Titan

Athena Coustenis. Encyclopedia of the Solar System. Editor: Lucy-Ann McFadden, et al., 2nd Edition, Elsevier, 2007.

Introduction

Titan’s Discovery, First Observations, and Models

Titan, Saturn’s biggest satellite (second in size among the satellites in our solar system), has attracted the eye of astronomers preferentially ever since its discovery by Dutch astronomer Christiaan Huygens on March 25, 1655. Titan orbits around Saturn at a distance of 1,222,000 km (759,478 mi) in a synchronous rotation, taking 15.9 days to complete. As Titan follows Saturn on its trek around the Sun, one Titanian year equals about 30 Earth years. The sunlight that reaches such distances is only 1/100th of that received by the Earth. Titan is therefore a cold and dark place, but a fascinating one.

It has been known for a long time that Titan possesses a substantial atmosphere: Catalan astronomer Jose Comas i Solà claimed in 1908 to have observed limb-darkening on Titan. Due to its thick atmosphere, Titan subtends 0.8 arcsec in the sky, and it was thought to be the largest of the satellites in the solar system. This explains the name it was given (following a proposition by Herschel, who suggested names of gods associated with Saturn for naming its satellites), until the advent of the Voyager missions that showed Ganymede to be a few kilometers larger. Today, we know that this massive atmosphere is the one most similar to the Earth’s among the other objects of our solar system as N2 is its major constituent and it is host to a complex organic chemistry.

In 1925, Sir James Jeans showed that Titan could have kept an atmosphere, in spite of its small size and weak gravity, because some of the constituents which could have been present in the proto-solar nebula (ammonia, argon, neon, molecular nitrogen and methane) would not escape. It was realized later that although ammonia (NH3) is in solid phase at the current Titan temperatures and could not in principle contribute to its present atmosphere, it could have evaporated in the early atmosphere and been converted into N2 at the end of the accretion period when the environment was warmer.

On the other hand, methane (CH4), the second most abundant constituent on Titan, is gaseous at present Titan’s atmospheric temperature range and, unlike molecular nitrogen, exhibits strong absorption bands in the infrared. These bands were first detected in 1944 by Gerard Kuiper of Chicago University. Ethane (C2H6), monodeuterated methane (CH3D), ethylene (C2H4) and acetylene (C2H2) were also discovered later.

Prior to spacecraft observations, two models were popular: a “thin methane” atmosphere model, which favored methane as the main component (about 90%) and predicted surface conditions of T = 86 K for 20 mbar as well as a temperature inversion in the higher atmospheric levels, illustrated by the presence of emission features of hydrocarbon gases in the infrared spectrum of Titan; and a “thick nitrogen” atmosphere model, which was based on the assumption that ammonia dissociation should produce molecular nitrogen (transparent in the visible and infrared spectrum) in large quantities and held that the surface temperature and pressure could be quite high (200 K for 20 bars). Independent of these two models, an explanation of the high observed ground temperatures was advanced: a pronounced greenhouse effect, resulting essentially from H2-H2 pressure-induced opacity at wavelengths higher than 15 μm. This opacity blocks the thermal emission reflected by the surface, thus creating a heat-up of the lower part of the atmosphere, as found on Earth.

Titan’s Exploration

Titan has since then been extensively studied from the ground and from space. In the latter case, Titan was “blessed” by several space mission encounters in the course of the planetary exploration in our solar system.

Although the Pioneer 11 spacecraft was the first to take a close look at the giant planets Jupiter and Saturn, it flew by Titan at a considerable distance of 363,000 km on September 2, 1979. The Voyager missions that followed were also dedicated to an extended study of the outer solar system. The Voyager 1 (V1) spacecraft (launched in 1977) arrived in the Saturnian system and made its closest approach of Titan on November 12, 1980, at a distance of only 6969 km (4394 miles) to the satellite’s center. Voyager 2 flew by Titan 9 months later but at a distance a hundred times greater (663,385 km) so that the Voyager 1 encounter was the closest a man-made machine ever came to Titan until 2004.

Titan’s visible appearance at the time was unexciting—an orange ball, completely covered by thick haze, which allowed no visibility of the surface (Fig. 1a). The most obvious feature seen by Voyager was a difference in the brightness of the two hemispheres. This difference is of the order of 25% at blue wavelengths and falls to a few percent in the ultraviolet and at red wavelengths. This so-called north-south asymmetry (NSA) is probably related to circulation in the atmosphere pushing haze from one hemisphere to the other. The altitude of unity vertical optical depth is of the order of 100 km. Also noticeable was a dark ring above the north (winter) pole. This feature, termed the polar hood, extending from 70° to 90° north latitude, was most prominent at blue and violet wavelengths, and it has since then been suggested that it may be associated with lack of illumination in the polar regions during the winter (since the subsolar latitude goes up to 26.4°) and/or subsidence in global circulation.

Besides the images, the Voyager instrument also allowed for the determination of the chemical composition and temperature structure. The latter and other basic parameters for Titan (Table 1) were provided by the radio-occultation Voyager experiment obtained by the Radio Science Subsystem (RSS). Titan’s surface radius was found to be 2575 ± 2 km, with a surface temperature of 94 ± 2 K and a pressure of about 1.44 bar.

After Voyager, scientists had to wait for about 25 years before getting another close look at Titan. Cassini-Huygens is a very ambitious mission, planned in the 1980s already. It is an extremely successful collaboration between ESA and NASA (with contribution from 17 countries), composed of an orbiter and a probe (Huygens). Although the mission’s objectives span the entire Saturnian system, Titan is a privileged target for Cassini (as for Voyager before it), and the mission is designed to address our principal questions about this satellite during its 6-year duration from 2004 onwards. The spacecraft is equipped with 18 science instruments (12 on the orbiter and 6 carried by the probe), gathering both remote sensing and in situ data. It communicates through one high-gain and two low-gain antennas. Power is provided through three radioisotope thermoelectric generators (or RTGs).

TABLE 1 Titan’s Orbital and Body Parameters, and Atmospheric Properties
Surface radius 2575 km
Mass 1.35 × 1023 kg (= 0.022 × Earth)
Mean density 1880 kg−3
Distance from Saturn 1.23 × 109m (= 20 Saturn radii)
Distance from Sun 9.546 AU
Orbital period around Sun 15.95 days
29.5 years
Obliquity 26.7°
Surface temperature 93.6 K
Surface pressure 1.467 bar

The 5650 kg (6 ton) Cassini-Huygens spacecraft was launched successfully on October 15, 1997, from the Kennedy Space Center at Cape Canaveral at 4:43 A.M. EDT. Because of its massive weight, Cassini could not be sent directly to Saturn but used the “gravity assist” technique to gain the energy required by looping twice around the Sun. This allowed it to also perform flybys by Venus (April 26, 1998, and June 24, 1999), Earth (August 18, 1999), and Jupiter (December 30, 2000). Cassini-Huygens reached Saturn in July 2004 and performed a flawless Saturn Orbit Insertion (SOI), becoming trapped forever in orbit like one of Saturn’s moons.

The Cassini instruments have since then returned a great amount of data concerning the Saturnian system. During its 4 year nominal mission, the Cassini orbiter will make about 40 flybys of Titan, some as close as 1000 km (Voyager 1 flew by at 4400 km) from the surface. Cassini will perform direct measurements with the visible, infrared, and radar instruments designed to perform in situ (on-site) studies of elements of Saturn, its atmosphere, moons, rings, and magnetosphere. One set of instruments studies the temperatures in various locations, the plasma levels, the neutral and charged particles, the surface composition, the atmospheres and rings, the solar wind, and even the dust grains in the Saturn system, while another performs spectral mapping for high-quality images of the ringed planet, its moons, and its rings.

Additionally, the mission saw the deployment of the European-built Huygens probe. After release from the Cassini orbiter, on December 25, 2004, this 300 kg probe plunged into Titan’s atmosphere on January 14, 2005, at 11:04 UTC and descended through it by means of several parachute brakes, which slowed the probe from supersonic speeds of 6 km/s during entry and down to 5 m/s at impact. The five batteries onboard the probe lasted much longer than expected, allowing Huygens to collect descent data for 2 hours and 27 minutes and surface data for 1 hour and 12 minutes. During its descent, Huygens’ camera returned more than 750 images, while the probe’s other instruments sampled Titan’s atmosphere to help determine its composition and structure. The telemetry data from Huygens was stored onboard Cassini‘s Solid State Recorders (SSRs) at a rate of 8 kbits/s, while the spacecraft was at an altitude of 60,000 km from Titan. Although some data from Huygens was lost during its transmission to Cassini through a stream called Channel A, in the end all of the measurements were recovered because Titan’s weak signal was captured by Earth-based radio telescopes!

As well as measuring the atmosphere and surface properties, the probe took samples of the haze and gases. These in situ measurements complement the remote-sensing data recorded from the orbiter.

The Cassini-Huygens mission has already provided a wealth of data. The analysis is in the first stages, and the Cassini orbiter promises to unveil yet more of Titan’s secrets in the years to come. What follows is an attempt to provide the reader with a precise account of current information on Titan’s environment from all available means of investigation.

The Atmosphere of Titan

The most interesting feature of Titan, as has been argued previously, is its amazing atmosphere, a close analog to the Earth’s primitive gas envelope according to some theories, but it is located almost ten times further away from the Sun.

Thermal Structure

The first definitive measurement of the atmospheric temperature structure was made by Voyager. The V 1/RRS radio-occultation experiment provided density and temperature profiles in Titan’s atmosphere from refractivity measurements. Titan’s temperature profile was measured in situ on January, 14, 2005 by the Cassini-Huygens Atmospheric Structure Instrument (HASI) at the probe’s landing site (15°S, 192°W) from 1400 km in altitude down to the surface, where 93.65 ± 0.25 K were measured for a surface pressure of 1467 ± 1 mbar. As Voyager did before, HASI found Titan’s atmosphere to exhibit the features that characterize the Earth’s thermal structure: the atmospheric layers include an exosphere, a mesosphere, a stratosphere and a troposphere, with two major temperature inversions at 40 and 250 km, corresponding to the tropopause and stratopause, associated with temperatures of 70.43 K (min) and 186 K (max), respectively. At the same time, the Composite Infrared Radiometer Spectrometer (CIRS) on the orbiter took spectra that confirmed the presence of a stratopause around 310 km of altitude for a maximum temperature of 186 K. Another inversion region, less contrasted than the previous ones and corresponding to the mesopause can be found at 490 km (for 152 K).

The HASI data furthermore yield more precise and new information on the upper part of the Titan atmosphere, the thermosphere, where several temperature fluctuations are observed due to dynamical (gravity and tidal) phenomena. Indeed, gravity waves signatures of 10-20 K in amplitude were recorded above 500 km around an average temperature of 170 K. HASI moreover found a lower ionospheric layer between 140 and 40 km, with electrical conductivity peaking near 60 km. A tentative detection of lightning is being investigated.

Besides the Huygens measurements, few constraints are available for the temperature structure in Titan’s higher atmosphere. The V1/UVS experiment recorded a temperature of 186 ± 20 K at 1265 km during a solar occultation for a methane mixing ratio of 8 ± 3% toward 1125 km, placing the homopause level at around 925 ± 70 km. A value of 183 ± 11 K near 450 km was derived from the July 3, 1989, stellar occultation of Titan. The occultation of star 28 Sgr by Titan was observed from places as widely dispersed as Israel, the Vatican, and Paris. This rare event provided information in the 250-500 km altitude range. A mean scale height of 48 km at 450 km altitude (∼3 mbar level) was inferred. This allowed the mean temperature to be constrained at that level to between 149 and 178 K.

From V1 infrared disk-resolved measurements, temperature latitudinal variations were already demonstrated to exist in Titan’s stratosphere. At that time, a maximal temperature decrease of 17 K at the 0.4-mbar level (225 km in altitude) was observed between 5°S (the warmest region in the Voyager data) and 70°N, whereas the temperature dropped by only 3 K from 5°S to 53°S. The coldest temperatures, found at high northern latitudes, were associated with enhanced gas concentration and haze opacity (as this may be caused by more efficient cooling) or/and dynamical inertia. CIRS mapped stratospheric temperatures over much of Titan in the latter half of 2004, when it was early southern summer on Titan (solstice was in October 2002). The warmest temperatures are near the equator. Temperatures are moderately colder at high southern latitudes, by 4-5 K near 1 mbar, but they are coldest at high latitudes in the north, where it is winter.

Titan was also found to have a quite extended ionosphere, due to the lack of a strong intrinsic global magnetic field. Charged particles in the rarified upper atmosphere are then exposed to bombardment by the solar wind and by particles precipitated from Saturn’s magnetosphere (creating an ionospheric layer between 700 and 2700 through which Cassini flew during some of its lower flybys of Titan), as well as by cosmic rays from outer space (producing a second layer between 40 and 140 km). Cassini found that more than 10% of the ionosphere is made up of ionized hydrocarbon molecules chemically similar to compounds such as ethylene, propyne, and diacetylene and that this population is lost to space at important rates.

Chemical Composition

Indeed, the nature of Titan’s atmosphere finally emerged as a combination of the two pre-Voyager models. Molecular nitrogen (N2, detected by the UV spectrometer) is by far the major component of the at mosphere (average of ∼95%). The presence of methane (the next most abundant molecule with abundances ranging from 0.5 to 3.4% in the stratosphere and from 4 to 8 % at the surface), traces of hydrogen, and a host of organic gases were inferred from emission bands observed in the infrared interferometer spectrometer (IRIS) spectra, which cover the 200-1500 cm−1 spectral region with a spectral resolution of 4.3 cm−1, and later confirmed in the Infrared Space Observatory (ISO) and CIRS observations, that afforded higher spectral resolution. In 1997 ISO Short Wavelength Spectrometer (SWS) spectra provided a good determination of the chemical abundance on Titan and also the first detection of water vapor in Titan’s atmosphere from 2 emission lines around 40 μm, for an associated mole fraction derived at 400 km of altitude of about 10−8. ISO also found the first hint of the presence of benzene (C6H6) at 674 cm−1 for a mole fraction on the order of a few 10−10. Since then, the benzene detection has been confirmed by Cassini/CIRS. The water vapor abundance, although seemingly small, implies a water influx on Titan significantly superior to what might be expected based on local and interplanetary sources alone (rather in favor of Saturn). By including the laboratory spectra of these gases in radiative transfer calculations, the abundances of all of these constituents can be estimated (Table 2). The spatial distribution (latitudinal and vertical) of these constituents was also retrieved. The vertical distributions generally increase with altitude, confirming the prediction of photochemical models that these species form in the upper atmosphere and then diffuse downward in the stratosphere. Below the condensation level of each gas, the distributions are assumed to decrease following the respective vapor saturation law.

Table 2 Chemical Composition of Titan’s Atmosphere Today from Cassini-Huygens Results Unless Otherwise Indicated
Constituent Mole Fraction (atm. altitude level)
a Increasing in the North.
b From ground-based heterodyne microwave observations.
c Only observed from the ground.
d From ISO observations.
e From Cassini and ground-based data.
Major
  Molecular nitrogen, N2 0.98
  Methane, CH4 4.9 × 10−2 (surface)
1.4-1.6 × 10−2 (stratosphere)
  Monodeuterated methane, CH3D 6 × 10−6 (in CH3D, in stratosphere.)
  Argon,36Ar 2.8 × 10−7
  40Ar 4.3 × 10−5
Minor
  Hydrogen, H2 ∼0.0011
  Ethane, C2H6 1.5 × 10−5 (around 130 km)
  Propane, C3H8 5 × 10−7 (around 125 km)
  Acetylene, C2H2 4 × 10−6 (around 140 km)
  Ethylene, C2H4 1.5 × 10−7 (around 130 km)
  Methylacetylene, CH3C2H 6.5 × 10−9 (around 110 km)a
  Diacetylene, C4H2 1.3 × 10−9 (around 110 km)a
  Cyanogen, C2N2 5.5 × 10−9 (around 120 km)a
  Hydrogen cyanide, HCN 1.0 × 10−7 (around 120 km)a
5 × 10−7 (around 200 km)b
5 × 10−6 (around 500 km)b
  Cyanoacetylene, HC3N 1× 10−9 (around 120 km)a
1 × 10−7 (around 500 km)b
  Acetonitrile, CH3CN 1 × 10−8 (around 200 km)c
1 × 10−7 (around 500 km)
  Water, H2O 8× 10−9 (at 400 km)d
  Carbon monoxide, CO 4 × 10−5 (uniform profile)e
  Carbon dioxide, CO2 1.5 × 10−8 (around 120 km)

Ground-based high-resolution heterodyne millimeter observations of Titan offered the opportunity to determine vertical profiles and partial mapping in some cases of HCN, CO, HC3N, and CH3CN, which showed that the nitrile abundances increase with altitude. Subsidence causes the abundance of these species to decrease in the lower atmosphere.

Curiously, the bulk composition of Titan was more difficult to determine than the abundances of the trace constituents. Cassini-Huygens finally allowed firm determinations for the major components: Huygens Gas Chromatograph Mass Spectrometer (GCMS) found a methane mole fraction of 1.41 × 10−2 in the stratosphere, increasing below the tropopause and reaching 4.9 5 10−2 near the surface, in good agreement with the stratospheric CH4 value inferred by CIRS on the Cassini orbiter (1.6 ± 0.5 × 10−2) and the surface estimate given by the Huygens Descent Images Spectral Radiometer (DISR) spectra (also 5%). The GCMS also saw a rapid increase of the methane signal after landing, which suggests that liquid methane exists on the surface, together with other trace organic species, including cyanogen, benzene, ethane, and carbon dioxide. The only noble gas detected to date is argon, found in the form of primordial 36Ar (2.8 × 10−7) and its radiogenic isotope 40Ar (4.32 × 10−5) by GCMS. The low abundance of primordial noble gases on Titan implies that nitrogen was originally captured as NH3 rather than N2. Subsequent photolysis may have created the N2 atmosphere we see today.

Isotopic ratios were determined from Cassini and Huygens instruments: 12C/13C (82.3 ± 1), 14N/15N (measured in situ in N2, 183 ± 5, which is 1.5 times less than on Earth) and D/H (measured in situ in H2, 2.3 ± 0.5 × 10−4, from the GCMS, and in CH4, 1.2 × 10−4, from remote sensing of infrared spectra recorded aboard the Cassini orbiter with CIRS). It is believed that nitrogen was initially brought in Titan in the form of NH3 and converted into N2 by photolysis of the early atmosphere. The measured 14N/15N implies that a substantial part of N2, from 2 to 10 times the mass of the early atmosphere, escaped over 4.5 billions of years. The D/H ratio is very important for Titan cosmogonical models. A lower value for D/H in methane (∼1.2 × 10−4) was found from the analysis of Cassini observations of the ν6 monodeuterated methane (CH3D) band at 8.6 μm, confirming a value found from Voyager data analyses. Both D/H values tend to suggest a deuterium enrichment in Titan’s atmosphere with respect to the proto-solar value as well as in that of the giant planets (D/H ∼ 2-3.4 × 10−5). The interpretation of this enrichment is related to the evolution of CH4 in the atmosphere. The key point is that CH4 is continuously photodissociated so that, in the absence of a substantial reservoir, it would entirely vanish from the atmosphere in 10-50 Myr. Imaging, infrared, and visible observations from the orbiter rule out the presence of a global ocean containing a large amount of CH4 on the surface of Titan. It is thus likely that methane outgasses from time to time from the interior of the satellite. Two scenarios for the origin of the internal CH4 have been proposed. One scenario advocates that CH4 was chemically produced from H2O and CO2 trapped in the planetesimals that formed Titan and which easily condensed in the solar nebula. However, this does not explain the detection of 36Ar in the atmosphere in an amount higher than that which could possibly have been trapped in the silicated core. A more plausible scenario argues that CH4 and 36Ar were present as ices or clathrates in the cool solar nebula and were incorporated in Titan planetesimals. This is consistent with the assumption that CH4 was enriched in deuterium by ion-molecules reactions in the presolar cloud, the resulting D/H in CH4 then being at least partly preserved in icy grains falling onto the solar nebula and—since no deuterium fractionation can occur in the interior—reflecting the value observed in the atmosphere of the satellite today.

Dynamical Processes

Zonal Circulation

At the time of the Voyager encounter, Titan’s northern hemisphere was coming out of winter. During the Cassini observations in 2006, Titan’s northern hemisphere was halfway into winter.

The general faintly banded appearance of Titan’s haze suggests rapid zonal motions (i.e., winds parallel to the equator). This impression is reinforced by the infrared temperature maps, which show very small contrasts in the longitudinal direction and rather large ones (of around 20 K) between the equator and the winter pole. The mean zonal winds inferred from this temperature field are weakest at high southern latitudes and increase toward the north, with maximum values at and mid-northern latitudes (20-40N) of about 160 m s−1. On Titan, pressure gradients are in cyclostrophic balance with centrifugal forces.

Stellar occultations are another indirect means to obtain the zonal winds. The atmospheric oblateness due to the zonal winds can be constrained from the analysis of the central flash, the increase of the signal at the center of the shadow (when the star is behind Titan) due to the focusing of the atmospheric rays at the limb. On July 3, 1989, Titan occulted the bright K-type star 28 Sgr, and fast zonal winds were derived close to 180 ms−1 at high southern latitudes and close to 100 ms−1 at low latitudes. Other occultations occurred on December, 20, 2001, and November, 14, 2003. They seem to suggest a seasonal variation with respect to 1989. In 2001, a strong 220 ms−1 jet was located at 60°N, with lower winds extending between 20°S and 60°S, and a much slower motion at midlatitudes. The CIRS data suggest that the strongest northern winds have migrated closer to the equator with respect to previous measurements, while the southern winds have weakened.

Space and occultation wind measurements could not provide the wind direction, a crucial factor for the Huygens probe mission, so different teams of ground-based observers tried to measure the zonal winds directly using alternative methods. The first measurement of prograde winds (in the sense of the rotation of the surface) was performed using infrared heterodyne spectroscopy of Doppler-shifted ethane emission lines. The measured winds were on the order of 210 ± 150 ms−1 between 7 and 0.1 mbar, a result that has since been refined. Other Doppler studies probing somewhat different levels also found prograde winds, using millimeter-wavelength interferometry of nitrile lines or high-resolution spectroscopy of Fraunhofer solar absorption lines in the visible. The recent advances in adaptive optics also allowed for the first detections of tropospheric clouds from the ground, mainly at circumpolar southern latitudes, but so far Titan winds remain poorly constrained due to the sparse data set of cloud positions. Better spatially resolved Cassini/International Space Station (ISS) observations only indicate slow eastward motions, which, extrapolated to the equator under the assumption of solid-body rotation, yield 19±15 ms−1 at around 25 km altitude. Finally, in 2005, the Huygens probe provided ground-truth measurements of the wind magnitude and direction in the lower stratosphere and troposphere. The Doppler wind experiment shows a marked decrease of winds with decreasing altitude, from 100 ms−1 at 140 km down to about nil at 80 km, then an increase up to 40 ms−1 at 60 km before decreasing again to null zonal velocity at the surface.

Latitudinal and Temporal Variations in the Atmosphere of Titan as Evidence of Meridional Circulation

Periodic change of Titan’s disk-integrated brightness has been monitored from Earth-based observations since the 1970s. Spatially resolved observations, starting with Voyager, have provided an interpretation of the periodic changes of the disk-integrated brightness as the combined action of the high inclination of the rotation axis and the seasonally varying north-south asymmetry. The NSA that Voyager 1 observed in 1980, with a darker northern hemisphere in visible light, has since been observed to reverse, as Titan’s season shifted from northern spring to present-day northern winter. When the Hubble Space Telescope (HST) first observed Titan in 1994, a little over a quarter of a Titan year after the Voyager encounters, the northern hemisphere was found to be brighter than the southern hemisphere. The turnover was later also found to occur gradually, starting at higher altitudes in the atmosphere.

Modeling with a two-dimensional general circulation model provided a qualitative description of the seasonal variations of the haze, where both the gradual inversion of the asymmetry and the detached haze layer can be explained by a seasonally varying Hadley circulation. The meridional wind in the upper branch of the Hadley cell is stronger close to the production zone (at 450 km) than below, and particles there are more rapidly transported toward the pole, where they sink. The asymmetry thus reverses first at higher altitudes. But this is not the only effect. As the season changes, shortly after equinox, the circulation reverses and an ascending motion sets in where the particles were previously descending. At the time of the transition, the polar haze, which was previously descending, is then redistributed about a scale height below the production zone, becoming physically separated from the freshly created particles aloft.

Meridional variations were also established for the gases in Titan’s stratosphere, and these are also tightly coupled with the circulation. The molecular abundances found by Cassini at this era indicate an enhancement for some species in the stratosphere at high latitudes, albeit not as dramatic as at the time of the Voyager encounter. The difference in magnitude between the Voyager 1 and the Cassini eras may be due to the difference in seasons, and it will be exciting to await the arrival of northern spring equinox toward the end of the Cassini mission and to measure the meridional variations then to see if we return to the IRIS inferences.

In the meantime, such latitudinal contrasts observed in the chemical trace species may be explained by invoking photochemical and dynamical reasons. The UV radiation from the Sun acts on methane and nitrogen to form radicals that combine into nitriles and the higher hydrocarbons. This production occurs in the mesosphere at high altitudes (above 300 km or 0.1 mbar). Eddy mixing transports these molecules into the lower stratosphere and troposphere where most of them condense. Photo dissociation by UV radiation occurs on timescales ranging from days to thousands of years. The combination of these processes leads to a vertical variation in the mixing ratio, which usually increases with height towards the production zone. Three-dimensional computation of actinic fluxes suggests that this mechanism alone cannot explain the latitudinal contrasts and that circulation must intervene. Simulations coupling photochemistry and atmospheric dynamics provide a consistent view: Competition between rapid sinking of air from the upper stratosphere in the winter polar vortex and latitudinal mixing controls the vertical distribution profiles of most species. The magnitude of the polar enrichment is controlled by down welling over the winter pole, which brings enriched air from the production zone to the stratosphere, and by the level of condensation. Short-lived species are more sensitive to the down welling due to steeper vertical composition gradients and exhibit higher contrasts.

In the stratosphere, the calculated radiative relaxation time is longer than the Titan season, so the temperature contrasts should be symmetric about the equator. That they are not indicates that the Hadley circulation must be connected with the lower atmosphere, where the time constant is much longer. This is consistent with the small thermal contrasts of 2-3 K in the troposphere, which suggest an efficient heat redistribution. Since Titan’s slow rotation and small radius rule out nonaxisymmetric processes, such as baroclinic eddies, as a preferred mechanism for heat transport, considerable meridional motions must be inferred. Latitudinal contrasts would be much larger if heat were not being transported poleward by Hadley advection.

Another phenomenon was first reported in 2001 from adaptive optics data taken in 1998. A diurnal change was found, manifested in an east-west asymmetry, with a brighter morning limb observed on Titan on several occasions. This dawn haze enhancement could be due to an accumulation of condensates during the Titan night (8 Earth days, though the superrotation of Titan’s atmosphere would lead to shorter nights for stratospheric clouds).

A Three-Dimensional View and Waves

Meridional contrasts are apparent in Titan’s atmospheric distributions of composition, haze, and temperature, and their seasonal variability is proof for a strong coupling with an underlying meridional circulation that has never been directly detected.

The superrotation observed in the stratosphere, a dynamical state in which the averaged angular momentum is much greater than that corresponding to corotation with the surface, is difficult to explain and has defied our understanding in the much better documented Venus case, the paradigm of a slowly rotating body with an atmosphere in rapid rotation. In recent studies, such a process has been identified under the form of planetary waves, forced by instabilities in the equator ward flank of the high-latitude jet. Two factors play a key role in facilitating the acceleration process. On the one hand, high altitude absorption processes decouple upper atmosphere dynamics from dissipation occurring at the surface layer, while on the other hand the slow rotation allows the Hadley cell to reach high latitudes by reducing centrifugal forces in the pole ward branch. A strong seasonal cycle due to Titan’s obliquity of 26.7° was also established: During most of the Titan year, the meridional motion is dominated by a large Hadley cell extending from the winter to the summer pole, with the symmetric two-cell configuration typical of equinoxes occurring only in a limited transition period. In the model, the jet is located close to 60° in the winter hemisphere, while the summer zonal circulation is close to solid body rotation.

The radiative time constant is long in the troposphere, but the surface has a smaller thermal inertia, so the surface temperature does respond to seasonal forcing, albeit by only a few Kelvin. This surface temperature variation is sufficient to reverse the circulation pattern of the Hadley circulation after the equinox when the Sun moves to the opposite hemisphere. Also the development of convective methane clouds is partly ascribed to seasonal surface heating. The reversal of the Hadley circulation may play an important role in the methane “hydrological” cycle because the vertical and horizontal transport of methane would vary seasonally.

Direct evidence for wave processes in Titan’s atmosphere remains scarce, despite their importance in the maintenance of superrotation. Because baroclinic processes are excluded, waves essentially barotropic in nature should be expected as the principal carrier of momentum from high to low latitudes. Modeling predicts wavenumber-2 waves with an amplitude of the zonal component about 10% of the mean wind speed, and in principle they can be inferred from horizontal maps of temperature and trace species exhibiting strong latitudinal contrasts. The first Cassini/CIRS temperature maps at 1.8 mbar do show spatial inhomogeneity, but long time series and better spatial coverage are needed to constrain spatial and temporal variations.

Another relevant nonaxisymmetric phenomenon in Titan’s troposphere is the gravitational tide exerted by Saturn. The eccentric orbit of Titan around Saturn gives rise to a tidal force, resulting in periodical oscillation in the atmospheric pressure and wind with a period of a Titan day (16 days), among which the most notable effect is the periodical reversal of the north-south component of the wind. In the lower atmosphere, the effect of this tide is modest, with a maximum temperature amplitude about 0.3 K and winds of 2m s−1.

Temperature inversions have been detected in both the Huygens HASI measurements and in stellar occultation data. Inversion layers were present close to 510 km altitude in HASI and 2003 occultation data, and at 425 and 455 km in 1989 occultation light curves. Vertical wavelengths were on the order of 100 km.

Haze and Clouds on Titan

It was recognized quite early that another important aspect of Titan’s atmosphere was the presence of aerosols. Pre-Cassini models treated the dissociation of methane molecules by solar actinic radiation, followed by chemical combination to heavier hydrocarbons that condense into particles. The cloud physics models with sedimentation and coagulation predicted a strong increase in haze density with decreasing altitude.

Titan’s Haze

The analysis of high-phase Voyager images indicated aerosol radii between 0.2 and 0.5 μm. These “smog” particles form a layer that enshrouds the entire globe of Titan and stretches from the surface to an altitude of about 200 km. A detached haze layer at 340-360 km altitude with large, compact, irregular dark particles was also found. The small haze particles required by Voyager measurements (radii less than or equal to 0.1 μm) produce a strong increase in optical depth with decreasing wavelength shortward of 1 μm. To fit the observations in the methane bands, it was necessary to remove the haze permitted by the cloud physics calculations at altitudes below about 70-90 km (called cut-off altitude) by invoking condensation of organic gases produced at high altitudes as they diffused down to colder levels. The condensation of many organic gases produced by photochemistry at high altitudes on Titan seemed consistent with this view. The next step in the development of Titan haze models included the use of fractal aggregate particles composed of several tens of small (0.06 μm in radius) monomers to produce strong linear polarization. Monomers composed of 45 aggregates with an effective radius of about 0.35 μm matched the Voyager observations.

Starting from the upper atmosphere, the Cassini ISS camera showed a faint thin haze layer that encircles the denser stratospheric haze (Fig. 1b) and could be the equivalent of the “detached haze layer” observed by Voyager 25 years ago, except for the difference in altitudes: The thin current haze layer is indeed located 150-200 km higher than the one seen by Voyager. Current models are still unable to render the complexity of seasonal phenomena or circulation patterns on Titan, which could be responsible for such an upward shift.

Cassini images also show a multilayer structure in the north polar hood region and, in some cases, at lower latitudes. These features could be due to gravity waves that have been detected on Titan at lower altitudes. Some of these layers may be related to the two global inversion layers observed in stellar occultations of Titan above 400 km in altitude.

The nature of the haze aerosols measured by Huygens/DISR during the descent through Titan’s lower atmosphere came as a surprise to scientists recalling the results from Pioneer and Voyager, as well as predictions by cloud physics models with sedimentation and coagulation. The new observations estimate the monomer radius to be 0.05 μm, in good agreement with previous values. However, contrary to previous assumptions, the DISR data seem to show that the size of the aggregate particles is several times as large as previously supposed.

In addition, measurements by the DISR violet photometer extend the optical measurements of the haze to wave-lengths as short as the band from 350 to 480 nm, also helping to constrain the size of the haze particles. The number density of the haze particles does not increase with depth nearly as dramatically as predicted by the older cloud physics models. In fact, the number density increases by only a factor of a few over the altitude range from 150 km to the surface. This implies that vertical mixing is much less than had been assumed in the older models where the particles were distributed approximately as the gas is with altitude. In any event, the clear space at low altitudes, which was suggested earlier, was not observed.

The methane mole fraction of 1.4-1.6% measured in the stratosphere by the CIRS and the GCMS is consistent with the DISR spectral measurements. At very low altitudes (20 m), DISR and the GCMS measured 5 ± 1% for the methane mole fraction.

Clouds

Cassini-Huygens has provided new information on the role of methane and the methane cycle in Titan’s atmosphere. The relative humidity of methane (about 50%) at the surface found by DISR and the evaporation witnessed by the GCMS show that fluid flows have existed and will probably again exist on the surface, implying precipitation of methane through the atmosphere.

Although some discussion took place as to whether Titan’s lower atmosphere could support convection and as to whether methane was supersaturated, there is clear evidence today that clouds exist in Titan’s troposphere, although in general they tend to appear higher than expected and are mostly restricted to high southern latitudes.

Methane clouds in Titan’s troposphere were first suspected from variability in the methane spectrum observed from the ground. Direct imaging of clouds on Titan has been achieved from Earth-based observatories since the turning of the century. Most of the currently detected clouds are located in Titan’s southern hemisphere, as expected given the season on Titan (summer in the south), which means that solar heating is concentrated there as are rising motions. Other than the large, bright South Pole system observed for the past 5 years or so, discrete clouds detected at midlatitudes are infrequent, small and short-lived (Cassini Visual and Infrared Mapping Spectrometer (VIMS) observations tend to indicate that they rise quickly to the upper troposphere and dissipate through rain within an hour). Keck and Gemini data indicate that they tend to cluster near 350°W and 40°S. They may be related to some surface-atmosphere exchange (such as geysering or cryovolcanism) because they don’t seem to be easily explained by a shift in global circulation. A dozen or so large-scale zonal streaks have also been observed by Cassini preferentially at low southern latitudes and mostly between 50 and 200°W.

The large south polar system has been visible consistently essentially in the near-infrared (at 2.12 μm for instance) since 1999, while no previous indication of it was ever reported. It was extremely bright in 2001–2002, and recent Cassini images have shown that it is disappearing (indeed it was visible only during the few first Titan flybys and not afterwards, see Fig. 5). Its shape is irregular and changing with time, recently resembling more a cluster of smaller-scale clouds than a large compact field. Should it prove that this system’s life was indeed on the order of 5-6 years (fairly close to a Titan season), stringent constraints can be retrieved on seasonal and circulation patterns on Titan. The cloud made a reappearance in 2006.

Note that DISR reported no definite detection of clouds during its descent through Titan’s atmosphere. However, the data are compatible with a thin haze layer at an altitude of 21 km, which could be due to methane condensation.

The Surface of Titan

To the eyes of the public and many scientists, the most important features revealed by the Cassini-Huygens mission were those found on Titan’s surface, finally observed in close-up by the orbiter since 2004 and even in situ conditions by the Huygens probe instruments on January 14, 2005. The spaceship has offered detailed views of Titan’s surface in the visible and the near-infrared with its camera, the mapping spectrometer, and radar. Descending through the atmosphere, the Huygens probe returned fantastic images of a first-seen domain, the farthest location a human-made vessel has ever landed upon. Although we still haven’t exactly determined the nature of all the surface constituents, the combination of the information retrieved by all the observing teams will eventually force Titan to uncover its mysterious soil. Undoubtedly the signs of dried lakes, volcanoes, and channels on Titan’s surface were unexpected. They offer an even more amazing view of a land much fantasized on.

Pre-Cassini Glimpses of an Exotic Ground

To the Voyager camuas, the surface of Titan was obscured by the dense haze in the atmosphere. Glimpses of what lay below were revealed afterwards by ground-based radar and infrared images from HST and ground-based observatories.

Theory argued that unless methane supersaturation conditions prevailed on Titan, the organics present in the atmosphere should condense at some level in the lower stratosphere and precipitate out, ending up on Titan’s surface and coat the ground in large proportions. Based on the surface conditions believed to prevail on Titan, liquid methane—and its principal by-product, ethane—is expected to exist and could even form an ocean, and in the troposphere, methane clouds (formed by saturation of methane gas) might cause rains. The degree of saturation in the lower atmosphere, however, was unknown, so the methane abundance was difficult to determine.

On the other hand, much of the outer part of the solid body of the satellite must, to be consistent with the observed mean density, consist of a thick layer of ice. The ethane ocean model, developed in 1983, was aesthetically appealing and compatible with all the Voyager-era data. It has since then long been abandoned in view of the spectroscopic and imaging evidence for a heterogeneous surface and the radar echoes indicating the presence of solid material.

Indeed, a shallow, global ocean was shown to be inconsistent with the constraints imposed by Titan’s orbital characteristics. The tidal action on an ocean less than 100 m deep would have dissipated Titan’s eccentricity of 0.03 (where 0 is circular and 1 is parabolic) long ago. Furthermore, the first remote-sensing technique to be used for sounding Titan’s surface, radar, indicated that the surface may be nonuniform but mostly solid with at most small lakes. Indeed, the radar echos obtained in 1990 using the National Radio Astronomy Observatory’s Very Large Array in New Mexico combined as a receiver of the signal transmitted to Titan by the NASA Goldstone radio telescope in California were among the first evidence against the global ocean model of the surface. Radar measurements from Arecibo Observatory in Puerto Rico in 2003, however, revealed a specular component at 75% (12 of 16) of the regions observed (globally distributed in longitude at about 26°S), which was interpreted as indicative of the existence of dark, liquid hydrocarbon on Titan’s surface. The idea of a widespread surface liquid was challenged in more recent observations from the ground, which failed to find any such signatures and proposed instead that very flat solid surfaces could be causing the radar evidence. The nature and extent of the exchange of condensable species between the atmosphere and the surface and the equilibrium which exists between the two is a key science topic.

More compelling evidence against a global hydrocarbon ocean on Titan came from spectroscopic data in the near-infrared (0.8-5 μm). This part of Titan’s spectrum, like that of the giant planets, is dominated by the methane absorption bands. At short (blue) wavelengths, light is strongly absorbed by the reddish haze particles. At red wavelengths, light is scattered by the haze, although the column optical depth is still high. In the near-infrared, the haze becomes increasingly more transparent (since the haze particles are smaller than the wavelength), although absorption by methane in a number of bands is very strong. Where the methane absorption is weak, clear regions or “windows,” situated near 4.8, 2.9, 2.0, 1.6, 1.28, 1.07, 0.94 and 0.83 μm, permit the sounding of the deep atmosphere and perhaps of the surface (Fig. 6). In between these windows, contrary to the giant planets, solar flux is not totally absorbed but scattered back through the atmosphere by stratospheric aerosols, especially at short wavelengths. The near-infrared spectrum is thus potentially extremely rich in information on the atmosphere and surface of Titan.

Titan’s near-infrared spectrum was used to investigate Titan’s surface in terms of detailed radiative transfer models of the near-infrared spectrum. This study indicated a surface albedo inconsistent with a global ocean and a surface reflectivity that showed a change in Titan’s albedo precisely correlated with Titan’s rotation.

The observations all agreed: The geometric albedo of Titan, measured over one orbit (16 days), shows significant variations indicative of a brighter leading hemisphere and a darker trailing one. The leading side corresponds to Titan’s Greatest Eastern Elongation (GEE) at about 90° Longitude of the Central Meridian (LCM—as opposed to geographical longitude, which is about 210°), when Titan rotates synchronously with Saturn; the trailing side is near 270° LCM or Greatest Western Elongation (GWE). The longitude at which this “bright” behavior is found was also subsequently identified in Titan images as a bright large area near the equator (see hereafter). At conjunctions (i.e., on the hemispheres facing Saturn and its opposite), the albedo was similar, of intermediate values between the maximum appearing near 120° LCM and the minimum near 230° LCM. As a consequence, Titan’s surface had then to be heterogeneous and rather “dry” with the hydrocarbon ocean stored in the porous, uppermost few kilometers of methane clathrate or water ice, “bed rock.”

The Titan surface spectrum seemed to indicate the presence of two lower-albedo regions near 1.6 and 2 μm (with respect to the continuum near 1 μm). These are also found in Hyperion and Callisto data where they are due to the water ice bands. The existence of a second (or more) surface component(s) was advocated by the orbital variations. It could be spectrally neutral or not and mixed with water ice zonally or intimately. Complex organics (tholins) show a neutral and fairly bright spectrum in the near-infrared, in agreement with high absolute albedos, but should be distributed uniformly with longitude. Hydrocarbon lakes or ices, silicate components, and other dark material are possible. Another possibility would be that the orbital variations may be due to longitudinal differences in the ice morphology (fresh or old, big or small particles, etc.).

Another technique, high-resolution imaging with the possibility to resolve Titan’s disk, offered further constraints on the Titan surface problem. Starting in 1994, two sets of data taken independently and with different methods were conclusively analyzed and presented to the public. The images showed clearly extensive quasi-permanent features, which were furthermore too bright to be hydrocarbon liquid. The heterogeneity of Titan’s surface, indicated in the near-infrared and with radar lightcurves, was graphically revealed by observations of Titan’s surface using the Hubble Space Telescope and adaptive optics technique.

On Titan images obtained with the Hubble Space Telescope, features were made discernible on Titan’s surface. Maps were produced of the surface in the 940 nm and 1070 nm windows, showing in more detail the bright leading and dark trailing sides, with notably a large (2500 × 4000 km) bright region, at 114°E and 10°S (nowadays known as Xanadu, this region has also a peculiar spectral behavior in that it appears bright at all investigated wavelengths (0.9, 1.1, 1.3, 1.6 and 2.0 μm), which may be indicative of an ice-covered mountain or something equivalent), as well as at a number of less bright regions. Subsequent HST data have confirmed the initial findings with more extensive mapping at 1.6 and 2.0 μm and allowed identification of spectrally distinct surface units, which may indicate regions of different composition.

At the same time, images taken using the adaptive optics system at the 3.6-m European Southern Observatory (ESO) Telescope at Chile, showed the same bright region at the equator and near 120° orbital longitude but also revealed a north-south hemispheric asymmetry apparent on Titan’s darker side. Adaptive optics is now a generally adopted method, and such systems exist in almost all the large Earth-based telescopes. Prior to the Cassini encounter, the adaptive optics system at the Canadian French Hawaiian Telescope on top of Mauna Kea and its twin at the Very Large Telescope (VLT) in Chile, as well as the Keck telescope, were applied to Titan and returned some of the most interesting and ground-breaking images of the satellite. The contrast on the adaptive optics images can achieve 50% under good observing conditions.

It was also then essentially demonstrated that Titan’s surface was much more complex than initially thought and that the “dark” hemisphere was—fortunately (because it was soon found out that the Huygens probe was not going to land where initially scheduled, close to the bright region, but rather on the trailing side)—not all that dark, showing some fine structure with bright areas.

The View from the Orbiter

The ISS and VIMS cameras confirmed these results and showed that the borders of these regions were linear but not smooth and that dramatic changes in surface albedo could be noted in the maps produced by these measurements. It is notable how well the distribution of bright and dark areas agrees among these three maps. The best resolution achieved by ISS was of a few kilometers on Titan’s surface. The large bright area around the equator first observed by the HST and the adaptive optics in 1994 was resolved and finely observed by Cassini instruments. It is centered at 10°S and 100°W and officially named Xanadu Regio. The midlatitude regions around the equator on Titan were found to be rather uniformly bright, while the southern pole is relatively dark. What exactly is causing the albedo variations is still uncertain. A plausible candidate for the darker regions could be accumulations of hydrocarbons (in liquid or solid form), precipitating down from the atmosphere.

These variations are more readily attributed to the presence on the surface of constituents with different albedos rather than topography, although contribution from the latter is also expected. The reason is that the Cassini camera observing at 0.94 μm cannot see shadows and also Titan’s icy bulk does not plead for high topographic structures on the surface (mountains should not exceed 3 km or so).

For the brighter regions, the task of interpreting the data is more difficult. It has been hypothesized that they could be associated with some topography and more exposed ice content, and this tends to be in agreement with findings by the Huygens/DISR instrument whose stereoscopic imaging revealed that the brighter terrain was also more elevated than the darker, smoother, and lower ice regions. The exact ice constituent that can satisfy the constraints imposed by all the observations is not easy to determine, hydrocarbon ice has been invoked on the basis of Xanadu appearing bright at all the near-infrared wavelengths observed to date.

A bright circular structure (about 30 km in diameter) found in the VIMS hyperspectral images is interpreted as a cryovolcanic dome in an area dominated by extension. The VIMS team hypothesized that the dry channels observed on Titan are related to upwelling “hot ice” and contaminated by hydrocarbons that vaporize as they get close to the surface (to account for the methane gas in the atmosphere), which are similar to those mechanisms operating for silicate volcanism on Earth (using tidal heating as an energy source) and which may lead to flows of non-H2O ices on Titan’s surface. Following such eruptions, methane rain could produce the dendritic dark structures seen by Cassini-Huygens. If these structures are indeed channels, they could have dried out due to the short timescale for methane dissociation in the atmosphere. Studying volcanism on Titan (if Cassini definitely yields evidence for it) is important to understand not only the thermal history of Titan (which must surely have evolved differently because it differs in its incorporation of volatiles from the Galilean satellites) but also how volatiles—in particular, methane—were delivered to the surface.

Titan’s present environment is very placid—tidal currents are weak; rainfall, if it occurs, is soft; and the diurnal temperature contrasts are small (and therefore winds are gentle). The solubility of ice in hydrocarbons is smaller than that of most rocks in water. Thus, except where the surface is more susceptible to erosion, due to organic deposits or perhaps water-ammonia ice, Titan’s topography should not be significantly modified by erosion.

The Cassini instruments have found no obvious evidence for a heavy craterization on the bright or the dark areas of Titan so far. A few features interpreted as impact craters have been announced to date: Cassini‘s RADAR and VIMS saw a 440-km diameter impact crater on Titan during two separate flybys in early 2005. The coloring of the feature indicates that its terrain is rough, with different material for the crater floor and the ejecta and tilted toward the radar during the observations. The multiringed impact basin was named Circus Maximus by the science team. A smaller crater of about 40 km was also observed, exhibiting a parabola-shaped ejecta blanket. In spite of the detection of a third crater-like feature, such formations, identified by the RADAR, VIMS, or the ISS are rare. This may mean that the surface of Titan is young (less than a billion years) or highly eroded/modified.

Other features observed by the Cassini orbiter include areas covered with analogs to terrestrial dunes in a set of linear dark features visible across a large part of the RADAR swath to the west of the large crater. These formations are aligned west to east covering hundreds of kilometers and rising to about 100 m. They are expected to have formed by a process similar to that on Earth, but the nature of this “sand” is quite different, consisting of fine grains of ice or organic material, rather than of silicates. The winds responsible for these structures (about 0.5 m/s on the surface) should primarily be attributable to the influence of Saturn, through tidal forces 400 times greater than on Earth and could easily move the Titanian “sand” in this world of low gravity.

Additionally, the RADAR onboard Cassini has discovered lakes sprinkled over the high northern altitudes of Titan (Fig. 8). In the images recorded, a variety of dark patches is observed, some of which extended outward (or inward) by means of channels, seemingly carved by liquid. The missing reservoir of liquid methane or ethane, which scientists have speculated on for a long time, may indeed—at least partly—be found in such areas.

In Situ Data: Landing on Titan

On January 14, 2005, the Huygens probe manufactured by ESA landed at 10.3°S and 192.3°W on Titan, providing the “ground truth” for the orbital measurements in terms of composition, structure, and geomorphology. The probe flew over an icy surface and then floated down and drifted eastward for about 160 km. Several of the instruments on board contributed to our knowledge of Titan’s surface conditions.

The HASI instrument measured the surface temperature and the pressure at the landing site to be 93.65 ± 0.25 K and 1467 ± 1 bar, respectively. The fact that the surface is solid but unconsolidated was verified by all the data. The first part of the probe to touch the surface was the Surface Science Pachage (SSP) penetrometer whose data are now interpreted as indicative of the probe first hitting one of the icy pebbles littering the landing area before sinking into the softer, darker ground material. The SSP detected the ground from 88 m in altitude by acoustic sounding, revealing a relatively smooth, but not flat surface for which our best current hypothesis is gravel, wet sand, wet clay, or lightly packed snow. With a landing speed of about 5 m/s the front of the probe followed and penetrated the surface, then slid slightly before settling to allow the DISR camera to take several pictures of a Mars-like landscape, complete with a dark riverbed and brighter pebbles.

No evidence for liquid was found at the Huygens landing site, but the surface is expected to be very humid because methane evaporation (a 40% increase of the abundance) was measured by the GCMS after landing. Thus, either the methane liquid reservoir may not be so far below the surface, but located instead in niches close to the exposed ground, or perhaps Huygens landed on Titan at a “dry” season when the rivers and lakes that may exist near the equator were empty but that could be flowing with hydro-carbons at a different era. Also, the presence of hydrocarbon lakes close to the North Pole, may also imply that there are seasonal phenomena that distribute the liquid on the ground. Nevertheless, Huygens landed on an organic-rich surface, with trace organic species such as cyanogens and ethane detected on the ground.

In spite of some misadventures (loss of the sun sensor measurements, of about half the images from Channel B and the probe’s erratic motion), the DISR imager and spectrometer gathered a precious set of data both in spectroscopy and imaging. Starting from the first surface image at 49 km, down to the unprecedented-quality snapshots of the Huygens landing site, and through the lamp-on data recorded below 700 m in altitude, this instrument played a decisive part in untangling the enigma of Titan’s surface morphology and lower atmospheric content. Panoramic mosaics constructed from a set of images taken at different altitudes show brighter regions separated by lanes or lineaments of darker material, interpreted as channels, which come in short stubby features or more complex ones with many branches (Fig. 9). This latter dendritic network can be caused by rainfall creating drainage channels, implying a liquid source somewhere or at some times on Titan’s surface. The former stubby channels are wider and rectilinear. They often start or end in dark circular areas suggesting dried lakes or pits. No obvious crater features were observed.

Stereoscopic analysis was performed on the DISR images indicating that the bright area cut with the dendritic systems is 50-200 m higher than the large darker plane to the south. If the latter feature is a dried lakebed, it seems too large by Earth standards to have been created by the creeks and channels seen on the images and could be due to larger rivers or a catastrophic event in the past. The dark channels visible in Fig. 9 could be due to liquid methane irrigating the bright elevated terrains before being carried through the channels to the region offshore in southeasterly flows. This migration toward the lower regions probably leads to water ice being exposed along the upstream faces of the ridges. The slopes are generally on the order of 30°. Some of the bright linear streaks seen on the images could be due to icy flows from the interior of Titan emerging through fissures.

The images taken after the probe had landed on Titan’s surface show a dark riverbed strewn with brighter round rocks. These “stones,” which are 15 cm in diameter at most, could possibly be hydrocarbon-coated water ice pebbles.

The spectra acquired during the descent gave information on the atmospheric properties (Table 1) and on the surface properties. Indeed, it was shown from spectral reflectance data of the region seen from the probe that the differences in albedo were related to differences in topography, which in turn can be connected to the spectral behavior of the ground constituents. Thus, the higher brighter regions were also found to be redder than the lowland lakebeds. The regions near the mouths of the rivers are also redder than the lake regions. The spectra taken by DISR are compatible with the presence of water ice on Titan’s surface, something that had already been suggested from ground-based observations. The most intriguing feature found in the spectra was, however, the featureless quasi-linear unidentified blue slope observed between 830 and 1420 nm. No combination of any ice and organic material from laboratory measurements has been adequate in reproducing this characteristic. The jury is still out on the constituent(s) that create(s) this signature.

Although many questions still remain about the sequence of flooding and the formation of all the complex structures observed by DISR, these data tend to clear the picture we have of Titan today and at the same time enhance the impression that by studying Saturn’s satellite we’re looking at an environment resembling the Earth more closely than any other place in our solar system.

No “little orange men” were photographed on Titan. The public is very interested about a possible past, present, or future life on Titan. One of the elements in the negative response (at least so far as the present or past life is concerned) was found by the GCMS in the 13C/14C isotopic ratio (around 82), which showed that no active biota exist on Titan and that the methane on Titan is not produced by life (a biological origin would have required the isotopic ratio to be in the 92-96 range).

The reality pictured by the Cassini-Huygens instruments went beyond anything that has been speculated about Titan’s surface. The diversity of the terrain includes impact craters, dark plains with some brighter flows, mysterious linear black features possibly related to winds, sand dunes, snow dunes and a host of possible actors (solids, winds, liquids, ices, volcanism, etc.). Titan has proven to be a much more complex world than originally thought and much tougher to unveil.

Looking Ahead

Much like Earth, a greenhouse effect exists on Titan; it is produced essentially by methane, with contributions by nitrogen and hydrogen, which have important consequences on the surface temperature. Methane is normally photolyzed in Titan’s atmosphere, and unless it can be replenished by a large reservoir on or beneath the surface, it is bound to disappear in a few million years. In such a case, the surface temperature would drop below the condensation point for nitrogen, and Titan’s atmosphere would collapse. Should the absorptivity of the surface increase subsequently (e.g., due to the accumulation of organics), the surface temperature might once again rise and cause the reevaporation of methane and nitrogen, thus rebuilding the atmosphere. Such cycles have been hypothesized to occur on Titan.

On the other hand, should the methane supply become abundant, a small perturbation in the solar flux received on Titan (such as is expected when the Sun becomes a red giant and then a dwarf) would produce a dramatic warming of the climate, raising the temperature on the surface and the pressure to values as high as 180 K (twice what we have today) for several bars. It is not inconceivable to imagine that some day in the distant future, conditions on Titan one day may very closely resemble those found on our own planet today.

In the meantime, the Cassini mission has demonstrated the complexity of this world and our need to further investigate it in order to better comprehend our solar system. Beyond the extended Cassini mission (2010), discussions on future missions to Titan are already underway.

Although future ideas for Titan missions are not mature (after all, Cassini is still on the spot), a prominent concept is the use of “aerobots,” or intelligent balloons, to explore a variety of Titan locations seems to be favored. Titan may have many more surprises in store for us.