Why is Venus Hot?

It seems like an easy question - Venus is closer to the Sun, and gets about twice as much solar radiation as the Earth does. It's not that simple though. Venus is a very shiny planet - it has an high albedo. Most of that solar radiation is just reflected back into space and plays no role in heating Venus up. On an area basis, Venus absorbs less energy from the Sun than Earth does.

The answer lies in the so-called greenhouse effect (I say so-called, because real greenhouses work mainly by trapping the warm air enclosed, rather than radiation). Venus makes it very hard for outgoing infrared radiation to escape. It does this by having a very thick and dense atmosphere, most of which is carbon dioxide (CO2).

It is slightly surprising that this should be so effective, since the CO2 spectrom has some prominent spectral holes through which you might expect a lot of hot thermal radiation to escape - the saturation effect. Two effects cooperate to close the holes. Spectral lines are broadened by the pressure of the hot, dense Venusian atmosphere. In addition, small amounts of water vapor and other compounds fill the spectral gaps.

The greenhouse effect gives Venus a temperature of almost 900 F - hot enough to melt lead. It's even hotter than Mercury, which is twice as close to the Sun.

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