The night sky: Titan an alien yet familiar world

This shows the night sky tonight at 7.30pm, directly overhead. South is at the top.

By Grant Christie

Recent discoveries are showing that Saturn's largest moon, Titan, is both an alien and a yet strangely familiar world. Astronomers have long believed that Titan must be very similar to how the Earth was when it had just formed.

Titan, discovered in 1655 by the Dutch astronomer Christiaan Huygens, is easy to spot with a small telescope.

Titan is the only moon in the solar system with a significant atmosphere, having a pressure 50 per cent greater than that on the Earth's surface.

Our first close-up views of Titan came from Voyagers 1 and 2 which flew by Saturn in 1980. They found that Titan's atmosphere is mainly nitrogen (like Earth) with 6 per cent argon, 2 per cent methane and trace amounts of other organic molecules. However, the thick, cloudy atmosphere prevented any direct views of the surface.

Starting in 2001, radar observations of Titan using the giant Arecibo radio telescope in Puerto Rico suggested that there were large very smooth areas that might be lakes. But any liquid would most likely have to be in the form of an oily organic soup because the temperature there is only about -150C.

Until the Cassini-Huygens craft arrived at Saturn in January 2004, Titan remained the largest unmapped surface in the solar system. The Huygens probe was released and, using a parachute, landed softly on the solid surface of Titan, sending back the first pictures.

The Cassini spacecraft has now begun using radar to map Titan's surface features, with the latest data confirming the existence of the very smooth regions.

The picture now emerging is of a world where a steady methane drizzle soaks the terrain. There are continents with mountains and valleys and streams of methane flowing into lakes. This alien landscape is illuminated by the weak light of the distant sun filtering through the smoggy haze and methane clouds.

Currently the planet Saturn is hidden from our view by the glare of the Sun. But the giant planet Jupiter is easily spotted in the early evening sky, high in the west. If the sky is clear tonight, the Moon will be right next to it.

Almost directly overhead is the landmark constellation of Scorpius and just to its east is the rich stellar region of Sagittarius. The Southern Cross and the Pointers are high in the south.

* Grant Christie is an astronomy researcher who writes a monthly column for the Herald.