Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 235

steady supplies of energy, too, although not the Sun's energy (so far
out, and beneath thick ice, this is a world of pitch darkness). Any sup-
ply would be of chemical energy generated far down on the ocean
floor, perhaps akin to the black smokers on the Earth's mid-ocean
ridges, around which entire chemosynthetic communities cluster.
There may, surprisingly, be free oxygen—for that molecule makes up
most of Europa's tenuous atmosphere, formed from the splitting of
water by radiation. Over time, sufficient amounts of this oxygen may
have been carried down into that deep ocean to allow aerobic metab-
olism in any organisms that might live there.
It will be a long time before we find out. There are plans for further
spacecraft missions, although it will be years before they arrive. There
are plenty of ideas (it is the funding that is the problem). One marvel-
lously creative idea that sadly has not gone much beyond the drawing
board is the sending out of a rocket ship, heated by nuclear power,
to melt through the ice (as a 'cryobot') and emerge as a submarine
(or 'hydrobot') in the waters below. Today's science fiction, hopefully,
will become science fact for our grandchildren.
It will be harder, on Jupiter's other Galilean moons Ganymede
and Callisto, to reach through the icy crust down to the liquid water
oceans that lie beneath. Ganymede, despite being the largest moon
in the solar system, with a liquid iron core, lies farther out from Jupi-
ter and so has a lower energy input from the tidal forces. The ancient
icy crust (making up around half of this moon's mass) is up to 1,000
kilometres thick. Part of it is highly cratered and truly ancient—
perhaps up to 4 billion years old—while part is younger, grooved
and striated like Europa (although it is more cratered, and hence
older); this doubtless reflects some phase of tectonics in Ganymede's
deep history. Somewhere within this icy crust, hundreds of kilo-
metres down, lies a layer of water that is salty enough for its electri-
cal conductivity to have been detected by the Galileo spacecraft.