Re: Food for thought - You are what you eat


"Phil Hays" <invalid@xxxxxxxxx> wrote in message
news:pan.2008.02.13.04.38.48.476595@xxxxxxxxxxxx
George wrote:


"Phil Hays" wrote:
George wrote:
How did Venus lose its water?

Short answer, the water (or more correctly the hydrogen) was lost to
space.

Earth and early Venus, the silicate + CO2 => carbonate reaction remove
most of the CO2 from the atmosphere. This reaction requires liquid
water. As the solar input increased, CO2 level drops to compensate.
Once
the solar input is large enough, water vapor forms a moist greenhouse
(surface
temperature ~100C, atmosphere has ~1 bar of water vapor. From such an
atmosphere, hydrogen easily escapes to space. After 10's of millions of
years, water is gone, CO2 can no longer be removed from the atmosphere.
After 100,s of millions of years, most of the carbon in the crust has
moved to the atmosphere. Two billion years from now, Earth will be just
like Venus, with a ~100 bar CO2 atmosphere.

Still longer answer, see Science special issue on Evolution of
Atmospheres.

So what initiated this process? Why did it not happen here on Earth
back then?

Sun is a fusing ball of hydrogen. As the hydrogen is converted to helium,
the Sun contracts and gets hotter to maintain balance between gravity and
internal pressure. As the output of the Sun increases with time, the
region where liquid water can exist on a Earth or Venus sized planet
moves
away from the Sun.


--
Phil Hays

Yet as far as we know, Venus, the Earth, and Mars have always been within
the Sun's habitable zone, and yet Venus is completely uninhabitable, and
Mars is very maginally so (if at all). I don't think variations in the
sun's output can explain what we see at Venus. Any other ideas?

George


.

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