False colour Voyager 1 image of Titan's haze layers.
TITAN - MOON OF SATURN
 
The Titans were a group of giants, the offspring of Uranus and Gaia. They fought with Zeus and the Olympian gods for rule of the heavens. Titan was discovered by Dutch astronomer and scientist Christiaan Huygens in 1655. Observations from Earth in the early part of the 20th century showed that Titan has an atmosphere. Telescopic studies had shown a darkening at the edge of the disk which could be ascribed to the presence of an atmosphere and in the 1940s Gerard Kuiper showed, using spectroscopy (analysis of light), that Titan has an atmosphere containing some methane.
 
Orbit
Titan orbits Saturn at a distance of 1.2 million kilometres. It has a very circular orbit, and is inclined only very slightly to the plane of Saturn's equator. Hubble Space Telescope observations have shown that Titan's rotation is synchronous, spinning once on its axis every orbit it makes of Saturn. It takes Titan slightly under 16 days to complete one orbit of Saturn.
 

Hubble Space Telescope image of Titan casting its shadow onto Saturn's atmosphere.
The orbits of Titan, Hyperion, Iapetus, and Phoebe.
 
Physical properties
Titan is 5150 kilometres across and the second largest satellite in the Solar System, dwarfing Saturn's other moons. It is larger than Mercury and Pluto. Titan was once thought to be the largest satellite but it was shown that Titan's atmosphere simply gives it a larger appearance. The solid globe of Titan is smaller than Ganymede which measures 5262 kilometres across. Titan is the densest of Saturn's moons. It has a density of 1880 kg m-3, indicating that although it is probably largely water ice, it contains a lot more rocky material than Saturn's other satellites which have a densities of about 1000 kg m-3 and are more icy.
 

Comparison between Titan and our Moon.
 
 
 
Magnetic field
No magnetic field has been detected.
 
Interior
Titan is probably differentiated into a layered planet and has a similar structure to Ganymede. It is sufficiently large to have developed a molten interior, allowing lighter materials to migrate to the surface. The rocky core is thought to occupy half of Titan's radius. Because Titan is quite big, pressures encountered within the moon are greater. This means that ices which have a more compact molecular arrangement can exist. There are probably layers of different types of ices wrapped around the core.
 
Atmosphere
Titan's atmosphere looks very thick and is indeed quite dense. In this respect, it is more like a planet than a typical moon. At the surface the pressure is 1.5 atmospheres or 1 1/2 times the atmospheric pressure on Earth at sea level. It has a denser atmosphere than Mercury, Pluto, and Mars. Although the atmosphere forms a dense cover, it traps little heat and Titan is very cold. At the surface the temperature is about 94 K or -180oC.
 

Titan's dense atmosphere hides details of its surface.
Temperature profile of Titan's atmosphere.
 
Titan's atmosphere is mostly nitrogen, about 94%, and it has been speculated that it could be like Earth's early atmosphere before oxygen was added by simple lifeforms. Titan also has a small percentage of methane. In addition to methane there are very small amounts of other organic compounds including ethane, acetylene, and propane. It is the hydrocarbon compounds like methane and ethane which give Titan its distinctive hue. Hydrogen cyanide and cyanogen are also present. The discovery of hydrogen cyanide is especially important because it goes into the manufacture of amino acids. Titan was considered once to be a candidate for extraterrestrial life but it is highly unlikely. Titan is extremely cold, and there is virtually no water.
 

Atmospheric composition.
 
The chemical reactions taking place in the atmosphere are not well understood. As the surface temperature of Titan is near the triple point of methane, it could exist as a solid, liquid, or a gas. It has been suggested that below the hazy upper layers of the atmosphere, there are clouds of ethane. Photochemical reactions (with sunlight) break apart methane making acetylene, ethylene and ethane, which it is thought to rain back down to the surface creating seas of liquid ethane. This is pure theory, however, and no one is sure. The reaction in which the methane is broken apart or "dissociated" is irreversible, and so it must somehow be replenished.
 

False colour Voyager 1 image of Titan's haze layers.
 
Titan at times orbits outside Saturn's magnetic field, depending on the strength of the solar wind which can affect the position of the magnetopause (the outer limit of the field). Titan lacks a magnetic field of its own which would protect it from radiation, so the solar wind can interact with Titan's atmosphere. Gases can be ionised, (stripped of their electrons), and blown off the top of the atmosphere. Gases at the top of Titan's atmosphere can escape easily because the moon has a fairly weak gravitational pull. The hydrogen molecules which escape from Titan form a thick, million kilometre wide, torus-shaped zone which rotates with the moon about Saturn. The torus, which extends inwards toward the orbit of Rhea, may also contain some oxygen.
 

The upper layers of Titan's atmosphere are stripped off by the solar wind.
 
Surface
Most of what we know of Titan was gleaned from the Voyager 1 encounter. The surface could not be seen through the thick orange tinted atmosphere. At first glance it appears very uniform with no patterns which might suggest a weather system or chemical variation across the surface.
 
Though Titan's surface appears to be, at first glance, uniformly orange and featureless, some very subtle variations have been seen. The main difference occurs between the northern and the southern hemisphere, though the temperature difference between the polar regions and the equator is very small, just a few degrees.
 
The northern hemisphere is slightly darker and redder than the southern. As a body with an atmosphere, Titan was a particularly important target for Voyager observations and many images were obtained. As Voyager approached, the north pole appeared slightly darker than the surrounding area. This cap formed a dark ring over the following months, as it spread outwards from the pole. Images which have been processed to enhance colour contrast reveal further faint bands. These colour differences are ascribed to variation, and change with time, in the composition of the atmosphere. Spectral studies show that there is more acetylene and methylacetylene at lower, equatorial, latitudes than at the poles.
 

Titan's atmospheric polar cap appears as a dark ring.
 
Since Titan's surface is cold and methane can exist as a liquid or a solid, the surface may look like nothing we have seen before. There has been all sorts of speculation on what the surface might look like. Some people envisage rivers of methane draining into hydrocarbon oceans. There could be frozen methane floating about on the surface, like the rafts of ice which once drifted about on Europa. An orange tinted landscape, bathed in hydrocarbon vapour and methane drizzle would give Titan's surface a very surreal quality indeed.
 
In recent years, with the help of the Hubble Space Telescope (HST) our knowledge of Titan's surface has made some advances, even though we cannot see any surface detail. In infrared light Titan's atmosphere is less opaque and HST images taken at these wavelengths can reveal vague surface markings. In the best HST images patchy bright and dark regions can be made out. The leading hemisphere, centred on the equator, is dominated by a particularly large bright patch which is suggestive of a continent.
 

Near-infrared imaging reveals surface features on the Saturn-facing hemisphere.

The bright surface patch on the leading hemisphere is about the same size as Australia.
 
 

Titan's atmosphere is transparent at infrared wavelengths, allowing the anti-Saturn surface to be seen.

Titan's trailing hemisphere solid surface, viewed in infrared by the Hubble Space Telescope.
 
The Cassini orbiter
The Cassini spacecraft launched in 1997 will rendezvous with Saturn in June 2004. The Cassini orbiter will use radar to map the solid surface of Titan, just like Magellan at Venus. Radar signals will penetrate the thick atmosphere and bounce back from the surface to the spacecraft - the strength of these signals will be used to create a picture of the surface texture and detail of landscape features.
 

Voyager 2 looks back at Titan in 1981 - the next spacecraft will visit in 2004.
 
The Cassini orbiter will release a probe named Huygens, which will take measurements and sample the atmosphere on its descent. One of the most exciting prospects of the mission is the return of close-up pictures of the surface as Huygens lands on Titan.