The Cassini-Huygens Mission to Titan, Saturn's Largest Moon

Let's visit the realms of science fiction for a moment. Imagine we had discovered another world, slightly smaller than our own, with an atmosphere consisting of mostly nitrogen similar to Earth. Imagine there were rain clouds in the sky with rivers and lakes on gently hilly continents surrounded by seas and oceans. Sounding a bit bland and Earth like? Okay then lets add some twists.

Voyager 2 found Titan completely covered in a permanent smog layer and could not see any details of the surface

First lets replace all the land with ice. This is a freezing world far from its Sun where ordinary water ice freezes as hard as rock and makes up all of the surface. Just like on Earth where dense metals sank to the centre forming a core with the lighter rock floating on the top, on our fantasy world the rock has sunk to the centre with the ice crust forming at the surface.

But what about the clouds and seas? If water freezes as hard as rock then what are these made of ? A perfect candidate is methane, the stuff that comes out of your gas hob at home. It condenses if it is cooled to around -160 degrees Celsius so if our world is cold enough we could have methane clouds raining liquid methane into methane rivers and seas.

Sounds like a very odd world doesn't it. Remarkably similar to Earth but at the same time very alien. Now if I told you that the Dutch astronomer Christiaan Huygens discovered our fantasy world in 1655 you could be forgiven for being a little surprised.

Titan, Saturn's largest moon, is actually larger than the planet Mercury and it has been the subject of much fascination since Jose Comas Sola produced sketches that hinted of the possibility of an atmosphere. In the early 80s both Voyager 1 and 2 spacecraft passed by Titan making measurements and taking images. Unfortunately they found Titan's surface completely obscured by a global covering of dense smog created by polymerization of atmospheric methane by the suns UV light.

In 2004 the Cassini orbiter with the Huygens probe arrived in the Saturnian system. With specially optimised optical systems Cassini allowed us to pierce the smog and produce high resolution images of Titan's surface for the first time. These images show bright continents with dark planes that are probably dry seabeds. At the south pole is a weather system of methane clouds with a lake that may actually contain liquid methane.

The Cassini orbiter used special optical systems to pierce Titans smog revealing bright continents, dark plains and methane clouds around the South Pole

On Christmas day 2004 the Huygens probe detached from Cassini and on January 14^th 2005 performed the most distant landing on another body ever attempted. During it's descent Huygens measured the composition of the tiny grains that made up the smog and sampled the atmosphere to determine how much methane was present. We found that the temperature on Titan reaches as low as -200 degrees Celsius with the surface at -180. Huygens took images during descent which show dark dry riverbeds and a coastline between a bright continent and a dark plain. Landing on the dark plain Huygens struck the surface at 5 metres per second (about the same speed as your PC would hit the floor if you dropped it from shoulder height) and continued to work perfectly allowing us to measure the hardness of the surface. We found that the surface "felt" like damp gravel or sand and images taken after landing show a gravely flat surface strewn with large rounded pebbles. After landing the heated internals of the probe were a fairly cosy 20 degrees Celsius with the outside at minus 180 and heat immediately began to be transmitted to the surface. After a few minutes we began to register methane evaporating from the area around the probe confirming that the gravel was damp with liquid methane.

As the Huygens probe approached the surface it saw gullies on the bright land. We believe these are dried up rivers opening onto a dried up lakebed

It is clear from the Cassini and Huygens data that fluid activity has played a significant role in shaping the surface of Titan, but to date we have been unable to absolutely confirm the existence of standing liquid on Titan. All the rivers and plains so far observed, with the possible exception of the south polar lake seem to be empty, however the Huygens probe has shown that some areas of the surface are damp. It may be that the rivers have been empty for millennia or it could simply be that we are currently in Titan's dry season. Either way continual monitoring from orbit by Cassini will show how Titan changes over the next 3 years or more and maybe soon we will have unlocked some of Titan's many mysteries.