Ocean Forms - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 40

scattering of lakes and ponds. Here there is much less scope to start
or maintain a hydrological cycle, or to sustain a grand planet-wide
cycle of plate tectonics. The water in such a pitiful scattering of
water bodies would not be the pure and limpid water of storybook
desert oases, but likely a brine so concentrated that it would suck
any scraps of moisture from the air—and be a huge challenge to the
workings of any biological cell that might try to survive in such
conditions.
These alternative visions of the world are by no means far-fetched—
as we shall see when we lift our eyes to the skies to scan the solar
system and the distances beyond (Chapters 9 and 10). On Earth our
present happy condition—as we perceive it, knowing little of any
other—may be put down to sheer chance at work within a cosmic
game of billiards. A few more or a few less large, water-bearing aster-
oids or comets impacting on the Earth could have made the difference
between a desert Earth and a land-less water world.
In the game of cosmic chance, the Earth ended up with just enough
water to irrigate the land and fertilize the seas and scrub the air of too
much carbon dioxide. There has been enough water, too, to kick-start
the Earth's plate tectonic machine. That last feature has been particu-
larly convenient, because it has given the Earth the basins to hold its
water in.
The Shape of the Oceans
If one was to take the solid Earth and simplify it to its absolute basics,
then it is, simply, a sphere. The waters on such an Earth would be
consistently some 2.5 kilometres deep. More precisely, the Earth is a
sphere that is slightly flattened at the poles, because its spin is causing
it to bulge out at the equator (the water would bulge slightly, too).
Let us look in just a little more detail. We have a world of two levels.
Most of the world's solid surface is 4 kilometres or thereabouts below