Among the organic composites, there is adenine, one of the 4 building blocs of the DNA molecule. This is why it is said that Titan's atmosphere is pre-biotic, it my resemble Earth's atmosphere before life appeared. The thickness of the atmosphere is 1270 km (a little larger than Earth) and the pressure on the surface is 1.5 times atmospheric pressure. The temperature on the ground is -180°C.

Just like on Earth, the atmosphere on Titan can be divided into different section as a function of how temperature evolves with altitude (image 3):

• Troposphere: the temperature decreases with altitude
• Stratosphere: the temperature rises with altitude
• Mesosphere: temperature decreases with altitude again
• Thermosphere: temperature rises again with altitude (on average up to 1000 m). This is the last layer of the atmosphere.

There is or has been a cycle similar to that of water on Earth but involving methane. There are methane clouds that condense to from rain. The methane then flows on the ground before evaporating beginning the cycle again. Due to the lower layers of the atmosphere, the surface is not visible in the visible wavelengths of the spectrum. (400 to 700 nm long wavelengths). On the other hand, in infra-red, (1 to 300 mm) or in radio wavelengths (over 10 cm), the reflected radiation from the surface can penetrated the atmosphere. The atmosphere is said to be transparent to infra-red and radio waves (images 1b and 1c and images 2a-d).

It has therefore been shown using Cassini's instruments that the surface in young with few craters. Some scenes look like river beds and catchment areas (network of streams and rivers that flow into each other) on Earth (images 2). But we don't know if these "rivers" and "lakes" are dry or still contain liquid methane. Cryovolcanic activity has also been seen. There are also dusty dunes made of hydrocarbons (image 2c) an mountains at least 1000 m high. The mountains were formed by the compression of a crust in the same way as the mountain ranges on Earth. These mountains are on a lighter part of the satellite (on radar images), called Xanadu (centre left one the image 1b).

On the edge of Xanadu, we can see strange circular spots. These are either craters created by an impact or calderas of cryovolcanoes. Other shiny circular spots, similar to volcanoes, have been found on radar images.

The Cassini-Huygens mission involved the Cassini probe that carried a small lander called Huygens. Huygens landed on Titan's surface after a descent, slowed by parachutes, of 2 hours and 30 minutes through the atmosphere. Once on the ground it photographed the immediate surroundings for a little over one hour. The ground is dark and dotted with rocks that are actually ice water blocs a few cm large. These blocs could have been smoothed by erosion due to liquid methane flowing over them. In some ways, the pictures taken by Huygens reminds us of the surface of Mars (image 4).

Titan had an evolution over which its internal structure changed quite a lot. It is mostly ice and so its structure today would be: (image 5):

• A rocky core
• A layer of ice under large pressures
• A layer of liquid water with added ammoniac
• A surface made of a layer of ice water

One scenario for the evolution of Titan (image 6) was recently suggested by researchers at Nantes University (Tobie et al. 2006):

The methane present in the atmosphere should disappear in a few tens of millions of years due to photochemistry. The atmosphere is continuously being renewed in methane. This could come from the inside Titan itself.

We believe that in the formation of Titan, the conditions were such that "clathrates" could have formed. These clathrates are very stable, cage shaped assemblies of water molecules that contain a methane molecule. The ice water in this form contains methane that it releases when these little cages are broken due to perturbations.

500 million years after its formation, Titan was still very hot and covered in liquid water; the clathrates rose to the surface to form a crust of ice 50 km thick. The crust preventes the heat from escaping out into space and the temperature of the internal ocean rises. The clathrates in contact with the ocean brake and liberate the methane which rises to the surface through fissures in the crust. The crust would also thin and after about 2 billion years due to heat accumulated in the core and that would be brutally ejected, the rest of the mass would melts and liberate a large quantity of methane which would condenses to rain and change the physical aspect of the surface. The ice crust is in consequence only 4 km deep.

This model agrees with the estimated age of 2 billion years of the surface of Titan.

After the internal heating due to the core decreases, methane cease to be added to the atmosphere for the next 1.5 billion years. The upper part of the internal ocean refreezes which induces an instability in the crust and the production of internal "solid convection". The colder ice sinks and the warmer ice rises creating hot spots and volcanoes spitting melted ice and methane. Methane is therefore once again added to the atmosphere and this process is what we could be seeing today.