Ordinary Meeting, 2002 November 30

The Cassini Mission

Mr Workman believed many amateurs might be asking why there was a need to send a new probe to Saturn when amateurs such as Damian Peach were obtaining such superb observations from the Earth. He pointed out that the ring structure is the most eye-catching feature for amateurs. Early observations had recorded them as a triple planet, and they had first been correctly identified in 1655 by Huygens. Two decades later in 1675, Cassini resolved the planet sufficiently well to see a gap between two apparently separate rings – the gap we now call the Cassini divide. The Berlin observatory identified another sub-gap within the A ring in 1837, but it was decided that it was sufficiently hard to resolve that it did not warrant a further division.

In contrast, the Voyager missions had revealed unexpected character on the surface of the planet itself. Its tremendous size dominates the dynamics of its gaseous surface. There is little large scale structure comparable to the Great Red Spot on Jupiter, but there is rich fine structure, with an average wind velocity in excess of 1000mph. These are the fastest in the solar system, and their energy source remains a mystery.

The Voyager probes had also overturned our previous picture of the rings. For example, it revealed that the Cassini divide was an area of low density, not a complete gap. It revealed that there are actually over 4000 closely-packed rings, not just four large structures as had previously been thought. The rings were believed to be composed of water ice and small pieces of rock, which ranged from the size of a sugar-lump to that of the lecture theatre. We had learnt from Voyager that rings are not the rare phenomenon once thought, but in fact all four gas giants exhibit them to some extent. Both Neptune and Jupiter have faint structures around them.

Further out in the Saturn system, the moons had produced numerous puzzles when Voyager resolved the first surface detail ever seen on them. Firstly, Mimas' landscape was dominated by an 80-mile crater. This was indicative of a particularly violent history, as the formation of such a crater in an object of 240-mile diameter would have brought it close to total destruction. The central peak of the impact crater rose several miles out of the surface.

In contrast, Enceladus had a bright surface of water ice which was virtually uncratered. This begged the question of where the craters had gone. The speaker favoured the explanation that vulcanism had eroded such features, although a more subtle alternative was that tidal gravitational forces on the moon from Saturn might heat it and cause surface melt. The liquid would flow up through cracks in the surface before resolidifying as a smooth top layer. A few fractures in the surface supported this idea of past breaking and rebonding, but were not sufficient to convince the speaker.

Mr Workman moved onto Diomine, which is believed to be composed of an inner rock core, surrounded by water ice. Overall the moon is rather lightweight – only twice the density of water. A curious feature of the surface is a number of white wisps and speckles, centred around the crater Amata. It is believed that the impact that created Amata also blew a large amount of liquid water melt off the surface. This subsequently resolidified in the vacuum of space before returning to the surface. Similar wisps are also seen on Rhea, a moon which the Cassini probe would spend some time imaging. It was the aim of the Cassini probe to orbit within the system for four years, rather than just fly past the system, as the Voyager probes had done.

Moving onto Iapetus, the speaker described a feature observed as long ago as 1671 by Cassini. The moon's two hemispheres have spectacularly different colourings. Furthermore, Voyager found that the shape of Iapetus is remarkably irregular for such a large object. Some people had proposed that a dark tar-like substance had oozed up through the surface, whilst others believed it had been accreted from elsewhere. The speaker favoured the opinion that the material had come from Phoebus, although the colour of the dark material on Iapetus did not quite match the hue of Phoebus.

Finally, the speaker described the moon commonly thought of as the king of Saturn's moons: Titan. The atmosphere is 94% nitrogen, and it appeared likely that the surface was liquid ethane or methane. Conditions in the atmosphere were ideal for liquification of such hydrocarbons. Infrared HST images hinted that light-coloured regions might be continents of solid hydrocarbon, although this remained an area of active research.

The Cassini mission would be in two-parts: there would be an orbiter and the Huygens landing probe, which would be destined for the atmosphere of Titan. The orbiter was exceptionally large – as high as a single-storey building – and weighing six tonnes. Sadly an atmospheric probe had not been included due to space constraints. Cassini had been launched in 1997 as a NASA/European collaboration. A recent encounter with Jupiter had been used to test the camera, which appeared to be working well. It would finally reach Saturn in 2004 July.

The first priority would be to obtain a stable orbit. Then, in 2005 January, the Huygens probe would be released into the atmosphere of Titan, marking the start of scientific investigation. A slight technical hitch had recently been identified with Huygens, although engineers were confident that the control software could be suitably modified. It was hoped that after penetrating the atmosphere, the probe would continue to function for around half an hour on the surface, and take two or three images of the landing site. A particularly interesting question would be whether Huygens would splat down or splash down on Titan.

It was hoped that the orbiter would complete around 80 orbits of Saturn, half of which would be close passes of Titan. A few questions of interest included the nature of the magnetic field, the composition of the ring particles, the atmospheric composition, as well as the many puzzling features of the moons. Between 2005 and 2008 around eight billion images would be taken. Depending upon the status of the mission in 2008, it could potentially extend until 2010.

The President thanked Mr Workman for his detailed report of an enthralling subject, before welcoming the afternoon's final speaker, Mr Chris Lintott, to speak about recent developments in cosmology.