Titan, the largest moon of the planet Saturn, was discovered in 1655 by Dutch astronomer Christiaan Huygens. He noted that unlike other moons he knew of, which were generally some shade of grey, Titan appeared pinkish in colour. We learned later that the odd colour is due to Titan being unique compared with other moons in the Solar System in that it has a dense, foggy atmosphere. Later measurements showed that Titan is big, with a diameter of 5,149 km, compared with our moon’s 3,475 km, and Earth’s 12,756 km. Titan is bigger than the planet Mercury, which has a diameter of 4,879 km. Our access to such accurate values comes as the result of centuries of careful work.
Its uniqueness guaranteed that it would get special attention from space probes. First it was Pioneer 11, which sped past in 1979. This measured Titan’s temperature and gave a few fuzzy images. Titan proved to be very cold, which considering its distance from the Sun was not overly surprising. Then in 1980 and 81, Voyagers one and two passed by on their epic voyages to explore the Solar System. However, the cameras could only show a brownish-pink ball; they could not penetrate the fog.
The game changed completely in January, 2004. The Cassini spacecraft, named after Giovanni Cassini, who first studied Saturn and its Moons, was intended to go into orbit around Saturn and by using rocket thrusters to change its orbit, to have multiple close fly-bys of anything of interest in the Saturn system, including of course Titan. However, to make things even better, Cassini had a passenger, another spacecraft, called Huygens. This spacecraft would soft land on Titan, sending back data on the way down and from surface.
On January 14 Huygens landed successfully. On the way down it sent pictures showing a landscape with lakes and drainage channels or rivers. It landed in what looked like a dry river bed, with rounded pebbles, obviously shaped by liquid. The results from Huygens and numerous flybys by Cassini, which was equipped to map Titan through the clouds, showed a world with an atmosphere, wind, rain, lakes and rivers. There are even deserts with huge sand dunes. However, the temperature reported by Huygens was -180 C. Obviously the liquid forming the rain, lakes and rivers cannot be water. From the composition of the atmosphere, liquid methane and ethane, both hydrocarbons, are good candidates. Just to make Titan seem even more exotic, the pebbles in the pictures sent back by Huygens are believed to be ice. At those temperatures, ice is just another rock mineral.
Current ideas are that life forms from a particular mixture of chemicals that new-born worlds get from the material between the stars. On Earth that mixture is long-gone, but it is still there on frigid Titan. It would be very interesting to see how far along the road to life Titan has got. It could be there are life forms on Titan, but at those temperatures they won’t be much like us. Our world would be an unsurvivable blast furnace to them, so I guess they won’t be invading our planet any time soon.
Mars and Saturn are low in the Southwest after dark. Saturn is still close to the star Spica. Jupiter rises around 10 p.m and Venus around 3 a.m. The Moon will be full on the 29th, the harvest Moon.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astro-physical Observatory, Penticton.