Ocean Origins on Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 26

light that shone from it. These showed that the water content of this
comet has a deuterium value very similar to that of the Earth's
oceans—and so could represent a potential source. 25
There is, therefore, a population of comets out there with the right
kind of chemistry to have supplied at least part of the world's water.
Thus, the late-ocean model for the Earth is viable once more. If there
is truth in it, then there are implications. These extend not just to such
matters as surface hydrology and the incubation of life. There may be
revolution, too, in the workings of the whole planet.
The Hydration of Earth
Time travelling to prehistoric Earth would be a fine thing. Of course,
in general it is a fine thing, and geologists do it every day as they walk
across strata that are dried and compressed sea floors and riverbeds,
beaches and deltas, swamps and forests. But for most of the first
billion years of history such things are absent, long erased from the
Earth's memory banks. Here, a latter-day Tardis really would come in
handy, for this first billion years set the course of our planet's history,
and we still know terribly little about it.
It has been called the Hadean, this early eon of Earth, after Hades,
brother of Zeus and Poseidon, and the fierce and unyielding (although
not unjust) god of the dead and of the underworld. To enter Hades'
realm, one had to cross the Acheron, the river of pain (in later ver-
sions it is the Styx). In our modern understanding, too, water is the
key to the underworld. Without water, that underworld cannot sculpt
our surface world. Therefore, once water was introduced to the Earth's
surface, it must descend, too, into the depths of the Earth, so it can do
its work.
How much do we know directly about the Hadean, and in particu-
lar, about those crucial early years in the aftermath of the impact of
Theia, and of the arrival (if such really happened) of the late water-rich