In message , Kate Brown
writes
On Fri, 26 Mar 2010, Alex Stephens Jr wrote
"Lawrence Jenkins" wrote in message
. ..
On this planet.
If not what are the most important factors that tilt the balance?
No.
Water, in all it's forms.

Which reminds me of a question I've long wanted to ask but been afraid
it was unanswerable or just daft. Does the quantity of water in and on
and round the planet remain the same, though in varying forms? Or do
some water molecules become irrevocably split, and others form newly
from handy bits of hydrgen and oxygen?

Water molecules can be created (e.g. by burning organic material) and
destroyed (e.g. photosynthesis). This is a cycle, and over the course of
a year the amount of water in the system averages out to roughly
constant.

Water molecules can be split by UV radiation in the upper atmosphere
(photodissociation) and the resulting hydrogen lost to space (because
the velocity of hydrogen atoms at exospheric temperatures is comparable
to the escape velocity. This leads to a reduction in the amount of water
on the earth, but because of the "cold trap" there's not much water in
the upper atmosphere, and consequently this is a slow process. But in
the case of Venus, the planet has lost most of its water in this manner.
--
Stewart Robert Hinsley