Methane in Oort cloud comets     

E.L. Gibb  M.J. Mumma  M.A. DiSanti 《Icarus》2003,165(2):391-406

We detected CH₄ in eight Oort cloud comets using high-dispersion (λ/Δλ∼2×10⁴) infrared spectra acquired with CSHELL at NASA's IRTF and NIRSPEC at the W.M. Keck Observatory. The observed comets were C/1995 O1 (Hale-Bopp), C/1996 B2 (Hyakutake), C/1999 H1 (Lee), C/1999 T1 (McNaught-Hartley), C/1999 S4 (LINEAR), C/2000 WM₁ (LINEAR), C/2001 A2 (LINEAR), and 153/P Ikeya-Zhang (C/2002 C1). We detected the R0 and R1 lines of the ν₃ vibrational band of CH₄ near 3.3 μm in each comet, with the exception of McNaught-Hartley where only the R0 line was measured. In order to obtain production rates, a fluorescence model has been developed for this band of CH₄. We report g-factors for the R0 and R1 transitions at several rotational temperatures typically found in comet comae and relevant to our observations. Using g-factors appropriate to T_rot as determined from HCN, CO and/or H₂O and C₂H₆, CH₄ production rates and mixing ratios are presented. Abundances of CH₄/H₂O are compared among our existing sample of comets, in the context of establishing their place of origin. In addition, CH₄ is compared to native CO, another hypervolatile species, and no correlation is found among the comets observed.  相似文献   

Laboratory experiments of Titan tholin formed in cold plasma at various pressures: implications for nitrogen-containing polycyclic aromatic compounds in Titan haze     

Hiroshi Imanaka  Bishun N. Khare  Emma L.O. Bakes  Dale P. Cruikshank  Takafumi Matsui 《Icarus》2004,168(2):344-366

Titan, the largest satellite of Saturn, has a thick nitrogen/methane atmosphere with a thick global organic haze. A laboratory analogue of Titan's haze, called tholin, was formed in an inductively coupled plasma from nitrogen/methane=90/10 gas mixture at various pressures ranging from 13 to 2300 Pa. Chemical and optical properties of the resulting tholin depend on the deposition pressure in cold plasma. Structural analyses by IR and UV/VIS spectroscopy, microprobe laser desorption/ionization mass spectrometry, and Raman spectroscopy suggest that larger amounts of aromatic ring structures with larger cluster size are formed at lower pressures (13 and 26 Pa) than at higher pressures (160 and 2300 Pa). Nitrogen is more likely to incorporate into carbon networks in tholins formed at lower pressures, while nitrogen is bonded as terminal groups at higher pressures. Elemental analysis reveals that the carbon/nitrogen ratio in tholins increases from 1.5-2 at lower pressures to 3 at 2300 Pa. The increase in the aromatic compounds and the decrease in C/N ratio in tholin formed at low pressures indicate the presence of the nitrogen-containing polycyclic aromatic compounds in tholin formed at low pressures. Tholin formed at high pressure (2300 Pa) consists of a polymer-like branched chain structure terminated with CH₃, NH₂, and CN with few aromatic compounds. Reddish-brown tholin films formed at low pressures (13-26 Pa) shows stronger absorptions (almost 10 times larger k-value) in the UV/VIS range than the yellowish tholin films formed at high pressures (160 and 2300 Pa). The tholins formed at low pressures may be better representations of Titan's haze than those formed at high pressures, because the optical properties of tholin formed at low pressures agree well with that of Khare et al. (1984a, Icarus 60, 127-137), which have been shown to account for Titan's observed geometric albedo. Thus, the nitrogen-containing polycyclic aromatic compounds we find in tholin formed at low pressure may be present in Titan's haze. These aromatic compounds may have a significant influence on the thermal structure and complex organic chemistry in Titan's atmosphere, because they are efficient absorbers of UV radiation and efficient charge exchange intermediaries. Our results also indicate that the haze layers at various altitudes might have different chemical and optical properties.  相似文献   

Mineralogical constraints on the high-silica martian surface component observed by TES     

Joseph R. Michalski  Michael D. Kraft  Lynda B. Williams 《Icarus》2005,174(1):161-177

The Thermal Emission Spectrometer (TES) has observed a high-silica material in the dark regions of Mars that is spectrally similar to obsidian glass and may have a volcanic origin. An alternate interpretation is that the spectrally amorphous material consists of clay minerals or some other secondary material, formed by chemical alteration of surface rocks. The regions where this material is observed (e.g., Acidalia Planitia) have relatively high spectral contrast, suggesting that the high-silica material exists as coarse particulates, indurated soils or cements, within rocks, or as indurated coatings on rock surfaces. The geologic interpretation of this spectral result has major implications for understanding magmatic evolution and weathering processes on Mars. One of the complications in interpreting spectral observations of glasses and clay minerals is that both are structurally and compositionally complex. In this study, we perform a detailed spectroscopic analysis of indurated smectite clay minerals and relate their thermal emission spectral features to structural and crystal chemical properties. We examine the spectral similarities and differences between smectite clay minerals and obsidian glass from a structural-chemical perspective, and make further mineralogical interpretations from previous TES results. The results suggest that neither smectite clays nor any clay mineral with similar structural and chemical properties can adequately explain TES observations of high-silica materials in some martian dark regions. If the spectrally amorphous materials observed by TES do represent an alteration product, then these materials are likely to be poorly crystalline aluminosilicates. While all clay minerals have Si/O ratios ?0.4, the position of the emissivity minimum at Mars suggests a Si/O ratio of 0.4-0.5. The spectral observation could be explained by the existence of a silica-rich alteration product, such as Al- or Fe-bearing opal, an intimate physical mixture of relatively pure silica and other aluminosilicates (such as clay minerals or clay precursors), or certain zeolites. The chemical alteration of basaltic rocks on Mars to phyllosilicate-poor, silica-rich alteration products provides a geologically reasonable and consistent explanation for the global TES surface mineralogical results.  相似文献   

Spectral comparison of heavily hydrated salts with disrupted terrains on Europa   总被引：1，自引：0，他引：1  

J.B. Dalton  O. Prieto-Ballesteros  C.S. Jamieson  R. Quinn 《Icarus》2005,177(2):472-490

Hydrated magnesium sulfate salts have been proposed as major components of the disrupted, reddish terrains on the surface of Europa. This is based on near-infrared reflectance spectra which contain distorted and asymmetric water absorption features typical of moderately hydrated materials such as hexahydrite (MgSO₄⋅6H₂O) and epsomite (MgSO₄⋅7H₂O). Hydrated magnesium sulfates having many waters of hydration could produce improved spectral matches. Here we present cryogenic laboratory spectra of highly hydrated sulfur-bearing salts, including hexahydrite, epsomite, bloedite (Na₂Mg(SO₄)₂⋅4H₂O), mirabilite (Na₂SO₄⋅10H₂O), sodium sulfide nonahydrate (Na₂S⋅9H₂O), supersaturated MgSO₄, NaHCO₃, and Na₂SO₄ brines, and magnesium sulfate dodecahydrate (MgSO₄⋅12H₂O). All have been measured under conditions of pressure and temperature appropriate to the surface environment of Europa. Novel methods for preparation, verification and analysis of MgSO₄⋅12H₂O, which is not stable at standard temperature and pressure (STP), are described. At 100 K, all of these materials exhibit distorted and asymmetric absorption features similar to those in the Europa observations, as well as several weaker, narrow absorptions having widths ranging from 15 to 80 nm. While the agreement with Galileo NIMS observations of dark terrains on Europa is indeed better for highly hydrated salts than for salts of lower hydration states, we conclude that none of these materials alone can account for all of the observed spectral character. As previously suggested, Europa's reddish material appears to be a complex mixture of sulfate hydrates and other materials.  相似文献   

Reanalysis of asteroid families structure through visible spectroscopy     

T. Mothé-Diniz  F. Roig 《Icarus》2005,174(1):54-80

The taxonomic properties of the main asteroid families are analyzed and discussed in the light of an updated definition of the families using a large proper elements database and the asteroids taxonomy derived from reflectance spectra recently obtained by two large visible spectroscopic surveys: the SMASS II and the S3OS2. Our analysis indicates that most families are quite homogeneous taxonomically and mineralogically—whenever there exists a mineralogical constraint—, being probably originated from homogeneous parent bodies. The exceptions are the Nysa family, that should likely be considered a clan, and the Eos family that encompasses a broad range of taxonomies, whose mineralogical relations cannot be completely ruled out. Only in a few cases the families may be taxonomically distinguished from the background population. That is the case of the Minerva/Gefion, Adeona, Dora, Merxia, Hoffmeister, Koronis, Eos, and Veritas families. Some of the families presented in this work show a larger spectral diversity than previously reported, as it is the case for the Maria and Koronis families. On the other hand, the Veritas family is found to be homogeneous, in sharp contrast with previous works. Mineralogical relations are reported whenever they could be found in the literature and we examine the possible constraints posed by the presence of different taxonomies in certain families.  相似文献   

A search for temporal variability in the surface chemistry of the icy Galilean satellites     

Deborah Domingue  Amanda Hendrix 《Icarus》2005,173(1):50-65

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   

Carbon monoxide fluorescence from Titan's atmosphere     

M.A. López-Valverde  E. Lellouch 《Icarus》2005,175(2):503-521

We report on the discovery of emissions due to carbon monoxide from Titan's atmosphere, from mid-infrared observations with the ISAAC spectrometer at the Very Large Telescope and covering the 4.50-4.85 μm range. We detected about 45 emission lines coinciding with CO ro-vibrational lines, including CO(1-0) (P18 to R11) and CO(2-1) (P11 to R11). We show that these emissions cannot be generated thermally but occur in non-LTE conditions, due to radiative de-excitation from the v=1 and v=2 CO levels after excitation at 4.7 and 2.3 μm by solar radiation. A complete fluorescence model is then developed, allowing to compute the state populations of the two most abundant CO isotopes and N₂(1). It includes absorption by CO and CH₄, and vibrational-thermal and vibrational-vibrational collisional exchanges with CO, N₂, CH₄, and H₂. Emerging radiances at the top of the atmosphere are evaluated with a line-by-line code and compared to observations. Contribution functions show that the CO emissions sound Titan's stratosphere: while the (1-0) lines generally probe two layers, located respectively at 100-250 km and 300-550 km, the (2-1) lines are sensitive to the intermediate layer at 150-300 km. A sensitivity study is performed to establish the effect of the main model parameters (temperature profile, collisional scenario, and CO stratospheric abundance) on the results. Models reproduce the essential structure of the observed emissions. The (1-0) fundamental band is generally well fit with a nominal CO mixing ratio of 32 ppm—as inferred in the troposphere from observations at 4.80-5.10 μm (Lellouch et al., 2003, Icarus 162, 126-143). However, this band is only weakly dependent on the CO abundance, and given temperature and collisional scenario uncertainties, it constrains the CO stratospheric mixing ratio only to within a factor of ∼3. In addition, the nominal model with 32 ppm CO underestimates the first hot (2-1) transition by approximately a factor of 2. This discrepancy can be resolved by a combined adjustment of collisional rates and an increased CO stratospheric ratio of 60 ppm, consistent with the determination of Gurwell and Muhleman (2000, Icarus 145, 653-656). In contrast, the CO vertical profile suggested by Hidayat et al. (1998, Icarus 133, 109-133), strongly depleted above 200 km, cannot match the data for any realistic collisional scenario, and is therefore not supported by our results.  相似文献   

MAWD observations revisited: seasonal behavior of water vapor in the martian atmosphere     

A.A. Fedorova  A.V. Rodin  I.V. Baklanova 《Icarus》2004,171(1):54-67

Viking/MAWD experiment and more recent MGC/TES observations have provided to date the most detailed information about the annual atmospheric water cycle on Mars. Their data agree in major details but still reveal some disagreements. These disagreements turn out to be most significant in the perihelion season and especially during the major dust storms. We consider the potential influence of aerosol scattering on 1.38 μm water retrieval under various types of observation geometry. In order to obtain new retrievals of water vapor abundance from MAWD data, we apply radiative transfer calculations. The resulting seasonal and spatial distribution of water turns out to be more consistent with TES results, implying a remarkable stability of the martian seasonal water cycle. Mapping data corresponding to particular seasons reveals a distinct wave structure in the global distribution of the water column. We interpret it as a manifestation of a strong control over the water cycle on Mars from the atmospheric circulation.  相似文献   

Optical characterization of laser ablated silicates     

Rosario Brunetto  Ted L. Roush  Vincenzo Orofino 《Icarus》2007,191(1):381-393

We perform an optical characterization of UV laser ablated silicates (olivine, pyroxene), starting from their reflectance spectra in the 0.3-2.5 μm spectral range. The goal is to provide useful tools to model space weathering effects on surfaces of asteroids and TNOs (trans-neptunian objects). We determine that the reddening and darkening spectral trend is compatible with the Hapke's space weathering model, using the optical constants of metallic iron in a silicate matrix. This result is supported by new magnetic susceptibility measurements on laser ablated orthopyroxene. We also investigate the potential contribution of formation of amorphous silicates in the process. Applying our results to silicate-rich surfaces in the Solar System, we investigate the possibility of a weathered olivine component on the surface of Centaur 5145 Pholus. Inclusion of this component slightly decreases the amount of complex organics and water ice from those previously estimated. Thus, the current Pholus spectrum is consistent with the presence of either unweathered or weathered olivine, or potentially both materials.  相似文献   

The distribution of water ice in the interior of Comet Tempel 1     

Jessica M. Sunshine  Olivier Groussin  Michael F. A'Hearn  Tony L. Farnham 《Icarus》2007,190(2):284-294

The Deep Impact flyby spacecraft includes a 1.05 to 4.8 μm infrared (IR) spectrometer. Although ice was not observed on the surface in the impact region, strong absorptions near 3 μm due to water ice are detected in IR measurements of the ejecta from the impact event. Absorptions from water ice occur throughout the IR dataset beginning three seconds after impact through the end of observations, ∼45 min after impact. Spatially and temporally resolved IR spectra of the ejecta are analyzed in conjunction with laboratory impact experiments. The results imply an internal stratigraphy for Tempel 1 consisting of devolatilized materials transitioning to unaltered components at a depth of approximately one meter. At greater depths, which are thermally isolated from the surface, water ice is present. Up to depths of 10 to 20 m, the maximum depths excavated by the impact, these pristine materials consist of very fine grained (∼1±1 μm) water ice particles, which are free from refractory impurities.  相似文献