November–December 2015 49OPPOSITE, TOP–BOTTOM: SCIENCE PHOTO LIBRARY; GETTY IMAGES; NASA / ABOVE: GETTY IMAGES
space missions – such as New Horizons to Pluto – could
power an ice-penetrating robot. By melting the ice below
it, the robot would slowly descend through Europa’s thick
ice crust, reporting back on what it finds – perhaps
emerging into the ocean beneath. Such heat-drilling is
already a proven technology on Earth – although without
the need for a nuclear heat source. That’s how dozens of
2.4km-deep holes were drilled into Antarctic ice for a
neutrino particle observatory at the South Pole.M
ORE EVIDENCE FOR a subsurface ocean comes
from Saturn’s moon Enceladus. This has an icy
surface like Europa, but with bluish-green
linear features dubbed ‘tiger stripes’ in the region around
its south pole. Erupting from these are geysers of ice that
were discovered in recent years by the international Cassini
probe, which has since flown through the ice-plumes and
detected mineral salts. This hints that a moon-wide liquid
reservoir from which the geysers emerge is in contact with
a rocky surface and not totally encased in ice.
The warmth that keeps Enceladus’s ocean liquid has
yet to be fully explained. Cassini observations show sur-
face ‘hot spots’ in its south polar region, with temperatures
up to –116oC, some 65oC above the ambient temperature.
Saturn’s tidal influence is part of the story, but Enceladus
seems warmer than can be explained by this alone.O
F ALL THE ICE-WORLDS of our Solar System,
none is more bizarre than Saturn’s biggest moon,
Titan (see page 51). It is the only known moon
with a thick, hazy atmosphere, which stabilises its surface
temperature at about –180oC and gives it some extraor-
dinary attributes. Most notable is that Titan is the only
place in the universe known to have seas both above and
below the surface.
A long-held suspicion that Titan might have pools of
liquid on its icy surface was confirmed by Cassini’s
smog-penetrating radar. By 2007 this had provided defin-
itive evidence of methane-filled lakes. Further verification
came from clever observations of sunlight glints. Located
mostly near Titan’s poles, the lakes pool in basins in the
ice ‘bedrock’. They are the only stable bodies of surface
liquid known anywhere in the universe beyond Earth.
Three of the lakes are particularly large – compara-
ble in area with North America’s Great Lakes – andGeyser moon. This illustration of jets of
water vapour and ice spewing from Saturn’s
moon Enceladus was inspired by images
taken by Cassini in 2011; experts now
believe there are up to 100 such geysers.The warmth that keeps
Enceladus’s ocean liquid has
yet to be fully explained.