The Gravity Assist Podcast is hosted by NASA's Director of Planetary Science, Jim Green, who each week talks to some of the greatest planetary scientists on the planet, giving a guided tour through the Solar System and beyond in the process. This week, he spoke to astrobiologist and planetary scientist David Grinspoon, of the Planetary Science Institute, about the second planet from the Sun: Venus, a world with surface temperatures hot enough to melt lead. As part of the discussion, they talk about Venus' volcanoes, its clouds of sulfuric acid and its runaway greenhouse effect. Was Venus once like Earth and what clues might it provide about the future of our own planet? They also explore Venus' backward rotation and its 'forever sunsets', plus the remarkable and heroic 'Apollo 11'-like story of the Akatsuki spacecraft.
You can listen to the full podcast here, or read the transcript below.
David, you did some early work on Venus for your PhD. What was that like? And, what did you do?
David Grinspoon: Well, when I was in grad school in the 1980s, there was this new idea about large impact events affecting planets. We had recently discovered this amazing fact that an event 65 million years ago had knocked out the dinosaurs and caused an extinction.
So, people were starting to wonder, "What else have large impacts done in the Solar System?" I had a couple of mentors who suggested that I look at some other atmospheres and I started to be drawn towards the fact that Venus is so similar to Earth, and yet so different, and wondering what large impacts did to Venus.
That led me to work on things like, what happens when comets hit Venus and what does that do to the amount of water in such a dry planet? What was the early climate of Venus like compared to Earth, under the influence of being pummeled by a lot of impacts?
It's basically how I learned to do climate modeling, by considering these weird scenarios of early atmospheres and what might have been happening to them under the influence of violent impacts.
Jim Green: Even though Venus is about the size of the Earth, it's very different in many ways. What do you think is happening on Venus that makes it so different?
David Grinspoon: Well, that's one of the very compelling questions about Venus, because it's so Earth-like in terms of its bulk properties. If you just ask the most basic questions, "What's the size of it? What's the mass of it? Where is it located in the Solar System?", you'd say, "Wow, it's just like Earth in all these ways."
Then you'd start looking at, [and asking] what's the environment like? And you'd go, "Whoa, that's not at all like Earth," because it's so incredibly hot and so incredibly dry. So, it's evolved in a different direction.
When you start talking about Venus today and comparing it to Earth, you're led to these questions of not just what's happening there now, but how has it evolved and how did those two planets head down such different paths? Obviously, for Venus, it's a story of somehow having lost its oceans and lost what we think of as a more pleasant climate a long time ago, and trying to understand that sort of divergence of conditions away from what we believe was more Earth-like a long time ago.
Jim Green: So, how does Venus help us understand what Earth's climate is like?
David Grinspoon: Well, it serves as an interesting laboratory for us to test a lot of our ideas about climate and atmospheric processes on a planet that, again, is somewhat Earth-like in some ways, and somehow very, very different in other ways. But, you know, with climate and the environment, a lot of it has the same physics and chemistry in different situations. Everybody knows about the greenhouse effect on Earth and how carbon dioxide is part of that. Well, picture a planet just like Earth, where the atmosphere is almost all carbon dioxide basically. What would that do to the climate? It's an interesting thought experiment, but it's also a real experiment because we have this planet next door, Venus, which is basically all carbon dioxide.
Many other aspects of Earth are exaggerated on Venus, too. You've heard about acid rain. The clouds on Venus are sulfuric acid, so it's an extreme case of acid rain. That's allowed us to study, again, an Earth environmental issue in an exaggerated form.
This is just something that kind of makes us smarter about the problems and the puzzles that we encounter on Earth, by seeing them in an altered, and sometimes exaggerated form, on a nearby planet.
Jim Green: Venus isn't actually that very far away from us, nor is it very far away from the Sun in the sense that our two planets have evolved, at the moment, very differently. What was Venus like early on in its evolution, do you think?
David Grinspoon: Well, that's a great question and, of course, that's one of the mysteries that compels us in our research and exploration to try to get a clearer picture of the earliest history of Venus. We have a lot of circumstantial evidence that leads us to believe it was a much more Earth-like planet a long time ago.
For instance, there's the fact that Venus is so incredibly dry today. In fact, if you add up the amount of water on Venus and compare it to what we think is the amount of water on Earth — I say it like that because we don't actually know how much water is hidden inside the Earth — but, it's something like 100,000 times as much water on Earth than on Venus, which is really strange, because we picture them probably having formed with roughly the same amount, because they formed nearby out of similar materials. So, we think Venus had oceans and we think it had a more pleasant climate, possibly even for life, early on, and we've got some other circumstantial evidence about that.
So, my best guess is that Venus was much more Earth-like early on. I would say an informed guess, but there's still a lot of mystery there and a lot of experiments [that] we'd like to do and measurements we'd like to make to try to pin down that early history much more clearly.
Jim Green: One of the things that's fascinated me about Venus is that it rotates in the opposite direction, and it rotates very slowly. How has that affected its evolution, do you think?
David Grinspoon: Yeah, it's an interesting fact that almost all the planets rotate in the same direction as Earth does. So, if you're standing on their surfaces, the Sun rises in the east and sets in the west, like we're used to.
If you were on the surface of Venus, assuming you could see the Sun, which would be hard because it's so cloudy there, [then] the Sun would actually rise in the west and set in the east. And it would do so very, very slowly, because the planet rotates incredibly slowly. [Editor's note: a 'solar day' on Venus lasts 116 Earth days.]
So, in fact, if you're on Venus, you could walk fast enough to keep the sunset in the same place. You could walk as fast as the Sun is moving around the planet. I did that calculation once and I was like, "Wow, well that would be kind of neat. You could watch the sunset forever just by walking."
How that fits into Venus' evolution is a fascinating question. We don't fully understand the cause of that. We surmise that it's related, both to the early impact history of Venus, just as Earth's rotation and Earth's moon are related to the early impact history of the Earth and setting the Earth spinning in a certain way.
You know, the planets formed by these big collisions and the final few were probably very violent. So the geometry of those final few collisions, which way they hit, probably really influenced that spin. But, on Venus, there's also the fact that we have this incredibly thick atmosphere, almost 100 times as thick as Earth's, and that can cause a drag on the rotation of the planet through what we call tides, atmospheric and solar tides, which are just these phenomena of the mass of the atmosphere itself actually pulling on the planet's rotation over a long period of time. So, that might have to do with how slowly it's rotating.
We're not sure about its total evolution of the rotation rate over time. As far as rotating in a backwards direction, if you will, we think that probably has to do with large impacts early on in its history, when it was still forming.
Jim Green: That's fascinating. Maybe that means that over time, Venus will end up being tidally locked with the Sun. And then, we're going to have a completely different environment, perhaps, with the extremes in temperature on both sides.
David Grinspoon: Yeah, that's really an intriguing possibility that Venus, moving so slowly, could be on its way to being tidally-locked, and that at some point, probably in the pretty distant future — I don't think we have to start revising our models too quickly — but, it could actually become a locked planet, like we think a lot of exoplanets are, where one side is permanently facing the star.
Jim Green: As you mentioned, Venus has this huge pressure and enormous clouds that are very opaque. But we have been able to penetrate through those. One mission was called Magellan. What was the most important set of observations that came from the Magellan spacecraft?
David Grinspoon: Magellan was an amazing mission. It really revolutionized our understanding of this neighboring planet. Before then, we had a couple of pictures of a couple of spots on the surface from Russian landers, which amazingly were able to operate under those extreme surface conditions briefly in the 1970s and the 1980s. [Editor's note: Magellan orbited Venus between 1990 and 1994.]
We also had a couple of vague images from radar of parts of the planet, but with Magellan, we were able to orbit and basically map almost the entire surface by using radar, which as you say, penetrates through those clouds. It does [a] sort of flash photography almost, bouncing radar off the surface, and then you see that image and you build up what the planet looks like.
That revolutionized our view of Venus in so many ways. One thing we learned was how volcanically interesting Venus is. Its surface is almost completely covered, in one way or another, with volcanic features, these broad, flat plains that we think of as flood basalts, like we have some areas on Earth. The Pacific Northwest comes to mind as one of these big flood basalt areas.
Then [there are] other kinds of volcanoes, these steep shield volcanoes, like Hawaiian style shield volcanoes. So I think of Venus almost as [a] volcano world. It immediately sharpened the question, is it still volcanically active?
And, ever since Magellan, we've been trying to nail that down. Again, we think we have some clues about that, but we don't have what we sometimes refer to as the smoking gun, telling us it's definitely volcanically active. Now that we know from Magellan there's volcanoes all over the place, that's sort of the next question. Are they still going?
Jim Green: Right. I mean, is the carbon dioxide in the atmosphere believed to be because of the volcanoes?
David Grinspoon: Well, over the long term, yes. I mean, that's the main way CO2 gets supplied to planetary atmospheres, but we don't really know if Venus requires an active supply now, because there may be nothing removing CO2 from the atmosphere.
On Earth we have this cycle where CO2 is supplied by volcanoes and, well, now by factories and cars, too, but, historically, by volcanoes. Then it's removed from the atmosphere by what we call weathering reactions, that are facilitated by water running over rocks and pulling CO2 out and making carbonate rocks.
Venus has no surface water, so it may not really have any way of removing CO2 from the atmosphere, but we do see other things in the atmosphere that may require an ongoing volcanic source. We see all these sulfur gases, SO2, sulfuric acid, and it may be that chemical reactions are always removing those [by] reacting with surface minerals. So, in fact, the sulfur gases we see on Venus today may require an ongoing source of volcanic gases.
Jim Green: Venus is an easy planet to see. It's close to the Sun most of the time. During its orbit around the Sun, it can be very bright because it can be close to the Earth. I know many cultures have been thinking about Venus and placing them in their own histories, and so it has a fascinating role from that perspective. Do you have any favorite stories about that?
David Grinspoon: Yeah. I'm fascinated by the fact that, as you say, nearly every culture, probably every culture, has stories about Venus, because it is so bright and also has such a different kind of behavior in the sky [compared to] a lot of other planets, because it's what we call an inferior planet, meaning that it orbits closer to the Sun than Earth. It just appears, for a few months, at sunset and then disappears, and then reappears for a few months at sunrise and then disappears. You'll never see it in the middle of the night in the midnight sky, like you might see Jupiter or Mars or Saturn. So, it sort of has this almost playful kind of behavior, where it kind of flirts with us and then disappears and then reappears on the other side [of the Sun]. A lot of cultures had stories that reflected that.
One area that I love about this sort of archaeoastronomy is the Mesoamerican stories about Venus. The Aztecs and the Toltecs and the Mayans were really astute and impassioned Venus observers. They were very good at predicting the motions of Venus and it was tied into their culture and their origin stories, their beliefs. The Mayans had these really neat stories about what they called Venus, Kukulkan. To the Aztecs, it was Quetzalcoatl, but it's basically the same character. Venus was the brother of the Sun. Together, Venus and the Sun would go into the underworld and do battle with the enemies of mankind. That kind of makes sense because you see Venus, you know, at sunset, sort of trailing the Sun and disappearing. So, they're going down to do battle. [There are] all these stories about Venus and the Sun together, beating back pestilence and disease and war and all these enemies of mankind, and basically making the world safe for humanity.
Jim Green: Well, in modern culture over the last 100 years or so there have been stories, like those by Edgar Rice Burroughs, inn which life on Venus could have arisen. Part of that comes from the increased scientific knowledge about Venus, like what happened in 1761 with the Venus transit. What happened during that transit?
David Grinspoon: I love that story. It's basically the discovery that Venus had an atmosphere, by a very astute observer in Saint Petersburg, Russia, an astronomer named Lomonosov. Venus does these transits in front of the Sun, as you mentioned, in this sort of weird pattern [where there are] pairs of transits eight years apart, separated by about a century. We've just had a two Venus transits in 2004 and 2012, and we won't [see one] again for almost another century. When it does [transit], you can learn a lot because you see the little disk of Venus, of course a little dot, passing across the Sun. Historically, we've learned a lot. It helped us figure out the size of the Solar System through geometry.
[In] this particular discovery that you mentioned, Lomonosov noticed that it wasn't just a circle moving across the Sun, [but] that there was this weird effect when Venus was just touching the Sun on one side and then the other. [It was] what he called 'the black drop': this weird sort of extension of the circle [of Venus' disk] into this strange sort of stretched-out shape, which we now associate with the refraction of [light] in the atmosphere of Venus that you can see in those moments when it's just touching the Sun. He was the first one to deduce that and he said Venus must have an atmosphere, and he was right.
Jim Green: And of course, that feeds into all kinds of stories. This is the first planet beyond Earth that was known to have an atmosphere, and so what would life be like, etc. So, indeed, Venus has been very important in our literature and culture.
David Grinspoon: Yeah, and it's not that long ago that we figured out, in some sense, the way Venus really is. A lot of the science fiction that I grew up reading was written when we still thought that Venus might be an oceanic planet. So, a lot of the science fiction stories that were written in the 1940s and 1950s, classic ones by Isaac Asimov and people like that, they still have this oceanic Venus and, you know, the Earth explorers are in submarines, exploring exotic creatures under the water there.
Jim Green: Today, there's one spacecraft that's orbiting Venus, but it seems like it's been a planet that's been observed by many different space agencies. Do you remember the one that's there now?
David Grinspoon: Yes. We are fortunate that our colleagues in the Japanese space agency (JAXA) have a spacecraft called Akatsuki. It's a word that means dawn in Japanese, which makes sense, given we are talking about Venus being the Morning Star. Akatsuki is actually a spacecraft that has a sort of heroic story, because it was launched in 2010 and made it to Venus just fine, in less than a year. But then it was supposed to burn its main engine for 12 minutes and go into orbit, and sadly, something went wrong with the main engine and it spun off helplessly around the Sun. Amazingly, it span off on a trajectory that was going to bring it back to Venus five years later. So, they had a chance to get it into orbit, but they didn't have a main engine. Then some very clever Japanese engineers figured out if they'd take these little maneuvering thrusters and fire them all in the same direction for 20 minutes, which they were not designed to do, that they might be able to get it into orbit. Of course, they couldn't practice it because they only had enough fuel to do it once. But it worked, and now Akatsuki is in orbit and making all kinds of neat observations of the atmosphere and the clouds and some observations of the surface at these wavelengths where you can peer a little bit through the clouds. It's a wonderful sort of a resurrected mission. [Japan at Venus: Photos from the Akatsuki Spacecraft's Mission]
Jim Green: You know, NASA's currently got some proposals for Venus in a set we call the New Frontiers call for missions. We're analyzing those and perhaps we'll be selecting a Venus mission, which would be pretty spectacular if we were able to do that.
David Grinspoon: That would be terrific. I'm hopeful that one of these years NASA will return to Venus. There's certainly a lot more left to explore there.
Jim Green: What would you do, from an exploration point of view? What would be the top science activities that we ought to be going after?
David Grinspoon: Venus is a challenging place to explore. Part of the reason we haven't been there more is that the surface environment is so severe, you can't see the surface from orbit, like you can from Mars, at least not in visible [light]. There are a few things that would really help us with our understanding of its history. One, is that we've never really gotten good measurements of the rare gases, what we call the noble gases, and their isotopes. Other things we would love to do are orbit again with a really much more sophisticated radar.
Now, we have these radar interferometers, which for instance, on Earth, can do these amazing measurements where you can actually see the motion of the San Andreas Fault, because they're so sensitive. If we had something like that on Venus, we could actually see if it's tectonically active, let alone map the surface in much more detail.
Of course, we would love to really land on Venus. The Soviets did it a long time ago. But, now, again with modern instrumentation and armed with the more sophisticated questions we have now based on what we've learned, we'd love to dig into the dirt and measure the minerals and really look for clues to the history of that surface and the history of climate and some of those big mysteries, [such as] what happened to the water on Venus? How long ago did Venus lose its water, and what has the climate done since then?
Jim Green: Unlike the Earth, Venus doesn't have a moon. Why do you think that is?
David Grinspoon: Our current understanding of why the Earth has its Moon, again, has to do with that early impact history, the sort of last stages of formation of the Earth [where] we think that a Mars-sized object hit Earth and in just the right way that it span a glowing ring of material into orbit around the Earth, which then coalesced to form the Moon. That was actually, if you think about it, a somewhat random and very specific event. If that impact hadn't happened, or it had a slightly different geometry, Earth might not have a Moon, or it might have several moons, like Mars.
So, the fact that Earth has its Moon, we think, comes down to this slightly chaotic and random event. It may just be as simple as Venus' final stages of formation happened a little bit differently from Earth's, in terms of the sizes and geometries of those last few big crashes that formed Venus.
Jim Green: We all get into this business some way or another, and I refer to that as that gravity assisted pulls you in. What's your story on that?
David Grinspoon: Well, I'm not one of these people that had to figure out what I was going to do and had many false starts. I feel like I was always on the sort of path, so I didn't need too many gravity assists. Partly, I was born at the right time. Literally, my earliest memory, my earliest vivid memory, is the Apollo 11 landing on the Moon. Yeah, I was in fourth grade and I was just so captivated. I think you'll find a lot of space scientists of my generation will say the same thing. Apollo was a big event for them.
Also, when I was young, I had some influential people and mentors in my life. Actually, it turns out one of my dad's best friends when I was little was Carl Sagan. They were both Harvard professors. This was before he [Sagan] was famous. He was just this cool guy we knew, who would lead these public observing nights at the Harvard Observatory and let us go run the controls at the planetarium. So, that was certainly an influence.
Then, when I went to college, I had some great mentors. In fact, [during] my first semester at Brown University, I took a class called 'Mars, the Moon and the Earth', with Jim Head, who is a little bit of a pied piper of planetary geology. He gets a lot of young students excited, and I was one of those.
And, then while I was in college I got a summer job, working for Professor Head, analyzing some of the new Viking images of Mars, which were just a few years old [at the time]. One thing led to another and I was just pulled in and captivated [by it] and [I] never lost that excitement.
Source: Nasa Astrobiology Magazine