Why do We Care About Venus?

To be fair, I suppose most people don't. However, as astronomy types we do. Today I'm going to show you a blatantly obvious but still often over looked fact about Venus. It pertains to the sweltering heat caused by the rather interesting atmosphere of Venus. Venus is indeed hotter than it should be, but not for the reason most people think.

Venus in true color. Source: wikipediaFile:Venus-real.jpg

Temperatures on Venus routinely exceed 400C, which is much hotter than Earth and even hotter than Mercury, even though Mercury is much closer to the sun. The commonly accepted reason for this is the rich carbon dioxide atmosphere, which helps hold in heat due to the greenhouse effect. This is true, but is not the whole story.

Mars and Venus have about the same percentage of carbon dioxide in their atmospheres, about 96% concentration. Mars receives about 25% as much sunlight as Venus does, being farther out. However, it is a rare day that Mars breaks the freezing point even at the equator, making it much colder than sheer solar irradiance would suggest. How then to account for this discrepancy?

Venus is not not only due to a greenhouse effect, but also because of the sheer mass of its atmosphere. The Venusian atmosphere weighs 93 times that of earth, and is thicker (in terms of the total height of the troposphere) to boot. In contrast the atmosphere of Mars is about 1% as dense as Earth's. Thus, while Venus and mars have the same concentration of greenhouse gasses, the total quantity of carbon dioxide in each atmosphere is very different. It is these huge differences in mass that account for the higher temperatures.

If you don't believe me, consider a jar containing 1 liter of air, and another containing 1 liter of water or some other liquid. Obviously, the liquid will have many times the mass of the air. If you heat both to the same temperature and let them cool, obviously the liquid will hold on to heat longer. This is because there are more molecules, which gives them less room to move around, which in turn means they can't dissipate energy as quickly.

If you were to take a jar of air at atmospheric pressure and another that was highly compressed and perform the same experiment, you would notice a similar effect.

Venus is so hot, not only because of the greenhouse effect, but also because of the sheer mass of its atmosphere. Greenhouse gasses exhibit a logarithmic effect, meaning the heating power caused by adding more to the atmosphere is subject to diminishing returns. Thus, another explanation is also required, and in this case it is thermal mass. The atmosphere of Venus is so dense and heavy that it can hold on to heat much better than any of its neighbors.

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