Philips Atlas of the Universe

THE SOLAR SYSTEM

 Titan’s surface from

Huygens. Drainage channels

visible near the top of this

image mosaic appear to lead

downhill into a dark lake bed.

Although the surface was

thought to be dry when the

image was taken, scientists

believe that liquid methane

flowed in the recent past.

 Titan from Voyager 2,

23 August 1981, from

2.3 million km (1.4 million

miles). The southern

hemisphere is lighter than

the northern; the difference

between the two is quite

possibly seasonal.

 Titan’s atmosphere from

Cassini, 31 March 2005,

range 9,500 km (5,900 miles).

Methane molecules are

broken down in the upper

atmosphere by solar

ultraviolet light. Lower down

is a smog of complex

organic molecules through

which only about 10 per cent

of the incoming sunlight

manages to penetrate.

 Surface details on Titan,

shown in infra-red from the

Hubble Space Telescope.

The bright area is about the

size of Australia; its nature

is uncertain.

becomes much more luminous than it is now, Titan could

become habitable. Unfortunately, there is a fatal objection

to this idea. Titan has a low escape velocity – only 2.4

kilometres (1.5 miles) per second, almost exactly the same

as that of the Moon – and it can hold on to its dense

atmosphere only because it is so cold; low temperatures

slow down the movements of atoms and molecules. Raise

the temperature, and Titan’s atmosphere will promptly

escape. In its way, Titan is as fascinating and puzzling a

place as any world in the Sun’s family.

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