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1.
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   

2.
Erika L. Barth  Owen B. Toon 《Icarus》2006,182(1):230-250
Theoretical arguments point to and recent observations confirm the existence of clouds in Titan's atmosphere, yet we possess very little data on their particle size, composition and formation mechanism. A time-dependent microphysical model is used to study the evolution of ice clouds in Titan's atmosphere. The model simulates nucleation, condensational growth, evaporation, coagulation, and transport of particles in a column of atmosphere. A variety of cloud compositions are studied, including pure ethane clouds, pure methane clouds, and mixed methane-ethane clouds (all with tholin cores). The abundance of methane cloud particles may be limited by the number of ethane coated tholin nuclei rather than the number of tholins with hydrocarbon coatings. However, even the condensation of methane onto these relatively sparse ethane/tholin cloud particles is sufficient to keep the methane close to saturation. Typical methane supersaturations are of order 0.06 on the average. For simulations which take into account recent lab measurements indicating it is relatively easy for methane to nucleate onto tholin particles without an ethane-layer present, the three types of clouds (methane, ethane, and mixed) exist simultaneously. Pure methane clouds are the most abundant cloud type and serve to lower the supersaturation to about 0.04. Cloud production does not require a continuous surface source of methane. However, clouds produced by mean motions are not the visible methane clouds seen in recent Cassini and ground-based observations. Ethane clouds in the troposphere almost instantaneously nucleate methane to form mixed clouds. However, a thin ethane ‘haze’ remains just above the tropopause for some scenarios and the mixed clouds at the tropopause remain ?50% ethane by mass. Also, evaporation of methane from the mixed cloud particles near the surface leaves a thicker layer of ethane cloud particles at ∼10 km. Nevertheless, the precipitation rate of methane to Titan's surface is between 0.001 and 0.5 cm/terrestrial-year, depending on various initial conditions such as critical saturation, size and abundance of cloud condensation nuclei, surface sources and eddy diffusion.  相似文献   

3.
The near-infrared spectrum of Titan, Saturn's largest moon and one of the Cassini/Huygens' space mission primary targets, covers the 0.8 to 5 micron region in which it shows several weak CH4 absorption regions, and in particular one centered near 2.75 micron. Due to the interference of telluric absorption, only part of this window region (2.9-3.1 μm) has previously been observed from the ground [Noll, K.S., Geballe, T.R., Knacke, R., Pendleton, F., Yvonne, J., 1996. Icarus 124, 625-631; Griffith, C.A., Owen, T., Miller, G.A., Geballe, T., 1998. Nature 395, 575-578; Griffith, C.A., Owen, T., Geballe, T.R., Rayner, J., Rannou, P., 2003. Science 300, 628-630; Geballe, T.R., Kim, S.J., Noll, K.S., Griffith, C.A., 2003. Astrophys. J. 583, L39-L42]. We report here on the first spectroscopic observations of Titan covering the whole 2.4-4.9 μm region by two instruments on board the Infrared Space Observatory (ISO) in 1997. These observations show the 2.75-μm window in its complete extent for the first time. In this study we have also used a high-resolution Titan spectrum in the 2.9-3.6 μm region taken with the Keck [Geballe, T.R., Kim, S.J., Noll, K.S., Griffith, C.A., 2003. Astrophys. J. 583, L39-L42; Kim, S.J., Geballe, T.R., Noll, K.S., Courtin, R., 2005. Icarus 173, 522-532] to infer information on the atmospheric parameters (haze extinction, single scattering albedo, methane abundance, etc.) by fitting the methane bands with a detailed microphysical model of Titan's atmosphere (updated from Rannou, P., McKay, C.P., Lorenz, R.D., 2003. Planet. Space Sci. 51, 963-976). We have included in this study an updated version of a database for the CH4 absorption coefficients [STDS, Wenger, Ch., Champion, J.-P., 1998. J. Quant. Spectrosc. Radiat. Transfer 59, 471-480. See also http://www.u-bourgogne.fr/LPUB/TSM/sTDS.html for latest updates; Boudon, V., Champion, J.-P., Gabard, T., Loëte, M., Michelot, F., Pierre, G., Rotger, M., Wenger, Ch., Rey, M., 2004. J. Mol. Spectrosc. 228, 620-634]. For the atmosphere we find that (a) the haze extinction profile that best matches the data is one with higher (by 40%) extinction in the atmosphere with respect to Rannou et al. (2003) down to about 30 km where a complete cut-off occurs; (b) the methane mixing ratio at Titan's surface cannot exceed 3% on a disk-average basis, yielding a maximum CH4 column abundance of 2.27 km-am in Titan's atmosphere. From the derived surface albedo spectrum in the 2.7-3.08 micron region, we bring some constraints on Titan's surface composition. The albedo in the center of the methane window varies from 0.01 to 0.08. These values, compared to others reported in the other methane windows, show a strong compatibility with the water ice spectrum in the near-infrared. Without confirming its existence from this work alone, our data then appear to be compatible with water ice. A variety of other ices, such as CO2, NH3, tholin material or hydrocarbon liquid cannot be excluded from our data, but an additional unidentified component with a signature around 2.74 micron is required to satisfy the data.  相似文献   

4.
Titan's bulk density along with Solar System formation models indicates considerable water as well as silicates as its major constituents. This satellite's dense atmosphere of nitrogen with methane is unique. Deposits or even oceans of organic compounds have been suggested to exist on Titan's solid surface due to UV-induced photochemistry in the atmosphere. Thus, the composition of the surface is a major piece of evidence needed to determine Titan's history. However, studies of the surface are hindered by the thick, absorbing, hazy and in some places cloudy atmosphere. Ground-based telescope investigations of the integral disk of Titan attempted to observe the surface albedo in spectral windows between methane absorptions by calculating and removing the haze effects. Their results were reported to be consistent with water ice on the surface that is contaminated with a small amount of dark material, perhaps organic material like tholin. We analyze here the recent Cassini Mission's visual and infrared mapping spectrometer (VIMS) observations that resolve regions on Titan. VIMS is able to see surface features and shows that there are spectral and therefore likely compositional units. By several methods, spectral albedo estimates within methane absorption windows between 0.75 and 5 μm were obtained for different surface units using VIMS image cubes from the Cassini-Huygens Titan Ta encounter. Of the spots studied, there appears to be two compositional classes present that are associated with the lower albedo and the higher albedo materials, with some variety among the brighter regions. These were compared with spectra of several different candidate materials. Our results show that the spectrum of water ice contaminated with a darker material matches the reflectance of the lower albedo Titan regions if the spectral slope from 2.71 to 2.79 μm in the poorly understood 2.8-μm methane window is ignored. The spectra for brighter regions are not matched by the spectrum of water ice or unoxidized tholin, in pure form or in mixtures with sufficient ice or tholin present to allow the water ice or tholin spectral features to be discerned. We find that the 2.8-μm methane absorption window is complex and seems to consist of two weak subwindows at 2.7 and 2.8 μm that have unknown opacities. A ratio image at these two wavelengths reveals an anomalous region on Titan that has a reflectance unlike any material so far identified, but it is unclear how much the reflectances in these two subwindows pertain to the surface.  相似文献   

5.
W. Macy 《Icarus》1979,40(2):213-222
Several models for the atmosphere of Uranus are considered. If the H2 abundance is less than 250 km-am and the internal heat source is only a few percent of the total emitted energy then the cloud at the base of the atmosphere may be composed of solid CH4 particles, while if the H2 abundance is greater than 250 km-am or if the internal heat source is near the current upper limit of 35% of the total emitted energy the cloud at the base of the atmosphere may be composed of either solid NH3 or H2S particles.  相似文献   

6.
Kevin H. Baines 《Icarus》1983,56(3):543-559
High-resolution (0.1-Å) spectra of the 6818.9-Å methane feature obtained for Jupiter, Saturn, and Uranus by K. H. Baines, W. V. Schempp, and W. H. Smith ((1983). Icarus56, 534–542) are modeled using a doubling and adding code after J. H. Hansen ((1969). Astrophys. J.155, 565–573). The feature's rotational quantum number is estimated using the relatively homogeneous atmosphere of Saturn, with only J = 0 and J = 1 fitting the observational constraints. The aerosol content within Saturn's northern temperate region is shown to be substantially less than at the equator, indicating a haze only half as optically thick. Models of Jupiter's atmosphere are consistent with the rotational quantum-number assignment. Synthetic line profiles of the 6818.9-Å feature observed on Uranus reveal that a substantial haze exists at or above the methane condensation region with an optical depth eight times greater than previously reported. Seasonal effects are indicated. The methane column abundance is 5 ± 1 km-am. The mixing ratio of methane to hydrogen within the deep unsaturated region of the planet is 0.045 ± 0.025, based on an H2 column abundance of 240 ± 60 km-am (W. H. Smith, W. Macy, and C. B. Pilcher (1980). Icarus43, 153–160), thus indicating that the methane comprises between one-sixth and one-half of the planet's mass. However, proper reevaluation of H2 quadrupole features accounting for the haze reported here may significantly reduce the relative methane abundance.  相似文献   

7.
To search for a possible atmosphere on Pluto and Triton, spectra of these objects as well as comparison stars were obtained with a three-stage Varo image tube for the spectral region from 6800 to 9000 Å. Ratio spectra indicate an absorption feature near 8900 Å, although the steeply diminishing response of the image tube at that wavelength casts some doubt on the reality of this feature. The feature appears more definitive in the spectrum of Pluto and less certain in the spectrum of Triton. The absorption was analyzed using our recently determined band-model parameters for methane. Under the assumption of a pressure higher than 0.01 atm an abundance of 3 m-amagat was determined. For pressures limited by the methane abundance itself, an abundance of 50 m-amagat and a pressure of 10?3 atm was derived (using g = 0.20 g⊕ for both Pluto and Triton). This pressure is close to the pressure that can be expected from the equilibrium vapor pressure of a methane frost. If the absorption at 8900 Å is spurious, our analysis will be applicable as an upper limit for the presence of methane gas on Pluto or Triton.  相似文献   

8.
Heating occurs in Titan's stratosphere from the absorption of incident solar radiation by methane and aerosols. About 10% of the incident sunlight reaches Titan's surface and causes heating there. Thermal radiation redistributes heat within the atmosphere and cools to space. The resulting vertical temperature profile is stable against convection and a state of radiative equilibrium is established. Equating theoretical and observed temperature profiles enables an empirical determination of the vertical distribution of thermal opacity. A uniformly mixed aerosol is responsible for most of the opacity in the stratosphere, whereas collision-induced absorption of gases is the main contributor in the troposphere. Occasional clouds are observed in the troposphere in spite of the large degrees of methane supersaturation found there. Photochemistry converts CH4 and N2 into more complex hydrocarbons and nitriles in the stratosphere and above. Thin ice clouds of trace organics are formed in the winter and early spring polar regions of the lower stratosphere. Precipitating ice particles serve as condensation sites for supersaturated methane vapor in the troposphere below, resulting in lowered methane degrees of supersaturation in the polar regions. Latitudinal variations of stratospheric temperature are seasonal, and lag instantaneous response to solar irradiation by about one season for two reasons: (1) an actual instantaneous thermal response to a latitudinal distribution of absorbing gases, themselves out of phase with the sun by about one season, and (2) a sluggish dynamical response of the stratosphere to the latitudinal transport of angular momentum, induced by radiative heating and cooling. Mean vertical abundances of stratospheric organics and aerosols are determined primarily by atmospheric chemistry and condensation, whereas latitudinal distributions are more influenced by meridional circulations. In addition to preferential scavenging by precipitating ice particles from above, the polar depletion of supersaturated methane results from periodic scavenging by short-lived tropospheric clouds, coupled with the steady poleward march of the continuously drying atmosphere due to meridional transport.  相似文献   

9.
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10.
3ew spectra of Titan centered at 7500 Å, at resolutions of 4 and 1 Å are presented. Weak absorptions coincident with features observed in the spectra of Uranus and Neptune are found. This observation suggests methane abundances in excess of 1 km-am, thereby emphasizing the complexity of line formation in Titan's atmosphere. The question of the total atmospheric pressure of Titan must be reexamined.  相似文献   

11.
Discovery by Cassini's plasma instrument of heavy positive and negative ions within Titan's upper atmosphere and ionosphere has advanced our understanding of ion neutral chemistry within Titan's upper atmosphere, primarily composed of molecular nitrogen, with ~2.5% methane. The external energy flux transforms Titan's upper atmosphere and ionosphere into a medium rich in complex hydrocarbons, nitriles and haze particles extending from the surface to 1200 km altitudes. The energy sources are solar UV, solar X-rays, Saturn's magnetospheric ions and electrons, solar wind and shocked magnetosheath ions and electrons, galactic cosmic rays (GCR) and the ablation of incident meteoritic dust from Enceladus’ E-ring and interplanetary medium. Here it is proposed that the heavy atmospheric ions detected in situ by Cassini for heights >950 km, are the likely seed particles for aerosols detected by the Huygens probe for altitudes <100 km. These seed particles may be in the form of polycyclic aromatic hydrocarbons (PAH) containing both carbon and hydrogen atoms CnHx. There could also be hollow shells of carbon atoms, such as C60, called fullerenes which contain no hydrogen. The fullerenes may compose a significant fraction of the seed particles with PAHs contributing the rest. As shown by Cassini, the upper atmosphere is bombarded by magnetospheric plasma composed of protons, H2+ and water group ions. The latter provide keV oxygen, hydroxyl and water ions to Titan's upper atmosphere and can become trapped within the fullerene molecules and ions. Pickup keV N2+, N+ and CH4+ can also be implanted inside of fullerenes. Attachment of oxygen ions to PAH molecules is uncertain, but following thermalization O+ can interact with abundant CH4 contributing to the CO and CO2 observed in Titan's atmosphere. If an exogenic keV O+ ion is implanted into the haze particles, it could become free oxygen within those aerosols that eventually fall onto Titan's surface. The process of freeing oxygen within aerosols could be driven by cosmic ray interactions with aerosols at all heights. This process could drive pre-biotic chemistry within the descending aerosols. Cosmic ray interactions with grains at the surface, including water frost depositing on grains from cryovolcanism, would further add to abundance of trapped free oxygen. Pre-biotic chemistry could arise within surface microcosms of the composite organic-ice grains, in part driven by free oxygen in the presence of organics and any heat sources, thereby raising the astrobiological potential for microscopic equivalents of Darwin's “warm ponds” on Titan.  相似文献   

12.
Laboratory spectra of methane-nitrogen mixtures have been recorded in the near-infrared range (1.0-1.65 μm) in conditions similar to Titan's near surface, to facilitate the interpretation of the DISR/DLIS (DISR—Descent Imager/Spectral Radiometer) spectra taken during the last phase of the descent of the Huygens Probe, when the surface was illuminated by a surface-science lamp. We used a 0.03 cm−1 spectral resolution, adequate to resolve the lines at high pressure (pN2∼1.5 bar). By comparing the laboratory spectra with synthetic calculations in the well-studied ν2+2ν3 band (7515-7620 cm−1), we determine a methane absorption column density of 178±20 cm atm and a temperature of 118±10 K in our experiment. From this, we derive the methane absorption coefficients over 1.0-1.65 μm with a 0.03 cm−1 sampling, allowing for the extrapolation of the results to any other methane column density under the relevant pressure and temperature conditions. We then revisit the calibration and analysis of the Titan “lamp-on” DLIS spectra. We infer a 5.1±0.8% methane-mixing ratio in the first 25 m of Titan's atmosphere. The CH4 mixing ratio measured 90 s after landing from a distance of 45 cm is found to be 0.92±0.25 times this value, thus showing no post-landing outgassing of methane in excess of ∼20%. Finally, we determine the surface reflectivity as seen between 25 m and 45 cm and find that the 1500 nm absorption band is deeper in the post-landing spectrum as compared to pre-landing.  相似文献   

13.
We present a quantitative analysis of CO thermal emissions discovered on the nightside of Titan by Baines et al. [2005. The atmospheres of Saturn and Titan in the near-infrared: First results of Cassini/VIMS. Earth, Moon, and Planets, 96, 119–147]. in Cassini/VIMS spectral imagery. We identify these emission features as the P and R branches of the 1-0 vibrational band of carbon monoxide (CO) near 4.65 μm. For CH3D, the prominent Q branch of the ν2 fundamental band of CH3D near 4.55 μm is apparent. CO2 emissions from the strong v3 vibrational band are virtually absent, indicating a CO2 abundance several orders of magnitude less than CO, in agreement with previous investigations. Analysis of CO emission spectra obtained over a variety of altitudes on Titan's nightside limb indicates that the stratospheric abundance of CO is 32±15 ppm, and together with other recent determinations, suggests a vertical distribution of CO nearly constant at this value from the surface throughout the troposphere to at least the stratopause near 300 km altitude. The corresponding total atmospheric content of CO in Titan is 2.9±1.5×1014 kg. Given the long lifetime of CO in the oxygen-poor Titan atmosphere (0.5–1.0 Gyr), we find a mean CO atmospheric production rate of 6±3×105 kg yr−1. Given the lack of primordial heavy noble gases observed by Huygens [Niemann et al., 2005. The abundances of constituents of Titan's atmosphere from the GCMS on the Huygens probe. Nature, 438, 779–784], the primary source of atmospheric CO is likely surface emissions. The implied CO/CH4 mixing ratio of near-surface material is 1.8±0.9×10−4, based on an average methane surface emission rate over the past 0.5 Gyr of 1.3×10−13 gm cm−2 s−1 as required to balance hydrocarbon haze production via methane photolysis [Wilson and Atreya, 2004. Current state of modeling the photochemistry of Titan's mutually dependent atmosphere and ionosphere. J. Geophys. Res. 109, E06002 Doi:10.1029/2003JE002181]. This low CO/CH4 ratio is much lower than expected for the sub-nebular formation region of Titan and supports the hypothesis [e.g., Atreya et al., 2005. Methane on Titan: photochemical-meteorological-hydrogeochemical cycle. Bull. Am. Astron. Soc. 37, 735] that the conversion of primordial CO and other carbon-bearing materials into CH4-enriched clathrate-hydrates occurs within the deep interior of Titan via the release of hydrogen through the serpentinization process followed by Fischer–Tropsch catalysis. The time-averaged predicted emission rate of methane-rich surface materials is 0.02 km3 yr−1, a value significantly lower than the rate of silicate lava production for the Earth and Venus, but nonetheless indicative of significant active geological processes reshaping the surface of Titan.  相似文献   

14.
A wide range of experiments has already been carried out to simulate the chemical evolution of Titan. Such experiments can provide useful information on the possible nature of minor constituents, mostly organic, likely to be present in Titan's atmosphere. Indeed, all but one of the organic compounds already detected in Titan's atmosphere have been identified in simulation experiments. The exception, C4N2, as well as other compounds expected in Titan from theoretical modeling, such as other N-organics, mainly CH2N2, and polyynes, namely C6H2, have never been detected in experimental simulation. It turned out that these compounds were thermally unstable, and the temperature conditions used during the simulation experiments (including conditions used for chemical analysis) were not appropriate. We have recently started a new program of simulation experiments using temperature conditions close to those of Titan's environment, more compatible with the build-up and detection of organics only stable at low temperature. Spark discharge of N2-CH4 gas mixtures was carried out at low temperature in the range of 100-150 K. The analysis of the obtained products was performed through FTIR, GC and GC-MS techniques. GC-peak identification was done owing to its mass spectrum and, in most cases, by comparison of the retention time and of the mass spectrum with standards. We report here the first detection in Titan's simulation experiments of C6H2. Its abundance is a few 10(-2) relative to C4H2. We also report a tentative identification of HC5N (to be confirmed by use of standard) with an abundance of a few 10(-2) relative to HC3N. The possible presence of HC5N suggested by our work provides the occurrence of very novel pathways in the formation of Titan's organic aerosols, involving not only C and H but also N atoms.  相似文献   

15.
《Icarus》1987,70(3):483-498
Absolute spectrophotometry of Pluto in the wavelength range of 5600 to 10,500 Å was obtained on 4 nights covering lightcurve phases of 0.18, 0.35, 0.49, and 0.98. The four phases included minimum light (0.98) and one near maximum light (0.49). The spectra reveal significant variations in the absorption depths of the methane bands at 6200, 7200, 7900, 8400, 8600, 8900, and 10,000 Å. The minimum amount of absorption was found to occur at minimum light. This variation would imply a 30° change in the column abundance of methane within 3 days. A model employing an anisotropic surface distribution of methane frost and a clear layer of CH4 gas was developed to explain the variation in absorption strength with rotational phase. The fit to the overall spectrum requires the presence of a frost with particle sizes on the order of a few millimeters. An upper limit of 5.5 m-am is derived for the one-way column abundance of CH4 gas. An equally good fit to the variation of the 7200-Å band is obtained if the atmosphere is removed from the model entirely.  相似文献   

16.
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17.
The descent imager/spectral radiometer (DISR) onboard the Huygens probe investigated the radiation balance inside Titan's atmosphere and took hundreds of images and spectra of the ground during the descent. The scattering of the aerosols in the atmosphere and the absorption by methane strongly influence the irradiation reaching the surface and the signals received by the various instruments. The physical properties of the surface can only be assessed after the influence of the atmosphere has been taken into account and properly removed. In the broadband visible images (660 to 1000 nm) the contrast of surface features is strongly reduced by the aerosol scattering. Calculations show that for an image taken from an altitude of 14.5 km, the corrected contrast is about three times higher than in the raw image.Spectral information of the surface by the imaging spectrometers in the visible and near infrared range can only be retrieved in the methane absorption windows. Intensity ratios from the methane windows can be used to make false color maps. The elevated bright ‘land’ terrain is redder than the flat dark ‘lake bed’ terrain.The reflectance spectra of the land and lake bed area in the IR are derived, as well as the reflectance phase function in the limited range from 20° to 50° phase angle. An absorption feature at 1.55 μm which may be attributed tentatively to water ice is found in the lake bed, but not in the land area. Otherwise the surface exhibits a featureless blue slope in the near-IR region (0.9-). Brightness profiles perpendicular to the coast line show that the bottoms of the channels of the large scale flow pattern become darker the further they are away from the land area. This could be interpreted as sedimentation of the bright land material transported by the rivers into the lake bed area. The river beds in the deeply incised valleys need not to be covered by dark material. Their roughly 10% brightness decrease could be caused by the illumination as illustrated by a model calculation. The size distribution of cobbles seen in the images after landing is in agreement with a single major flooding of the area with a flow speed of about .  相似文献   

18.
Titan is one of the primary scientific objectives of the NASA–ESA–ASI Cassini–Huygens mission. Scattering by haze particles in Titan's atmosphere and numerous methane absorptions dramatically veil Titan's surface in the visible range, though it can be studied more easily in some narrow infrared windows. The Visual and Infrared Mapping Spectrometer (VIMS) instrument onboard the Cassini spacecraft successfully imaged its surface in the atmospheric windows, taking hyperspectral images in the range 0.4–5.2 μm. On 26 October (TA flyby) and 13 December 2004 (TB flyby), the Cassini–Huygens mission flew over Titan at an altitude lower than 1200 km at closest approach. We report here on the analysis of VIMS images of the Huygens landing site acquired at TA and TB, with a spatial resolution ranging from 16 to14.4 km/pixel. The pure atmospheric backscattering component is corrected by using both an empirical method and a first-order theoretical model. Both approaches provide consistent results. After the removal of scattering, ratio images reveal subtle surface heterogeneities. A particularly contrasted structure appears in ratio images involving the 1.59 and 2.03 μm images north of the Huygens landing site. Although pure water ice cannot be the only component exposed at Titan's surface, this area is consistent with a local enrichment in exposed water ice and seems to be consistent with DISR/Huygens images and spectra interpretations. The images show also a morphological structure that can be interpreted as a 150 km diameter impact crater with a central peak.  相似文献   

19.
Methane is key to sustaining Titan's thick nitrogen atmosphere. However, methane is destroyed and converted to heavier hydrocarbons irreversibly on a relatively short timescale of approximately 10-100 million years. Without the warming provided by CH4-generated hydrocarbon hazes in the stratosphere and the pressure induced opacity in the infrared, particularly by CH4-N2 and H2-N2 collisions in the troposphere, the atmosphere could be gradually reduced to as low as tens of millibar pressure. An understanding of the source-sink cycle of methane is thus crucial to the evolutionary history of Titan and its atmosphere. In this paper we propose that a complex photochemical-meteorological-hydrogeochemical cycle of methane operates on Titan. We further suggest that although photochemistry leads to the loss of methane from the atmosphere, conversion to a global ocean of ethane is unlikely. The behavior of methane in the troposphere and the surface, as measured by the Cassini-Huygens gas chromatograph mass spectrometer, together with evidence of cryovolcanism reported by the Cassini visual and infrared mapping spectrometer, represents a “methalogical” cycle on Titan, somewhat akin to the hydrological cycle on Earth. In the absence of net loss to the interior, it would represent a closed cycle. However, a source is still needed to replenish the methane lost to photolysis. A hydrogeochemical source deep in the interior of Titan holds promise. It is well known that in serpentinization, hydration of ultramafic silicates in terrestrial oceans produces H2(aq), whose reaction with carbon grains or carbon dioxide in the crustal pores produces methane gas. Appropriate geological, thermal, and pressure conditions could have existed in and below Titan's purported water-ammonia ocean for “low-temperature” serpentinization to occur in Titan's accretionary heating phase. On the other hand, impacts could trigger the process at high temperatures. In either instance, storage of methane as a stable clathrate-hydrate in Titan's interior for later release to the atmosphere is quite plausible. There is also some likelihood that the production of methane on Titan by serpentinization is a gradual and continuous on-going process.  相似文献   

20.
The processes of dissociation and dissociative ionization of molecular nitrogen by solar UV radiation and by the accompanying flux of photoelectrons, as well as sputtering of the atmosphere by fluxes of magnetospheric ions and pick-up ions, are the main sources of translationally excited (hot, or suprathermal) nitrogen atoms and molecules in Titan's upper atmosphere. Since Titan does not possess an intrinsic magnetic field, ions from Saturn's magnetosphere can penetrate into the outer layers of Titan's atmosphere and sputter atoms and molecules from the atmosphere in momentum-transfer and charge exchange collisions. Atmospheric sputtering by corotating nitrogen ions and carbon-containing pick-up ions, as well as photodissociation-related losses, was considered previously by Lammer and Bauer (1993) and Shematovich et al. (2001, 2003). In this paper we investigate the processes of the formation and evolution of the fraction of suprathermal nitrogen atoms and molecules in the transition region of Titan's upper atmosphere using the previously developed Monte Carlo model for hot satellite and planetary coronas (Shematovich, 1999, 2004). It is established that the suprathermal nitrogen fraction in the transition region of Titan's upper atmosphere includes nitrogen atoms and molecules but the suprathermal nitrogen concentration is relatively small owing to high rates of escape from the atmosphere and to the efficient thermalization of suprathermal nitrogen at the altitudes of the relatively dense lower thermosphere. However, the scale height for suprathermal nitrogen in the transition region is much higher than that for the ambient atmospheric gas. Therefore, suprathermal nitrogen becomes one of the dominant components in the outer exosphere.  相似文献   

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