Titan's Troposphere Research Articles

On the H2 abundance and ortho-to-para ratio in Titan's troposphere

We have analyzed spectra recorded between 50 and 650 cm−1 by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft at low and high emission angles to determine simultaneously the H2 mole fraction and ortho-to-para ratio in Titan's troposphere. We used constraints from limb spectra between 50 and 900 cm−1 and from in situ measurements by the Huygens probe to characterize the temperature, haze and gaseous absorber profiles. We confirm that the N2-CH4 collision-induced absorption (CIA) coefficients used up to now need to be increased by about 52% at temperatures of 70–85 K. We find that the N2-N2 CIA coefficients are also too low in the N2 band far wing, beyond 110 cm−1, in agreement with recent quantum mechanical calculations. We derived a H2 mole fraction equal to (0.88 ± 0.13) × 10−3, which pertains to the ~1–34 km altitude range probed by the S0(0) and S0(1) lines. This result agrees with a previous determination based only on the H2-N2 dimer transition in the S0(0) line, and with the in situ measurement by the Gas Chromatograph Mass Spectrometer (GCMS) aboard Huygens. It is 3–4 times smaller than the value measured in situ by the Ion Neutral Mass Spectrometer (INMS) of Cassini at 1000–1100 km. The H2 para fraction is close to equilibrium in the 20-km region. CIRS spectra can be fitted assuming ortho-to-para (o-p) H2 thermodynamical equilibrium at all levels or a constant para fraction in the range 0.49–0.53. We have investigated different mechanisms that may operate in Titan's atmosphere to equilibrate the H2 o-p ratio and we have developed a one-dimensional model that solves the continuity equation in presence of such conversion mechanisms. We conclude that exchange with H atoms in the gas phase or magnetic interaction of H2 in a physisorbed state on the surface of aerosols are too slow compared with atmospheric mixing to play a significant role. On the other hand, magnetic interaction of H2 with CH4, and to a lesser extent N2, can operate on a timescale similar to the vertical mixing time in the troposphere. This process is thus likely responsible for the o-p equilibration of H2 in the mid-troposphere implied by CIRS measurements. The model can reproduce the inferred o-p ratio in the 20-km region, assuming low atmospheric mixing in the troposphere down to 15–20 km and conversion rates with CH4 or N2 slightly larger than obtained from an extrapolation of natural ortho-para conversion rate measured in gaseous hydrogen.

Titan's Methane Weather

Conditions in Titan's troposphere are near the triple point of methane, the second most abundant component of its atmosphere. Our understanding of Titan's lower atmosphere has shifted considerably in the past decade. Ground-based observations, Hubble Space Telescope images, and data returned from the Cassini and Huygens spacecraft show that Titan's troposphere hosts a methane-based meteorology in direct analogy to the water-based meteorology of Earth. What once was thought to be a quiescent place, lacking in clouds or localized weather and changing only subtly on long seasonal timescales, is now understood to be a dynamic system with significant weather events regularly occurring against the backdrop of dramatic seasonal changes. Although the observational record of Titan's weather covers only a third of its 30-year seasonal cycle, Titan's atmospheric processes appear to be more closely analogous to those of Earth than to those of any other object in our solar system.

The abundance of H2 in Titan's troposphere from the Cassini CIRS investigation

We derived the mole fraction of H2 in the troposphere of Titan from an analysis of the H2–N2 dimer S0(0) rotational transition seen near 355cm−1 in spectra recorded by the Composite Infra-Red Spectrometer (CIRS) on the Cassini spacecraft between March 2006 and February 2007. We find a globally-averaged value of (9.6±2.4)×10−4, consistent with the earlier determination based on the Voyager IRIS dataset, and with the value obtained from the Gas Chromatograph Mass Spectrometer (GCMS) experiment on the Huygens probe, but a factor of ∼4 lower than that measured by the Ion and Neutral Mass Spectrometer (INMS) in the 1025–1475km altitude range. Interestingly, the latitudinal distribution of H2 appears to be non-uniform, with a mole fraction above 50°N in excess of 30–70% (±40%) relatively to the globally-averaged value. Such a non-uniform distribution is unexpected since H2 is thought to be chemically inert in the lower atmosphere of Titan. It is proposed that the observed enhancement in the north polar region is caused by downwelling occurring during the winter season in the polar vortex. This interpretation is dependent on the existence of a vertical gradient of the H2 concentration in the stratosphere, as suggested by the differences between the INMS values and those measured by GCMS and CIRS, although such a gradient is not consistent with the present understanding of dynamics and chemistry in Titan's atmosphere.

METHANE-NITROGEN BINARY NUCLEATION: A NEW MICROPHYSICAL MECHANISM FOR CLOUD FORMATION IN TITAN'S ATMOSPHERE

It is known that clouds are present in the troposphere of Titan; however, their formation mechanism, particle size, and chemical composition remain poorly understood. In this study, a two-component (CH{sub 4} and N{sub 2}) bin-microphysics model is developed and applied to simulate cloud formation in the troposphere of Titan. A new process, binary nucleation of particles from CH{sub 4} and N{sub 2} gases, is considered. The model is validated and calibrated by recent laboratory experiments that synthesize particle formation in Titan-like environments. Our model simulations show that cloud layers can be formed at about 20 km with a particle size ranging from one to several hundred {mu}m and number concentration 10{sup -2} to over 100 cm{sup -3} depending on the strength of the vertical updraft. The particles are formed by binary nucleation and grow via the condensation of both CH{sub 4} and N{sub 2} gases, with their N{sub 2} mole fraction varying from 30% in the condensation growth stage. The locally occurring CH{sub 4}-N{sub 2} binary nucleation mechanism is strong and could potentially be more important than the falling condensation nuclei mechanism assumed in many current models.

Electrical properties of ions in the atmosphere of Titan

The Permittivity, Wave and Altimetry (PWA) package on the Huygens probe measured atmospheric electrical activity during its 14 January 2005 descent into Titan's atmosphere. Electrical mobilities of positive ions have been estimated using a combination of predictions of mass from the available chemical models, and the atmospheric properties measured by Huygens. Disc-shaped relaxation probes were used on the PWA to measure air conductivity. In this paper we present the extension of an inversion technique, originally developed to extract ion mobility information from cylindrical relaxation probe data, to the disc geometry. The maximum distance from which typical positive ion species are influenced by the electric field of the relaxation probe in Titan's troposphere and stratosphere is calculated, and found to be ∼10μm. However, loss of positive ion data from the Huygens probe prevents the application of the new technique. Based on the Huygens data, evidence for, and possible characteristics of, a global electric circuit on Titan are also discussed.

The dynamics of Titan's troposphere

While the Voyager mission could essentially not reveal the dynamics of Titan's troposphere, useful information was obtained by the Cassini spacecraft and, particularly, by the Huygens probe that landed on Titan's surface; this information can be interpreted by means of numerical models of atmospheric circulation. The meridional circulation is likely to consist of a large Hadley circulation asymmetric about the equator, but is susceptible to disruption by turbulence in clouds. The zonal wind in the troposphere is comparable to or even weaker than that in the terrestrial troposphere and contains zones of easterlies, much in contrast to the super-rotating stratosphere. Unique to Titan is the transition from a geostrophic to cyclostrophic wind balance in the upper troposphere. While Earth-like storm systems associated with baroclinic instability are absent, Saturn's gravitational tide introduces a planetary wave of wavenumber 2 and a periodical variation in the wind direction in the troposphere. Unlike on Earth, the wind over the equatorial surface is westerly. The seasonal reversal in the Hadley circulation sense and zonal wind direction is predicted to have a substantial influence on the formation of dunes as well as variation of Titan's rotation rate and length of day.

Titan's Tropical Storms in an Evolving Atmosphere

The Huygens probe landed in a damp lake bed fed by fluvial channels, indicative of past rainfall. Such washes, interspersed with vast dunes, are typical of Titan's tropical landscape. Yet, Cassini-Huygens measurements reveal a highly stable tropical atmosphere devoid of deep convective storms, and the formation of washes in dune fields is not understood. Here we examine the effects of seasonal variations in humidity, surface heating, and dynamical forcing on the stability of Titan's troposphere. We find that during the probe landing, the middle troposphere was weakly unstable to convection, consistent with the tenuous cloud detected at 21 km. Yet the tropical atmosphere, at any season, is too stable to produce deep convective storms. Convection in the tropics remains weak and confined to altitudes below ~30 km, unless the humidity is increased below 9 km altitude. Solar heating is insufficient to significantly humidify the tropical atmosphere. The large polar lakes are seasonably stable, and the methane column abundance measured by Huygens typical of the tropical atmosphere. Our study indicates the presence of distinct polar and equatorial climates. It also suggests that fluvial features in the tropics do not result from recent seasonal rainstorms, and thereby supports other origins such as geological seepage, cryovolcanism, or a wetter climate in the past.

HST spectral imaging of Titan's haze and methane profile between 0.6 and 1 μm during the 2000 opposition

We report on the analysis of high spatial resolution visible to near-infrared spectral images of Titan at L s = 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 CH 4–N 2 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.

Widespread Morning Drizzle on Titan

Precipitation is expected in Titan's atmosphere, yet it has not been directly observed, and the geographical regions where rain occurs are unknown. Here we present near-infrared spectra from the Very Large Telescope and W. M. Keck Observatories that reveal an enhancement of opacity in Titan's troposphere on the morning side of the leading hemisphere. Retrieved extinction profiles are consistent with condensed methane in clouds at an altitude near 30 kilometers and concomitant methane drizzle below. The moisture encompasses the equatorial region over Titan's brightest continent, Xanadu. Diurnal temperature gradients that cause variations in methane relative humidity, winds, and topography may each be a contributing factor to the condensation mechanism. The clouds and precipitation are optically thin at 2.0 micrometers, and models of "subvisible" clouds suggest that the droplets are 0.1 millimeter or larger.

Near-surface winds at the Huygens site on Titan: Interpretation by means of a general circulation model

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.

Methane drizzle on Titan

Saturn's moon Titan shows landscapes with fluvial features suggestive of hydrology based on liquid methane. Recent efforts in understanding Titan's methane hydrological cycle have focused on occasional cloud outbursts near the south pole or cloud streaks at southern mid-latitudes and the mechanisms of their formation. It is not known, however, if the clouds produce rain or if there are also non-convective clouds, as predicted by several models. Here we show that the in situ data on the methane concentration and temperature profile in Titan's troposphere point to the presence of layered optically thin stratiform clouds. The data indicate an upper methane ice cloud and a lower, barely visible, liquid methane-nitrogen cloud, with a gap in between. The lower, liquid, cloud produces drizzle that reaches the surface. These non-convective methane clouds are quasi-permanent features supported by the global atmospheric circulation, indicating that methane precipitation occurs wherever there is slow upward motion. This drizzle is a persistent component of Titan's methane hydrological cycle and, by wetting the surface on a global scale, plays an active role in the surface geology of Titan.

GCM simulation of balloon trajectories on Titan

Possible trajectories of passive balloons in Titan's troposphere are simulated with the instantaneous wind field predicted by a GCM (general circulation model). In most areas the basic motion of a balloon is a predominantly eastward or westward drift, depending on altitude, latitude and season of the balloon release point. Some meridional oscillation is always superposed on this basic motion, resulting in a wavy trajectory, with a maximum extent (of 40 ∘ ) at high latitudes of the winter hemisphere. As a general rule, the meridional oscillation can be maximised if a balloon is deployed in altitudes and latitudes where the mean zonal wind is eastward and smaller than the phase speed of Saturn's gravitational tide on Titan. A balloon's groundtrack does not repeat as it makes successive circuits around Titan, but rather makes a spiral or braided pattern. The summer pole is rather difficult to access for a balloon not directly introduced there because of small meridional oscillation, while the winter pole can be readily accessed and left several times. A preferred zonal drift direction can be achieved by choosing a proper altitude and hemisphere, but choosing a preferred meridional direction is not possible.

Discovery of Temperate Latitude Clouds on Titan

Until now, all the clouds imaged in Titan's troposphere have been found at far southern latitudes (60°-90° south). The occurrence and location of these clouds is thought to be the result of convection driven by the maximum annual solar heating of Titan's surface, which occurs at summer solstice (2002 October) in this south polar region. We report the first observations of a new recurring type of tropospheric cloud feature, confined narrowly to ~40° south latitude, which cannot be explained by this simple insolation hypothesis. We propose two classes of formation scenario, one linked to surface geography and the other to seasonally evolving circulation, which will be easily distinguished with continued observations over the next few years.

Energy dissipation of possible Titan lightning strokes

The search for lightning on Saturn's satellite Titan is one scientific target of the Cassini/Huygens mission. Although Voyager 1 did not detect any radio emissions caused by Titan lightning during its flyby in November 1980, one cannot generally rule out their existence, because of low flash rates or ionospheric radiation blockade. Recently, Tokano et al. (Planet. Space Sci. 49 (2001a) 539) have developed a thundercloud model in Titan's troposphere favoring the existence of Titan lightning due to negatively charged clouds causing temporary electric fields sufficient to initiate cloud-to-ground lightning strokes. In the present investigation we estimate the amount of energy dissipation of such lightning strokes by electrostatic energy considerations similarly to those by Cooray (J. Geophys. Res. 102(D17) (1997) 21,401). The analysis is based on the cloud charge distribution given by Tokano et al. (2001a), which has a monopole structure or a dipole structure depending on the electrification mechanism. It consists of horizontally homogenous charge layers, whose charge densities depend on the altitude above ground. As results we get the typical charge lowered in a possible Titan lightning stroke and the amount of energy dissipation. For a simulated Titan monopolar cloud charged by electron attachment we found that cloud-to-ground strokes lower about 30C of charge and dissipate energies about 1010J. For the modelled bipolar clouds charged by collisional charging these values are a few C of lowered charge and about 108–109J of dissipated energy, which are quite similar to typical Earth values. These energies are substantially higher than the energies suggested by Desch and Kaiser (Nature 343 (1990) 442), who concluded from the Voyager data that discharges might be frequent but weak (<106J). We shortly discuss the detection capability of the Cassini/RPWS (Radio and Plasma Wave Science experiment) for possible Titan lightning strokes taking into account the wave attenuation in the frequency range up to 16MHz during the propagation through Titan's ionosphere as calculated by Schwingenschuh et al. (Adv. Space Res. 28(10) (2001) 1505).

A model of Titan's haze of fractal aerosols constrained by multiple observations

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 kg m −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.

High-Resolution 3 Micron Spectroscopy of Molecules in the Mesosphere and Troposphere of Titan

A high-resolution 2.9-3.5 μm spectrum of Titan has revealed a surprisingly strong emission band of hydrogen cyanide (HCN) and strong absorption bands of acetylene (C2H2) and monodeuterated methane (CH3D), in addition to methane (CH4) emission and absorption known from previous observations. Whereas the C2H2 and CH3D absorptions are largely in the troposphere, the HCN emission must occur at higher altitudes. However, the intensity of the HCN band is inconsistent with our current understanding of the upper atmosphere of Titan.

Nitriles produced by ion chemistry in the lower ionosphere of Titan

Titan, the largest satellite of Saturn, possesses a dense N2‐CH4 atmosphere rich in organics with a complex chemistry that could be similar to Earth's primordial atmosphere. Due to its massive atmosphere and the long distance to the Sun, cosmic rays are the main energetic source of the lower part of the atmosphere. Cosmic radiation can reach Titan's troposphere (z &lt; 40 km) and activate the chemistry at depths where UV light and high‐energy electrons cannot penetrate. The interaction of cosmic rays with the neutral constituents of Titan produces a complex ion chemistry that leads to the formation of a high variety of massive ions and organic molecules. The low temperatures and high pressures prevailing in Titan's atmosphere result in a high efficiency of three‐body reactions and on the formation of ion clusters. Association of neutral molecules with HCNH+, CH3CNH+, and HC3NH+ produces ions containing the CN group, and the electron recombination of these clusters can produce a high variety of N‐containing compounds. Here we propose schemes to synthesize nitrile‐type molecules from the dissociative recombination ion clusters produced by cosmic rays. Our ion chemistry model can explain the existence of some of the nitriles found in laboratory studies, such as propenenitrile, propanenitrile, butanenitrile, pentanenitrile, and 10 more as well as predict the existence of new species. Experimental simulations of Titan's atmosphere yield the formation of a high number of species, which are not considered in the chemical schemes of current photochemical models.

Three-Dimensional Modeling of the Tropospheric Methane Cycle on Titan

The tropospheric methane cycle on Titan including advection, diffusion, condensation, precipitation, evaporation, and surface source is simulated by a three-dimensional general circulation model. The model takes into account the different nucleation of methane in Titan's troposphere, and assumes condensation whenever the saturation ratio (relative humidity) exceeds a critical value of either 100 or 150%. The simulation with the latter critical saturation is shown to be more compatible with the Voyager data. The meridional circulation accounts for a seasonal methane transport from the winter to summer hemisphere in the mid-troposphere, causing a higher supersaturation in late summer and early autumn. Condensation preferentially occurs in this season at 15 km altitude at low latitudes. The occasional character may be explained by the high critical saturation ratio, which cannot be often exceeded. Falling rain evaporates during the descent, so no rain arrives at the surface. The hydrological cycle of methane on Titan takes place mostly within the atmosphere. The latitudinal methane distribution near the surface cannot be reproduced by transport, evaporation, condensation, and combinations of them, indicating some undetermined surface processes affecting the methane distribution. The zonal and meridional circulation is sensitive to the methane distribution.

Modelling of thunderclouds and lightning generation on Titan

The likelihood of lightning generation in Titan's troposphere is investigated by theoretical consideration and a numerical one-dimensional time-dependent thundercloud model. The main cloud electrification hypotheses proposed for terrestrial thunderstorms are examined taking into account recent knowledge concerning Titan's atmospheric state, especially condensation, and the lower ionosphere. Titan's thunderclouds may be quite rare because of the difficult methane nucleation and low convection energy, but once moist convection is triggered, a massive cloud containing slowly falling solid and liquid methane particles may be expected. In this case the cloud rapidly attaches a large amount of free electrons, which are abundant in Titan's troposphere due to the low abundance of electrophilic species. The entirely negative space charge in the cloud causes within a few minutes a temporary maximum electric field of 2×10 6 V m −1 which is sufficient to initiate 20 km long negative cloud-to-ground lightning strokes in Titan's lower troposphere. The collisional charging, on the other hand, appears to be less efficient since the charge transfer itself may be limited at Titan's cold temperatures and no substantial charge redistribution takes place in the cloud due to the weak updraft and gravitation.

Temperature Lapse Rate and Methane in Titan's Troposphere

We have reanalyzed the Voyager radio occultation data for Titan, examining two alternative approaches to methane condensation. In one approach, methane condensation is facilitated by the presence of nitrogen because nitrogen lowers the condensation level of a methane/nitrogen mixture. The resulting enhancement in methane condensation lowers the upper limit on surface relative humidity of methane obtained from the Voyager occultation data from 0.7 to 0.6. We conclude that in this case the surface relative humidity of methane lies between 0.08 and 0.6, with values close to 0.6 indicated. In the other approach, methane is allowed to become supersaturated and reaches 1.4 times saturation in the troposphere. In this case, surface humidities up to 100% are allowed by the Voyager occultation data, and thus the upper limit must be set by other considerations. We conclude that if supersaturation is included, then the surface relative humidity of methane can be any value greater than 0.08—unless a deep ocean is present, in which case the surface relative humidity is limited to less than 0.85. Again, values close to 0.6 are indicated. Overall, the tropospheric lapse rate on Titan appears to be determined by radiative equilibrium. The lapse rate is everywhere stable against dry convection, but is unstable to moist convection. This finding is consistent with a supersaturated atmosphere in which condensation—and hence moist convection—is inhibited.