Undreamed Shores - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 258

with a planet composed of a rocky core and a thin envelope of light
gases: or, more intriguingly, a rocky core with an envelope of super-
critical water. If the latter is the case, then 55 Cancri e has the weirdest
oceans of all: oceans of something that is not quite water, and not
quite steam.
About one-fifth of the mass of 55 Cancri e consists of light materi-
als, most probably including water. If so, this is supercritical water at
high temperature and pressure, existing simultaneously as a liquid
that is able to dissolve minerals, and as a gas that is able to diffuse
through solids. There are some analogies for this kind of supercritical
water on Earth. These are mainly to be found in steam turbines, or in
the kinds of industrial processes that remove the caffeine from coffee
beans by steam heating. More naturally, they may occur in volcanoes
deep below the sea. At shallow depths, water issuing from submarine
volcanoes blasts its way to the surface as steam. But deep under the
sea, 3 kilometres down, the water is under enormous pressure and it
emerges as streams of supercritical, mineral-rich water—as black
smokers, that is—at over 375 degrees Celsius. Entire oceans may exist
in this form on 55 Cancri e.
Water Worlds
55 Cancri e is a water world, of a sort. Earth is not a water world and
never will be. Even if the world warmed sufficiently to melt all of the
ice of Antarctica and Greenland, of the Himalayas and the Alps and the
Cordillera, sea levels would rise by only 70 metres. This is enough to
inundate many of the world's great cities, granted: London, Shanghai,
and New York would all be beneath the waves. But it would by no
means submerge the world completely. On Earth, water accounts for
just some 0.05 per cent of Earth's mass. There are worlds, though,
where more than 10 per cent of the mass may be water, and where
oceans may be hundreds of kilometres deep. Their abyssal depths