Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 241

Also circling around Saturn is the tiny moon Enceladus, discovered
by William and Caroline Herschel with their '40 foot' telescope in
Slough (see Chapter 1). This moon is just under 500 kilometres in
diameter—a similar size to the small American state of Maine, or the
European country of Belgium. Despite its diminutive size, it is among
the most interesting of all Saturn's myriad small worlds. Speeding
past this moon in 2005, Cassini discovered something remarkable:
water-rich plumes, more than a dozen altogether, shooting material
into space from its southern polar terrain and feeding a giant plume
that extends thousands of kilometres into space. This water contributes
material to one of Saturn's famous rings, the 'E-ring', lacing it with an
icy spray containing sodium salts.
There seems to be a pressurized, salty ocean below the surface of
this moon, 149 heated by the tidal pressures exerted by the neighbour-
ing moon Dione, and by radioactive elements within the silicate core
of Enceladus itself. More importantly, Cassini recorded other materials
in the watery plumes, including some of the basic chemicals that are
needed for life: simple organic compounds, nitrogen, and methane
(which, just perhaps, might be a product of biological activity). The
watery world that is likely present below the southern polar region of
Enceladus is seen as one of the best prospects for extraterrestrial life.
Beyond Saturn lie the last of the official planets, Uranus and Nep-
tune. Both are regarded as 'ice giants', named after mythological
characters ( Jupiter's grandfather and the Roman god of the sea respec-
tively, although Uranus only just escaped being called Georgium
Sidus, after the famously mad King George III). In both planets, small
metallic and rocky cores are overlain by thick icy mantles of water,
ammonia, and methane, in turn covered by dense atmospheres of
hydrogen, helium, and ammonia—the last of these giving them their
beautiful blue colour. There is plenty of water here, but nothing that
we would regard as oceans. Both, though, have many moons, mostly