submersible design. The liquid is cold
(94 K; –290 degrees F), so just staying
warm will draw a large part of a probe’s
energy and dictate its structure. Another
problem is how Titan’s atmosphere reacts
with its methane seas. On Earth, subma-
rines can use air to fill their tanks and
regulate buoyancy. But the nitrogen that
makes up the majority of Titan’s atmo-
sphere is soluble in liquid methane, so it
has less power to make the sub buoyant.
If designers use nitrogen for f lotation, the
gas will be effective only at limited
depths. The other option is to use a noble
gas, such as neon.
Nitrogen dissolved in methane pres-
ents another problem, Lorenz explains.
“If you have a patch of your submersible
which is leaking heat, you could raise the
temperature of that liquid enough that it
reduces the amount of nitrogen that can
dissolve. Think how much CO2 you can
dissolve in water on Earth (imagine a
soda bottle), and that’s the picture. You
could get bubbles on [the outside of]
your submarine. That’s not going to
make the thing sink, but it reduces move-
ment, and they might inf luence a side-
scan sonar to image the seabed. This is a
problem that just doesn’t happen on a
terrestrial submarine.”
Another probe under consideration is
more akin to a dinghy. Known as an
unmanned surface vehicle (USV), this
robot would f loat atop the surface of
Titan’s sea rather than diving below.
Lorenz says, “It simplifies things to have a
capsule that just f loats and doesn’t have to
do buoyancy control. You can imagine a
propelled vehicle — a boat — that would
be interesting.” But a boat loses the ability
to profile the liquid column to see if the
mix of methane and ethane is stratified.
Does Ligeia Mare have an ethane-rich
layer 100 meters down at the
bottom, something like the
anoxic layer found at the
bottom of the Black Sea?
Researchers see evi-
dence of evaporites —
minerals left over after a
body of water evapo-
rates — on the shores of
Titan’s seas, as if the
seas have dried out and
SeaBED, captured
here on a test
run under the
Antarctic ice, is
an autonomous
underwater
vehicle that can
hover over Earth’s
seafloor at depths
of up to 6,000 feet
(2,000 meters).
KLAUS MEINERS/AUSTRALIAN
ANTARCTIC DIVISION ROV TEAM