Water in the Cosmos - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 15

temperatures—because of the near-vacuum pressure—well below
zero degrees Celsius.
It is a kind of temperature ladder. How high up on it a planet is
gives some idea of the ambient conditions, when the main phase of
planet building was over, once the planetary nebula had cleared. 12 The
Earth seems, overall, depleted in elements on the lower rungs of
the ladder. This depletion is relative to what scientists take to be an
average of the solar system's building blocks. A measure of such an
average can be found in the composition of certain types of meteor-
ites, particularly those termed chondrites, which appear to represent
the stuck-together dust of the primitive nebula. Another measure is in
the composition of the Sun, where the concentration of the elements
(other than hydrogen and helium) can be measured by spectroscopic
analysis of its bright photosphere.
These measures, when compared with estimates of the Earth's bulk
composition, suggest that our planet has relatively less of such ele-
ments as potassium and zinc than the solar system average. This, in
turn, suggests that, while the bulk of the Earth was forming amid the
clash of planetesimals and embryo planets, these elements were still
in a hot gas phase, at temperatures of several hundred degrees Celsius,
and being swept outwards by the solar wind. If the primeval Earth
could have difficulty in holding on to zinc and potassium, it is hard to
see how it could have retained much water—a much more volatile
material—as it formed.
There is some evidence, therefore (and yes, it is controversial), that
the proto-Earth was dry. How, then, did it acquire its oceans, to
become a blue planet?