Titan's Troposphere Research Articles

Steady-state model for methane condensation in Titan's troposphere

In an attempt to better understand the physical basis for the large degrees of methane supersaturation inferred to be in Titan's troposphere from recent analyses of Voyager 1 IRIS data, a steady-state model has been developed in which the loss of methane at every level due to condensation and precipitation is exactly balanced by the replacement of methane vapor through eddy diffusion. Ethane ice crystals precipitating from the lower stratosphere serve as nucleation sites for methane condensation in the troposphere. Free parameters include a normalized methane abundance, eddy diffusion coefficient, and particle flux (equivalently an initial ethane particle radius); the downward ethane molecular flux is fixed from photochemical theory. Solutions are obtained for the altitude-dependences of methane particle size, degree of methane supersaturation, and methane mole fraction. It is found that the large degrees of methane supersaturation inferred from IRIS data require rather low initial ethane particle fluxes, corresponding to ethane particle radii ∼100 μm or so. A logical corollary to the theory suggests Titan's troposphere is stable against moist convection. From other analyses it is inferred that condensation and precipitation may occur cyclically with the season, and preferentially at the spring poles. It follows that there should be latitudinal gradients for both the degree of methane supersaturation in the upper troposphere, and methane mole fraction near the surface, with both quantities decreasing from equator to pole. This is consistent with the latitude-dependence for these quantities inferred from Voyager IRIS data.

Gaseous abundances and methane supersaturation in Titan's troposphere

Various properties of Titan's troposphere are inferred from an analysis of Voyager 1 infrared spectrometer (IRIS) data between 200 and 600 cm −1. Two homogeneous spectral averages acquired at widely separated emission angles are chosen for the analysis. Both data sets are associated with northern low latitudes very close to that of the radio science ingress occultation point. Solutions require simultaneous nonlinear least-squares fits to the two IRIS data sets, coupled with iteration of the radio occultation refractivity data. Values and associated 1-σ uncertainties of several parameters are inferred from our analysis. These include mole fractions for molecular hydrogen (∼0.0011), argon (small), and methane near the surface (∼0.057). Solutions are also obtained for the hydrogen para-fraction (close to equilibrium, with considerable uncertainty), air temperature near the surface (∼93 K), surface temperature discontinuity (∼1 K), and maximum degree of methane supersaturation in the upper troposphere (∼ 1.5). Actual values for the above-mentioned parameters depend on the amount of ethane cloud near the tropopause. There is no evidence for methane clouds in the upper troposphere, nor is their presence compatible with large degrees of supersaturation. A wave number dependence for the stratospheric haze opacity is inferred similar to that found for a polymeric residue created in laboratory discharge experiments. This haze appears to be uniformly distributed with latitude between altitudes of 40 and 160 km, provided those nighttime data at southern high latitudes that are subject to possible systematic calibration errors are discounted. Assuming uniform haze distribution, both the air temperature and methane vapor mole fraction near the surface are symmetrically distributed about the equator, with lower values at higher latitudes. Either the tropopause temperature or the maximum degree of methane supersaturation is asymmetrically distributed about the equator. In either case, the data are consistent with a decrease of methane supersaturation toward the poles, which suggests an increase in mean annual precipitation at high latitudes compared with the equatorial region. If methane vapor is in saturation equilibrium with Titan's surface, the derived latitudinal gradient of the near surface methane vapor mole fraction implies that the liquid content of the surface is ethane-enriched near the poles.

Corona discharge in a simulated Titan's atmosphere

The atmosphere of Titan, the largest satellite of Saturn, is basically composed by molecular nitrogen (75-99%), argon (~2%), methane (0.5-3.7%), hydrogen (0.2-0.6%), and traces of other molecules, such as hydrocarbons, N-organics and O-compounds (Raulin, 1992). These minor compounds are essentially the products of an active N2-CH 4 chemistry. Several experiments have been designed to study the chemistry of Titan's troposphere and stratosphere initiated by different types of energetic sources. The aim of them was to simulate the irradiation of Titan by solar UV photons, Saturn magnetospheric electrons and protons, interplanetary electrons, and cosmic rays. These phenomena initiate chemical reactions leading to the formation of a great variety of organic compounds. However, these experiments were conducted in the context of prebiotic chemistry on Earth and when the analysis of the Voyager IRIS data was done, some important discrepancies were found. It was the case of an absence of HCN, C2N2 or amines in some, or the overproduction of saturated hydrocarbons in others (Cabane, 1995). Furthermore, some were conducted at CH 4 abundance greater than those applicable to Titan conditions. There is another feature in Titan that calls attention. No terrestrial-like lightning activity was observed during the Voyager 1 flyby on November 1980, in spite of the facts that there is a very large electron conductivity in Titan's atmosphere and that nitrogen and other atmospheric constituents on Titan do not form any negative ions (Grard, 1995). Nevertheless, it is known that Titan is covered by methane clouds, and electrical activity could be postulated to exist in those clouds. No lightning or sparks but corona discharge instead, could interact with the gases and give rise to complex organic molecules. The aim of this work is to perform a simulation experiment of this environment. A mixture of methane (10%), argon (2%) and nitrogen at 500 Torr, was prepared with ultrahigh purity gases using a mass flow measuring and control system. The corona discharge was produced with a Tesla coil at a frecuency of 0.3 MHz. The gas mixture was irradiated during 10.5 hrs. The resulting products were analyzed by a GC/MS/IR system with computerized data acquisition, analysis, and data base search capabilities. A PoraPLOTQ column, for hydrocarbon analysis, was used. Mass and infrared spectral data base matches were confirmed by visual inspection. The products detected are shown in figure 1. They include low molecular weight saturated and unsaturated hydrocarbons and nitriles.

Moist convective clouds in Titan's atmosphere

We examine moist convection as a plausible mechanism for driving large vertical motions and mixing in the troposphere of Titan. The model used here is based on that ofStoker[1986] for Jupiter — a one dimensional diagnostic model of cumulus clouds developed to calculate cloud temperature and vertical velocity profiles ‐ incorporating an improved treatment of the drag effect of condensate on the buoyancy of the plumes. We find that under conditions plausible for the near surface of Titan, moist convection can occur with vertical velocity in excess of 1–13 m/s but that such plumes occupy only a very small fraction of the surface area of Titan. Nonetheless, we also find that such plumes may be the primary means of moving methane and other trace species from the surface into Titan's upper troposphere.

Plasma discharge in N 2 + CH 4 at low pressures: Experimental results and applications to Titan

We report the yields of gaseous hydrocarbons and nitriles produced in a continuous flow, low-dose, cold plasma discharge excited in a 10% CH 4, 90% N 2 atmosphere at 295 K and pressures p of 17 and 0.24 mbar, and use the results to compute expected abundances of minor constituents in Titan's atmosphere. These experiments are, by design, relevant to the atmospheric chemistry induced by cosmic rays in Titan's troposphere and (at the lower pressure) to chemistry initiated by Saturnian magnetospheric electrons and other charged particle sources which excite stratospheric aurorae. At p = 17 mbar, 59 gaseous species including 27 nitriles are detected in overall yield 4.0 (C + N) atoms incorporated into products per 100 eV (heV). At p = 0.24 mbar, 19 species are detected, including six nitriles and three other unidentified N-bearing compounds; the yield is 0.79 (C + N)/heV, a mild decrease with pressure. The types of molecules formed change more markedly, with high degrees of multiple bonding at 0.24 mbar prevailing over more H-saturated molecules at 17 mbar. The molecules and yields at 0.24 mbar bear a striking resemblance to the minor constituents found in Titan's atmosphere, all of which are abundant products in the laboratory experiment. Using the altitude-integrated flux of charged particle energy deposition at Titan, the laboratory yields at p = 0.24 mb, and a simple eddy mixing model, we compute absolute stratospheric column abundances and mole fractions. These are found to be in very good agreement with the Voyager IRIS observations. Except for the primarily photochemical products, C 2H 6 and C 3H 8, the match is much better than that obtained by photochemical-kinetic models, demonstrating that properly designed laboratory experiments are directly applicable to modeling radiation-chemical processes in planetary atmospheres. On the basis of this agreement we expect CH 3CN (ethanenitrile = acetonitrile) CH 2CHCHCH 2 (1,3-butadiene), CH 2CCH 2 (1,2-propadiene = allene), and CH 2CHCCH (1-buten-3-yne) to be present at mol fractions X > 10 −9, and CH 2CHCN (propenenitrile), CH 3CHCH 2 (propene), and CH 3CH 2CN (propanenitrile) at X > 10 −10 in Titan's atmosphere.

The atmosphere of Titan: An analysis of the Voyager 1 radio occultation measurements

Two coherently related radio signals transmitted from Voyager 1 at wavelengths of 13 cm (S-band) and 3.6 cm (X-band) were used to probe the equatorial atmosphere of Titan. The measurements were conducted during the occultation of the spacecraft by the satellite on November 12, 1980. An analysis of the differential dispersive frequency measurements did not reveal any ionization layers in the upper atmosphere of Titan. The resolution was approximately 3 × 10 3 and 5 × 10 3 electrons/cm 3 near the evening and morning terminators, respectively. Abrupt signal changes observed at ingress and egress indicated a surface radius of 2575.0 ± 0.5 km, leading to a mean density of 1.881 ± 0.002 g cm −3 for the satellite. The nondispersive data were used to derive profiles in height of the gas refractivity and microwave absorption in Titan's troposphere and stratosphere. No absorption was detected; the resolution was about 0.01 dB/km at the 13-cm wavelength. The gas refractivity data, which extend from the surface to about 200 km altitude, were interpreted in two different ways. In the first, it is assumed that N 2 makes up essentially all of the atmosphere, but with very small amounts of CH 4 and other hydrocarbons also present. This approach yielded a temperature and pressure at the surface of 94.0 ± 0.7°K and 1496 ± 20 mbar, respectively. The tropopause, which was detected near 42 km altitude, had a temperature of 71.4 ± 0.5°K and a pressure of about 130 mbar. Above the tropopause, the temperature increased with height, reaching 170 ± 15°K near the 200-km level. The maximum temperature lapse rate observed near the surface (1.38 ± 0.10°K/km) corresponds to the adiabatic value expected for a dry N 2 atmosphere—indicating that methane saturation did not occur in tbis region. Above the 3.5-km altitude level the lapse rate dropped abruptly to 0.9 ± 0.1°K/km and then decreased slowly with increasing altitude, crossing zero at the tropopause. For the N 2 atmospheric model, the lapse rate transition at the 3.5-km level appears to mark the boundary between a convective region near the surface having the dry adiabatic lapse rate, and a higher stable region in radiative equilibrium. In the second interpretation of the refractivity data, it is assumed, instead, that the 3.5 km altitude level corresponds to the bottom of a CH 4 cloud layer, and that N 2 and CH 4 are perfectly mixed below this level. These assumptions lead to an atmospheric model which below the clouds contains about 10% CH 4 by number density. The temperature near the surface is about 95°K. Arguments concerning the temperature lapse rates computed from the radio measurements appear to favor models in which methane forms at most a limited haze layer high in the troposphere.