Why is Venus's atmosphere so hot?

B. Elliott

Someone not believing in the greenhouse effect would be akin to someone not believing in magnetism. The greenhouse effect is simply the name given to what happens when a planets atmosphere absorbs and holds energy... which every atmosphere does, to an extent. The differences lie in how much heat is withheld.

From the Venus Wikipedia link...

bassplayer142

Thank you very much. I scanned the article but I guess I missed that.

sanman

Anyway, has anybody seen these pictures from the surface of Venus?

http://wanderingspace.net/2006/11/th...enus-revealed/

Pretty cool, if I may say. This is the first time I've seen them.
They were taken from one of the Soviet-launched Venera missions to Mars during the 70s.

LowlyPion

You might better push a bowling ball from a mile away with a single strand of wet angel hair pasta as sharpshooting a stream of nanoparticles with a laser beam across the vast expanse of solar winds with the intent to inject them in the Venusian stratosphere.

As a side note just how long might you think it would take such particles to make this fantasy trip you envision anyway? Surely you weren't thinking the speed of light as their cruising speed.

B. Elliott

Your welcome. The basic concept is very simple, but naturally as you start to investigate individual contributing factors, it get's complicated.

Since I didn't, Here's a good general site that touches on a few of the specifics...

http://www.ucar.edu/learn/1_3_1.htm

chazzone

The "greenhouse effect" is a misnomer. A greenhouse works because there is an actual physical barrier to trap warm air. This does not occur in an atmosphere. While water vapor can hold on to some heat for a very short time, it is quickly radiated away from the planet. In fact recent data has shown that heat is escaping Earth at a much faster rate than had been imagined/calculated. CO2, especially in the incredibly small amounts that are contained in Earth's atmosphere, has virtually no impact on our planet's ability to slow heat loss.

Drakkith

You're going to need to provide a reliable reference, as that goes against everything I've ever heard of in regards to the greenhouse effect for Earth and Venus.

chazzone

I suppose rational thought and basic thermodynamics is too much to ask?

Science is not based on consensus, and no matter how many people get it wrong, it will never be right. There are plenty of people out there who have apparently never really considered what is actually happening here, but if there is something specific that you don't understand, I'll be happy to assist.

Jamie Kern

When light is absorbed by a particle, it may be re-emitted in any direction. That is the essence of the greenhouse effect: a balance between how much light gets from space to the surface, is then absorbed by the surface and re-emitted as infrared light, and then absorbed and re-emitted by particles in the atmosphere. Individual molecules of CO2 and H2O do not hold on to the heat long at all. But they re-emit a percentage of it back down toward the planet instead of out to space, which results in a higher average temperature than there would be without any greenhouse gas. This is a relatively simple concept. This is why temperatures on Earth do not drop to 100 K at night.

The more particles present in the atmosphere that can absorb and re-emit infrared light, the more times an infrared photon will be absorbed and re-emitted before escaping to space. It just slows the cooling process down.

Conceptually, this is similar to the "random walk" of photons in the radiation zone of the Sun, which is supposed to cause light to take hundreds of thousands of years to escape the Sun's innards. If that layer were less dense, or contained fewer particles capable of absorbing those photons (somehow!), the process would not take as long, and the Sun's interior would release energy faster - therefore cooling it faster as well. (And yes, I realize this would effect energy production as well - just trying to draw a parallel with the concept, not start a discussion on fusion rates vs. inward pressure in the Sun).

6.28318531

I think maybe you are confused ;

quoting wikipedia
If we didn't have the "greenhouse effect" It would be nice and cold,

chazzone

The problem is that, as usual, wikipedia isn't really accurate. There is zero direct heating of the Earth from the Sun.

That's ZERO, nadda, zilch.

100% of the heat on the Earth comes from shortwave radiation, also known as visible and UV light, being absorbed by matter on the surface.

The atmospheric gases ad virtually no heating as they allow light to pass through unimpeded.

Water vapor pretty much holds on to the IR/longwave radiation/heat for a very short time, and the photons that release it are bounced back into space in nanoseconds. In fact, a photon leaving Earth is more than 3/4's of its way to the Moon in just one second.

But hey, don't take my word for it. You can easily check out the temperature of space just outside our atmosphere. It's a balmy 14 dregrees Kelvin. Not much heat out there, huh?

Too many people sleeping through science class, and not challenging their profs when they say something ridiculous.

Jamie Kern

Or not listening to their professors when they have already explained the part you think is missing.

The Sun's light, much of which does indeed penetrate the Earth's atmosphere without being absorbed, strikes the Earth's surface itself. Some of it is absorbed and re-emitted as infrared light, which we feel as heat. It then travels from the surface of the Earth upward, encountering various greenhouse particles as it goes.

See my previous post, a few above this, for the more complete explanation. I would feel silly quoting my own post for you.

Drakkith

I'm sorry this is incorrect. In reality much of the entire solar spectrum makes it to Earth's surface. There are bands where nearly 100% of the incoming radiation is absorbed, but much of it makes it through. See below:



From wikipedia on infrared: The Earth's surface and the clouds absorb visible and invisible radiation from the sun and re-emit much of the energy as infrared back to the atmosphere. Certain substances in the atmosphere, chiefly cloud droplets and water vapor, but also carbon dioxide, methane, nitrous oxide, sulfur hexafluoride, and chlorofluorocarbons,[31] absorb this infrared, and re-radiate it in all directions including back to Earth. Thus the greenhouse effect keeps the atmosphere and surface much warmer than if the infrared absorbers were absent from the atmosphere.

As it states, much of the energy absorbed by the Earth and it's atmosphere, whether it is from long or short wavelengths, is emitted back out as infrared radiation. So pretty much all of that big chunk of radiation in the 350-900 nm range that isn't reflected back out into space is absorbed and emitted as infrared. Since certain molecules absorb infrared they act just like a blanket does and the result is an increase in the temperature of the Earth.



As you can see on this graph, the amount of infrared that is actually released into space is only about 15-30% depending on the season. In the "Total Absorption and Scattering" part, the gray areas are what is NOT transmitted.

The curved lines in the top picture are the total blackbody radiation for the appropriate temperatures that are labeled for both the Sun and the Earth. They are approximately the spectrum that each body emits. The red is the solar spectrum reaching us, while the blue is Earth's outgoing thermal spectrum.

Except for all that radiation that gets released and almost immediately gets absorbed by those same greenhouse gases.

Drakkith

Let's take this to an extreme and assume that ALL radiation below the visible part of the spectrum is unable to penetrate the atmosphere. So that radiation is now absorbed by the atmosphere, heating it up just like normal while the visible light makes it through. When the atmosphere and surface emit radiation back out it just gets reabsorbed again! (One key thing is to remember that gases can transmit heat directly by contact, so they don't need to release radiation to heat up their surroundings) Only near the very top of the atmosphere where the air is very very thin can any infrared radiation make it back out, resulting in about 95% of the outgoing radiation simply being absorbed again.

The result of all this is that the atmosphere and the surface just keep heating up until the temperature is high enough to make its black body spectrum emit enough energy in the visible range (since that is the only part that can make it out) to equal the incoming energy from the Sun. That's about...2,000 k?

chazzone

I read your post, and I completely understand the process.

chazzone

I'm not sure of why you are using an example that doesn't exist in reality.

100% of all IR is eventually re-radiated back into space, and most of it within a very short time frame.

The IR from one molecule does not just bounce around and heat up another, since it is always re-emitted at a lower energy/longer wavelenght, and gasses only resonate in a very narrow frequency band.
This is why the idea of CO2 as a major greenhouse gas on Earth, is so ridiculous. Water already covers the same wavelength as CO2, and is a thousand time more prevalent in Earth's atmosphere. So, it relegates CO2 to an irrellevant status. It's like heating a BB to red hot, and throwing it into an olympic sized swimming pool. The effect is so negligible as to be nonexistant.

I'm willing to accept that it has a greater role to play on Venus, but Venus' albedo is .9 (where earth's is between .3 and .36. There simply isn't enough light getting to Venus' surface to create the kind of heat observed.

Drakkith

The same reason we simplify things in basic physics to perfect machines and frictionless surfaces, to help people understand the basics.

Eventually sure, not within a very short time frame though.

You are forgetting that gases bounce around and transfer energy and are able to emit thermal radiation in a broad band because of this.

You must be looking at a different chart than I am. CO2 absorbs wavelengths from 15-20 μm at nearly 100%. Water vapor fall off in the 15-18 μm range. In addition it absorbs wavelengths of 4-5 μm where water vapor absorbs almost none of that.

The greenhouse effect on Venus is substantially stronger than here on Earth and Venus receives double the irradiance that Earth does. That's how it is able to accumulate so much heat even though it's albedo is so high. Note that Venus relies strongly on the sulfur dioxide, water vapor, and sulfuric acid in it's clouds for it's greenhouse effect. (Ref: http://www.imcce.fr/vt2004/en/fiches/fiche_n13_eng.html)