A moon with atmosphere: Saturn's biggest moon, Titan, has an atmosphere more like ours than any other body in the Solar System. Why?

27 September 1991

Ever since we began to explore the Solar System, we have searched for
another Earth. But one by one, the planets have disappointed us: Mars, whose
‘canals’ once hinted at intelligent beings, is a cold, cratered world, and
Venus, whose clouds might have hidden oceans and life, is far too hot to
have either. Although scientists no longer expect to find intelligent life
on other planets around our Sun, they still want to find worlds like Earth.
Their aim is to understand more about the early history of our planet, before
life began to reshape its surface layers.

Their best candidate lies not among the planets but in their satellites:
Saturn’s moon Titan, a world that at first glance seems anything but earthly.
Titan lies over a billion kilometres from the Sun and has a temperature
of -179 °C. But the temperature is the key; it has frozen the satellite
into the past. Titan has a dense nitrogen atmosphere containing many organic
compounds. Scientists believe that present-day Titan resembles ancient Earth
so much that through it they can study the conditions that fostered the
development of life on Earth.

Titan is appropriately named: at 5150 kilo-metres across, not only is
it the biggest moon of Saturn but it is also the second biggest moon of
the 61 known in the Solar System. Titan dwarfs the Moon (which is 3476 kilometres
in diameter), surpasses the planets Mercury (4878 kilometres) and Pluto
(2300 kilometres), and rivals Mars (6787 kilometres). Titan is also fast,
compared to our Moon. At 1 221 850 kilometres from Saturn, a separation
more than three times the …

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