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Some key tourist attractions:

Hydrocarbon Lakes, Rivers and Seas

Liquid hydrocarbons, such as methane and ethane, are to Titan what water is to Earth. Orange-yellow clouds  
rich in hydrocarbons appear to rain down and periodically fill the many hydrocarbon lakes and rivers on Titan's
surface, most of which are currently dry. Click on the images below to enlarge:
Star System Sol - Planet System Saturn - Titan: The Smog World
Orb type: Titan is Saturn's largest satellite and is really a planet in its own right, which orbits Saturn rather than
having its own orbit about the Sun: it is essentially a secondary planet.

Mean radius :2575 km
Orbit: 1 222 thousand km from Saturn (Saturn orbits the Sun between 9.01 and 10.04 AU (one AU is one
Astronomical Unit, which is the average distance of the Earth from the Sun or 149 597 870 km). Orbits Saturn
once every 15.95 days.
Atmosphere: thick, mostly nitrogen (98.4%) some methane and traces of various other hydrocarbons, including
ethane and propane, acetylene, diacetylene, methylacetylene and cyanoacetylene and traces of carbon
monoxide, carbon dioxide, argon, cyanogen and hydrogen cyanide with a surface pressure 1.6 times greater
than on Earth. The thick, hazy and orange atmosphere gives Titan a yellow, orange or reddish appearance
from space, and most of the surface details are hidden beneath the haze.
Surface temperature: 94 K (minus 179 degrees C).
Surface composition: the outer crust consists mostly of water ice, along with frozen hydrocarbons.
Titan is so cold that many chemicals which are liquid on Earth, are totally frozen, so water exists as ice, and
many chemicals that are gases on Earth are cold liquids on Titan, so methane and ethane are liquids. Many
hydrocarbons also freeze solid on Titan, forming hydrocarbon ices. The water and hydrocarbon ices cover the
surface of Titan in orange-yellow material and behave as rocks (water ice becomes like rock when extremely
cold) and many have been sculpted and rounded down to pebbles by the hydrocarbon rivers.

Hydrocarbons form a persistent yellow-orange haze of what is essentially natural smog. Clouds also occur on
Titan. Extensive lakes, seas and river basins of liquid methane and ethane cover much of Titan's surface.

Cryovolcanoes

There is evidence that cryovolcanoes eject water and ammonia ices, possibly from a subsurface ocean, which
may be liquid or frozen slush. On Earth, magma beneath the Earth is a hot deformable plastic solid, since
despite the enormous temperatures the high pressures prevent the magma from liquefying. Only when pockets
of magma work their way up through the crust does the magma liquefy, as the pressure drops, and hence it
erupts as liquid lava. Ice is rather different. Unlike rocks, which tend to solidify under pressure, water ice tends
to liquefy under high pressure. This raises the possibility that liquid water nearing the surface of Titan would
solidify as the pressure drops. In any case the drop in temperature will eventually solidify any liquid water. Upon
solidifying, water expands and this could have a rather explosive effect, propelling slush and ice from
cryovolcanoes with considerable force.

Sand Dunes

These are not made from silicon sand, as are sand dunes on Earth, but probably from ice grains and grains of
hydrocarbons and other organics. These dunes are probably built up by the strong tidal winds of Titan. On
Earth, the Moon's gravity causes tidal forces that tug on the oceans, moving them back and forth. On Titan, the
closeness of Saturn with its strong gravitational field, means that tides are 400 times stronger than those
caused by the Moon on Earth. The strong tidal winds blow predominantly from west to east and gather the sand
into long parallel dunes running from west to east.
Ice Mountains

Chains of mountains made up of ice are quite abundant on Titan, some mountains are 1500 metres high and a
chain 150 km long is known. These mountains are thought to have formed from upwelling water-ammonia
magma, not hot magma, but cryomagma at subzero temperatures, but nevertheless warmer than the
surrounding surface. Mountain chains on Titan also give rise to fixed bands of clouds, as prevailing winds blow
from west to east and rise up over the mountains, forcing methane droplets to condense from the rising air as it
cools.

Is there life on Titan?

Unknown. Even if the liquid hydrocarbons on Titan could function in place of water in living cells, the
temperatures would possibly be too low to sustain cellular chemistry, since chemical reactions occur more
slowly at low temperatures. However, maybe enzyme catalysts could overcome this barrier, but even so, the
pace of life and its subsequent evolution would probably be very slow under such conditions.

It is far more likely that if life exists on Titan, that it exists in a much warmer subsurface ocean, which may be of
hydrocarbons or a mixture of ammonia and water. If subsurface conditions are warm enough to maintain liquid
water, and they probably are, then conditions are probably suitable for life, but whether or not such life has
evolved is another issue. However, it is worth noting that life exists on Earth in the coldest bodies of liquid water
found to occur there. For example, Antarctic hypersaline (very salty) lakes, which are saturated with calcium
chloride salt contain thriving bacteria. The salt lowers the freezing point of the water to 222 degrees kelvin (-51
degrees centigrade).

Another factor to deal with is the high pressure that might exist at the bottom of the subsurface ocean, if indeed
such an ocean exists. Such an ocean would make Titan a type of ice planet, similar to Europa (and our
Seraf-9). The pressure at the bottom of say a 200 km ocean beneath 30 kilometres of ice, on a Titan-like world,
would be 4.5 kbar (45 000 atmospheres, atm, or 4 500 MPa), compared to 1.1 kbar (11 000 atmospheres or
1100 MPa) at the bottom of Earth's oceans, but at the surface pressures would be 1 kbar. Note that a 200 km
ocean on Earth would have a bed pressure of about 20 000 atmospheres or 2 000 MPa =  20 kbar, but the
gravity on Titan is only about 10% that of Earth, so on Titan the pressure would be about 2 kbar, so our
estimate seems about right).

Note: New evidence does indeed suggest that Titan has a subsurface ocean. Beneath its lakes, seas and rivers
of methane and ethane and its icy crust of water and ethane ice there appears to be a highly fluid mantle.
Observations from the Cassini probe orbiting Titan has revealed substantial shifts in surface features in a
relatively short period of time - as if the icy crust and its lakes were slipping or floating on a liquid ocean,
presumed to be primarily water. It is not yet known how deep this apparent ocean is, and indeed whether it is an
ocean of water. Many other
ice 'planets' like Europa and some of the smaller satellites of Jupiter and Saturn
quite possibly have liquid water subsurface oceans. In some cases these oceans may extend to considerable
depths. Bearing in mind that the main sources of heat and fuel on such worlds may be hydrothermal vents on
the ocean bed, the pressure that any organisms exploiting such sources must be able to withstand becames an
important issue in astrobiology.

Now, pressure does not impose a problem on the structure of living things since the bodies of deep sea
creatures have a similar internal pressure to the external pressure of their surroundings and it is the pressure
differential that causes structures to collapse or explode. However, many biochemical reactions do involve a
slight change in volume. Clearly certain chemical reactions involve a great change in volume, as when
trinitrotoluene (TNT) decomposes to yield lots of nitrogen and carbon monoxide gases. Since gases have a
much greater volume than solids this causes a massive increase in pressure which is why this reaction is
explosive! However, even reactions in which the reactants and products remain in the same physical state,
such as in aqueous solution, there can still be very slight volume changes due to the changes in atomic
configuration. These volume changes are so small that you would not notice the increase or decrease in
volume that these chemical reactions may cause if they occurred inside your cells. However, when the external
pressure is around 1 kbar (10 000 atm), as it is in the deepest ocean trenches on Earth, these biochemical
reactions start to become significantly affected - biochemical reactions that normally result in a very slight
increase in volume are inhibited.

For example, the protein
actin is found in animal cells, including the cells of fish, and a key part of the cell's
skeleton (cytoskeleton). Actin can exist as a solution of separate actin monomers, like pieces in a lego set, or it
can polymerise (the lego bricks join together) to form rods that act as struts, levers and beams, allowing the cell
to move and change shape. This polymerisation process requires a slight increase in volume (by 63 to 139
ml/mol in terrestrial vertebrates, such as humans, at atmospheric pressure). Fish that live at depth have
modified their actins to reduce this pressure increase - the increase is about 60 ml/mol in the shallow water fish
Coryphaenoides acrolepis and 10 ml/mol in the deeper water fish Coryphaenoides armatus which lives at 1900
to 4800 m depth (19 to 48 MPa pressure). The rates of important biochemical reactions are also affected by
high pressure, for example the enzyme LDH (lactate dehydrogenase) is important in vertebrates, including fish,
is vital for anaerobic respiration (as when sprinting or burst swimming) and produces lactic acid. However, in
Sebastolobus alascanus, which occurs mostly at depths less than 500 m, this enzyme is inhibited by 17% at a
pressure of 34 MPa, whereas the LDH of
Sebastolobus altivelis, which occurs mostly at depths greater than 500
m, is only inhibited by 11% at 34 MPa. Thus, certain enzymes from deep water fish are more tolerant to high
pressure than those from shallower water fish. (Ref. A. Gibbs, Biochemistry at Depth, 1997, in Deep-Sea
Fishes, Academic Press).

At really immense pressures, i.e., those greater than 100 MPa (= about 1000 atmospheres = 1 kbar),
corresponding to over 10 000 metres depth on Earth, proteins break down and lose their function. This
corresponds to the very bottom of the deepest ocean trenches on Earth (the Marianas Trench extends down to
11 000 metres). However, live on Earth thrives at these depths and must narrowly avoid this severe problem.
To live at greater pressures would require considerable changes to protein chemistry. Whether or not Titan has
an ocean deep enough (considering its weaker gravity) to produce such pressures remains to be seen.

The Structure of Titan
Titan
Caves on Titan
Above: a model of ice caves on Titan. An orange hydrocarbon mist hangs over the orange
hydrocarbon pool.
Titan structure
The diagram above shows a model for the structure of Titan. This model is based on theory and observational
data.

The outermost crust is represented as a thin layer of water ice (Ice I) and hydrocarbon ice in this model. It is
possible, however, that the crust is a mixture of rock and ice. Beneath this thin layer (shown in white) is
possibly an ocean of water and ammonia, shown in blue and assumed to be 200 kilometres in depth. An
alternative model would be of a solid rocky layer permeated by subterranean lakes and rivers. Beneath this
ocean is depicted a thick water ice layer (Ice VI and ammonia dihydrate ice) shown in translucent white. In the
centre is thought to be a solid rocky core, made principally of silicates. Hydrocarbons, like methane, may be
outgased from the hot silicate core and erupt on to the surface, forming the surface lakes and clouds and
haze of hydrocarbons that have been observed on Titan. Liquid water and ammonia may possibly also erupt
onto the surface, but this liquid would rapidly freeze on the cold surface.

An alternative model is also considered likely. In this alternative, the whole of Titan is solid silicate rock
covered by a surface of water and hydrocarbon ices and liquid hydrocarbon lakes. The hydrocarbons may mix
beneath the surface with an 'ocean' of groundwater, possibly consisting of water, ammonia and hydrocarbons.
Whether the subsurface ocean is a series of liquid filled rocky or icy chambers, or whether it is a complete
liquid ocean is not known, however, the presence of liquid hydrocarbons on the surface of Titan is indicative
of a huge subsurface reservoir of some sort.

The key points to note are:

1. The surface of Titan contains lakes of liquid hydrocarbon and water/hydrocarbon ices that play the role of
'rocks'.
2. There is almost certainly a subsurface reservoir of liquid.
3. This subsurface reservoir could be liquid hydrocarbon rivers and lakes, or an ocean of water and ammonia
or some mixture of the two. It may consist of a free ocean beneath an icy crust or a series of chambers carved
into ice or rock.
4. It will be much warmer in these subterranean locations than on the surface, so if life exists on Titan, it would
probably be in these subsurface reservoirs.
5. Life on Earth is theoretically able to thrive in such conditions, but whether or not life has evolved despite
probable favourable conditions for its existence is another issue.
6. Although the pressures would be enormous at the bottom of the ocean, pressures at the top would be
similar to those at the bottom of Earth's ocean, where life thrives.

Click here for a general discussion about
ice planets.