Why is Venus so hot?

Venus, the second planet from our Sun is a hellish place. Despite being further away than Mercury, the planet is several hundred degrees hotter. At 735K (that’s 462ºC or 863F in old money) the air is hot enough to melt lead and once you start looking at the planet a bit more closely, the reason becomes clear.

Venus's thick atmosphere

Venus’s thick atmosphere prevents astronomers from seeing the planet’s surface. Credit: NASA

Venus’s has one heck of an atmosphere. The surface pressure is around 92 times that found on Earth and it’s made out of some pretty nasty stuff: 96.5% carbon dioxide, 3.5 nitrogen, laced with sulfur dioxide,  sulphric acid and a few other trace elements.

It’s no great secret that carbon dioxide is a green house gas – it lets light and heat in, but not out again. Venus is an example of what can happen if you let carbon dioxide build up in your atmosphere to ridiculous levels.

But what I always wondered was why does Venus have such a thick atmosphere in the first place? In most respects the planet is much like Earth. It’s slightly smaller, with around 80% of Earth’s mass, meaning its gravity is pretty much the same. It’s at 0.7 AU, one AU being the distance between the Earth and the Sun, so it’s not that much closer and from most observations seems to be pretty similar to the Earth. At first glance its warmer atmosphere and lower gravitational pull should mean atmospheric molecules are more likely to escape. So what the hell happened?

The surface of Venus

Using radar, scientists have managed to get a good idea of what Venus’s surface looks like. Credit: Venus

The answer is all to do with that magical substance that has defined the hunt for life in the Galaxy: water. Venus doesn’t have any, or at least not much. It has trace amounts in the atmosphere, around 20 parts per million, but compared to the 40,000 parts per million of water found in our atmosphere it’s practically nothing.

But it used to. As I said, Venus is very similar to Earth and almost certainly formed in much the same way. The Earth has water, as did Mars in the beginning for that matter. It stands to reason that Venus had its fair share too, though we don’t currently have any evidence as no Venusian lander has lasted longer than two hours on the surface.

At some point though the temperature on the planet reached a tipping point and it began to lose its oceans. The Sun has been warming at a rate of a few percent every billion years. On Earth this hasn’t appeared to have effected us too much, but Venus receives twice the energy from the Sun. The change in temperature was too much, eventually the oceans began to evaporate and as that happened, things began to go oh so terribly wrong.

Water’s of Venus

The evaporating water saturated the atmosphere. In the high atmospheric levels the Sun’s radiation started to break apart the water into hydrogen and oxygen. The light hydrogen floated away into the deep dark depths of space while the heavier oxygen does what oxygen always does, and reacted with absa-bloody-everything, most notably any carbon hanging around to create carbon dioxide.

Not only that but as the water evaporated the planet’s mantel began to dry out. Plate tectonics require liquid water to absorb minerals, act as a lubricant and so on. Without it, everything just seizes up, and that’s exactly what happened on Venus. Volcanic activity is another way that carbon dioxide gets sucked up and stored in rocks, being re-released when volcanoes do their thing.

All of this lead to rising carbon dioxide levels in the atmosphere, which caused the planet to begin heating causing the oceans to evapourate and eventually boil. To make matters even worse, water vapour is itself a green house gas, insulating a planet. Once the water began to be lost, Venus never stood a chance. On Earth the interaction between water, rock and carbon dioxide help to regulate our nice, temperate climate [1]. With this thrown out of kilter, everything went awry. The end result is a planet no one ever wants to go on holiday to.

Venus surface by Venera 13

The Russian Venera programme has sent several probes to Venus, including several landers. The missions were the first to land on another planet, but none lasted longer than two hours. However they did manage to send back several images of the planet surface, including this image taken by Venera 13. Credit: Roscosmos

[1] The Earth is pretty good at regulating its temperature. It does this through the interactions of volcanism, carbon dioxide and water. Volcanic activity is constantly catching and releasing carbon dioxide in the Earth’s crust. If the global temperature drops, water freezes, stopping it from absorbing carbon dioxide, which creates a green house effect, which heats the planet and melts the ice. Global temperatures get too high, the ice caps melt, the atmosphere becomes more humid and all that lovely liquid water sucks up the carbon dioxide and temperatures drop again. It should also be noted that while the planet is doing this, there are colossal civilisation ending, mass extinction educing storms and weather pattern shifts, which is why ‘global warming’ does not mean ‘a bit sunnier in the Summer’.

 

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