Geoscience Reference
In-Depth Information
H. G. Wells was one of the few who imagined things differently. In
his novel
In the Days of the Comet
, humanity was transformed by the
passing of a comet. As it passed, the air's nitrogen was not destroyed
but was changed into a breathable gas that promoted well-being and
goodwill to one's fellow humans. This led to a utopian society in
which there was—a little daringly for the time—free love, to help ease
body and soul. Such delights may be beyond the powers of even extra-
terrestrial geochemistry to achieve, but if comets were a key factor in
bringing water to the Earth, then Wells was right to paint them as the
source of life and love, rather than of doom and despair.
There has been a problem with seeing comets as ocean providers,
though, that relates to the fundamental mechanics of how the solar
system evolved. That comets are now coming into consideration sug-
gests that there is something important about those mechanics that
we have not understood properly.
This problem relates to how the chemistry of the Earth's water tal-
lies with what we know of water elsewhere in the solar system. The
hydrogen and oxygen that make up water are both made up of differ-
ent isotopes—that is, forms of those elements in which the number
of protons is constant but the number of neutrons differs. Hydrogen
mostly comprises a single proton and an electron, but there is also
the hydrogen isotope known as deuterium, in which the proton in the
nucleus is accompanied by a neutron, to give the atom double the mass
(there is also another isotope, tritium, with two neutrons, but this
isotope is radioactive, decaying over time into an isotope of helium
23
).
Most oxygen is made up of
16
O (with eight protons and eight neutrons
in the nucleus), but there are also small but variable amounts of
17
O
(with nine neutrons) and
18
O (with ten neutrons).
24
When water vapour condenses to form ice, the temperature at
which this happens helps govern which isotopes are brought out of
the gas phase. As the temperature drops, the resulting ice tends to
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