PDA

View Full Version : Venus and the greenhouse effect



loandbehold
2004-Apr-06, 12:05 PM
I have a question about the greenhouse effect on Venus, and I was wondering if anybody here could help me. I guess like most people, I always thought that the high surface temperatures on Venus were due to the greenhouse effect of carbon dioxide. However, I was reading a page (http://www.eia.doe.gov/cneaf/pubs_html/attf94_v2/chap2.html) which claims:


Scientists have been concerned with the greenhouse effect regarding not only the quality of life on Earth, but also its effect on other planets. Twenty-six million miles from Earth, in an orbit much closer to the Sun, Venus spins through space with a furnace-like surface temperature of more than 800o Fahrenheit (F) (426.5o Celsius [C]), which is much hotter than its proximity to the Sun would explain. Scientists used to believe that Venus fell victim to the greenhouse effect because 96 percent of its atmosphere is carbon dioxide, with nitrogen accounting for almost all the remainder [26]. It is now generally agreed within the planetary atmospheres community that carbon dioxide alone would lead to an average temperature of less than 25oC. The primary reason that Venus is warmer than this is the presence of sulfuric acid cloud cover over the entire planet, extending from about 50 kilometers to 70 kilometers from the surface.

(Bolding mine). So it's claiming that most of the effect is due to sulphuric acid. I'm a bit puzzled since it seems like a pretty reliable site, and I've tried searching other sites and have got conflicting answers. Can anybody here shed any light on this?

Thanks in advance.

Glom
2004-Apr-06, 12:12 PM
I think part of the problem is that the sulphuric acid clouds contain the heat within the planet. Venus is incredibly volcanically active. Heat is released into and contained within the atmosphere. Venus is trapped in a giant pressure cooker.

bobjohnston
2004-Apr-06, 08:27 PM
I'm not aware of anything to support the claim in that EIA article (a claim like that deserved a footnote, and there was none). From what I see, planetary climatologists still blame Venus' temperatures on CO2 greenhouse. The cloud cover cools the planet, it doesn't warm it: Venus' clouds reflect about 70% of the sunlight it receives, so without the clouds Venus would absorb about three times as much solar energy. One simulation of the atmosphere of Venus suggests that its surface temperature would be about 180 degrees C hotter without the cloud cover.

What is true is that Venus' climate is being studied by models that consider gases like sulfur dioxide and the interaction of gases with surface materials, or formation of clouds. These impact the temperature, but not in the way that the EIA article suggests.

It's misleading to think of clouds or atmospheres as "trapping" heat, whether the heat is from outside or inside. We trap heat on Earth (mostly) to prevent cooling by convection, but planets are in space and there is no medium for convection. The only way to remove heat from a planet is radiation. Gases that are transparent at some wavelengths and absorbtive in others, like CO2 and H2O, alter the radiative balance of a planet. Specifically, what they do is force the planet to a higher temperature than otherwise to achieve a balance between incoming and outgoing energy. Clouds generally can't do this, because usually they are more uniformly opaque across a wide range of wavelengths of light.

Brady Yoon
2004-Apr-07, 04:02 PM
I think part of the problem is that the sulphuric acid clouds contain the heat within the planet. Venus is incredibly volcanically active. Heat is released into and contained within the atmosphere. Venus is trapped in a giant pressure cooker.

Are you sure about this? I thought Venus was geologically dead for about 600 million years now. Doesn't it go through cycles every so often that melt its entire crust?

bobjohnston
2004-Apr-07, 06:27 PM
[quote]I thought Venus was geologically dead for about 600 million years now. Doesn't it go through cycles every so often that melt its entire crust?

A good point. We don't have concrete evidence for much current volcanic activity on Venus (although there are some interesting hints). The suggestions of occasional active periods relates to the atmosphere in some models, if I remember right.

loandbehold
2004-Apr-08, 01:53 PM
Thanks for your answers. It seems that I was right to be suspicious.

Maybe I'll send them an e-mail asking about this- at least they might provide a footnote for this assertion.

bobjohnston
2004-Apr-08, 04:44 PM
If they respond to you, it would be interesting to hear what they say!