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1.
《Planetary and Space Science》2007,55(13):2010-2014
Since the discovery of the main composition of Titan's atmosphere, many laboratory experiments have been carried out to reproduce its chemical evolution, particularly the formation of organic haze particles found throughout this atmosphere. Some of these simulations have produced solid products—referred to as Titan's tholins—that are assumed to have properties similar to those of Titan's aerosols. In the present work, we focus on the possible isotopic fractionation of carbon during the processes involved in the formation of Titan's tholins. Initial 12C/13C isotopic ratios measured on tholins made in the laboratory using cold plasma discharges are presented. Measurements of isotopic enhancement in 13C (δ13C), both on tholins and on the initial gas mixture (N2:CH4 (98:2)) used to produce them do not show any clear deficit or enrichment in 13C relative to 12C in the lab-made tholins compared to the initial gas mixture. Preliminary data recovered from the Aerosol Collector Pyrolyzer (ACP) experiment of the Huygens probe suggests that Titan's aerosols may also be exempt of carbon isotopic enrichment. This observation creates possibilities for deeper analysis of ACP experiment data.  相似文献   

2.
Nitrile ions are abundant in Titan's upper atmosphere and are expected to be lost mainly via dissociative recombination with free electrons. We review in this paper a series of experimental results on the dissociative recombination reactions of nitrile ions known/expected to be present in Titan's upper atmosphere. The experiments were all performed at the heavy ion storage ring CRYRING in Stockholm, Sweden, and the results presented here include information on rate coefficients at electron temperatures relevant for Titan's upper atmosphere as well as information on the product branching fractions of the reactions. We discuss implications of the results for Titan's atmosphere. As an example the presented results support a statement by Krasnopolsky (2009) that nitriles do not degrade to yield N2 again in Titan's atmosphere, indicating that condensation and polymerization with precipitation to the surface are their ultimate fate.  相似文献   

3.
《Planetary and Space Science》2007,55(14):2128-2136
We propose a new approach to study the chemical complexity of Titan's atmosphere. We have developed an evolutionary algorithm-based model that simulates the evolution of interacting elements with different valences. This abstract model mimics a C–H–O–N system that might get an insight into the general properties of the chemistry of Titan's atmosphere. Comparison with detailed models like photochemical models is discussed to evaluate limitations and benefits of each approach. Comparison with observations suggests that Titan's atmosphere might self-organize to produce hydrocarbons with distributions that follow a power-law relation. If confirmed, this property makes possible some prediction about the abundance of heavy hydrocarbons in the atmosphere of Titan.  相似文献   

4.
《Planetary and Space Science》2007,55(10):1470-1489
Uncertainties carried by the different kinetic parameters included in photochemical models of planetary atmospheres have rarely been considered even if they are supposed to be contributing mostly to the inconsistencies between observations and computed predictions. In this paper, we report the first detailed analysis of the propagation of uncertainties carried by the reaction rate coefficients included in an up-to-date photochemical model of Titan's atmosphere. Monte Carlo calculations performed on these reaction rate coefficients have been used to introduce their uncertainties and to investigate their significance on the photochemical modeling of Titan's atmosphere. Crude approximations in the implemented physical processes have been adopted to limit the number of free parameters. This allows us to pinpoint specifically the importance of chemical processes uncertainties in Titan's photochemical models and to evaluate their chemical robustness. First implications of this preliminary study related to purely chemical rate coefficient uncertainties are discussed. They are important enough to question indeed any comparisons between theoretical models with observations as well as any potential conclusions subsequently inferred. Since the latest missions, such as Cassini–Huygens, are likely to induce an ever-increasing interest for such kind of comparing studies, our conclusions show that it is crucial to reform the way we think of, and use, current photochemical models to understand the processes occurring in the atmospheres of the outer Solar System.  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.
V.G. Teifel 《Icarus》1983,53(3):389-398
Modeling of the geometric albedo of Uranus in and near prominent methane absorption bands between 0.5 and 0.9 μm indicates that the visible atmosphere probably consists of a thin aerosol haze layer (τscat ? 0.3?0.5; ωH ? 0.95) above an optically thick, semi-infinite Rayleigh scattering atmosphere. A significant depletion of methane gas above the haze layer is indicated. The mixing ratio of methane in the lower atmosphere is consistent with a value of CH4/H2 ? 3 × 10?3, comparable to those derived for Jupiter and Saturn.  相似文献   

7.
We report observations of Titan's high-altitude exosphere detected out to about 50,000 km altitude. The observations were made by the Ion Neutral Camera (INCA) on board the Cassini spacecraft. INCA detects energetic neutral atoms (ENA) that are formed when the ambient magnetospheric ions charge exchange with Titan's neutral atmosphere and exosphere. We find that Titan's exospheric H2 distribution follows closely a full Chamberlain distribution including ballistic, escaping and satellite distributions. As expected, neutral densities are dominated by a satellite distribution above about 10,000 km. The maximum detectable extent of the exosphere (～50,000 km) coincides with the radius of the Hill sphere of gravitational influence from Saturn. While we find no direct indications of a neutral Titan torus with densities greater than about 1000 cm?3, we observe interesting asymmetries in the distribution that warrants further investigation. Based on these findings we compute the average precipitating ENA flux to be about 5×106 keV/(cm2 s), or 8×10?3 erg/(cm2 s), which is directly comparable to that of precipitating energetic ions (Sittler, et al., 2009) and slightly higher than that of solar EUV (Tobiska, 2004). Thus, the energy deposited by precipitating ENAs must also be taken into consideration when studying the energy balance of Titan's thermosphere.  相似文献   

8.
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   

9.
One of the key components controlling the chemical composition and climatology of Titan's atmosphere is the removal of reactive atomic hydrogen from the atmosphere. A proposed process of the removal of atomic hydrogen is the heterogeneous reaction with organic aerosol. In this study, we investigate the effect of heterogeneous reactions in Titan's atmospheric chemistry using new measurements of the heterogeneous reaction rate [Sekine, Y., Imanaka, H., Matsui, T., Khare, B.N., Bakes, E.L.O., McKay, C.P., Sugita, S., 2008. Icarus 194, 186-200] in a one-dimensional photochemical model. Our results indicate that 60-75% of the atomic hydrogen in the stratosphere and mesosphere are consumed by the heterogeneous reactions. This result implies that the heterogeneous reactions on the aerosol surface may predominantly remove atomic hydrogen in Titan's stratosphere and mesosphere. The results of our calculation also indicate that a low concentration of atomic hydrogen enhances the concentrations of unsaturated complex organics, such as C4H2 and phenyl radical, by more than two orders in magnitude around 400 km in altitude. Such an increase in unsaturated species may induce efficient haze production in Titan's mesosphere and upper stratosphere. These results imply a positive feedback mechanism in haze production in Titan's atmosphere. The increase in haze production would affect the chemical composition of the atmosphere, which might induce further haze production. Such a positive feedback could tend to dampen the loss and supply cycles of CH4 due to an episodic CH4 release into Titan's atmosphere.  相似文献   

10.
We report a wide-ranging study of Titan's surface temperatures by analysis of the Moon's outgoing radiance through a spectral window in the thermal infrared at 19 μm (530 cm?1) characterized by lower atmospheric opacity. We begin by modeling Cassini Composite Infrared Spectrometer (CIRS) far infrared spectra collected in the period 2004–2010, using a radiative transfer forward model combined with a non-linear optimal estimation inversion method. At low-latitudes, we agree with the HASI near-surface temperature of about 94 K at 10°S (Fulchignoni et al., 2005). We find a systematic decrease from the equator toward the poles, hemispherically asymmetric, of ～1 K at 60° south and ～3 K at 60° north, in general agreement with a previous analysis of CIRS data (Jennings et al., 2009), and with Voyager results from the previous northern winter. Subdividing the available database, corresponding to about one Titan season, into 3 consecutive periods, small seasonal changes of up to 2 K at 60°N became noticeable in the results. In addition, clear evidence of diurnal variations of the surface temperatures near the equator are observed for the first time: we find a trend of slowly increasing temperature from the morning to the early afternoon and a faster decrease during the night. The diurnal change is ～1.5 K, in agreement with model predictions for a surface with a thermal inertia between 300 and 600 J m?2 s?1/2 K?1. These results provide important constraints on coupled surface–atmosphere models of Titan's meteorology and atmospheric dynamic.  相似文献   

11.
Buu N. Tran  John J. Chera 《Icarus》2003,162(1):114-124
The photochemical flow reactor (D.W. Clarke et al., 2000, Icarus 147, 282-291) has been modified to minimize the incorporation of oxygen and other impurities in the photoproducts. A mixture of gases that approximate their mixing ratios on Titan (N2, CH4, H2, C2H2, C2H4, and HC3N) (0.98, 0.018, 0.002, 3.5 × 10−4, 3 × 10−4, 1.7 × 10−5, respectively) was irradiated in the flow photochemical reactor using a 185-nm source to give a Titan haze analog as a solid product. X-ray photoelectron spectroscopy (XPS) gave a composition of 93.3% C, 5.3% N, and 1.4% O. Of the 93.3% carbon, high-resolution XPS revealed that 81.2% was present as CH, CC, and CC groups, 12.1% may be CO, CN, CN, CN, and/or CN groups, 5.3% as a CN group. The peak for N was symmetrical and was assigned to the CN while that for oxygen was assigned to the CO and/or the CO group. Some of these assignments were confirmed by FTIR spectroscopy. The polymeric product had a C:N ratio of 17.6, which is significantly greater than that for Titan haze analogs prepared in discharge reactions. When the polymer was exposed to air for seven days the oxygen content increased by 6% along with an increase in the infrared absorption at 1710 cm−1 assigned to the CO group of a ketone. The oxidation is attributed to the reaction of oxygen with free radicals trapped in the polymer matrix. It is proposed that the photochemical initiation of Titan haze formation from compounds formed from starting materials formed high in Titan’s atmosphere is a more plausible model than haze formed in reactions initiated by solely by discharges. These data will be helpful in the interpretation of the data returned from the Huygens probe of the Cassini mission.  相似文献   

12.
Abstract— A simple thermal model is developed to determine the temperature history of the inlet tube of the Huygens probe gas chromatograph mass spectrometer (GCMS) after its fortuitous emplacement on the surface of Saturn's moon Titan. The model parameters are adjusted to match the recorded temperature history of a nearby heater, taking into account heat losses by conduction to the rest of the probe and to Titan's cold atmosphere. The model suggests that after impact when forced convective cooling ceased, the inlet temperature rose from ?110 K to an asymptotic value of only ?145 K. This requires that the inlet was embedded in a surface that acted as an effective heat sink, most plausibly interpreted as wet or damp with liquid methane. The data appear inconsistent with a tar or dry, fine‐grained surface, and the inlet was not warm enough to devolatilize methane hydrate.  相似文献   

13.
Titan in the past, just as Ganymede, had a massive ice envelope subjected to volumetric electrolysis under the action of unipolar electric current generated through the interaction of the satellite with Saturn's magnetosphere. The electrolysis products concentration required to cause explosion could become accumulated only under conditions of an exponential decay of Saturn's magnetic field in time (with τ? ≈ 0.55 Gyr) which implies a relict nature of the field and agrees with the present ideas on the planet's structure. The explosion of the electrolysis products contained in the ice envelope resulted in Titan's having lost ～13% of its mass in the form of gas (mainly of water vapor) and solid ice fragments, as well as in the appearance on Titan of an atmosphere (of volatile products from incomplete combustion of hydrogen and hydrocarbons) and a deep (～1000 km) ocean of liquid water. The presence of liquid water on Titan's surface is confirmed by an analysis of the available microwave measurements of brightness temperature. The condensation of the water vapor lost by Titan produced the visible inner rings of Saturn while large solid fragments of the ice envelope govern their dynamics. These are also located in the gap between Rhea and Titan (the G ring?). Most of the ice fragments were swept out from Saturn's system through perturbations by Titan. They made up a reservoir of cometary nuclei beyond Jupiter's orbit. Arguments are presented in favor of a recent (3–10 thousand years ago) explosion of Titan. Some implications from these concepts, lending themselves to observational testing, are pointed out.  相似文献   

14.
《Planetary and Space Science》2007,55(13):1877-1885
Cassini/Huygens, a flagship mission to explore the rings, atmosphere, magnetic field, and moons that make up the Saturn system, is a joint endeavor of the National Aeronautics and Space Administration, the European Space Agency, and Agenzia Spaziale Italiana. Comprising two spacecraft—a Saturn orbiter built by NASA and a Titan entry/descent probe built by the European Space Agency—Cassini/Huygens was launched in October 1997. The Huygens probe parachuted to the surface of Titan in January 2005. During the descent, six science instruments provided in situ measurements of Titan's atmosphere, clouds, and winds, and photographed Titan's surface. To correctly interpret and correlate results from the probe science experiments, and to provide a reference set of data for ground-truth calibration of orbiter remote sensing measurements, an accurate reconstruction of the probe entry and descent trajectory and surface landing location is necessary. The Huygens Descent Trajectory Working Group was chartered in 1996 as a subgroup of the Huygens Science Working Team to develop and implement an organizational framework and retrieval methodologies for the probe descent trajectory reconstruction from the entry altitude of 1270 km to the surface using navigation data, and engineering and science data acquired by the instruments on the Huygens Probe. This paper presents an overview of the Descent Trajectory Working Group, including the history, rationale, goals and objectives, organizational framework, rules and procedures, and implementation.  相似文献   

15.
《Planetary and Space Science》2007,55(13):1990-2009
This study aims at interpreting the zonal and meridional wind in Titan's troposphere measured by the Huygens probe by means of a general circulation model. The numerical simulation elucidates the relative importance of the seasonal variation in the Hadley circulation and Saturn's gravitational tide in affecting the actual wind profile. The observed reversal of the zonal wind at two altitudes in the lower troposphere can be reproduced with this model only if the near-surface temperature profile is asymmetric about the equator and substantial seasonal redistribution of angular momentum by the variable Hadley circulation takes place. The meridional wind near the surface is mainly caused by the meridional pressure gradient and is thus a manifestation of the Hadley circulation. Southward meridional wind in the PBL (planetary boundary layer) is consistent with the near-surface temperature at the equator being lower than at mid southern latitudes. Even small changes in the radiative heating profile in the troposphere can substantially affect the mean zonal and meridional wind including their direction. Saturn's gravitational tide is rather weak at the Huygens site due to the proximity to the equator, and does not clearly manifest itself in the instantaneous vertical profile of wind. Nevertheless, the simulated descent trajectory is more consistent with the observation if the tide is present. Because of a different force balance in Titan's atmosphere from terrestrial conditions, PBL-specific wind systems like on Earth are unlikely to exist on Titan.  相似文献   

16.
We use Titan's geometric albedo to constrain the vertical distribution of the haze. Microphysical models incorporating fractal aggregates do not readily fit the methane features at 0.62 μm band and the dark 0.88 μm of the albedo spectrum simultaneously. We take advantage of this apparent discrepancy to constrain the haze vertical profile.We used the geometric albedo and several results and constraints from other works to better constrain the vertical haze extinction profile, especially in the low stratosphere. The objective of this model is to give a solution that simultaneously fits the main constraints known to apply to the haze.We find that the haze extinction increases with decreasing altitude with a scale height about equal to the atmospheric scale height down to 100 km. Below this altitude, extinction must decrease down to 30 km. This is necessary in order to have enough haze to sustain a relatively high albedo (0.076) in the dark 0.88 μm methane band and to show the 0.62 μm band in the haze continuum. We set the haze production rate around 7×10−14 kgm−2 s−1, and the aerosols production altitude around 400 km (or at pressure 1.5 Pa).The physical processes which generate such a profile are not clear. However, purely one-dimensional effects such as condensation, sedimentation, and rainout can be ruled out, and we believe that this relative clearing in Titan's troposphere and lower stratosphere is due to particle horizontal transport by the mean circulation.  相似文献   

17.
The strong 14C increase in the year AD 774/5 detected in one German and two Japanese trees was recently suggested to have been caused by an impact of a comet onto Earth and a deposition of large amounts of 14C into the atmosphere (Liu et al. 2014). The authors supported their claim using a report of a historic Chinese observation of a comet ostensibly colliding with Earth's atmosphere in AD 773 January. We show here that the Chinese text presented by those authors is not an original historic text, but that it is comprised of several different sources. Moreover, the translation presented in Liu et al. is misleading and inaccurate. We give the exact Chinese wordings and our English translations. According to the original sources, the Chinese observed a comet in mid January 773, but they report neither a collision nor a large coma, just a long tail. Also, there is no report in any of the source texts about “dust rain in the daytime” as claimed by Liu et al. (2014), but simply a normal dust storm. Ho (1962) reports sightings of this comet in China on AD 773 Jan 15 and/or 17 and in Japan on AD 773 Jan 20 (Ho 1962). At the relevant historic time, the Chinese held that comets were produced within the Earth's atmosphere, so that it would have been impossible for them to report a “collision” of a comet with Earth's atmosphere. The translation and conclusions made by Liu et al. (2014) are not supported by the historical record. Therefore, postulating a sudden increase in 14C in corals off the Chinese coast precisely in mid January 773 (Liu et al. 2014) is not justified given just the 230Th dating for AD 783 ± 14. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

18.
Darrell F. Strobel 《Icarus》2010,208(2):878-886
The third most abundant species in Titan’s atmosphere is molecular hydrogen with a tropospheric/lower stratospheric mole fraction of 0.001 derived from Voyager and Cassini infrared measurements. The globally averaged thermospheric H2 mole fraction profile from the Cassini Ion Neutral Mass Spectrometer (INMS) measurements implies a small positive gradient in the H2 mixing ratio from the tropopause region to the lower thermosphere (∼950-1000 km), which drives a downward H2 flux into Titan’s surface comparable to the H2 escape flux out of the atmosphere (∼2 × 1010 cm−2 s−1 referenced to the surface) and requires larger photochemical production rates of H2 than obtained by previous photochemical models. From detailed model calculations based on known photochemistry with eddy, molecular, and thermal diffusion, the tropospheric and thermospheric H2 mole fractions are incompatible by a factor of ∼2. The measurements imply that the downward H2 surface flux is in substantial excess of the speculative threshold value for methanogenic life consumption of H2 (McKay, C.P., Smith, H.D. [2005], Icarus 178, 274-276. doi:10.1016/j.icarus.2005.05.018), but without the extreme reduction in the surface H2 mixing ratio.  相似文献   

19.
Abstract— Asteroids tens to hundreds of meters in diameter constitute the most immediate impact hazard to human populations, yet the rate at which they arrive at Earth's surface is poorly known. Astronomic observations are still incomplete in this size range; impactors are subjected to disruption in Earth's atmosphere, and unlike the Moon, small craters on Earth are rapidly eroded. In this paper, we first model the atmospheric behavior of iron and stony bodies over the mass range 1–1012 kg (size range 6 cm‐1 km) taking into account deceleration, ablation, and fragmentation. Previous models in meteoritics deal with rather small masses (<105–106 kg) with the aim of interpreting registered fireballs in atmosphere, or with substantially larger objects without taking into account asteroid disruption to model cratering processes. A few earlier attempts to model terrestrial crater strewn fields did not take into account possible cascade fragmentation. We have performed large numbers of simulations in a wide mass range, using both the earlier “pancake” models and also the separated fragments model to develop a statistical picture of atmosphere‐bolide interaction for both iron and stony impactors with initial diameters up to ?1 km. Second, using a compilation of data for the flux at the upper atmosphere, we have derived a cumulative size‐frequency distribution (SFD) for upper atmosphere impactors. This curve is a close fit to virtually all of the upper atmosphere data over 16 orders of magnitude. Third, we have applied our model results to scale the upper atmosphere curve to a flux at the Earth's surface, elucidating the impact rate of objects <1 km diameter on Earth. We find that iron meteorites >5 times 104 kg (2.5 m) arrive at the Earth's surface approximately once every 50 years. Iron bodies a few meters in diameter (105–106 kg), which form craters ?100 m in diameter, will strike the Earth's land area every 500 years. Larger bodies will form craters 0.5 km in diameter every 20,000 years, and craters 1 km in diameter will be formed on the Earth's land area every 50,000 years. Tunguska events (low‐level atmospheric disruption of stony bolides >108 kg) may occur every 500 years. Bodies capable of producing hazardous tsunami (?200 m diameter projectiles) should strike the Earth's surface every ?100,000 years. This data also allows us to assess the completeness of the terrestrial crater record for a given area over a given time interval.  相似文献   

20.
The spectrometers of the Cassini mission to the Saturn system have detected haze layers reaching up to 800 km in Titan’s atmosphere. Knowledge of the complex refractive index (k) of the haze is important for modeling the surface and atmosphere of Titan and retrieving some information about the functional groups present in the aerosols. Plasma discharges or ultraviolet radiation are commonly used to drive the formation of solid organics assumed to be good analogs of the Titan aerosols. [Tran, B.N., Ferris, J.P., Chera, J.J., 2003a. The photochemical formation of a Titan haze analog. Structural analysis by X-ray photoelectron and infrared spectroscopy. Icarus 162, 114-124; Tran, B.N., Force, M., Briggs, R., Ferris J.P., Persans, P., Chera, J.J., 2008. Photochemical processes on Titan: Irradiation of mixtures of gases that simulate Titan’s atmosphere. Icarus 177, 106-115] reported the index of refraction of analogs synthesized by far ultraviolet irradiation of various gas mixtures. k was determined in the 200-800 nm wavelength range from transmission and reflection spectroscopy. However, this technique is limited by (i) uncertainties in the absorption values because of the small amounts of organics available, (ii) light scattering by the surface roughness and particulates in the sample. These limitations prompted us to perform new measurements using photothermal deflection spectroscopy (PDS), a technique based on the conversion of absorbed light into heat in the material of interest. By combining traditional spectroscopy (λ < 500 nm) and PDS (λ > 500 nm), we determined values of k over the 375-1550 nm range. k values as low as 10−4 above 1000 nm were determined. This is one order of magnitude lower than the measurements generally used as a reference for Titan’s aerosols analogs [Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callicott, T.A., Williams, M.W., 1984. Optical-constants of organic Tholins produced in a simulated Titanian atmosphere—from soft-X-ray to microwave-frequencies. Icarus 60(1), 127-137]. We recommend that these results were used in models to describe the optical properties of the aerosols produced in Titan’s stratosphere.  相似文献   

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