Was Venus once a watery world?

Space scientists at the University of Cambridge have, they think, answered a long-standing question about our near-neighbour, Venus:  whilst it’s a hothouse today with a surface temperature sufficient to melt lead and sulphuric acid for rain, given its similarity to Earth, was it once a lot wetter, like we are? By looking at the gases that the Venusian volcanoes release, Tereza Constantinou has been able to work out how much water is really there inside the planet. And the picture it paints is not of a once watery world…

Tereza - Earth and Venus are often thought of as sister planets. That's because they're very similar in mass, radius, density and distance from the sun. But now, they've ended up looking really, really different. You've got Earth, where we've got oceans, we've got a nice climate, it's quite comfortable. However, on Venus, the surface conditions are extreme. You've got an atmospheric pressure at 90 bar, so that is 90 times the pressure that you experience when you're standing on earth, and that is the equivalent of being about a kilometre underwater. Imagine the pressure you'd feel in your ears from that, that is extreme. Then you've got the toxic atmosphere made of carbon dioxide and you've got sulphuric acid clouds. So this is vastly different from what it would be like on Earth. However, there's been this theory suggesting that Venus was once very much like Earth: it had oceans, a cool, temperate climate, but that is really unknown. Even scientists are still debating to this day whether that was the case. That's what we were seeking out to answer, trying to see if there's any evidence in what Venus looks like today that speaks to the climate past, that tells us whether Venus ever had oceans.

Chris - We struggled to do something similar for our own planet, that we are on, that's a lot less inhospitable. So how can you get to the bottom of those questions on Venus?

Tereza - It has been very tricky so far. The way it's been done for Venus is using climate modelling. That's very much what people would use when you're checking the weather for here on Earth, but doing that for an early Venus: seeing if it was ever cool and temperate, cold enough essentially to have liquid water at its surface. But we wanted to approach the problem a slightly different way. We wanted a more direct test, something to do with what Venus is like today. That is a very clear clue as to whether the past of Venus ever had oceans.

Chris - It's almost like, you know where we are today, you're going to wind the clock back using some kind of model of how things evolve and change to work out what it would've been like back in the day?

Tereza - That's essentially what has been done so far. But we wanted to do something else. We wanted to see if there was any clue within Venus as it is today that was a signature of this past. Essentially, we wanted to see if Venus in its present day has any clues about it ever having oceans. So, for example, on Mars, just by looking at the surface, you can see where water has shaped surface features. So, much like flowing water through soil or sand will shape channels and valleys, we see that on Mars. However, Venus's surface is really young in geological terms, even though it's a few hundred million years old. But essentially there was a large volcanic event that released lots of lava, covering most of the planet's surface, so if there was any evidence of flowing of water - like valleys - it is now covered, so completely erased. We've had to come up with entirely different ways to figure this out. Different clues to look for.

Chris - And what are they? How do you do it?

Tereza - The way we did it is we looked into volcanism on Venus to find out how much water is inside the planet. Now this is a very key clue to whether Venus ever had any oceans in its past. Studying the composition of the gases being released by volcanoes on Venus, we can see how much water is inside the planet. Now imagine a volcanic eruption here on earth. If you've ever seen any photos of them, you essentially see these large billowing clouds coming out. Most of that is water, and this is directly coming from the inside of the planet where the lava comes from. We wanted to do the same for Venus by studying the position of the gases coming out of Venus's clouds. We wanted to see if there was any water, or rather how much water there is inside the planets and this was a very key signature about the climate's past.

Chris - How are you actually making those measurements though?

Tereza - It would be great if we could send a probe now and study Venus's clouds, and there are plans in the future. There are probes being sent to Venus to do that, but obviously we haven't done that yet. In fact, any probes that we sent in the past died when they reached the surface due to the extreme pressures and temperatures. So we did it slightly differently. Most of the data that we have about Venus or the thing we understand the most about Venus is its atmospheric composition. So we modelled that atmospheric composition. We studied the chemistry of the atmosphere, and we looked at different gases being destroyed in the atmosphere through chemistry, which must be restored by volcanism to maintain atmospheric stability. So we use that to kind of study what kind of gas you would need to be added into the atmosphere to sustain what it is like today.

Chris - So you can basically see the atmosphere from far away. You don't need a probe to do that, and you can work out what's in it and therefore you can work out what must be going into it and what's coming out of it. So therefore you get that dynamic of what the volcanoes must be doing and therefore the amount of water can be inferred indirectly.

Tereza - Exactly. So we've been observing Venus for decades now. We've sent missions there that took some measurements of what the atmospheric composition is, and we've also used telescopes on the ground here on Earth to observe Venus and, again, study the composition of the atmosphere. That's exactly what we've used.

Chris - What does that reveal then? Is it much wetter like the Earth is, or were we wrong?

Tereza - It is really, really dry. There is so little water coming out of the volcanoes on Venus, and this is vastly different to Earth where you've got lots of water. Most of the gases released from the volcanoes are water. But that's not what we found for Venus. In fact, there was very little water, meaning the planet itself has very little water as a whole.

Chris - How do you reconcile that? Because you mentioned when you started you said, 'look, Venus is almost a sister planet to the Earth. There are so many similarities and we know the Earth's got loads of water. It had some to start with, loads of asteroids and comets rained down on it when it was young and they put water here.' Why wasn't the same happening to Venus then?

Tereza - I think Venus ended up being too close to the sun so that it took too long to cool down and solidify into this rocky body that we now know. All the water that the planet might have had only remained as steam in the atmosphere. This gave the water enough time to be dissociated. So this meant incoming radiation from the sun was strong enough to be able to break apart molecules in the atmosphere. Water was broken up into hydrogen and oxygen. Given Venus is closer to the sun, it's got more incoming radiation, and hydrogen is a very light molecule. It escapes your atmosphere at very high rates. Now, if the planet never cooled down fast enough to condense that water and trap it within your system, the water will be exposed to the sun in the atmosphere. It would dissociate and leave the planet, leaving behind a very dry and desiccated planet.