Water: Why Venus Doesn’t have it

In an earlier blog, we looked at why Mars doesn’t have any water. How about our neighbour on the other side – Venus – why doesn’t it have any either? The knee jerk answer (it’s closer to the sun, so it’s hotter) doesn’t hold up on closer inspection. In his book, H2O, Philip Ball points out that Venus’ surface temperature is 500˚C, far, far more than what its proximity to the sun should dictate. So what happened to Venus?

 

Well, the volcanoes of Venus released a huge amount of CO2. This led to the greenhouse effect, which then caused whatever water was on the planet to evaporate. It isn’t as well known, but water vapour too is a greenhouse gas, i.e., it raises the temperature. A vicious loop had been set off: the evaporated water added to the greenhouse effect, leading to even faster evaporation of water:

“With nothing to hold this process in check, the entire oceans boiled dry… This is the greenhouse effect taken to its terrifying conclusion.”

Further, even the evaporated water vapour soon escaped from Venus’ atmosphere. After all, like Mars, its gravity was weaker than Earth’s.

 

Why didn’t early Earth have the same fate as Venus? The Earth was farther away from the sun, thus a bit cooler, which allowed just enough room for negative feedback loops to kick in to stop the greenhouse effect from spiralling out of control. Here’s what happened on Earth.

 

The first steps were same as Venus: the volcanoes spewed out CO2, which led to global warming, which led ocean water to evaporate. But then, since the Earth was farther from the sun (and thus cooler), the clouds could condense and rain (the famous water cycle). The rain in turn reduced the water vapour in the air (and thus the greenhouse effect). Even better, the liquid oceans could dissolve parts of the free CO2, thus reducing the CO2 and their greenhouse effect.

 

So you can see why Ball says, half tongue-in-cheek:

“Once the oceans of Venus evaporated, it lost forever its means of salvation from the inferno.”