Titan
is the only large moon in the Saturnian system and it is the only moon
in the Solar System with a thick atmosphere. Not only that, the atmosphere
shows signs of containing many elements and compounds that are the
basis of organic chemicals. For this reason it is a very interesting
place in our the Search for Life. As Saturn is twice
as far from the Sun as is Jupiter, it receives only one-quarter of the
solar radiation that Jupiter receives. This means it is very
very cold here, but because the atmosphere of Titan is so thick it absorbs
whatever solar radiation is available and uses it to drive chemical processes
in the atmosphere and on the surface. This means that weather
and complex chemistry occur on Titan, even if this is rather slow.

The
depth of its atmosphere is ten times Earth's, due not only to the higher
pressures, but also the low temperatures and low gravity. The atmosphere
is extremely dense and an opaque layer of haze, about 200 km (125 miles)
above the surface, effectively obscures any attempt at optical observation.
In fact it is the only major body in the Solar System whose surface we
have never observed at high resolution. Within the last few years,
though, the Hubble Space Telescope infra-red cameras have been able to
penetrate the cloud and we now have some coarse resolution images, that
show the largest features on the surface.

Images
of Titan were taken by the Voyagers. All they showed was a uniformly-smooth
orangey ball - the dense atmosphere. Spectral analysis, both from
the spacecraft and from Earth, revealed the atmosphere to be largely Nitrogen
- 97%. Trace amounts of a cocktail of organic gases are present,
and these give Titan its characteristic color - Methane, Ethane,
Ethene, Propane and half-a-dozen others. Hydrogen and Carbon
Mon- and Di-oxide are also there and significant amounts of water vapor
were detected by ESA's Infrared Space Observatory in 1997.
The organic compounds are in a reduced rather than oxidized form.
Also, and this came as a surprise to many planetary scientists, the nitrogen
is not contained in the form of ammonia as it is in the gas giant planets
- Jupiter and Saturn.

At
present, two explanations are offered for the lack of ammonia. The
first suggests that when Titan formed it was so cold that all the nitrogen
was locked up in various ices as a clathrate
- a solid material with tiny cavities containing the gas at the atomic
level. On melting through radiogenic
(heat produced through the radio-active decay of heavy elements such as
uranium), or by tidal heating, the nitrogen was released. Ammonia
must also have been present, but this has not been released in this way.

The
second explanation, which is most favored at present, suggests an unstable
active atmosphere that recycles the nitrogen though the agency of ultraviolet
solar radiation. This liberates hydrogen which then escapes into
space. There is good evidence for this, as Titan orbits in a thin
halo of hydrogen. This was detected by the Voyagers. The process
is time-limited, because Titan has only so much hydrogen, so once this
is all liberated to space the chemistry will cease.

The
mechanism works through ultraviolet disassociation of the ammonia.
Basically, one of the three hydrogen atoms gets knocked off the ammonia
- NH3
- by a photon. This leaves a radical, NH2
and the single H escapes away to space.

Through
a complex chain of chemical processes, various organic hydrocarbons found
in the atmosphere are produced through a myriad of chemical reactions
between the NH radicals and carbon mon- and
di-oxide. The processes also provide for the possibility of long
carbon chain molecules forming and these could produce interesting organic
compounds, which in some cases could be the precursors
of life.

All
this complex chemistry suggests that the chemical precursors of life are
present on Titan. However, Titan's problem for life may be that
it is, and always was, too cold for life as we understand it to exist.
Titan is regarded, by optimistic life-scientists and planetary specialists,
as a natural laboratory to study the origins of life. It may be
in a state like the Earth was four-billion or more years ago when similar
organic chemistry was taking place here, though it will be much colder.

Conditions
on Titan are such that methane could exist in all the three physical states:
vapor, liquid and solid, for the temperatures and pressures are close
to methane's triple-point. It has been speculated that if
ammonia rain and snow fall, great ice cliffs could form near the poles
and oceans may exist.

The
Hubble Space Telescope has been able to map Titan, and has found highlands
and lowlands, but does an ocean of liquid ammonia exist?

If
life did exist on Titan, what might it be like? Titan's chemistry
may be active enough to be able to create amino acids, and other basic
building blocks of organic chemistry may be present. The big problem
for life at these low temperatures and low energy levels is the rate of
the process. The speed of chemical reactions drop off exponentially
with temperature. If life exists on Titan, then the rate of
metabolism will be inordinately slow, which raises the key question "Will
it therefore be detectable?"

That
is very difficult to answer, not only because of the slow rates of reaction
and process, but because it is highly unlikely that liquid water
could be present. This is unlike the case of Europa, where
temperatures could be higher. But it is interesting to note that
ammonia - NH3
- is a polar solvent like water, but only liquid
at much lower temperatures than occur on Earth. Titan is the one
place we know of where this may occur and ammonia could take the place
of water as the life-giving solvent. There is a problem, though.

Water
has another highly unique characteristic. It assists in the working
of enzymes, which are organic catalysts. Enzymes are critical in
allowing amino acids, the main building-blocks of life, to react with
other chemicals both quickly and efficiently.

Additionally,
nearly all liquids as they solidify do so from below. Generally,
the solid state of a substance is denser than the liquid state.
So assuming the presence of a gravitational field, when a solid forms
or solidifies it does so on the bottom. Water is virtually
unique in the way it solidifies. It forms ice and this floats on
the surface of the liquid. A layer of ice creates an insulating
blanket, which assists in preventing water below from freezing as well,
unless very much colder conditions are encountered (ie, much colder than
would be necessary to freeze bottom-solidifying material).

This
allows living organisms to continue functioning below a water-ice layer,
while conditions above are too severe for survival. In the Arctic
for instance, the air temperature may be sixty below, but the water temperature
under the ice is a pleasant freezing point, more or less.
Similarly, ice will melt fairly readily when warmer conditions prevail.
Bottom-forming solids may never melt out (convection heat tends to rise),
but continue to build up by solid layering, as warm and cold conditions
cycle. Ammonia is a bottom-solidifying material and this factor
would not help life to flourish on Titan.