ASTRONOMY (HERSCHEL)

Titan: an earth in deep freeze

Malcolm Wright

Huygens Project Team

1 April 2005

Saturn has many moons, two of which - Enceladus and Mimas - were discovered by William Herschel in 1789. Cassini had already discovered three moons - Dione, Rhea and Tethys - in 1672 and 1684. However, the first to be discovered was Titan, by Christiaan Huygens in 1655. It is appropriate that the spacecraft, launched on 15 October 1997 should be named Cassini and that it should carry a probe, destined to land on Titan, called Huygens. At 2 tons, the Cassini probe is the most complex spacecraft to date. It arrived at Saturn in July 2004 and is expected to operate for about four years. The Huygens probe it carried was built by the European Space Agency and carried 6 science instruments designed to study the content and dynamics of Titan's atmosphere and collect data including cameras pointing in 6 different directions to obtain images on the surface.

Titan is second only to Ganymede, one of the Galilean moons of Jupiter, in size. It is actually larger than the planet Mercury and almost as large as Mars. It takes almost 16 days to orbit Saturn and at 8th magnitude can be seen from Earth by a sharp-eyed observer with a good pair of binoculars. However, even in the best Earth-based telescopes it appears featureless. Its size and its distance from the Sun suggested that it might have an atmosphere and this was confirmed in 1903 by the observation of ‘limb-darkening’ by the Spanish astronomer J. Comas Solá. In 1944/5 Gerard P. Kuiper showed spectroscopically that Titan had an atmosphere containing methane.When the space probe Voyager I flew by Titan, it revealed that the N and S hemispheres were of a different colour and had a dark ring round the pole but observed no surface details. The temperature at the surface was found to be that of liquid nitrogen (-190°C) and the pressure 1½ that of the Earth’s. After a seven-year journey bolted to the side of the Cassini Orbiter, Huygens was set free on 25 December 2004 when it coasted for 21 days en route to Titan. The diagram shows the planned stages in its descent through the atmosphere.It descended slowly to allow it to collect data and also to allow landing before losing contact. There was an ethane/ethyne haze up to 120 km and methane/nitrogenclouds 40km above the surface, under which there was methane rain.The stratopause at 1 millibar (mb) had a temperature of 170K (-103°C), followed by the tropopause at 128 mb 100°C cooler. However, at the surface it was 13°C warmer and the pressure had risen to about 1500 mb.The first photographs were taken at a height of 16 km - below the clouds. They were taken in visible light by cameras underneath the descending Huygens probe. The photographs are combined as a mosaic in the picture [above left].These show the possibility of mountains, rivulets and a sea. The photographs to the left were taken at 8 km altitude and the sinuous branched features are very much like rivers running towards a coastline off of which there are patches of ‘fog’. The channels are probably rivers of liquid methane with tributaries from methane springs.On landing a penetrometer, part of the SSP (Surface Science Package), recorded a 5 kg force over a time period of 50 milliseconds. This enabled the nature of the soil to be analyzed in comparison with Earth-based experiments. The probe was tilted at 12° to the vertical. The nature of the soil indicates that it would only rain methane every 100 (Earth) years but when it did it would be torrential, though falling like snow, and last for a few years! The low temperature slows the rain cycle from hours (on Earth) to years (on Titan).The picture to the left was taken after landing and shows a terrain similar to that of Mars (The full-colour original is orange above and below the horizon - again, not unlike that of Mars). The ‘rocks’ consist almost entirely of water-ice on which there are dark deposits which were analyzed by a mass-spectrometer to be complex organic deposits (though insufficiently complex for life!).The six large rocks in the foreground are within 85 cm of the camera, the furthest measuring 15 cm across. The dark rock beyond the clear area and to the left is 240 cm away and measures 13 cm across.The speaker went on to show Cassini pictures of Saturn showing cloud belts similar to Jupiter with similar vortices and eddies. The true colour of the planet is lemon-yellow. Other Cassini pictures showed other satellites of Saturn and details of the rings with small satellites herding the F-ring.It was a great privilege to have first-hand knowledge of the most recent space exploration from someone who had been a member of the project team developing the Huygens probe at Bristol at the end of the last century.

Richard H Phillips