedo map of Iapetus derived from Cassini VIMS and ISS and Voyager ISS data in conjunction with Cassini CIRS temperature data to reevaluate surface thermal inertia across Iapetus. Results showed the thermal inertia of the dark terrain varies between 11 and 14.8 J m−2 K−1 s−1/2 while the light material varies between 15 and 25 J m−2 K−1 s−1/2. Using an approximation to the thermal properties of the dark overburden derived from our thermal inertia results, we can implement our thermal model to provide estimates on the dark material thickness, which was found to lie between 7 cm and 16 cm. In order to develop an accurate global thermal model, a weighted function that approximates the surface thermal inertia across Iapetus was developed and verified via our measurements. The global bolometric Bond albedo map, surface thermal inertia map, and the thermal model are then used to synthesize global temperature maps that may be used to study the stability of volatiles.  相似文献   

7.
Using the Hubble Space Telescope's Space Telescope Imaging Spectrograph we have obtained for the first time spatially resolved 2000-3000 Å spectra of Io's Prometheus plume and adjoining regions on Io's anti-jovian hemisphere in the latitude range 60° N-60° S, using a 0.1″ slit centered on Prometheus and tilted roughly 45° to the spin axis. The SO2 column density peaked at 1.25×1017 cm−2 near the equator, with an additional 5×1016 cm−2 enhancement over Prometheus corresponding to a model volcanic SO2 output of 105 kg s−1. Apart from the Prometheus peak, the SO2 column density dropped fairly smoothly away from the subsolar point, even over regions that included potential volcanic sources. At latitudes less than ±30°, the dropoff rate was consistent with control by vapor pressure equilibrium with surface frost with subsolar temperature 117.3±0.6 K, though SO2 abundance was higher than predicted by vapor pressure control at mid-latitudes, especially in the northern hemisphere. We conclude that, at least at low latitudes on the anti-jovian hemisphere where there are extensive deposits of optically-thick SO2 frost, the atmosphere is probably primarily supported by sublimation of surface frost. Although the 45° tilt of our slit prevents us from separating the dependence of atmospheric density on solar zenith angle from its dependence on latitude, the pattern is consistent with a sublimation atmosphere regardless of which parameter is the dominant control. The observed drop in gas abundance towards higher latitudes is consistent with the interpretation of previous Lyman alpha images of Io as indicating an atmosphere concentrated at low latitudes. Comparison with previous disk-resolved UV spectroscopy, Lyman-alpha images, and mid-infrared spectroscopy suggests that Io's atmosphere is denser and more widespread on the anti-jovian hemisphere than at other longitudes. SO2 gas temperatures were in the range of 150-250 K over the majority of the anti-jovian hemisphere, consistent with previous observations. SO was not definitively detected in our spectra, with upper limits to the SO/SO2 ratio in the range 1-10%, roughly consistent with previous observations. S2 gas was not seen anywhere, with an upper limit of 7.5×1014 cm−2 for the Prometheus plume, confirming that this plume is significantly poorer in S2 than the Pele plume (S2 /SO2<0.005, compared to 0.08-0.3 at Pele). In addition to the gas absorption signatures, we have observed continuum emission in the near ultraviolet (near 2800 Å) for the first time. The brightness of the observed emission was directly correlated with the SO2 abundance, strongly peaking in the equatorial region over Prometheus. Emission brightness was modestly anti-correlated with the jovian magnetic latitude, decreasing when Io intersected the torus centrifugal equator.  相似文献   

8.
To settle the question of disulfur monoxide and sulfur monoxide deposition and occurrence on Io's surface, we performed series of laboratory experiments reproducing the condensation of S2O at low temperature. Its polymerization has been monitored by recording infrared spectra under conditions of temperature, pressure, mixing with SO2 and UV-visible radiation simulating that of Io's surface. Our experiments show that S2O condensates are not chemically stable under ionian conditions. We also demonstrate that SO and S2O outgassed by Io's volcanoes and condensing on Io's surface should lead to yellow polysulfuroxide deposits or to white deposits of S2O diluted in sulfur dioxide frost (i.e., S2O/SO2 < 0.1%). Thus S2O condensation cannot be responsible for the red volcanic deposits on Io. Comparison of the laboratory infrared spectra of S2O and polysulfuroxide with NIMS/Galileo infrared spectra of Io's surface leads us to discuss the possible identification of polysulfuroxide. We also recorded the visible transmission spectra of sulfur samples resulting from polysulfuroxide decomposition. These samples consist in a mixture of sulfur polymer and orthorhombic sulfur. Using the optical constants extracted from these measurements, we show that a linear combination of the reflectance spectra of our samples, the reflectance spectrum of orthorhombic S8 sulfur and SO2 reflectance spectrum, leads to a very good matching of Io's visible spectrum between 330 and 520 nm. We conclude then that Io's surface is probably mainly composed of sulfur dioxide and a mixture of sulfur S8 and sulfur polymer. Some polysulfuroxide could also co-exist with these dominant components, but is probably restricted to some volcanic areas.  相似文献   

9.
The available full-disk reflectance spectra of Io in the range 0.3 to 2.5 μm have been interpreted by comparison with new laboratory spectra of a wide variety of natural and synthetic mineral phases in order to determine a surface compositional model for Io that is consistent with Io's other known chemical and physical properties. Our results indicate that the dominant mineral phases are sulfates and free sulfur derived from them, which points toward a low temperature and initially water-rich surface assemblage. Our current preferred mineral phase mixture that best matches the Io data and is simultaneously most consistent with other constraints, consists of a fine-grained particulate mixture of free sulfur (55 vol%), dehydrated bloedite [Na2Mg(SO4)2·xH2O] (30 vol%) ferric sulfate [Fe2(SO4)3·xH2O] (15 vol%), and trace amounts of hematite [Fe2O3]. Other salts may be present, such as halite and sodium nitrate, as well as clay minerals. Such a model is consistent with a probable pre- and post-accretion thermal history of Io-forming material and Io's observed Na emission and other properties. These results further support the evaporite surface hypothesis of Fanale et al'; while not precluding the presence of certain silicate phases such as montmorillonite.The average surface of Io's leading hemisphere appears to contain less free sulfur and more salts and to be finer grained than that of the trailing hemisphere. Since Io is immersed in Jupiter's magnetosphere, irradiation damage effects from low-energy proton bombardment were studied. Irradiation damage of lattices is estimated to be a relatively minor but operative process on the surface of Io; irradiation darkening by sulfate reduction to free sulfur and by F-center production in salts may be partly responsible for the differences in albedo of leading and trailing hemispheres and equatorial and polar regions of Io, but slight regional differences in relative intrinsic phase concentration on the surface may likewise account for these global variations in albedo.Possible unusual surface properties predicted by this model include: posteclipse darkening in certain wavelenghts, limb brightening in certain wavelengths, and unusual surface electrical properties. Further refinement of Io's surface composition model and better understanding of surface irradiation effects will be possible when observational data in the range 0.20 to 0.30 μm are obtained and when improved spectra in the range 0.30 to 5.0 μm are obtained having increased spectral, spatial, and temporal resolution.  相似文献   

10.
Using the SPICAV-UV spectrometer aboard Venus Express in nadir mode, we were able to derive spectral radiance factors in the middle atmosphere of Venus in the 170-320 nm range at a spectral resolution of R ? 200 during 2006 and 2007 in the northern hemisphere. By comparison with a radiative transfer model of the upper atmosphere of Venus, we could derive column abundance above the visible cloud top for SO2 using its spectral absorption bands near 280 and 220 nm. SO2 column densities show large temporal and spatial variations on a horizontal scale of a few hundred kilometers. Typical SO2 column densities at low latitudes (up to 50°N) were found between 5 and 50 μm-atm, whereas in the northern polar region SO2 content was usually below 5 μm-atm. The observed latitudinal variations follow closely the cloud top altitude derived by SPICAV-IR and are thought to be of dynamical origin. Also, a sudden increase of SO2 column density in the whole northern hemisphere has been observed in early 2007, possibly related to a convective episode advecting some deep SO2 into the upper atmosphere.  相似文献   

11.
We analyze observations taken with Cassini’s Visual and Infrared Mapping Spectrometer (VIMS), to determine the current methane and haze latitudinal distribution between 60°S and 40°N. The methane variation was measured primarily from its absorption band at 0.61 μm, which is optically thin enough to be sensitive to the methane abundance at 20-50 km altitude. Haze characteristics were determined from Titan’s 0.4-1.6 μm spectra, which sample Titan’s atmosphere from the surface to 200 km altitude. Radiative transfer models based on the haze properties and methane absorption profiles at the Huygens site reproduced the observed VIMS spectra and allowed us to retrieve latitude variations in the methane abundance and haze. We find the haze variations can be reproduced by varying only the density and single scattering albedo above 80 km altitude. There is an ambiguity between methane abundance and haze optical depth, because higher haze optical depth causes shallower methane bands; thus a family of solutions is allowed by the data. We find that haze variations alone, with a constant methane abundance, can reproduce the spatial variation in the methane bands if the haze density increases by 60% between 20°S and 10°S (roughly the sub-solar latitude) and single scattering absorption increases by 20% between 60°S and 40°N. On the other hand, a higher abundance of methane between 20 and 50 km in the summer hemisphere, as much as two times that of the winter hemisphere, is also possible, if the haze variations are minimized. The range of possible methane variations between 27°S and 19°N is consistent with condensation as a result of temperature variations of 0-1.5 K at 20-30 km. Our analysis indicates that the latitudinal variations in Titan’s visible to near-IR albedo, the north/south asymmetry (NSA), result primarily from variations in the thickness of the darker haze layer, detected by Huygens DISR, above 80 km altitude. If we assume little to no latitudinal methane variations we can reproduce the NSA wavelength signatures with the derived haze characteristics. We calculate the solar heating rate as a function of latitude and derive variations of ∼10-15% near the sub-solar latitude resulting from the NSA. Most of the latitudinal variations in the heating rate stem from changes in solar zenith angle rather than compositional variations.  相似文献   

12.
W.M Grundy  L.A Young  E.F Young 《Icarus》2003,162(1):222-229
New 0.8- to 2.4-μm spectral observations of the leading and trailing hemispheres of the uranian satellite Ariel were obtained at IRTF/SpeX during 2002 July 16 and 17 UT. The new spectra reveal contrasts between Ariel’s leading and trailing hemispheres, with the leading hemisphere presenting deeper H2O ice absorption bands. The observed dichotomy is comparable to leading-trailing spectral asymmetries observed among jovian and saturnian icy satellites. More remarkably, the trailing hemisphere spectrum exhibits three narrow CO2 ice absorption bands near 2 μm. This discovery of CO2 ice on one hemisphere of Ariel is its first reported detection in the uranian system.  相似文献   

13.
We present values from the Cassini Visual and Infrared Mapping Spectrometer (VIMS) of four fundamental disk-integrated spectrophotometric properties (bolometric Bond albedo, solar phase curve, phase integral, and geometric albedo at 7-15 different wavelengths in the λ = 0.35-5.1 μm range) for five mid-sized saturnian icy satellites: Rhea, Dione, Tethys, Mimas, and Enceladus. These values, which include data from the period 2004-2008 and add to past VIMS phase curves, include opposition surge effects at down to fractions of a degree in solar phase angle for several moons and extend to over double the solar phase angle coverage of the Voyager mission. We also present new rotational light curves for Rhea and Dione at 7 near-infrared bands not previously available in ground-based or spacecraft studies. The bolometric Bond albedos we derive are as follows: 0.48 ± 0.09 (Rhea), 0.52 ± 0.08 (Dione), 0.61 ± 0.09 (Tethys), 0.67 ± 0.10 (Mimas), and 0.85 ± 0.11 (Enceladus). We also provide breakdowns of the major photometric quantities in both leading and trailing hemispheres. These refined parameters can be used to construct future bolometric Bond albedo maps that will contribute to surface composition identification studies, as well as models of volatile transport and sublimation. Through such applications, these data will help to determine the physical properties of surface particles, how the E-ring affects the inner saturnian moons, what is responsible for the dark albedo patterns seen on Tethys, and if these moons (e.g., Dione) are geologically active.  相似文献   

14.
We present a detailed analysis of the variations in spectral properties across the surface of Saturn’s satellite Dione using Cassini/VIMS data and their relationships to geological and/or morphological characteristics as seen in the Cassini/ISS images. This analysis focuses on a local region on Dione’s anti-saturnian hemisphere that was observed by VIMS with high spatial resolution during orbit 16 in October 2005. The results are incorporated into a global context provided by VIMS data acquired within Cassini’s first 50 orbits. Our results show that Dione’s surface is dominated by at least one global process. Bombardment by magnetospheric particles is consistent with the concentration of dark material and enhanced CO2 absorption on the trailing hemisphere of Dione independent of the geology. Local regions within this terrain indicate a special kind of resurfacing that probably is related to large-scale impact process. In contrast, the enhanced ice signature on the leading side is associated with the extended ejecta of the fresh impact crater Creusa (∼49°N/76°W). Although no geologically active regions could be identified, Dione’s tectonized regions observed with high spatial resolution partly show some clean H2O ice implying that tectonic processes could have continued into more recent times.  相似文献   

15.
We present individual spectra 0.8-2.5 μm of the leading and trailing hemispheres of Enceladus obtained with the CorMASS spectrograph on the 1.8 m Vatican Advanced Technology Telescope (VATT) at the Mount Graham International Observatory. While the absorption bands of water ice dominate the spectrum of both hemispheres, most of these bands are stronger on the leading hemisphere than the trailing hemisphere. In addition, longward of 1 μm, the continuum slope is greater on the leading hemisphere than the trailing hemisphere. These differences could be produced by the presence of particles on the trailing side that are smaller and/or microstructurally more complex than those on the leading side, consistent with the preferential erosion or structural degradation of regolith particle grains on the trailing side by magnetospheric sweeping. We also explore compositional differences between the two hemispheres by applying Hapke spectrophotometric mixture models to the spectra whose components include water ice and ammonia hydrate (1% NH3⋅H2O). We find that spectral models which include as much as 25% by weight ammonia hydrate intimately mixed with water ice and covering 80% of the illuminated area of the satellite fit the observed spectrum of both the leading and trailing hemispheres. Areal (checkerboard) mixing models of ammonia hydrate and water ice fit the leading hemisphere with 15% of the surface comprised of ammonia hydrate and the trailing hemisphere with 10% ammonia hydrate. Therefore, while these spectral data do not contain an unambiguous detection of ammonia hydrate on Enceladus, our spectral models do not preclude the presence of a modest amount of 1% NH3⋅H2O on both hemispheres. We examine spectral differences and similarities between both hemispheres and the tenuous E ring within which Enceladus orbits. The spectral resolution (R=λ/Δλ) of these CorMASS data (R∼300) is comparable to but nevertheless higher than that of the Visual-Infrared Mapping Spectrometer (VIMS) (R=225) onboard the Cassini spacecraft.  相似文献   

16.
The purpose of this study was to determine if any temporal variability in the broadband UV spectral properties of the icy Galilean satellites exists, and if so, to characterize its spatial distribution as a function of longitude in order to attempt to correlate any temporal changes with satellite surface interactions with the space environment. The temporal time period examined is between 1978-1984 (referred to as the 1980s data) and those from 1995-1996 (referred to as the 1990s data). The plausible temporal characteristics detected appear to vary from one satellite to the next. For Europa possible temporal variations are concentrated on the leading, anti-jovian quadrant. Example broadband UV spectra show Europa's spectral slope decreases (and darkens) with time on the leading and anti-jovian hemispheres, but remains essentially constant with time on the trailing hemisphere. The data quality does not support any definitive temporal changes for Ganymede. Possible temporal changes seen in the Callisto data set are concentrated on the jovian hemisphere. Example broadband UV spectra for Callisto show no definitive change in slope with time. The hypothesis is that these temporal differences in UV spectral properties are caused by variations in the surface ice chemistry due to temporal variability in the space environment. It is postulated that the UV spectral changes suggested for Europa may be linked to changes in H2O2 concentrations, whereas the changes on Callisto may be linked to variability in SO2 concentration.  相似文献   

17.
For many years an ongoing research program performed at our laboratory has had the aim to investigate the implantation of reactive ions in ices relevant to planetology by using IR spectroscopy. We present new results obtained by implanting 200 keV sulfur ions into water ice at 80 K. We have looked at the formation of sulfur-bearing molecules such as sulfuric acid, sulfur dioxide and hydrogen sulfide. We find that hydrated sulfuric acid is formed with high yield (0.65±0.1 molecules/ion). An upper limit to the production yield of SO2 (Y0.025 molecules/ion) has been estimated; no hydrogen sulfide has been detected. The formation of hydrogen peroxide is confirmed. Ozone is not detected. The results are discussed relevant to the inquiry on the radiolytic sulfur cycle considered responsible for the formation of sulfur-bearing molecules on the surfaces of the Galilean satellites. We demonstrate that sulfur implantation efficiently forms hydrated sulfuric acid whose observed abundance is explained as caused by an exogenic process. It is more difficult to say if the observed sulfur dioxide is quantitatively supported by only sulfur implantation; additional experimental studies are necessary along with direct observations, especially at UV wavelengths such as those that could be performed by instruments on board Hubble Space Telescope or by the forthcoming World Space Observatory (WSO/UV).  相似文献   

18.
First measurements of SO2 and SO in the Venus mesosphere (70-100 km) are reported. This altitude range is distinctly above the ∼60-70 km range to which nadir-sounding IR and UV investigations are sensitive. Since July 2004, use of ground-based sub-mm spectroscopy has yielded multiple discoveries. Abundance of each molecule varies strongly on many timescales over the entire sub-Earth Venus hemisphere. Diurnal behavior is evident, with more SO2, and less SO, at night than during the day. Non-diurnal variability is also present, with measured SO2 and SO abundances each changing by up to 2× or more between observations conducted on different dates, but at fixed phase, hence identical sub-Earth Venus local times. Change as large and rapid as a 5σ doubling of SO on a one-week timescale is seen. The sum of SO2 and SO abundances varies by an order of magnitude or more, indicating at least one additional sulfur reservoir must be present, and that it must function as both a sink and source for these molecules. The ratio SO2/SO varies by nearly two orders of magnitude, with both diurnal and non-diurnal components. In contrast to the strong time dependence of molecular abundances, their altitude distributions are temporally invariant, with far more SO2 and SO at 85-100 km than at 70-85 km. The observed increase of SO2 mixing ratio with altitude requires that the primary SO2 source be upper mesospheric photochemistry, contrary to atmospheric models which assert upward transport as the only source of above-cloud SO2. Abundance of upper mesospheric aerosol, with assumption that it is composed primarily of sulfuric acid, is at least sufficient to provide the maximum gas phase (SO + SO2) sulfur reported in this study. Sulfate aerosol is thus a plausible source of upper mesospheric SO2.  相似文献   

19.
We present a detailed study of an Iapetus mosaic of VIMS data with high spatial resolution (0.5 × 0.5° or ∼6.4 km/pixel). The spectra were taken in August 2007 and provide the highest VIMS spatial resolution data for this object during Cassini’s primary mission. We analyze this set of data using a statistical clustering approach to reduce the analysis of a large number of data (∼104 spectra from 0.35 to 5.10 μm) to the study of seven representative groups accounting for 99.6% of the surface covered by the original sample. We analyze the spectral absorption bands in the spectra of the different clusters indicative of different composition over the observed surface. We find coherence between the distribution of the clusters and the geographical features on the surface. We give special attention to the study of the water ice and CO2 bands. We find that CO2 is widespread over the entire surface being studied, including the bright and dark areas on Iapetus’ surface, and is probably trapped at the molecular level with other materials. The strength of the CO2 band in the areas where both, H2O- and carbon-bearing materials exist, gives support to the hypothesis that this volatile is formed on the surface of Iapetus as a product of irradiation of these two components. Finally, we also compare the Iapetus CO2 with that on other satellites confirming, that there are evident differences on the center, depth and width of the band on Iapetus and Phoebe, where CO2 has been suggested to be endogenous.  相似文献   

20.
Io’s sublimation-driven atmosphere is modeled using the direct simulation Monte Carlo (DSMC) method. These rarefied gas dynamics simulations improve upon earlier models by using a three-dimensional domain encompassing the entire planet computed in parallel. The effects of plasma heating, planetary rotation, inhomogeneous surface frost, molecular residence time of SO2 on the exposed (non-volatile) rocky surface, and surface temperature distribution are investigated. Circumplanetary flow is predicted to develop from the warm dayside toward the cooler nightside. Io’s rotation leads to a highly asymmetric frost surface temperature distribution (due to the frost’s high thermal inertia) which results in circumplanetary flow that is not axi-symmetric about the subsolar point. The non-equilibrium thermal structure of the atmosphere, specifically vibrational and rotational temperatures, is also examined. Plasma heating is found to significantly inflate the atmosphere on both the dayside and nightside. The plasma energy flux causes high temperatures at high altitudes but plasma energy depletion through the dense gas column above the warmest frost permits gas temperatures cooler than the surface at low altitudes. A frost map (Douté, S., Schmitt, B., Lopes-Gautier, R., Carlson, R., Soderblom, L., Shirley, J., and the Galileo NIMS Team [2001]. Icarus 149, 107-132) is used to control the sublimated flux of SO2 which can result in inhomogeneous column densities that vary by nearly a factor of four for the same surface temperature. A short residence time for SO2 molecules on the “rock” component is found to smooth lateral atmospheric inhomogeneities caused by variations in the surface frost distribution, creating an atmosphere that looks nearly identical to one with uniform frost coverage. A longer residence time is found to agree better with mid-infrared observations (Spencer, J.R., Lellouch, E., Richter, M.J., López-Valverde, M.A., Jessup, K.L, Greathouse, T.K., Flaud, J. [2005]. Icarus 176, 283-304) and reproduce the observed anti-jovian/sub-jovian column density asymmetry. The computed peak dayside column density for Io assuming a surface frost temperature of 115 K agrees with those suggested by Lyman-α observations (Feaga, L.M., McGrath, M., Feldman, P.D. [2009]. Icarus 201, 570-584). On the other hand, the peak dayside column density at 120 K is a factor of five larger and is higher than the upper range of observations (Jessup, K.L., Spencer, J.R., Ballester, G.E., Howell, R.R., Roesler, F., Vigel, M., Yelle, R. [2004]. Icarus 169, 197-215; Spencer et al., 2005).  相似文献   

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