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From: Wolfgang Köberer
Date: 2004 May 13, 18:21 +0200
Frank R wrote
“Water is a potent greenhouse gas (much more
important than carbon dioxide)”
Thanks for the hint, Frank. So I`ll revv up my SUV, because
– as you point out: “the runaway greenhouse effect from the
evaporation of the Earth's oceans will begin in about 500 million years”.
Good to know and great for my conscience.
Von:
Gesendet: Donnerstag, 13. Mai 2004
07:34
An:
NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Betreff: Re: Venus
Robert E wrote:
"While I agree with the greenhouse gas theory of Venus, I indicated
to my friend that the idea of Venus supporting life is preposterous; for even
without the thick atmosphere its proximity to the sun guarantees daytime
temperature that would be far too hot for humans."
It is considered very likely that Venus once had oceans and those oceans may
well have supported a diverse biology. But it's all gone now... About a billion
years ago (give or take a few years), as the Sun's luminosity slowly increased,
the oceans of Venus began to evaporate at an accelerating rate. Water is a
potent greenhouse gas (much more important than carbon dioxide) so increasing
water vapor induces positive feedback warming the planet even more and
evaporating still more water in a geologically short period of time. Soon the
oceans were gone. The water itself is believed to have photo-dissociated with
most of the hydrogen escaping into space. The loss of the oceans would have
shut down the plate tectonics engine on Venus (the plates require hydrated
minerals to keep moving). Without the ability to release heat through the
rolling convection of plate tectonics, the planet experienced a meltdown. The
entire crust of Venus appears to have been resurfaced about 500 million years
ago --there are no old craters on Venus. In the process, carbonate minerals
were baked out releasing vast quantities of carbon dioxide.
Lucky that's not us, huh? Well, guess what... The Sun's luminosity is still
increasing. As stars age, they accumulate helium "ash" in their cores
which reduces the efficiency of their fusion reactions and requires higher
temperatures, and thus higher luminosity, to maintain equilibrium. The fate of
Venus is a prelude to the Earth's likely fate. And it's not all that far away.
By many estimates, the runaway greenhouse effect from the evaporation of the
Earth's oceans will begin in about 500 million years. And by 500 million years
after that the surface of the Earth, like the surface of Venus, will be
completely destroyed and repaved by the meltdown of the entire surface.
So... don't get into any long-term property leases.
Disclaimer: needless to say, the above is the best hypothesis of current
planetary science. It's well-supported by evidence but we don't have much
videotape from Venus a billion years ago. The whole story might be way off.
For more fuel for your debate with your friend I suggest you hit google and
search on "life on venus" (include the quotation marks). The topic
was in the news just a couple of weeks ago.
By the way, you calculate the equilibrium temperature of a planet by equating
incident energy from sunlight to radiant energy via Stefan-Boltzmann. For the
Earth, you get something like 260 kelvin and for Venus it's around 310 kelvin.
Note that the radiative equilibrium temperature scales as 1/sqrt(r) not 1/r^2
which is what you might think at first (r being distance from the Sun).
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois