On Venus, though, this doesn’t appear to occur. So why is the connection between tectonic and geologic activity to the Venusian atmosphere and environment so important?
“We don’t know, actually,” said Dr. Ghail. “And that is one of the things that we want to try and understand. But what we do know is that when we look at the upper atmosphere of Venus, we see over long periods of time, from Pioneer Venus through Venus Express, we see occasional big spikes in the sulfur dioxide concentration of the upper atmosphere.”
“We don’t know what’s driving that. And one of the possibilities is that they are the products of the volcanic plumes. And when you look at the size of those spikes in sulfur dioxide, the big that was picked up by Pioneer Venus, that’s roughly equivalent to the amount of sulfur dioxide that was released by Pinatubo, if you remember that eruption in the early 1990s. It was a big volcano. And it had a huge impact on our atmosphere, it dropped global temperature by something like a quarter of a degree C for a couple of years” Dr. Ghail said.
“So we only have one bit of that story. We only have what happens in the upper atmosphere. We don’t know where it comes from. We’d like to know if that comes from volcanic plumes, and if so how it gets through the atmosphere, how it gets through the clouds.”
Volcanic plumes, according to Dr. Ghail, are very important to understanding certain environmental processes on Venus. However, how volcanic gasses from eruptions are sustained in the atmosphere is still a mystery that EnVision is hoping to explore.
“So if we can see what’s coming out of the centers of [volcanic] plumes, and so on, then we can see, how good is that process that’s sustaining the clouds? What timescale does it work on? We have a carbon dioxide cycle on the Earth, well, a carbon cycle. We have a sulfur cycle. We have lots of cycles on the Earth. And over millions of years, that balance changes.”
“If those same cycles take place on Venus — as they could well do, there’s no reason to think that they wouldn’t — that could have really profound effects on Venus. So whereas we might change in temperature by five, ten degrees between a hothouse world and an icehouse world, Venus might change by several hundred degrees. If the clouds start to thin and disappear, the temperature could go up by three or four hundred degrees C. So it’s a really big effect, and it has a really important consequence on the history of Venus.”
Once again comparing Earth to Venus, the two planets could have once had similar pasts. Venus could have once been wet like Earth is today, and could still be wet to this day (although not to the extent that Earth is). Venus also could have been extremely active like Earth possibly was in the Cretaceous period.
“We’ve seen that activity on the Earth in the Cretaceous. And earlier in Earth history. So again, we see evidence of things like that on Venus. So, did Venus suffer one or two of these big events and that just tripped it over the balance, but a billion years ago it had oceans and it maybe even had life?”
EnVision won’t be the only spacecraft at Venus trying to solve these complex questions though. NASA’s newly announced VERITAS Discovery class mission is very similar to EnVision in that it also plans to map the surface of Venus and examine various surface features for clues to what happened.
EnVision and VERITAS teams have hoped for years that the two missions would get to fly in sequence with one another due to their complimentary nature, and were taken aback when they were both selected to fly at Venus.
“With a sort of faint hope, forlorn hope almost, we have argued for the last three or four years, that VERITAS and EnVision are very complementary missions and would be great to fly in sequence. They’d be a bit like Mars Global Surveyor and Mars Reconnaissance Orbiter. That’s kind of how you can think of these two missions,” said Dr. Ghail.
So, how will these two missions work together?
“VERITAS is really a sort of global survey mission, focused on geophysics. It’s going to get really good gravity. Actually, better gravity than EnVision would get. It’s going to get global topography at really good resolution, so we’ll be able to put those two things together to really understand the interior and how they planet works as a whole. And it will get much better global imagery than Magellan got, about three times better resolution,” Dr. Ghail said.
“But in a sense, a bit like MGS, it’s black and white. It’s just a base image. Where EnVision comes in, a bit like MRO, is as a targeted mission. We are looking for specific things. We’re focused on volcanism. We’re focused on the tesserae regions, these ancient continental regions of the planet. And we’re going to throw everything at them. As I say, we’ve got a whole suite of different instruments that are designed to help us understand them.”
The component that separates the two missions the most, though, is their radars — and how they’ll be used at Venus.
“But the radar itself, which is probably where people think, oh, one radar’s like any other, and they’re not. The radar is quite different. Our radar does something like polarimetry. Which is, it’s crude to say it’s like a color photo compared to a black and white one, but it’s kind of that thing. If you look at a polarimetric image of the surface, you get a lot more understanding of what’s going on. You can see differences in grain type. Differences in surface texture. So you can pick out lava flows, or channels, or different plains units much more easily than you can in a single sort of black and white radar image.”
“But we’re also getting the raw radar data. That was one of the key things that we’re aiming to do. And the reason for that is one of the key goals of EnVision, which was looking for activity. We planned to do interferometry. VERITAS is going to do twelve 200×200 km² areas of repeat pass interferometry. Just sort of one-off, just to see what it can find. EnVision was really designed to do multiple repeat-pass interferometry, year after year after year for six Venus cycles, about four years of Earth time, to really look for change.”
For EnVision, using an advanced rocket like Arianespace’s upcoming Ariane 6 means getting more accurate orbits that will allow EnVision to observe Venus’ surface more than VERITAS.
“Now, at the moment, because of constraints with the launcher — we have a new launcher in Europe called the Ariane 62, and it’s still in the testing phase — we don’t have on paper the capability of doing RPI, but we fully expect that to come back. And we will get that for about 10 or 20 percent of the surface of Venus,” Dr. Ghail said.
Furthermore, to Dr. Ghail, the two missions’ approaches to observing Venus and spacecraft design are different, but, they work well together.
“And that will really set us apart. But I think even without it, that sort of targeted approach, the focus on particular areas and the search for activity, are things that VERITAS is not doing. And then there’s a range of observations, such as the subsurface sounder, that are not on VERITAS at all. So I think they are quite complementary. They are quite distinct and different philosophies of mission, but designed to work together.”
However, VERITAS won’t be the only other mission headed to Venus.
NASA also announced, alongside VERITAS, that they’ve selected DAVINCI+ to travel to Venus. DAVINCI+ will fly straight into Venus’ atmosphere and collect information on Venus’ atmospheric contents and surface features while performing an atmospheric entry.
“DAVINCI+ I think is an absolutely essential measurement to make if you want to understand the early history of Venus particularly,” Dr. Ghail said. “We can only do so much from orbit. As I said, we sort of sat down and thought, what are all the things that we can do from orbit with a spacecraft?”
“And we’re trying to do most of them. There are things we can’t do. We can’t sample the atmosphere. We can’t make very careful isotopic measurements. So we can’t detect how much deuterium there is in the atmosphere, for example. And those are really important things to get a measurement of. And so DAVINCI+ is going to do those measurements. It’s going to do them very, very precisely.”
“But it’s like a line through the atmosphere, it’s a one-shot, one measurement thing. And to know what it really means, we have to put those measurements into some kind of context. And that is part of what the atmospheric package on EnVision is going to do. We’re not going to be able to do all of the measurements that DAVINCI is going to make, but for example, one of the features that DAVINCI’s really going to focus on is water and the deuterium ratio in water.”
“And we are going to be looking at water at all levels of the atmosphere, on a spatial scale. So we can put that line measurement, that pinpoint, very, very precise measurement, into context. And so it can ground-truth our global observations, and we can give context to DAVINCI+’s observations.”
Overall, the three missions each have their differences but will work very nicely together around Venus.
“In a sense, as I think I explained earlier, I think VERITAS and EnVision are the other way around. VERITAS is going to do the global, kind of big-scale understanding of how Venus works as a planet, and we are going to be doing the targeted ground-truthing in a sense, albeit from orbit measurements that tell us where and when Venus is active and how it’s behaving.”
Then there were 3! Congratulations to @ESA on the announcement of their EnVision mission to Venus! We are excited to collaborate with you on this bold endeavor and to work in concert with @NASASolarSystem’s DAVINCI+ and VERITAS. It's a Venus hat-trick! https://t.co/BZaY31dLTM pic.twitter.com/YQj5IQ75m5
— Thomas Zurbuchen (@Dr_ThomasZ) June 10, 2021
“And so I think these three missions do compliment each other really quite well.”
Overall, EnVision, and other future Venus missions like DAVINCI+ and VERITAS, mark the beginning of a new and advanced era of Venusian exploration.
The exploration of Venus came to a sort of halt after Magellan entered Venus’ atmosphere on October 13, 1994. Following Magellan’s entry, much of the planetary science in our solar system shifted focus to Mars, as more and more missions were being sent to explore the red planet that may have supported past or present life.
“The small bodies in our solar system, up to the size of Mars which is only ten percent of the mass of the Earth, we really understand those planets and we understand the differences between them, because we have put a lot of resources into observing them, making good measurements, really getting a picture for what their histories have been.”
But Venus has always been Earth’s sister planet — and Earth houses life. Could Venus have once housed it? We don’t know, and we haven’t been able to explore the thought more due to the lack of missions to the planet.
“We also, I think, now, are starting to understand just how special our planet is. It has life on it. And as we discover other Earth-sized planets around other stars, of course, we’re asking the question, do they have life? And we’re starting to realize that actually there’s a huge number of things that are really quite unique about our planet in theory. The origin of the moon, from modeling simulations and so on, only about two percent of cases of giant impacts generate a moon. So there could be a thousand Earths with no moon. How different would it be?”
“Well, we have one next door. It’s called Venus.”
“And what we would like to know, and I think what our mission is going to tell, this series of missions really, is going to tell us about Venus, is how another large Earth-sized planet works. When we know how it works, we’ll be able to see what features it has in common with the Earth.”
To Dr. Ghail, we can’t begin to fully understand Venus until we know what the planet is like, and that’s what these missions are setting out to do.
“So maybe plumes are common to all big terrestrial planets. Maybe convection and some kind of plate-like process, but not necessarily plate tectonics as we understand it, is what happens on these big planets. They have mobile lithospheres, if I can put it that way. Maybe they all have big atmospheres, because they can retain all these volatiles. All of these things are things that I think we can only really start to answer the questions to when we understand Venus. And it’s the only other Earth-sized planet that we have access to.”
“So I think that’s one of the big arguments for this suite of spacecraft to be going to Venus in the next decade.”
“I think you’re going to see more and more things coming out about Venus. We’ve had a string of things coming out recently in a science sense as well as the mission sense, and I think that’s great. But I think, I would really, really want to encourage people who perhaps haven’t thought about studying planets. Because if you study Mars, you’re automatically studying astrobiology, or water, or something like that. Or maybe you’re interested in volcanoes, or continents, or tectonics.”
“But there’s this whole other world, especially if you’re an Earth scientist, there’s this whole other world to go and explore. And I think that’s one of the really exciting things that we can now go and do.”
(Lead render of EnVision – via ESA)