48 Australian Geographic
H
ERE’S SOMETHING TO think about the
next time you’re on the water. It doesn’t
matter whether you’re on a lake or the
ocean; or whether you’re floating on a
yacht, an ocean liner, or even just on your
back. Take a look at that animated, sparkling surface that
surrounds you. It’s the exquisite boundary between our
planet’s liquid shroud and its gaseous envelope, and it
stays there because the water and the atmosphere are in
a state of perfect equilibrium.
In the grand scheme of things, that’s a very special
circumstance – so rare that, at present, we know of only
two places in the universe where it is found. One is here
on Earth and the other is...well, we’ll get to that.
It’s not so long ago that our forebears took seas and
oceans completely for granted, assuming they would be
commonplace on other worlds. The darker regions of
our own Moon – easily visible to the unaided eye – have
long been known as ‘maria’, Latin for seas. Some later
sky-gazers were so fixated on the immutable perfection
of the cosmos that they rationalised the Moon into hav-
ing a mirror-like surface that reflected an image of our
oceans back to us. But the invention of the telescope in
the early 1600s quickly demonstrated that the lunar
maria were quite unlike anything on Earth. We now know
they are frozen lava flows – seas of a very different kind.
With the dawn of the space age, our horizons broad-
ened to include not just the planets of the Solar System,
but also their moons. Fly-bys of Jupiter and Saturn by
the Pioneer and Voyager spacecraft in the 1970s allowed
accurate estimates of the densities of their larger moons,
suggesting a high proportion of ice in their make-up.
Speculation quickly arose about the possibility of
oceans existing beneath their icy surfaces – something
that would have seemed like pure science fiction a decade
earlier. Today’s investigations extend even further into
the realm of science fiction, with some querying whether
living organisms might have developed within these sub-
surface oceans – a question we are yet to answer.T
HE PLANET JUPITER boasts more than 60
moons, and the four largest are significant worlds
in their own right. Three – Europa, Ganymede
and Callisto – may have a thick surface layer of ice over-
laying a liquid-water ocean, all overlaying a rocky core.
How do we know they have oceans under their sur-
faces? The major evidence comes from their magnetic
properties, since their water generates a weak magnetic
field in response to Jupiter’s extreme magnetism. That,
together with the detection of hydrogen atoms by NASA’s
Galileo spacecraft (in orbit at Jupiter from 1995 to 2003),
suggests large bodies of liquid water. It is the gravitational
pummelling of the ice-moons by massive Jupiter’s tidal
forces, pulling and pushing on their rocky cores, that
creates frictional heat and keeps the oceans liquid.
The demonstrated existence of large bodies of liquid
water within the Solar System’s ice-moons excites
astrobiologists looking for proof of living organisms,Once thought of as a
massive lump of rock and
ice, Jupiter’s Callisto may
have a subterranean ocean
layer – a feature suggested
by its low density.CALLISTO
This large Jovian
moon may have a salty
ocean, holding twice as
much water as Earth’s,
beneath its kilometres-
thick icy exterior.EUROPA
because water is essential for life on Earth. It is indeed
possible that life may, for example, have evolved in
Europa’s mineral-rich ocean nurtured by the tidal heat-
ing that keeps it liquid.
A first step in finding out whether any of Jupiter’s
moons offer a habitat suitable for life is already on the
drawing board. The European Space Agency’s (ESA)
JUICE (JUpiter ICy moons Explorer) mission is slated
for launch in 2022 to visit Ganymede, Europa and
Callisto. The spacecraft won’t land, but the exploration
of their subsurface oceans is high on the list of mission
objectives. This will be achieved by imagery, laser altim-
etry and ice-penetrating radar, together with spectrum
analysis of the surface to determine composition.
Beyond that, a NASA concept study has considered a
robotic lander to investigate the rusty brown cracks that
criss-cross Europa’s surface. This is to see what has been
dredged up from the depths below, and how it has sur-
vived Jupiter’s harsh radiation environment.
It has even been suggested that a small nuclear heat
source, like those used for power-generation on deepFRED WATSON is our regular astronomy columnist and the
astronomer-in-charge of the Australian Astronomical Observatory.ENCELADUS
Saturn’s Enceladus is
mostly rock and ice,
but appears to have an
internal ocean encircling it.CRACKS CAUSE SURFACE GEYSERSMETAL CORE
ROCKY MANTLE
LIQUID WATER
ICE CRUSTCORE OF ROCK
AND ICELIQUID OCEAN
LAYER DIRTY ICE CRUSTROCKY INTERIOR
LIQUID OCEAN LAYERHIDDEN SEAS