Recent Knowledge: Atmosphere of Titan
Composition
The atmosphere of Titan was noticed for the first time in 1907 by Josep Comas i Solà and spectroscopically identified in 1944 by Gerard Kuiper. At this event methane (CH4) was detected although it is not the main constituent of Titan’s atmosphere. The first close investigation of the atmosphere took place in 1980 during the fly-by of the Voyager 1 spacecraft. Remote sensing revealed that Titan possesses a dense atmosphere of roughly 1500 hPa (1.5 bar) with molecular nitrogen (CH4) as the main constituent.
Chemical composition of the atmosphere
| Stratosphere | Near the Surface | ||
|---|---|---|---|
| 95 % | N2 | 95 % | N2 |
| 2 % | CH4 | 5 % | CH4 |
| 0.1 % | H2 | ||
| 1-2×10-5 | C2H6 | ||
| 2-4×10-6 | C2H2 |
Present understanding is that nitrogen was probably formed initially from ammonia that degassed from the interior. There is evidence that Titan’s atmosphere may have even denser than today and a substantial fraction of the atmospheric molecules already escaped to space.
The source of methane, the second most abundant constituent of the atmosphere, is assumed at the surface. Methane likely was directly trapped during the formation of Titan and stored in near-surface methane clathrates. It is unknown whether methane is continuously or episodically degassed to the atmosphere. Given its limited chemical lifetime methane has to be replenished form the surface in order to maintain the atmosphere. Liquid hydrocarbon lakes in the polar region cannot be regarded as the only source because they merely represent a reservoir of the hydrological cycle. However, the lakes can significantly affect the atmospheric composition via chemical equilibrium with the atmosphere.
Characteristic of Titan is the existence of a plethora of organic compounds (hydrocarbons, nitriles), which are generated from nitrogen and methane by photochemical reactions. Polymerization and condensation of organic molecules give rise to the aerosols (haze layer) that cover the entire globe in the stratosphere and reduce the visibility in the visible spectrum. Oxygen compounds are only present as minor species and formed either by geological processes (e.g. volcanism) or entry from space.
Titan is the only satellite in the Solar System with a dense atmosphere. There is no simple answer for the question as to why other moons of similar size such as the Jupiter moons Ganymede or Callisto have only tenuous atmospheres unlike Titan.
Thermal Structure
The thermal structure of the lower atmosphere was already investigated by radio signals of Voyager 1. The only continuous profile of temperature and pressure from 1400 km altitude down to the surface was obtained in January 2005 by the European Huygens Probe.
The neutral atmosphere of Titan can be subdivided by the temperature profile into the troposphere, stratosphere, mesosphere and thermosphere quite like the Earth’s atmosphere.
The temperature in the troposphere drops from 93 K at the surface down to 70 K at the tropopause (~40 km). Near the surface the temperature profile approximately follows the dry-adiabatic lapse rate. Greenhouse effect by a combination of N2, CH4 und H2 and heat exchange with the surface are relevant for the heat budget in the troposphere. The temperature at the tropopause is only 7 K higher than the freezing point of nitrogen, the main constituent of the atmosphere. Only the greenhouse effect and an efficient horizontal heat transport prevent the temperature at the poles from dropping below the freezing point of nitrogen and from freezing out of the atmosphere as a whole. Contrary to previous understanding the near-surface temperature is subject to a seasonal variation, which is small by terrestrial standards, though.
The temperature in the stratosphere steeply increases up to 180-190 K at 250 km altitude. The absorption of visible light by aerosols is responsible for this warming, so the stratospheric aerosols play a similar role as the ozone layer on Earth. The latitudinal and seasonal variation of temperature is substantially more pronounced in the stratosphere. Measurements by the Cassini infrared spectrometer indicate temperature contrasts of up to 30 K between equator and the winter pole, while the temperature difference amounts to only 3 K at the surface.
The temperature profile in the mesosphere is controlled by absorption of sunlight and thermal emission by methane, ethane and acetylene. Observational data do not clearly reveal a stratopause. The measured temperature profile in the middle atmosphere (stratosphere and mesosphere) exhibits a strong wavy structure that cannot be explained by radiation alone. This is likely to reflect dynamic processes such as gravity wave or tides.
In the thermosphere heating by absorption of solar EUV radiation by N2 is predominant. Sometimes energetic particles from the Saturnian magnetosphere contribute to heating as well.
Atmospheric Dynamics
In contrast to the thermal structure and composition, information about the winds on Titan has been quite poor until the turn of the millennium. There was indirect evidence of very strong winds in the stratosphere, but even the wind direction was unknown for some years. Meanwhile, ground-based telescopes can observe frequency shifts of the emission of selected molecules in the stratosphere and from these data the wind direction and speed in certain altitudes can be estimated. Such data point to strong westerlies of 100 to 200 m/s.
The Huygens Probe, which descended into the atmosphere, measured the prevailing winds in situ along the descent trajectory. Huygens confirmed the stratospheric westerlies of this order of magnitude. However, a marked anomaly was detected in the lower stratosphere, where the wind speed decreased to close to zero. The wind speed decreased towards the surface and a layer with weak easterlies was detected. Temperature measurements in the stratosphere by the infrared spectrometer of Cassini are found to be consistent with wind speeds of up to 200 m/s.
The strong stratospheric winds and the circumstance that these westerly winds dominate virtually the entire globe are referred to as superrotation. The superrotation is recognized as one of the major tasks in planetary meteorology. The exact mechanism of the maintenance of such strong winds on Titan is not exhaustively resolved. However, the rotation rate of Titan is supposed to play a major role.
Due to the slow rotation of Titan the Coriolis force is relatively unimportant for meteorology. The prevailing equilibrium for stratospheric winds is the cyclostrophic balance between the meridional pressure gradient force and the centrifugal force. Considering the quadratic dependence of the centrifugal force on the wind speed, both westerlies and easterlies could equally satisfy this equilibrium. In this equilibrium there is no necessity to reverse the zonal wind direction upon seasonal reversal of the pressure gradient force, unlike on Earth. Once established, westerlies could be remain westerlies eternally.
The slow rotation of Titan also has a large influence on other aspects of Titan’s meteorology. The thermally direct circulation (Hadley circulation) extends from one pole to the other because the cell is not subdivided into several cells as a result of the weak Coriolis force. For this reason frontal systems, which cause variable weather at mid latitudes of the Earth, are unlikely. There is ample observational evidence of the existence of the Hadley circulation on Titan. The near-surface wind mostly blows from the winter hemisphere to the summer hemisphere, but not from west to east or from east to west as it is the case at mid and low latitudes of the Earth, respectively. Consequently, very efficient heat transport in north-south direction can occur. There is only a very small surface temperature contrast of 3 K between the equator and pole. The situation in the remainder of the troposphere is similar. However, locally larger temperature contrasts can develop under certain circumstances, particularly in the vicinity of polar seas. They can generate sea or land breezes.
A special feature of Titan’s troposphere is the tidal wind caused by Saturn. The cause for the tidal wind is the elliptical orbit of Titan and thus libration and variation of the distance between Saturn and Titan. The tidal manifests itself as a planetary wave of wavenumber 2, which is superposed on the background winds and propagates with half the orbital speed of Titan eastward without modifying the shape. Given the lacking low pressure systems of mid latitudes the largest surface pressure variation on short timescales is caused by the tidal force. It is, however, a formidable task to verify the tidal wind by observations.
Hydrological Cycle of Methane
Clouds and lakes near the south pole (image credit: NASA/JPL/Space Science Institute, Turtle et al., 2009)
Titan is the first authentic place with a hydrological cycle that is not based on water but another species: methane. The first ideas about the methane hydrological cycle came up right after the Voyager mission in 1980 when the temperature and pressure condition as well as the approximate composition of the troposphere became clear. There was, however, uncertainty as to whether there is evidence of clouds in the observed spectra, methane condensation is possible at all under Titan’s conditions and what would initiate condensation.
Thanks to numerous ground-based observations that are being carried out since some years, there is no doubt any more that at least occasionally large convective methane clouds develop. Most clouds have been seen near the summer pole and they morphologically resemble cumulonimbi on Earth, although the physicochemical properties of liquid methane are undoubtedly different from those of water or ice. The reason for the large abundance of clouds near the summer pole may be the high surface temperature or convergence of moist air at the summer pole. Some observations also point to a possible relationship between the polar seas and clouds. Occasionally a few new dark features appeared on the surface in the vicinity of some clouds. These dark features are possibly ponds filled by falling rain from these clouds.
No optically thick clouds were observed anywhere during the landing of the Huygens probe. There was also no strong rainfall. However, there is evidence that Huygens descended through optically thin clouds and drizzle. The upper part of the cloud consists of frozen methane, whereas the lower part contains liquid methane with substantial fractions of nitrogen and ethane. Depending on the humidity and composition of the troposphere, raindrops could reach the surface. In this way a closed hydrological cycle between the atmosphere, surface and subsurface is possible.