University of Winnipeg

The Venus Paradox

The Venus Paradox

The planet Venus is hot. Really hot. Its average surface temperature is over 460 °C, compared to Earth’s 14 °C. This might not come as a surprise: Venus is, after all, much closer to the sun than we are.

So, the paradox is not that Venus is so hot, it’s that Venus is also so bright.

Venus is the third brightest object in the sky (next to the sun and the moon). It is so bright, it can cast shadows at night. Its radiance is partly because Venus is so close to Earth. Mars, however, is about the same distance away from us but is much, much dimmer in the night sky. Venus is very bright because its sky is blanketed by thick clouds made of sulphuric acid, which are yellow and highly reflective.  And that’s where things get interesting. Those sulphuric clouds reflect so much sunlight that Venus should be much colder than Earth.

So, what’s really behind Venus’ blast-furnace-like climate? Venus’ super thick atmosphere is 96% carbon dioxide by volume, compared to Earth’s 0.3%, meaning the greenhouse effect on Venus is through the roof. This is the real reason Venus is so hot – not simply because it is closer to the sun, but because of its super-concentration of greenhouse gases.

There are many interesting lessons to learn by analyzing our solar system’s neighbour.

1 the incredibly large effect an atmospheric gas in seemingly small quantities can have on a planet’s climate. On Venus, the concentration of sulfur dioxide (the gas that makes Venus yellow) in the atmosphere is only about 150 parts per million. But that’s still enough to turn Venus into the most reflective planet in the sky, and should have made it an icy wasteland even though it’s 50 million kilometers closer to the sun than we are. For comparison, Earth has more than twice the concentration of carbon dioxide in the atmosphere than there is sulfur dioxide on Venus. Small concentrations of gases can have huge effects.

2 Venus is a dramatic example of what happens to a planet when its atmosphere is made up almost entirely of greenhouse gases. Some astronomers hypothesize that billions of years ago, before Venus’ atmosphere had so much carbon dioxide built up, the surface of Venus could have supported liquid water. Today, however, Venus is a raging furnace, and any leftover water is forever trapped as a vapour in the atmosphere.

The Prairie Climate Centre

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Ryan Smith, Prairie Climate Centre


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