Ocean Origins on Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 19

the orbiting, spinning Earth-Moon system. However, evidence has
been building that it does not explain everything. Computer models
clearly show that if a Mars-sized body impacts on the Earth with the
kind of glancing blow that would have given us our spin, then most of
the splash material that goes on to become the Moon should come
from the impactor . The trouble is that the chemistry of the rocks of the
Earth and Moon are uncannily similar, as regards fundamental fea-
tures such as the ratios of isotopes of oxygen, silicon, tungsten, and
titanium; those of Mars we know to be very different (as regards oxy-
gen isotopes, for example, Mars and Earth rocks differ by a factor of
about 50).
This problem is now exercising the minds of quite a few scientists. 15
Variations on a theme have been suggested. One new model is an
impact on Earth that had already been spinning rapidly because of a
previous impact, and so, torn apart, could fling out a mass of its own
material. Another postulates a clash of equals that are destroyed and
then recombined into the Earth and Moon. Yet another idea is to have,
post-collision, a long-lasting cloud of hot vapour in which the chem-
istry from the two bodies would mix, before the final condensation
into refashioned Earth and brand-new Moon.
By whichever scenario, this was an impact of almost unimaginable
violence. The material that coalesced to form the Moon was initially
incandescent rock vapour and spray, from which most volatile com-
ponents (such as water) would have vapourized and been driven off,
ultimately to get carried away by the solar wind. The Moon is now, for
most practical intents and purposes, bone-dry, both in its lack of sig-
nificant surface water, and in the pristine state of the minerals within
samples of Moon rock. Over billions of years, these minerals have
never been chemically weathered, suggesting an absence of water. 16
Afterwards the Earth, too, was a glowing magma ocean, perhaps
to a depth of 1,000 kilometres or more. It had been refashioned to