50 Australian Geographic
CARLOS CLARIVAN / SCIENCE PHOTO LIBRARYecho ancient terminology by being described as maria.
Some 30 smaller lakes, ranging in length from a few kilo-
metres up to 200km, have also been identified. Most are
thought to be fed by methane rain draining into
river-like features. But a few in Titan’s equatorial region- in places where the ice bedrock is porous – could be
fed by springs from a hydrocarbon ‘water table’. Depths
vary from 2–3m for the smallest lakes, to 10s of metres
for the polar seas, with a maximum depth of more than
200m for Ligeia Mare, Titan’s second-biggest sea.
The hydrocarbon seas hold many mysteries. What are
the temporary surface features that have been observed
in the three largest? Some scientists believe they are the
surface ripples reflecting Cassini’s radar signals. But
methane icebergs, which form on or near the surface and
then sink as the conditions change, might also be
responsible. It is thought cyclones may occur during
Titan’s frigid summer. And strong currents in the
so-called Throat of Kraken (a narrow neck of liquid in
Kraken Mare, Titan’s largest sea) may even generate
spectacular whirlpools.
Because observations have suggested a rich chemistry
of ‘organic’ (carbon-containing) compounds on Titan’s
surface, some scientists believe this distant moon is an
analogue for the early Earth – with an atmosphere
similar to that here before life evolved. Others go further,
proposing there could already be life forms thriving in
the hydrocarbon lakes. These would be quite different
from the water-based life we see on Earth and use liquid
methane as their working fluid, breathing hydrogen and
feeding on acetylene. Tantalisingly, both chemicals are
depleted to low levels in Titan’s atmosphere.
How could we explore the potentially rich submarine
environment of a world that is 1.4 billion kilometres from
Earth? The answer could be close. We already have much
experience using unmanned underwater vehicles (UUVs)
in our own planet’s oceans – for scientific exploration,
military purposes, resource surveys and, most recently,
searching for the wreckage of downed flight MH370.
UUVs provide a sound basis for further development.
A UUV for exploring Kraken Mare, for example,
would be feasible, although it would need an onboard
nuclear generator for power. Its sleek form would hide
an array of specially developed sensors (see opposite),
including chemical and biological samplers, imagers and
sonar. To relay data to and from such a submersible would
require it to work, as the Mars rovers have done, in con-
junction with an orbiting spacecraft.
A Kraken Mare submersible has already been the sub-
ject of a detailed NASA study, with thought given to such
subtleties as dumping excess heat from the craft into its
liquid methane-ethane surroundings. But there are still
open questions. How, for example, would you deliver
it from Titan’s orbit to sea level without damaging delicate
equipment? The moon’s murky atmosphere might help.
A winged atmospheric entry vehicle, not unlike the USTHE ICE-MOONS
OF JUPITER
EUROPA
Diameter 3130km
Orbits Jupiter in 3.55 days- Relatively young surface criss-crossed by linear
cracks, marked by rust-coloured deposits. - The cracks have recently been shown to be
consistent with deposits of salt, coloured by the
intense radiation generated by Jupiter’s
enormous magnetic fi eld. - This suggests a briny subsurface ocean leaking
through surface cracks and leaving behind the
coloured residues after evaporation.
GANYMEDE
Diameter 5268km
Orbits Jupiter in 7.15 days- Largest of all the moons in
the Solar System (400km wider
than the planet Mercury). - Unique in having a magnetic
fi eld, which gives rise to
auroral displays. - Shift in latitude of auroras
suggests existence of a saline
ocean under the surface.
CALLISTO
Diameter 4806km
Orbits Jupiter in 16.7 days
- Has the most heavily cratered
surface in the Solar System. - Indicates great age, and no
surface regeneration since the
‘late heavy bombardment’
period 3.8 billion years ago. - Low density suggests the
presence of an ocean beneath
the icy surface.
CALLISTOGANYMEDEEUROPAIOJovian Satellites. The ‘Galilean
moons’, discovered in 1610,
were the fi rst objects found
orbiting other worlds; they are
among the biggest objects in
the Solar System after the plan-
ets. Io in contrast to the other
three, which have abundant
water-ice, is one of the driest
objects in the Solar System.