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1.
In Titan's atmosphere consisting of N2 and CH4, large amounts of atomic hydrogen are produced by photochemical reactions during the formation of complex organics. This atomic hydrogen may undergo heterogeneous reactions with organic aerosol in the stratosphere and mesosphere of Titan. In order to investigate both the mechanisms and kinetics of the heterogeneous reactions, atomic deuterium is irradiated onto Titan tholin formed from N2 and CH4 gas mixtures at various surface-temperatures of the tholin ranging from 160 to 310 K. The combined analyses of the gas species and the exposed tholin indicate that the interaction mechanisms of atomic deuterium with the tholin are composed of three reactions; (a) abstraction of hydrogen from tholin resulting in gaseous HD formation (HD recombination), (b) addition of D atom into tholin (hydrogenation), and (c) removal of carbon and/or nitrogen (chemical erosion). The reaction probabilities of HD recombination and hydrogenation are obtained as ηabst=1.9(±0.6)×10−3×exp(−300/T) and ηhydro=2.08(±0.64)×exp(−1000/T), respectively. The chemical erosion process is very inefficient under the conditions of temperature range of Titan's stratosphere and mesosphere. Under Titan conditions, the rates of hydrogenation > HD recombination ? chemical erosion. Our measured HD recombination rate is about 10 times (with an uncertainty of a factor of 3-5) the prediction of previous theoretical model. These results imply that organic aerosol can remove atomic hydrogen efficiently from Titan's atmosphere through the heterogeneous reactions and that the presence of aerosol may affect the subsequent organic chemistry.  相似文献   

2.
Titan, Saturn's largest moon, has a thick nitrogen/methane atmosphere. The temperature and pressure conditions in Titan's atmosphere are such that the methane vapor should condense near the tropopause to form clouds. Several ground-based measurements have observed sparse cloud-like features in Titan's atmosphere, while the Cassini mission to Saturn has provided large scale images of the clouds. However, Titan's cloud formation conditions remain poorly constrained. Heterogeneous nucleation (from the vapor phase onto a solid or liquid aerosol surface) greatly enhances cloud formation relative to homogeneous nucleation. In order to elucidate the cloud formation mechanism near the tropopause, we have performed laboratory measurements of the adsorption of methane and ethane onto solid organic particles (tholins) representative of Titan's photochemical haze. We find that monolayers of methane adsorb onto tholin particles at saturation ratios less than unity. We also find that solid methane nucleates onto the adsorbed methane at a saturation ratio of S=1.07±0.008. This implies that Titan's methane clouds should form easily. This is consistent with recent measurements of the column of methane ruling out excessive methane supersaturation. In addition, we find ethane adsorbs onto tholin particles in a metastable phase prior to nucleation. However, ethane nucleation onto the adsorbed ethane occurs at a relatively high saturation ratio of S=1.36±0.08. These findings are consistent with the recent report of polar ethane clouds in Titan's lower stratosphere.  相似文献   

3.
Simon Petrie 《Icarus》2004,171(1):199-209
We report results of quantum chemical calculations of Mg+/ligand bond dissociation energies involving ligands identified as major constituents of Titan's upper atmosphere. Trends identified in these results allow elucidation of the important bimolecular and termolecular reactions of Mg+, and of simple molecular ions containing Mg+, arising from meteoric infall into Titan's atmosphere. Our study highlights, and includes calculated rate coefficients for, crucial ligand-switching and ligand-stripping reactions which ensure that a dynamic equilibrium exists between atomic and molecular ions of Mg+. Neutralization of ionized meteoric Mg is expected to produce the radical MgNC in high yield. The highly polar MgNC radical should provide an excellent nucleation site for condensation of polar (e.g., HCN, CH3CN, and HC3N) and highly unsaturated (e.g., C2H2, C4H2, and C2N2) neutrals at comparatively high altitude, leading to precipitation of Mg-doped tholin-like material. The implications for Titan's prebiotic chemical evolution, of the surface deposition of such material (which may feasibly contain magnesium porphyrins, or other bioactive Mg-containing complexes) remain to be assessed.  相似文献   

4.
C.M. Anderson  E.F. Young  C.P. McKay 《Icarus》2008,194(2):721-745
We report on the analysis of high spatial resolution visible to near-infrared spectral images of Titan at Ls=240° in November 2000, obtained with the Space Telescope Imaging Spectrograph instrument on board the Hubble Space Telescope as part of program GO-8580. We employ a radiative transfer fractal particle aerosol model with a Bayesian parameter estimation routine that computes Titan's absolute reflectivity per pixel for 122 wavelengths by modeling the vertical distribution of the lower atmosphere haze and tropospheric methane. Analysis of these data suggests that Titan's haze concentration in the lower atmosphere varies in strength with latitude. We find Titan's tropospheric methane profile to be fairly consistent with latitude and longitude, and we find evidence for local areas of a CH4-N2 binary saturation in Titan's troposphere. Our results suggest that a methane and haze profile at one location on Titan would not be representative of global conditions.  相似文献   

5.
A prominent feature of Titan's atmosphere is a thick haze region that acts as the end product of hydrocarbon and nitrile chemistry. Using a one-dimensional photochemical model, an investigation into the chemical mechanisms responsible for the formation of this haze region is conducted. The model derives profiles for Titan's atmospheric constituents that are consistent with observations. Included is an updated benzene profile that matches more closely with—recent ISO observations (Icarus 161 (2003) 383), replacing the profile given in the benzene study of Wilson et al. (J. Geophys. Res. 108 (2003) 5014). Using these profiles, pathways from polyynes, aromatics, and nitriles are considered, as well as possible copolymerization among the pathways. The model demonstrates that the growth of polycyclic aromatic hydrocarbons throughout the lower stratosphere plays an important role in furnishing the main haze layer, with nitriles playing a secondary role. The peak chemical production of haze layer ranges from 140 to 300 km peaking at an altitude of 220 km, with a production rate of 3.2×10−14 gcm−2 s−1. Possible mechanisms for polymerization and copolymerization and suggestions for further kinetic study are discussed, along with the implications for the distribution of haze in Titan's atmosphere.  相似文献   

6.
A global-mean model of coupled neutral and ion chemistry on Titan has been developed. Unlike the previous coupled models, the model involves ambipolar diffusion and escape of ions, hydrodynamic escape of light species, and calculates the H2 and CO densities near the surface that were assigned in some previous models. We tried to reduce the numbers of species and reactions in the model and remove all species and reactions that weakly affect the observed species. Hydrocarbon chemistry is extended to C12H10 for neutrals and C10H+11 for ions but does not include PAHs. The model involves 415 reactions of 83 neutrals and 33 ions, effects of magnetospheric electrons, protons, and cosmic rays. UV absorption by Titan's haze was calculated using the Huygens observations and a code for the aggregate particles. Hydrocarbon, nitrile, and ion chemistries are strongly coupled on Titan, and attempt to calculate them separately (e.g., in models of ionospheric composition) may result in significant error. The model densities of various species are typically in good agreement with the observations except vertical profiles in the stratosphere that are steeper than the CIRS limb data. (A model with eddy diffusion that facilitates fitting to the CIRS limb data is considered as well.) The CO densities are supported by the O+ flux from Saturn's magnetosphere. The ionosphere includes a peak at 80 km formed by the cosmic rays, steplike layers at 500-700 and 700-900 km and a peak at 1060 km (SZA = 60°). Nighttime densities of major ions agree with the INMS data. Ion chemistry dominates in the production of bicyclic aromatic hydrocarbons above 600 km. The model estimates of heavy positive and negative ions are in reasonable agreement with the Cassini results. The major haze production is in the reactions C6H + C4H2, C3N + C4H2, and condensation of hydrocarbons below 100 km. Overall, precipitation rate of the photochemical products is equal to 4-7 kg cm−2 Byr−1 (50-90 m Byr−1 while the global-mean depth of the organic sediments is ∼3 m). Escape rates of methane and hydrogen are 2.9 and 1.4 kg cm−2 Byr−1, respectively. The model does not support the low C/N ratio observed by the Huygens ACP in Titan's haze.  相似文献   

7.
Previous studies of the photochemistry of small molecules in Titan’s atmosphere found it difficult to have hydrogen atoms removed at a rate sufficient to explain the observed abundance of unsaturated hydrocarbons. One qualitative explanation of the discrepancy nominated catalytic aerosol surface chemistry as an efficient sink of hydrogen atoms, although no quantitative study of this mechanism was attempted. In this paper, we quantify how haze aerosols and macromolecules may efficiently catalyze the formation of hydrogen atoms into H2. We describe the prompt reaction model for the formation of H2 on aerosol surfaces and compare this with the catalytic formation of H2 using negatively charged hydrogenated aromatic macromolecules. We conclude that the PRM is an efficient mechanism for the removal of hydrogen atoms from the atmosphere to form H2 with a peak formation rate of ∼ 70 cm−3 s−1 at 420 km. We also conclude that catalytic H2 formation via hydrogenated anionic macromolecules is viable but much less productive (a maximum of ∼ 0.1 cm−3 s−1 at 210 km) than microphysical aerosols.  相似文献   

8.
Panayotis Lavvas 《Icarus》2009,201(2):626-633
By comparing observations from the Cassini imaging system, UV spectrometer, and Huygens atmospheric structure instrument, we determine an apparent radius of ∼40 nm, an imaginary index <0.3 at 187.5 nm and a number density of ∼30 particles cm−3 for the detached haze layer at 520 km in Titan's mesosphere. We point out that the detached haze layer is coincident with a local maximum in the measured temperature profile and show that the temperature maximum is caused by absorption of sunlight in the detached haze layer. This rules out condensation as the source of the layer. The derived particle size is in good agreement with that estimated for the size of the monomers in the aggregate particles that make up the main haze layer. Calculations of the sedimentation velocity of the haze particles coupled with the derived number density imply a mass flux , which is approximately equal to the mass flux required to explain the main haze layer. Because the aerosol size and mass flux derived for the detached layer agree with those determined for the main layer, we suggest that the main haze layer in Titan's stratosphere is formed primarily by sedimentation and coagulation of particles in the detached layer. This implies that high-energy radical and ion chemistry in the thermosphere is the main source of haze on Titan.  相似文献   

9.
The new one-dimensional radiative-convective/photochemical/microphysical model described in Part I is applied to the study of Titan's atmospheric processes that lead to haze formation. Our model generates the haze structure from the gaseous species photochemistry. Model results are presented for the species vertical concentration profiles, haze formation and its radiative properties, vertical temperature/density profiles and geometric albedo. These are validated against Cassini/Huygens observations and other ground-based and space-borne measurements. The model reproduces well most of the latest measurements from the Cassini/Huygens instruments for the chemical composition of Titan's atmosphere and the vertical profiles of the observed species. For the haze production we have included pathways that are based on pure hydrocarbons, pure nitriles and hydrocarbon/nitrile copolymers. From these, the nitrile and copolymer pathways provide the stronger contribution, in agreement with the results from the ACP instrument, which support the incorporation of nitrogen in the pyrolized haze structures. Our haze model reveals a new second major peak in the vertical profile of haze production rate between 500 and 900 km. This peak is produced by the copolymer family used and has important ramifications for the vertical atmospheric temperature profile and geometric albedo. In particular, the existence of this second peak determines the vertical profile of haze extinction. Our model results have been compared with the DISR retrieved haze extinction profiles and are found to be in very good agreement. We have also incorporated in our model heterogeneous chemistry on the haze particles that converts atomic hydrogen to molecular hydrogen. The resultant H2 profile is closer to the INMS measurements, while the vertical profile of the diacetylene formed is found to be closer to that of the CIRS profile when this heterogenous chemistry is included.  相似文献   

10.
Benzene has recently been observed in the atmosphere of Jupiter, Saturn and also Titan. This compound is required as a precursor for larger aromatic species (PAHs) that may be part of aerosol particles. Several photochemical models have tried to reproduce the observed quantities of benzene in the atmospheres of Jupiter (both low- and high-latitudes regions), Saturn and Titan. In this present work, we have conducted a sensitivity study of benzene and PAHs formation, using similar photochemical schemes both for Titan and Jupiter (low-latitudes conditions). Two different photochemical schemes are used, for which the modeled composition fairly agrees with observational constraints, both for Jupiter and Titan. Some disagreements are specific to each atmospheric case, which may point to needed improvements, especially in kinetic data involved in the corresponding chemical cycles. The observed benzene mole fraction in Titan's stratosphere is reproduced by the model, but in the case of Jupiter, low-latitudes benzene abundance is only 3% of the observed column density, which may indicate a possible influence of latitudinal transport, since abundance of benzene is much higher in auroral regions. Though, the photochemical scheme of C6 compounds at temperature and pressure conditions of planetary atmospheres is still very uncertain. Several variations are therefore done on key reactions in benzene production. These variations show that benzene abundance is mainly sensitive to reactions that may affect the propargyl radical. The effect of aerosol production on hydrocarbons composition is also tested, as well as possible heterogenous recombination of atomic hydrogen in the case of Titan. PAHs are a major pathway for aerosol production in both models. The mass production profiles for aerosols are discussed for both Titan and Jupiter. Total production mass fluxes are roughly three times the one expected by observational constraints in both cases. Such comparative studies are useful to bring more constraints on photochemical models.  相似文献   

11.
The formation of organic compounds in the atmosphere of Titan is an ongoing process of the generation of complex organics from the simplest hydrocarbon, methane. Solar radiation and magnetosphere electrons are the main energy sources that drive the reactions in Titan's atmosphere. Since energy from solar radiation is 200 times greater than that from magnetosphere electrons, we have investigated the products formed by the action of UV radiation (185 and 254 nm) on a mixture of gases containing nitrogen, methane, hydrogen, acetylene, ethylene, and cyanoacetylene, the basic gas mixture (BGM) that simulates aspects of Titan's atmosphere using a flow reactor [Tran, B.N., Ferris, J.P., Chera, J.J., 2003a. Icarus 162, 114-124; Tran, B.N., Joseph, J.C., Force, M., Briggs, R.G., Vuitton, V., Ferris, J.P., 2005. Icarus 177, 106-115]. The present research extends these studies by the addition of carbon monoxide and hydrogen cyanide to the BGM. Quantum yields for the loss of reactants and the formation of volatile products were determined and compared with those measured in the absence of the hydrogen cyanide and carbon monoxide. The GCMS analyses of the volatile photolysis products from the BGM, with added hydrogen cyanide, had a composition similar to that of the BGM while the photolysis products of the BGM with added carbon monoxide contained many oxygenated compounds. The infrared spectrum of the corresponding solid product revealed the absorption band of a ketone group, which was probably formed from the reaction of carbon monoxide with the free radicals generated by photolysis of acetylene and ethylene. Of particular interest was the observation that the addition of HCN to the gas mixture only resulted in a very small change in the C/N ratio and in the intensity of the CN frequency at 2210 cm−1 in the infrared spectrum suggesting that little HCN is incorporated into the haze analog. The C/N ratio of the haze analogs was found to be in the 10-12 range. The UV spectra of the solid products formed when HCN or CO added to the BGM is similar to the UV absorption formed from the BGM alone. This result is consistent with absence of additional UV chromophores to the solid product when these mixtures are photolyzed. The following photoproducts, which were not starting materials in our photochemical studies, have been observed on Titan: acetonitrile, benzene, diacetylene, ethane, propene, propane, and propyne.  相似文献   

12.
A solar occultation by Titan's atmosphere has been observed through the solar port of the Cassini/VIMS instrument on January 15th, 2006. Transmission spectra acquired during solar egress probe the atmosphere in the altitude range 70 to 900 km at the latitude of 71° S. Several molecular absorption bands of CH4 and CO are visible in these data. A line-by-line radiative transfer calculation in spherical geometry is used to model three methane bands (1.7, 2.3, 3.3 μm) and the CO 4.7 μm band. Above 200 km, the methane 2.3 μm band is well fit with constant mixing ratio between 1.4 and 1.7%, in agreement with in situ and other Cassini measurements. Under 200 km, there are discrepancies between models and observations that are yet fully understood. Under 480 km, the 3.3 μm CH4 band is mixed with a large and deep additional absorption. It corresponds to the C-H stretching mode of aliphatic hydrocarbon chains attached to large organic molecules. The CO 4.7 μm band is observed in the lower stratosphere (altitudes below 150 km) and is well fit with a model with constant mixing ratio of 33±10 ppm. The continuum level of the observed transmission spectra provides new constraints on the aerosol content of the atmosphere. A model using fractal aggregates and optical properties of tholins produced by Khare et al. [Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W., 1984. Icarus 60, 127-137] is developed. Fractal aggregates with more than 1000 spheres of radius 0.05 μm are needed to fit the data. Clear differences in the chemical composition are revealed between tholins and actual haze particles. Extinction and density profiles are also retrieved using an inversion of the continuum values. An exponential increase of the haze number density is observed under 420 km with a typical scale height of 60 km.  相似文献   

13.
H.G. Roe  I. de Pater 《Icarus》2004,169(2):440-461
All previous observations of seasonal change on Titan have been of physical phenomena such as clouds and haze. We present here the first observational evidence of chemical change in Titan's atmosphere. Images taken during 1999-2002 (late southern spring on Titan) with the W.M. Keck I 10-meter telescope at 8-13 μm show a significant accumulation of ethylene (C2H4) in the south polar stratosphere as well as north-south stratospheric temperature variation (colder at poles). Our observations restrict this newly discovered south polar ethylene accumulation to latitudes south of 60° S. The only other observations of the spatial distribution of C2H4 were those of Voyager I, which found a significant north polar accumulation in early northern spring. We see no build-up in the north, although the highest northern latitudes are obstructed from view in the current season. Our observations constrain any unobserved north polar accumulation of C2H4 to north of 50° N latitude. Comparison of the Voyager I results with our new results show seasonal chemical change has occurred in Titan's atmosphere.  相似文献   

14.
Mid-infrared limb spectra in the range 600-1400 cm−1 taken with the Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraft were used to determine vertical profiles of HCN, HC3N, C2H2, and temperature in Titan's atmosphere. Both high (0.5 cm−1) and low (13.5 cm−1) spectral resolution data were used. The 0.5 cm−1 data gave profiles at four latitudes and the 13.5 cm−1 data gave almost complete latitudinal coverage of the atmosphere. Both datasets were found to be consistent with each other. High temperatures in the upper stratosphere and mesosphere were observed at Titan's northern winter pole and were attributed to adiabatic heating in the subsiding branch of a meridional circulation cell. On the other hand, the lower stratosphere was much colder in the north than at the equator, which can be explained by the lack of solar radiation and increased IR emission from volatile enriched air. HC3N had a vertical profile consistent with previous ground based observations at southern and equatorial latitudes, but was massively enriched near the north pole. This can also be explained in terms of subsidence at the winter pole. A boundary observed at 60° N between enriched and un-enriched air is consistent with a confining polar vortex at 60° N and HC3N's short lifetime. In the far north, layers were observed in the HC3N profile that were reminiscent of haze layers observed by Cassini's imaging cameras. HCN was also enriched over the north pole, which gives further evidence for subsidence. However, the atmospheric cross section obtained from 13.5 cm−1 data indicated a HCN enriched layer at 200-250 km, extending into the southern hemisphere. This could be interpreted as advection of polar enriched air towards the south by a meridional circulation cell. This is observed for HCN but not for HC3N due to HCN's longer photochemical lifetime. C2H2 appears to have a uniform abundance with altitude and is not significantly enriched in the north. This is consistent with observations from previous CIRS analysis that show increased abundances of nitriles and hydrocarbons but not C2H2 towards the north pole.  相似文献   

15.
David E. Woon  Jin-Young Park 《Icarus》2009,202(2):642-680
Barrierless reactions between unsaturated hydrocarbons and the ethynyl radical (C2H) can contribute to the growth of organic particulates in the haze-forming regions of Titan's atmosphere as well as in the gas giants and in the interstellar medium. We employed a combination of quantum chemistry and statistical rate theories to characterize reactions between ground state C2H and seven alkenes of the general structure R1R2C′C″R3R4 containing up to six carbons. The alkenes included ethene (C2H4); propene (C3H6); 1-butene, 2-butene, and isobutene (C4H8); trimethylethene (C5H10); and tetramethylethene (C6H12). Density functional theory calculations at the B3LYP/6-31 + G∗∗ level were used to characterize the adducts, isomers, products, and the intervening transition states for the addition-elimination reactions of all seven species. A multiple-well treatment was then employed to determine the outcome distributions for the range of temperatures and pressures relevant to Titan's atmosphere, the interstellar medium, and the outer atmospheres of the gas giants. Finally, trajectory calculations using an ROMP2 potential energy surface were used to calculate kinetic rates for the ethene + C2H reaction, where the agreement between the computed and measured values is very good. At low pressure and temperature, vinyl acetylene is a dominant product of several of the reactions, and all of the reactions yield at least one dominant product with both a double and a triple CC bond.  相似文献   

16.
The main gas-phase constituents of Titan's upper atmosphere, N2 and CH4, are photolyzed and radiolyzed by solar photons and magnetospheric electrons, respectively. The primary products of these chemical interactions evolve to heavier organic compounds that are likely to associate into the particles of haze layers that hide Titan's surface. The different theories and models that have been put forward to explain the characteristics and properties of the haze composites require a knowledge of their optical properties, which are determined by the complex refractive index. We present a new set of values for refractive index n and extinction coefficient k calculated directly from the transmittance and reflectance curves exhibited by a laboratory analogue of Titan's aerosols in the 200-900 nm range. Improvements in the aerosol analogue quality have been made. The effects of variables such as the uncertainty in sample thickness, aerosol porosity, and amount of scattered light on the final n and k values are assessed and discussed. Within the studied wavelength domain, n varies from 1.53 to 1.68 and k varies from 2.62×10−4 to 2.87×10−2. These final n and k values should be considered as a new reference to modelers who compute the properties of Titan's aerosols in trying to explain the atmospheric dynamics and surface characteristics.  相似文献   

17.
We investigate the chemical transition of simple molecules like C2H2 and HCN into aerosol particles in the context of Titan's atmosphere. Experiments that synthesize analogs (tholins) for these aerosols can help illuminate and constrain these polymerization mechanisms. Using information available from these experiments, we suggest chemical pathways that can link simple molecules to macromolecules, which will be the precursors to aerosol particles: polymers of acetylene and cyanoacetylene, polycyclic aromatics, polymers of HCN and other nitriles, and polyynes. Although our goal here is not to build a detailed kinetic model for this transition, we propose parameterizations to estimate the production rates of these macromolecules, their C/N and C/H ratios, and the loss of parent molecules (C2H2, HCN, HC3N and other nitriles, and C6H6) from the gas phase to the haze. We use a one-dimensional photochemical model of Titan's atmosphere to estimate the formation rate of precursor macromolecules. We find a production zone slightly lower than 200 km altitude with a total production rate of 4×10−14 g cm−2 s−1 and a C/N?4. These results are compared with experimental data, and to microphysical model requirements. The Cassini/Huygens mission will bring a detailed picture of the haze distribution and properties, which will be a great challenge for our understanding of these chemical processes.  相似文献   

18.
Using a one-dimensional model, we investigate the hydrogen budget and escape to space in Titan’s atmosphere. Our goal is to study in detail the distributions and fluxes of atomic and molecular hydrogen in the model, while identifying sources of qualitative and quantitative uncertainties. Our study confirms that the escape of atomic and molecular hydrogen to space is limited by the diffusion through the homopause level. The H distribution and flux inside the atmosphere are very sensitive to the eddy diffusion coefficient used above altitude 600 km. We chose a high value of this coefficient 1 × 108 cm2 s−1 and a homopause level around altitude 900 km. We find that H flows down significantly from the production region above 500 km to the region [300-500] km, where it recombines into H2. Production of both H and H2 also occurs in the stratosphere, mostly from photodissociation of acetylene. The only available observational data to be compared are the escape rate of H deduced from Pioneer 11 and IUE observations of the H torus 1-3 × 109 cm−2 s−1 and the latest retrieved value of the H2 mole fraction in the stratosphere: (1.1 ± 0.1) × 10−3. Our results for both of these values are at least 50-100% higher, though the uncertainties within the chemical schemes and other aspects of the model are large. The chemical conversion from H to H2 is essentially done through catalytic cycles using acetylene and diacetylene. We have studied the role of this diacetylene cycle, for which the associated reaction rates are poorly known. We find that it mostly affects C4 species and benzene in the lower atmosphere, rather than the H profile and the hydrogen budget. We have introduced the heterogenous recombination of hydrogen on the surface of aerosol particles in the stratosphere, and this appears to be a significant process, comparable to the chemical processes. It has a major influence on the H distribution, and consequently on several other species, especially C3H4, C4H2 and C6H6. Therefore, this heterogenous process should be taken into account when trying to understand the stratospheric distribution of these hydrocarbons.  相似文献   

19.
Simulations of Titan's atmospheric transmission and surface reflectivity have been developed in order to estimate how Titan's atmosphere and surface properties could affect performances of the Cassini radar experiment. In this paper we present a selection of models for Titan's haze, vertical rain distribution, and surface composition implemented in our simulations. We collected dielectric constant values for the Cassini radar wavelength (∼2.2 cm) for materials of interest for Titan: liquid methane, liquid mixture of methane-ethane, water ice, and light hydrocarbon ices. Due to the lack of permittivity values for Titan's haze particles in the microwave range, we performed dielectric constant (εr) measurements around 2.2 cm on tholins synthesized in laboratory. We obtained a real part of εr in the range of 2-2.5 and a loss tangent between 10−3 and 5×10−2. By combining aerosol distribution models (with hypothetical condensation at low altitudes) to surface models, we find the following results: (1) Aerosol-only atmospheres should cause no loss and are essentially transparent for Cassini radar, as expected by former analysis. (2) However, if clouds are present, some atmospheric models generate significant attenuation that can reach −50 dB, well below the sensitivity threshold of the receiver. In such cases, a 13.78 GHz radar would not be able to measure echoes coming from the surface. We thus warn about possible risks of misinterpretation if a “wet atmosphere” is not taken into account. (3) Rough surface scattering leads to a typical response of ∼−17 dB. These results will have important implications on future Cassini radar data analysis.  相似文献   

20.
Titan's tholins are used as analogs of Titan's aerosols and N-rich organic solids present on many icy surfaces. However, it is not clear whether or not they are relevant analogs, and which kind of tholins should be used among a wide set available in literature. This paper presents reflectance spectral data of two tholins selected as end-members of a series of samples covering a very wide range of continuous chemical and optical properties. These samples were formed under experimental conditions fairly consistent with Titan's stratosphere. A general framework for using these laboratory data to the analysis of spectral observation of Titan's surface or other objects is suggested. Furthermore, the study reports the first in situ unambiguous identification of aromatics compounds and evidences variations in the sp2 carbon structure, which controls the absorption properties in the visible/NIR. These results also point out it is very unlikely to derive quantitative chemical information (e.g., N content, sp2/sp3 ratio) from remote sensing reflectance data.  相似文献   

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