Philips Atlas of the Universe

Titan

A

part from the Galileans, Titan was the first planetary

satellite to be discovered – by Christiaan Huygens, in

1656. It is actually larger than the planet Mercury, though
      not so massive. In 1944 G.P. Kuiper showed spectroscopi-
      cally that it has a dense atmosphere, which proved to con-
      sist mainly of nitrogen, with a good deal of methane. The
      Voyagers could do no more than image the upper clouds,
      and so Titan was a prime target for the Cassini-Huygens
      mission. After a 7-year journey Cassini, carrying Huygens,
      reached Saturn in late 2004.
      On 14 January 2005 the Huygens spacecraft made a
      controlled landing on Titan; it had been released from the
      Cassini probe on 25 December 2004, and made an auto-
      matic landing, involving parachutes. The descent took
      2
      hours through Titan’s dense atmosphere, during which
      measurements were made; the touchdown speed was less
      than 20 km/h (13 mph). After arrival, Huygens sent back
      data for 72 minutes – far longer than had been expected.
      Huygens came down on a thin crust, and settled down
      some centimetres below on spongy, hydrocarbon material
      with about the consistency of wet sand. The heat of
      Huygens’ batteries caused some of the frozen surface to
      ‘boil’, causing puffs of methane. Titan is unique. There are
      icy pebble-sized objects near the landing site; the site was dry

ATLAS OF THE UNIVERSE

Moon

Titan

Earth

Comparative size of

Titan as against that of the

Earth and the Moon. Unlike

our Moon, Titan has an

atmosphere.

 Titan from Cassini, in a

mosaic of nine images

acquired on 26 October 2004,

during the spacecraft’s first

close fly-by. Bright clouds

can be seen, as well as some

surface features.

 The surface of Titan as

recorded by Huygens after

its descent through the

moon’s dense atmosphere.

The rocks in the foreground

are only about 15 cm

(6 inches) across.

when Huygens arrived, but had been wet very recently.

Methane rain lands on the icy uplands and washes the dark

organic material off the hills; this is transported down to the

plains in drainage channels, and eventually disappears. The

hills themselves are of ‘dirty water ice’. Huygens imaged

snaking, branching river tracks; there are water-ice volca-

noes, and liquid certainly flowed soon before the probe land-

ed. There must be liquid a few centimetres below the surface.

The ‘mud’, as it was described, seems to be a mixture

of sand, methane and complex organic molecules that

form in the upper atmosphere. According to Martin

Tomasko (University of Arizona): ‘This smog falls out of

the atmosphere and settles on everything. Then methane

rain comes, washes it off the ice ridges and into rivers,

then out into the broad plain where the rain settles into the

ground and dries up. We are seeing evidence of Earth-like

processes, but with very exotic materials.’ Methane is con-

stantly being destroyed and turned into complex chemical

smog, so there must be some source inside Titan to replen-

ish the atmosphere. Some gases, such as argon, are absent.

All the ingredients for life exist on Titan, but it seems

virtually certain that the very low temperature has prevented

life from appearing there. There have been suggestions

that in the far future, when the Sun swells out and

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