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    Re: Venus
    From: Frank Reed CT
    Date: 2004 May 13, 01:33 EDT
    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
       
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