Imagine an alien landscape; it is illuminated by a dim twilight. The sky is brownish, and the air is filled with an orange-brown mist. It is cold, about -180 degrees Celsius, but despite the low temperature, a thin rain is falling. At those temperatures it is not water; the rain is made up of droplets of liquid ethane and methane. Ahead, and stretching as far to the left and right as we can see are huge dunes, up to two kilometres high: bigger versions of the ones we see in deserts here on Earth. We will need spacesuits to protect us from the cold, and also from the atmosphere, which is mainly nitrogen, together with a fog of assorted hydrocarbons. There is no detectable oxygen. We pick up a handful of “sand”, and find it to be a mixture of ice and hydrocarbons. Of course this is not Earth, or Mercury, Venus, Mars or the Moon. We are on Titan, the largest moon of Saturn, sixth planet out from the Sun; Earth is the third.
Titan is easily visible through telescopes, as a pinkish/brownish starlike object close to the planet Saturn, which is a moderately bright object in the eastern sky these evenings, close to Spica, the brightest star in the constellation of Virgo. Through even a small telescope Saturn is utterly unmistakable; it is the only planet in the Solar System surrounded by a spectacular set of rings.
Titan was discovered in 1655 by Christiaan Huygens. It was established pretty soon that Titan is unique. It is the only moon in the Solar System that has a significant atmosphere. In fact Titan is enveloped in a layer of orange-brown fog so thick that from space we never see its surface. When a Voyager spacecraft flew past, it just showed a ball of brownish fog.
This situation remained until the arrival of the Cassini spacecraft in 2004. Cassini was designed to spend years exploring Saturn and its moons, and is equipped with a radar mapping system that can penetrate Titan’s clouds. The spacecraft also carried another spacecraft, the lander Huygens. As Cassini approached Titan, Huygens detached and successfully landed on the surface of Titan, taking pictures all the way down. It revealed land and what looked like lakes and seas of liquid hydrocarbons. It landed on a dry streambed, and sent back pictures of rocks smoothed not by water, but by liquid methane and ethane. On Titan water is just another solid, rock-forming mineral.
While Huygens busied itself with its little bit of Titan’s surface, Cassini used its radar to penetrate the fog and map that strange world. There are volcanoes, mountains, plains and close to the equator, a huge system of dunes, each stretching up to hundreds of kilometres from end to end.
With all that organic chemistry going on, the idea of life on Titan is an intriguing one, but a definitive answer will probably have to wait until we turn our imaginary visit into a real one.
Venus still dominates the western sky after sunset. Mars is high in the South, close to the brightish star Regulus, in Leo; Saturn is in the eastern sky, quite close to Spica, the brightest star in Virgo. The Moon will be full on the 5th.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton, B.C.