index ... Thoughts on Gas Giant Atmospheres
by Roger Bourke White Jr., copyright May 2006
Venus has a reputation for being hot, really hot! And the reputation is well deserved. Its surface temperature is roughly 750 degrees Kelvin (500C, 900F) which is hot enough to melt lead. That's hot, all right! But that is comparing surface temperatures between Earth and Venus, and there are other ways of comparing temperatures. One of those is comparing the atmospheric temperatures. Comparing atmospheric temperatures is useful if you want to know how much difference there is in the greenhouse effect between Earth and Venus.
Greenhouse effect is the warming of a planet’s surface caused by the difference in the way atmospheres treat light radiation and infrared radiation. Light can pass through greenhouse gases without being affected, infrared radiation gets blocked. It’s like the difference between seeing the sun a sunny day and a cloudy day--for infrared, carbon dioxide always acts like cloudy. So the light part of sunlight can penetrate to the earth’s surface quickly and easily and warm it, but the infrared created by this warmed earth’s surface is much slower at leaving. This causes the greenhouse effect temperature rise.
Greenhouse effect is in the news in the 2010’s because it is part of the climate change controversy. And related, popular science writers tell us that runaway greenhouse effect is the main culprit behind Venus's blazing hot surface temperature compared to Earth's.
But is that really so?
How much more is the greenhouse effect on Venus than on Earth?
To answer these questions, you have to compare atmospheric temperatures.
Venus's atmosphere is hotter than Earth's, but it is also thicker and composed mostly of carbon dioxide rather than nitrogen (the pressure of nitrogen in both atmospheres is close to the same). Being mostly carbon dioxide has little direct effect on the temperature, but being thick has a lot of effect. Venus's atmosphere has 93 times Earth's pressure at the planet's surface, and that high pressure affects the temperature, a lot. That high-pressure atmosphere means that comparing Earth's surface temperature to Venus's surface temperature is an apples-to-oranges comparison when you are talking about how hot the two atmospheres are.
A more apples-to-apples comparison of atmospheres is to compare Venus's air temperature at the 1 Bar level (50KM above the surface) to Earth's temperature at the 1 Bar level (at the surface). Earth's average surface temperature is 288 Kelvin while Venus's 1 Bar temperature is 360 Kelvin. Venus's atmosphere is hotter than Earth's by 72 degrees Celsius (72C/161F). This is quite a bit smaller than the roughly 500 degree difference in surface temperatures, but not even all of this is caused by runaway greenhouse effect. (this information about Venus' 1 Bar temperature comes from the NASA site Tables of Atmospheric Parameters for the Planets with thanks to Lyle Huber)
A still more apples-to-apples comparison is to factor out the temperature rise due to the extra solar radiation Venus gets. Venus’s position closer to the sun allows it to get 1.9 times more radiation than Earth does.
If you were to set Venus's atmospheric temperature high enough for it to lose radiation at 1.9 times Earth's rate, what temperature would you have to take it to? (Let’s continue to use the 1 Bar level for this question.)
To find out, you use the Stefan-Boltzmann law. That law states that the energy radiated by a planet (any "black body", actually) will equal a constant, “sigma", times the temperature forth-powered (E radiated = Sigma * T^4 (Kelvin)). Note that radiation goes up as the fourth power of temperature, so the planet temperature does not have to rise much to double its radiation rate. Earth's average surface temperature is 288K, so Venus's 1 Bar temperature, to radiate off the extra solar energy, would have to rise to 340K.
360K is actual temperature of Venus at 1 Bar. So 20C/68F (360K - 340K) is how much of Venus's excess heat is due to having more greenhouse effect, and all other miscellaneous effects, than Earth does.
Only 20 degrees, the difference between a freezing winter day and a comfortable summer day on Earth. That's a long way from the 500C degrees of difference that comes from comparing surface temperatures.
Let’s summarize those differences.
Earth's surface is at 288K and Venus's surface is at 755K. To get Venus's temperature down to Earth's temperature take out:
• 386 degrees for adiabatic expansion from 93 atmospheres to 1 atmosphere (the cooling that happens as the pressure of a gas drops)
• 52 degrees for Venus's extra solar radiation
• 20 degrees for Greenhouse effect, and all other plus and minus effects (such as thick cloud cover)
It turns out that most of Venus's "extra" surface heat is due to the adiabatic compression caused by the high pressure, the next most is caused by extra solar radiation, and greenhouse effect is a distant third.
First, we Earthlings need not fear that a runaway greenhouse effect is going to stoke Earth's surface to 700+ degrees Kelvin, like Venus's is now. That will only happen if Earth's atmospheric pressure at the surface rises dramatically, and that will happen only if the surface temperature rises high enough for the surface to start baking carbon dioxide out of limestone -- basically, having the surface act like a planet-wide lime kiln.
Second, whatever caused Venus to end up with such a different atmosphere from Earth's must have taken place at much lower temperatures and pressures than Venus is currently experiencing. Once again, whatever caused the difference between Earth and Venus either had to prevent Venus's calcium deposits from steadily absorbing the atmosphere's carbon dioxide and becoming limestone, or had to reverse that process later in the planet's history (the lime kiln effect I just talked about). One difference between Earth and Venus is plate tectonics. Perhaps Earth's plate tectonics steadily cycled calcium up to the surface, and Venus's surface never had a similar effect.
Or, you can look at this difference from the reverse point of view: Earth had something special happen in its early history that allowed a lot of calcium oxide to come in contact with a lot of carbon dioxide and steadily leech it from the atmosphere. Perhaps the early oceans of Earth allowed this to happen, and Venus had no comparable early oceans.
Third, Venus can be looked upon as a "gas giant core". Venus is a gas giant type planet, except that it lacks the thick covering of hydrogen and helium that make up most of the atmosphere of the outer gas giant planets. Perhaps the heat of the inner solar system kept that final step of collecting a huge gas covering from happening, perhaps something else. Had Venus collected a thick blanket of hydrogen and helium, the surface temperature would be even hotter, and that high surface temperature would have allowed the hydrogen to chemically combine with both the nitrogen and the carbon dioxide. The result would have been large quantities of ammonia, methane, and water, which are the constituents of the clouds of the outer gas giants. It is likely that the outer gas giants got their clouds of ammonia, methane, and water by having their hydrogen mix with carbon dioxide and nitrogen at a furnace-hot rocky core surface similar to Venus's.
Venus's atmosphere is hotter than Earth's, but the part of that excess heat that is due to Venus having more greenhouse effect than Earth is only about 20 degrees. The rest of the difference is due to the high pressure of the atmosphere at the surface and the more intense radiation Venus receives from the sun. Runaway greenhouse effect does not explain how Venus came to be Venus and Earth came to be Earth. We need to look elsewhere to explain that difference.
We also need to be aware that Venus's planetary evolution may be the more average case and Earth's evolution is the exception. It may be that in most star systems, the inner planets are either Mars/Mercury/Moon style with vanishingly thin atmospheres, or Venus types with thick atmospheres, and medium thick Earth types come up only when the circumstances are very special.
This essay was inspired by a rather unusual chain of thought. In 2005 I went to see that science disaster movie, Day After Tomorrow, about an Instant Ice Age befalling Earth. In it, the chief scientist claims that at the center of a huge hurricane-like storm, stratospheric air is falling straight to the Earth's surface and producing a deadly insta-freeze. "Ha!" thinks I, “that would never happen because the stratospheric air would warm as it compresses, and actually come to the surface hotter than the air that was currently there!" A year later, I put this line of thought together with Venus and came up with the foundation for this essay. This essay lead to the essay on the “ooze zone” in gas giants, and that essay lead to the “Pressure Point” story in Honeycomb Comet.
Where threads of science thinking can lead to is fascinating.
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